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DART’s Ejecta and Planetary Defense

I’m glad to see the widespread coverage of the DART mission results, both in terms of demonstrating to the public what is possible in terms of asteroid threat mitigation, and also of calming overblown fears that we have too little knowledge of where these objects are located. DART (Double Asteroid Redirection Test) was a surprisingly demonstrative success, shortening the orbit of the satellite asteroid Dimorphos by an unexpectedly large value of 33 minutes. The recoil effect from the ejection of asteroid material, perhaps as high as 0.5% of its total mass, accounts for the result.

Watching the ejecta evolve has been fascinating in its own right, as the interactions between the two elements of the binary asteroid come into play along with solar radiation pressure. Asteroids have previously been observed that displayed a sustained tail, as Dimorphos did after impact, and the DART results suggest that the hypothesis of similar impacts on these objects is correct. Thus we learn valuable lessons about how asteroids behave when impacted either by technologies or by natural objects. We can expect the study of ‘active asteroids’ to get a boost from the success of this mission.

The two images below are from the Hubble instrument, which observed the development of Dimorphos’ tail. Jian-Yang Li (Planetary Science Institute) is lead author of a recent paper in Nature on the evolution of the ejecta. Li comments on the interplay between the gravity of Dimorphos and parent asteroid Didymos as well as the pressure of sunlight in the first two and a half weeks after the impact. Bear in mind that an impact on a single as opposed to a binary asteroid would not display such complex effects. The presence of Didymos was indeed useful:

“A simple way to visualize the evolution of the ejecta is to imagine a cone-shaped ejecta curtain coming out from Dimorphos, which is orbiting Didymos. After about a day, the base of the cone is slowly distorted by the gravity of Didymos first, forming a curved or twisted funnel in two to three days. In the meantime, the pressure from sunlight constantly pushes the dust in the ejecta towards the opposite direction of the Sun, and slowly modifies and finally destroys the cone shape. This effect becomes apparent after about three days. Because small particles are pushed faster than large particles, the ejecta was stretched towards the anti-solar direction, forming streaks in the ejecta.”

Image: Ejecta from Dimorphos 4.7 days (above) and 8.8 days (below) after impact, taken on October 1 and October 5, 2022, respectively. The Sun is at the 8 o’clock direction. The ejecta is pushed by the sunlight towards the 2 o’clock direction and increasingly stretched to form streaks. Credit: NASA, ESA, STScI, Jian-Yang Li (PSI), Image Processing: Joseph DePasquale.

So we’ve learned that slamming a 570 kilogram spacecraft into this type of asteroid at something over 22,000 kilometers per hour can alter its orbital speed. Data from the Light Italian CubeSat for Imaging of Asteroids (LICIACube) is part of the current analysis, while we’ll learn yet more about the effects of the impact from the European Space Agency’s Hera mission, which will survey both Didymos and Diomorphos, focusing on the crater left by DART and the changes to the mass of the impacted asteroid.

The matter of locating those hazardous asteroids that have yet to be identified is now highlighted by what we can consider the next planetary defense mission, the Near-Earth Object Surveyor, planned for a 2028 launch. The mission will carry a 50 centimeter diameter telescope operating at two infrared wavelengths, conducting a multi-year survey in search of near-Earth objects larger than 140 meters. The goal is to find 90 percent of such objects coming within 48 million kilometers of Earth’s orbit. The observation strategy employed should allow accurate enough determination of asteroid orbits to allow them to be found again and their trajectories tracked.

Image: Near-Earth asteroids and the possibilities of impact. Credit: NASA.

The paper on DART, one of five papers recently published in Nature on the mission, is Jian-Yang Li et al., “Ejecta from the DART-produced active asteroid Dimorphos,” Nature 01 March 2023 (abstract).


Re-thinking the Early Universe?

I hadn’t intended to return so quickly to the issue of high-redshift galaxies, but SPT0418-47 jibes nicely with last week’s piece on 13.5 billion year old galaxies as studied by Penn State’s Joel Leja and colleagues. In that case, the issue was the apparent maturity of these objects at such an early age in the universe.

Today’s work, reported in a paper in The Astrophysical Journal Letters, comes from a team led by Bo Peng at Cornell University. It too uses JWST data, in this case targeting a previously unseen galaxy the instrument picked out of the foreground light of galaxy SPT0418-47. In both cases, we’re seeing data that challenge conventional understanding of conditions in this remote era. This is evidence, but of what? Are we wrong about the basics of galaxy formation? Do we need to recalibrate the models we use to understand astrophysics at high-redshift?

SPT0418-47 is the galaxy JWST was being used to study, an intriguing subject in its own right. This is an infant galaxy still forming stars in the early universe, observable through the bending of its light by a foreground galaxy to form an Einstein ring. In other words, we’re seeing gravitational lensing at work here, magnifying the young galaxy’s light, out of which information can be extracted about the primordial object. And within that light, astronomers have now found a second galaxy which manifested itself in two places in the ring.

Image: This is Figure 1 from the paper. Caption: Figure 1. Left: Hα pseudo-narrowband image of the SPT0418 system, averaged over the channels including the Hα emission in the original spectral cube. The strongly lensed ring and the two newly discovered sources (SE-1 and SE-2) are highlighted by a red annulus and gray and black ellipses, marked as “A,” “B,” and “C,” respectively. The lensing galaxy is shown as the central bright source. The 835 μm continuum is plotted as the thin black contours, with the levels 2, 4, 8, 16, 32 × σ where σ = 56.7 μJy beam −1. Right: the spectra of the three sources integrated over the regions highlighted in the left panel, using the same color scheme. The spectrum for the ring is scaled by a factor of 0.1 for clarity. The small black bar below the Hα line marks the wavelength coverage of the pseudo-narrowband image. The potentially detected lines are marked by vertical dotted lines. Credit: The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acb59c.

ALMA (the Atacama Large Millimeter/submillimeter Array) data could do no more than hint at the background galaxy’s existence, but working with spectral data from JWST’s NIRSpec instrument, Peng discovered the new light source within the Einstein ring. The unexpected find was a galaxy being gravitationally lensed by the same foreground galaxy that had made SPT0418-47 available for study, though considerably fainter.

What stands out here is the analysis of the chemical composition of the new galaxy’s light, which shows strong emission lines from hydrogen, nitrogen and sulfur atoms whose redshift showed the object to be about 10 percent of the age of the universe. The new galaxy, dubbed SPT0418-SE, appears to be close enough to SPT0418-47 that the two galaxies will interact with each other, making the duo a case study for galactic mergers. All of which is helpful, but here again we run into a fascinating problem. The newly discovered galaxy shows levels of metallicity comparable to our Sun.

It’s a conundrum. The Sun drew on earlier stellar generations to build up elements heavier than helium and hydrogen, and the Sun is roughly 4.6 billion years old. Amit Vishwas (Cornell Center for Astrophysics and Planetary Sciences) is second author on the paper:

“We are seeing the leftovers of at least a couple of generations of stars having lived and died within the first billion years of the universe’s existence, which is not what we typically see. We speculate that the process of forming stars in these galaxies must have been very efficient and started very early in the universe, particularly to explain the measured abundance of nitrogen relative to oxygen, as this ratio is a reliable measure of how many generations of stars have lived and died.”

But let’s turn back a minute, for we’re looking at two early galaxies, and it’s intriguing that SPT0418-47, the first of these, shows its own anomalies. Data from ALMA allow astronomers to see that although 12 billion years old, this object has a more mature structure than would be expected. No spiral arms are apparent, but a rotating disk and bulge are found, with stars packed tightly around the galactic center. Simona Vegetti (Max Planck Institute for Astrophysics), co-author on the 2020 paper on SPT0418-47 (citation below), had this to say three years ago:

“What we found was quite puzzling; despite forming stars at a high rate, and therefore being the site of highly energetic processes, SPT0418-47 is the most well-ordered galaxy disc ever observed in the early Universe. This result is quite unexpected and has important implications for how we think galaxies evolve.”

Image: Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, have revealed an extremely distant and therefore very young galaxy that looks surprisingly like our Milky Way. The galaxy is so far away its light has taken more than 12 billion years to reach us: we see it as it was when the Universe was just 1.4 billion years old. It is also surprisingly unchaotic, contradicting theories that all galaxies in the early Universe were turbulent and unstable. This unexpected discovery challenges our understanding of how galaxies form, giving new insights into the past of our Universe. Credit: Rizzo et al./European Southern Observatory.

So the new galaxy, SPT0418-SE, adds to earlier evidence that the early universe was considerably less chaotic than we once thought. The new paper summarizes the issue with reference to the unexpectedly strong emission lines found in the data:

This spectroscopic study of a z > 4 galaxy opens up many questions, including the spatial arrangement and stellar/gas/metallicity distribution of the companion; the merging hypothesis of SPT0418-47; the dark-matter halo of the system; the overdensity of this potentially crowded field; reconciling the relatively high chemical abundances with the short formation time and the moderate stellar mass for the whole system; and interpreting the small [N ii] 122 and 205 μm luminosities in the context of either a soft radiation field and/or a high N/O.

But again that note of high-redshift caution that I mentioned last week:

We attempt to reconcile the high metallicity in this system by invoking early onset of star formation with continuous high star-forming efficiency or by suggesting that optical strong line diagnostics need revision at high redshift. We suggest that SPT0418-47 resides in a massive dark-matter halo with yet-to-be-discovered neighbors.

Clearly scientists will be looking hard at how high-redshift targets are interpreted even as they continue to hypothesize about astrophysical mechanisms and star formation efficiency to explain seemingly mature objects at this early era. The game is afoot, as Sherlock Holmes used to say, and we’re a long way from reaching firm conclusions. The data are going to start coming fast and furious as we keep mining JWST and using ALMA to examine the universe in this early stage, as witness the image below, which I found just this morning. It shows us another remarkable object.

Image: The radio telescope array ALMA has pin-pointed the exact cosmic age of a distant JWST-identified galaxy, GHZ2/GLASS-z12, at 367 million years after the Big Bang. ALMA’s deep spectroscopic observations revealed a spectral emission line associated with ionized Oxygen near the galaxy, which has been shifted in its observed frequency due to the expansion of the Universe since the line was emitted. This observation confirms that the JWST is able to look out to record distances, and heralds a leap in our ability to understand the formation of the earliest galaxies in the Universe. Credit: NASA / ESA / CSA / T. Treu, UCLA / NAOJ / T. Bakx, Nagoya U.

The paper is Bo Peng et al., “Discovery of a Dusty, Chemically Mature Companion to a z ∼ 4 Starburst Galaxy in JWST ERS Data,” The Astrophysical Journal Letters 944 No. 2 L36 (17 February 2023). Full text. The paper on SPT0418-47 is Rizzo et al., “A dynamically cold disk galaxy in the early Universe,” Nature 584 (12 August 2020), pp. 201–204. Abstract. The GHZ2/GLASS-z12 paper is Bakx et al., “Deep ALMA redshift search of a z 12 GLASS-JWST galaxy candidate,” Monthly Notices of the Royal Astronomical Society Volume 519, Issue (4 March 2023), pp. 5076–5085 (abstract).


High Redshift Caution

When something turns up in astronomical data that contradicts long accepted theory, the way forward is to proceed with caution, keep taking data and try to resolve the tension with older models. That would of course include considering the possibilities of error somewhere in the observations. All that is obvious enough, but a new paper on JWST data on high-redshift galaxies is striking in its implications. Researchers examining this primordial era have found six galaxies, from no more than 500 to 700 million years after the Big Bang, that give the appearance of being massive.

We’re looking at light from objects 13.5 billion years old that should be anything but mature, if compact, galaxies. That’s a surprise, and it’s fascinating to see the scrutiny to which these findings have been exposed. The editors of Nature have helpfully made available a peer review file containing back and forth comments between the authors and reviewers that give a jeweler’s eye look at how intricate the taking of high-redshift measurements can be. Reading this material offers an inside look at how the scientific community tests and refines its results enroute to what may need to be a modification of previous models.

It’s the availability of that peer review file that, as much as the findings themselves, occasions this post, as it offers laymen like myself a chance to see the scientific publication process at work. That cannot be anything but salutary in an era when complicated ideas are routinely pared into often misleading news headlines.

Image: Images of six candidate massive galaxies, seen 500-700 million years after the Big Bang. One of the sources (bottom left) could contain as many stars as our present-day Milky Way, according to researchers, but it is 30 times more compact. Credit: NASA, ESA, CSA, I. Labbe (Swinburne University of Technology). Image processing: G. Brammer (Niels Bohr Institute’s Cosmic Dawn Center at the University of Copenhagen). All Rights Reserved.

One note of caution emerges in the abstract to this work: “If verified with spectroscopy, the stellar mass density in massive galaxies would be much higher than anticipated from previous studies based on rest-frame ultraviolet-selected samples.”

That’s a pointer to what seems to be needed next. Penn State’s Joel Leja modeled the light from these objects, and I like the openness to alternative explanations that he injects here:

“This is our first glimpse back this far, so it’s important that we keep an open mind about what we are seeing. While the data indicates they are likely galaxies, I think there is a real possibility that a few of these objects turn out to be obscured supermassive black holes. Regardless, the amount of mass we discovered means that the known mass in stars at this period of our universe is up to 100 times greater than we had previously thought. Even if we cut the sample in half, this is still an astounding change.”

So the question of mass looms just as large as the formation process even if these do not turn out to be galaxies. No wonder Leja says the research team has been calling the six objects ‘universe breakers.’ On the one hand, the question of mass gets into fundamental issues of cosmology and the models that have long served astronomers. If galaxies actually form at this level at such an early time in the universe, then the mechanisms of galaxy formation demand renewed scrutiny. Leja is suggesting that a spectrum be produced for each of the new objects that can confirm the accuracy of our distance measurements, and also demonstrate what these ‘galaxies’ are made up of.

The paper on this remarkable finding itself continues to evolve. It’s Labbé et al., “A population of red candidate massive galaxies ~600 Myr after the Big Bang.” Nature 22 February 2023 (abstract). Note this editorial comment from the abstract page: “We are providing an unedited version of this manuscript to give early access to its findings. Before final publication, the manuscript will undergo further editing.” I’d like to read that final edit before commenting any further.


How Common Are Planets Around Red Dwarf Stars?

We’re beginning to learn how common planets are around stars of various types, but M-dwarfs get special attention given their role in future astrobiological studies. As I’ve just been talking about CARMENES, the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs program, I’ll fold in today’s news about their release of 20,000 observations covering more than 300 stars, for we can mine some data here about planet occurrence rates.

59 new planets turn up in the spectroscopic data gathered at the Calar Alto Observatory in Span, with about 12 thought to be in the habitable zone of their star. I’ll await with interest our friend Andrew LePage’s assessment. His habitable zone examinations serve as a highly useful reality check.

I mentioned spectrographic data above. The CARMENES instruments are built for optical as well as near-infrared studies, and have been used to explore nearby red dwarfs and their possible planets since 2015. The original project ended at the end of 2020, covering the data released here, although later observations are continuing, so we can expect further releases. Remember, this is radial velocity rather than transit work. CARMENES offers an accuracy of about 1 meter per second, meaning astronomers can measure the tiny motions induced by an orbiting planet with that degree of sensitivity. At this level, low-mass stars yield up Earth-sized planets.

The new CARMENES planets appear in the graphic below, drawn from publication of the project’s first large dataset as published in Astronomy & Astrophysics. 19,633 spectra for a sample of 362 targets were collected. The current paper covers only the visible-light data, with the infrared data to be made public when processing is complete. CARMENES has now observed about half of all nearby red dwarfs, limited of course by the fact that its location in Spain precludes southern hemisphere observations. A bit more on the original 362 target stars:

The sample was designed to be as complete as possible by including M dwarfs observable from the Calar Alto Observatory with no selection criteria other than brightness limits and visual binarity restrictions. To best exploit the capabilities of the instrument, variable brightness cuts were applied as a function of spectral type to increase the presence of late-type targets. This effectively leads to a sample that does not deviate significantly from a volume-limited one for each spectral type. The global completeness of the sample is 15% of all known M dwarfs out to a distance of 20 pc and 48% at 10 pc.

Image: An animated illustration of the CARMENES planets. All planets discovered with the same method as CARMENES, but with other instruments, are shown as grey dots. With the data collected in the period 2016-2020, CARMENES has discovered and confirmed 6 Jupiter-like planets (with masses more than 50 times that of the Earth), 10 Neptunes (10 to 50 Earth masses) and 43 Earths and super-Earths (up to 10 Earth masses). The vertical axis indicates what star type the planets orbit around, from the coolest and smallest red dwarfs to brighter and hotter stars (the Sun would correspond to the second from the top). The horizontal axis depicts the distance from the planet to the star by showing the time it takes to complete the orbit. Planets in the habitable zone (blue-shaded area) can harbor liquid water on their surface. Credit: © Institut d’Estudis Espacials de Catalunya (IEEC).

As to the title of today’s post, the authors have calculated new planet occurrence rates from a subset of the overall sample, with low-mass planets tagged at 1.06 planets per star in periods of 1 day to 100 days, “and an overabundance of short-period planets around the lowest-mass stars of our sample compared to stars with higher masses.” Nearly every M-dwarf, in other words, appears to host at least one planet. The long-period giant planet occurrence rate comes in at 3%.

CARMENES Legacy Plus, which continues the original project, began in 2021 and continues to observe the same stars at least until the end of this year. Juan Carlos Morales is a researcher at IEEC (Institut d’Estudis Espacials de Catalunya):

“In order to determine the existence of planets around a star, we observe it a minimum of 50 times. Although the first round of data have already been published to grant access to the scientific community, the observations are still ongoing.”

The paper is Ribas et al., “The CARMENES search for exoplanets around M dwarfs Guaranteed time observations Data Release 1 (2016-2020),” Astronomy & Astrophysics Vol. 670, A139 (22 February 2023). Full text.


Uranus Orbiter and Probe: Implications for Icy Moons

What do you get if you shake ice in a container with centimeter-wide stainless steel balls at temperature of –200 ˚C? The answer is a kind of ice with implications for the outer Solar System. I just ran across an article in Science (citation below) that describes the resulting powder, a form of ‘amorphous ice,’ meaning ice that lacks the familiar crystalline arrangement of regular ice. There is no regularity here, no ordered structure. The two previously discovered types of amorphous ice – varying by their density – are uncommon on Earth but an apparently standard constituent of comets.

The new medium-density amorphous ice may well be produced on outer system moons, created through the shearing process that the researchers, led by Alexander Rosu-Finsen at University College London, produced in their lab work. There is a good overview of this water ‘frozen in time’ in a recent issue of Nature. The article quotes Christoph Salzmann (UCL), a co-author on the Science paper:

The team used a ball mill, a tool normally used to grind or blend materials in mineral processing, to grind down crystallized ice. Using a container with metal balls inside, they shook a small amount of ice about 20 times per second. The metal balls produced a ‘shear force’ on the ice, says Salzmann, breaking it down into a white powder.

Firing X-rays at the powder and measuring them as they bounced off — a process known as X-ray diffraction — allowed the team to work out its structure. The ice had a molecular density similar to that of liquid water, with no apparent ordered structure to the molecules — meaning that crystallinity was “destroyed”, says Salzmann. “You’re looking at a very disordered material.”

Disruptions in icy surfaces caused by the process would have implications for the interface between ice and liquid water that is presumed to exist on moons like Europa and Enceladus. The surface might be given to disruptions that would expose ocean beneath.

What goes on in the icy moons of the outer system is always of interest, especially given the astrobiological possibilities, and it was probably the thought of an ice giant orbiter at Uranus that triggered my interest in the amorphous ice issue. Kathleen Mandt (Johns Hopkins University Applied Physics Laboratory) just wrote the former up in Science as well, noting how little we’ve learned since the solitary Voyager flyby of the planet in 1986. In addition to planetary structure and atmosphere, we could do with a lot more information about its moons and their possible liquid water oceans.

Image: This 2006 image taken by the Hubble Space Telescope shows bands and a new dark spot in Uranus’ atmosphere. Credit: NASA/Space Telescope Science Institute.

The planetary science decadal survey released in 2022, called Origins, Worlds, and Life, which reviewed over 500 white papers and 300 presentations over the course of its 176 meetings, flagged the need for such a mission in the coming decade, a planetary flagship mission as a next step forward after Europa Clipper. I don’t want to downplay the role of such a mission in deepening our understanding of ice giant formation and migration, not to mention the Uranian atmosphere, but the moon system here has proven an extreme challenge for observers. Its study becomes a major driver for the Uranus Orbiter and Probe (UOP) mission:

The system’s extreme obliquity…limits visibility of the moons to one hemisphere during southern and northern summers. Voyager 2 could only image the moons’ southern hemispheres, but what was seen was unexpected. The five largest moons, predicted to be cold dead worlds, all showed evidence of recent resurfacing, suggesting that geologic activity might be ongoing. One or more of these moons could have potentially habitable liquid water oceans under an ice shell, making them “ocean worlds.” Ariel, the most extensively resurfaced moon, is a strong ocean worlds candidate along with the two largest moons, Titania and Oberon… UOP will image and measure the composition of the full surfaces of the moons to search for ongoing geologic activity, and measure whether magnetic fields vary in their interiors owing to the presence of liquid water.

Miranda, Ariel, Umbriel, Titania and Oberon, the five largest moons of Uranus, could all contain subsurface oceans (and I don’t want to leave out of the UOP story the fact that it would carry a Uranus atmospheric probe designed to reach a depth of at least 1 bar in pressure, a fascinating investigation in itself). The decadal survey has recommended a launch by 2032 to take advantage of a Jupiter gravity assist that would allow arrival before the northern autumn equinox in 2050 for better study of the moon system. “The space science community has waited more than 30 years to explore the ice giants,” writes Mandt, “and missions to them will benefit many generations to come.”

The first of today’s papers is Rosu-Finsen et al., “Medium-density amorphous ice,” Science Vol. 379, No. 6631 (2 February 2023), pp. 474-478 (abstract). The Mandt article is “The first dedicated ice giants mission,” Science Vol. 379, No. 6633 (16 February 2023), pp. 640-642 (full text).


Wolf 1069b: Why System Architecture Matters

Let’s look at a second red dwarf planet in this small series on such, this one being Wolf 1069b. I want to mention it partly because of the prior post on K2-415b, where we had the good fortune to be dealing with a transiting world around an M-dwarf that should be useful in future atmospheric characterization efforts. Wolf 1069b, by contrast, was found by radial velocity methods, and I’m less interested in whether or not it’s in a ‘habitable’ orbit than in the system architecture here, which raises questions.

This work, recounted in a recent paper in Astronomy & Astrophysics, describes a planet that is not just Earth-sized, as is K2-415b, but roughly equivalent to Earth in mass, making a future search for biosignatures interesting once we have the capability of collecting photons directly from the planet. If the planet has an atmosphere, argue the authors of the paper, its surface temperature could reach 13 degrees Celsius, certainly a comfortable temperature for liquid water. A putative atmosphere would also shield the world from harmful radiation from the host star, although Wolf 1069 appears so far to be an unusually quiescent M-dwarf.

In fact, the lack of distorting surface activity on the star makes possible a high degree of accuracy in the radial velocity measurements here. The data, pulled in by one of the two CARMENES spectrographs, were taken by Diana Kossakowski (Max Planck Institute for Astronomy in Heidelberg), who is lead author of the paper on this work, and colleagues. The CARMENES instruments operate with the 3.5-metre telescope of the Calar Alto Observatory near Almería in southern Spain, and Kossakowski and team have been working the numbers on Wolf 1069 for the past four years.

Image: The figure shows measurements of the velocities at which the star Wolf 1069 moves towards or away from us by the mean. The measuring points were arranged in such a way that they depict the orbital period of the planet. This shows the tiny but significant variation in motion caused by the planet 1.3 times the mass of Earth orbiting in 15.56 days, and is illustrated by the gray line with the black dots. Credit: © D. Kossakowski et. al. from A&A 2023).

CARMENES is itself a research consortium (the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs program). The eleven German and Spanish institutions involved are focusing on Earth-like exoplanets near M-dwarfs, in other words, and I think we can expect the first doubtlessly controversial findings related to biomarkers will emerge on such worlds.

Wolf 1069b is on a 15.6 day orbit around an M-dwarf about 30 light years away in Cygnus. That distance is, of course, intriguing as we build the catalog for nearby worlds for future study of biomarkers or, one day, probes; the planet counts as the sixth-closest Earth mass world in a habitable zone orbit (the others are Proxima Centauri b, GJ 1061 d, Teegarden’s Star c, and GJ 1002 b and c). Keep in mind that only 1.5 percent of all the more than 5000 exoplanets yet detected have masses below two Earth masses – K2-415b, for all its interest, evidently weighs in at three.

Tidal lock is likely, though perhaps not a show-stopper for life, especially if the early indications of Wolf 1069’s low levels of activity are born out by future observation, and if an atmosphere is indeed present (without one, the authors estimate, the surface temperature would be 250 K, or -23 °C, as opposed to the + 13 °C mentioned above). So that interesting scenario of daylight (or night) that goes on forever emerges here.

Image: Simulated surface temperature map of Wolf 1069 b, assuming a modern Earth-like atmosphere. The map is centered at a point that always faces the central star. The temperatures are given in Kelvin (K). 273.15 K corresponds to 0 °C. Liquid water would be possible on the planet’s surface inside the red line. Credit: © Kossakowski et al. (2023) / MPIA.

But it’s something that Max Planck Institute for Astronomy scientist Remo Burn said that catches my eye:

“Our computer simulations show that about 5% of all evolving planetary systems around low-mass stars, such as Wolf 1069, end up with a single detectable planet. The simulations also reveal a stage of violent encounters with planetary embryos during the construction of the planetary system, leading to occasional catastrophic impacts,”

That’s a noteworthy thought, for such impacts could generate a planetary core that remains liquid today, resulting in a global magnetic field that would offer further shielding effects from stellar activity. The question would be whether Wolf 1069b really is alone, and on this the results are simply not in. What the researchers have been able to do is to exclude additional planets of Earth mass or more and orbital periods of less than 10 days. What they cannot do yet is rule out planets on wider orbits.

If alone around its star, Wolf 1069b is the only one of the six Earth-mass planets in habitable zones nearest to Earth that is found without an inner planet keeping it company. Note that the mass of Wolf 1069 is 0.167±0.011 solar masses. And now let’s turn to the paper:

This notion is supported by the works of Burn et al. (2021), Mulders et al. (2021), and Schlecker et al. (2021), where we expect a lower planet occurrence rate for stars with M* < 0.2 M than for stars with 0.2 M < M* < 0.5 M for both the pebble and core accretion scenarios.

The authors run this out on a rather lengthy speculative thread:

Granted, these are theoretical predictions as more observation-based evidence is required to confirm this, and Wolf 1069b could still be accompanied by closer-in and outer planets. Nevertheless, the concept that only one planet survives is predicted by formation models if there were at least one giant impact at the late stage. This would enhance the chance of having a massive moon similar to the Earth and might also stir up the interior of the planet to prevent stratification and sustain a magnetic field (e.g., Jacobson et al. 2017). As remote as this appears, the search for exo-moons is no longer so far-fetched in recent times (e.g., Martínez-Rodríguez et al. 2019; Dobos et al. 2022).”

In the absence of data on these matters, speculation is welcome, but I can only imagine that when we get the right instrumentation online to make direct observations of planets like Wolf 1069b, we’re going to find more than our share of surprises. Whether or not an exo-moon hinting at an impact hinting at a magnetic field is one of them remains to be seen. A lot of ‘ifs’ creep into discussions of ‘habitable’ worlds. Would a tidally locked red dwarf planet look something like the speculation we see below?

Image: Artist’s conception of a rocky Earth-mass exoplanet like Wolf 1069 b orbiting a red dwarf star. If the planet had retained its atmosphere, chances are high that it would feature liquid water and habitable conditions over a wide area of its dayside. Credit: © NASA/Ames Research Center/Daniel Rutter.

The paper is Kossakowski et al., “The CARMENES search for exoplanets around M dwarfs Wolf 1069 b: Earth-mass planet in the habitable zone of a nearby, very low-mass star,” Astronomy & Astrophysics Vol. 670, A84 (10 February 2023). Full text.


The Relevance of K2-415b

I want to mention the recent confirmation of K2-415b because this world falls into an interesting category: Planets with major implications for studying their atmospheres. Orbiting an M5V M-dwarf every 4.018 days at a distance of 0.027 AU, this is not a planet with any likelihood for life. Far from it, given an equilibrium temperature expected to be in the range of 400 K (the equivalent figure for Earth is 255 K). And although it’s roughly Earth-sized, K2-415b turns out to be at least three times more massive.

What this planet has going for it, though, is that it transits a low mass star, and at 70 light years, it’s close. Consider: If we want to take advantage of transmission spectroscopy to study light being filtered through the planetary atmosphere during ingress and egress from the transit, nearby M-dwarf systems make ideal targets. Their habitable zones are close in, so we get frequent transits around small stars. But the number of Earth-sized transiting worlds around nearby examples of such stars is small, totalling 14 in eight 8 different systems (limiting the range to 30 pc, or roughly 100 light years). That includes the high-priority seven around TRAPPIST-1.

Image: This is Figure 3 from the paper. Caption: Sensitivity plot (5 σ contrast curve) for K2-415 in the K0 band based on the combined IRCS image. The inset shows the zoomed image of the target with a FoV of 4” × 4”. Credit: Hirano et al.

As we move into ever deepening searches for atmospheric biomarkers, worlds like these are going to be in the vanguard, fodder for the emerging class of 30-meter telescopes and, of course, space-based observatories like the James Webb Space Telescope. We’re going to get to know this small group of worlds well as this decade continues, which will also greatly assist us in understanding how M-dwarf planets evolve. When it comes to atmospheres around small red stars, there are no guarantees.

After all, these stars especially in their youth are known to be intense emitters of extreme ultraviolet and X-ray radiation, with striking levels of flare activity. Can an atmosphere survive this bombardment, or an intense stellar wind? Will these planets evolve secondary atmospheres through surface outgassing from volcanoes and interactions with magma? Doubtless we’ll find examples of various scenarios, which should deepen our knowledge of how the atmospheres of habitable worlds emerge.

It will also be interesting to see if the apparent and recently noted (in TESS data) lack of detected planets around the lowest mass stars will be supported by later datasets. K2-415b forces that question, in that it orbits one of the lowest mass stars hosting an Earth-sized planet. Counting planets of any size, only ten transiting planet-hosting stars are cooler than K2-415. One of these is TRAPPIST-1.

K2, the extended Kepler mission, observed K2-415b, an unusually close system at 70 light years given that of the original Kepler target stars, fewer than one percent were closer than 600 light years (we have much to thank K2 for as it moved out of the original Kepler field, a classic case of making a virtue out of necessity). The follow-up campaign described in today’s paper validates the planet. And the authors note that future radial velocity studies will be on the lookout for additional planets in this system.

This too could get interesting. Lead author Teruyuki Hirano (Graduate University for Advanced Studies, Japan) and colleagues explain::

K2-415b is located slightly inner of the classical habitable zone (Kopparapu et al. 2016) based on the insolation flux onto the planet, but an outer planet, if any, in the system with a slightly longer period (e.g., 10 − 15 days) could sit inside the habitable zone. Little is known on the properties of multi-planet systems around the lowest mass stars (< 0.3 M), but assuming that their properties are similar to those in the “Kepler-multi” systems, the planets could have a typical spacing of ∼ 20 mutual Hill radii (Weiss et al. 2018). Recently, Hoshino & Kokubo (2023) also showed that the typical orbital spacing of planets formed by giant impacts is ∼ 20 mutual Hill radii independently of stellar masses through N-body simulations. Thus, it is quite possible that a secondary planet having ∼ 1 M has an orbital period of 10 − 15 days.

The paper is Hirano et al., “An Earth-sized Planet around an M5 Dwarf Star at 22 pc,” accepted for publication at the Astrophysical Journal and available as a preprint.


A Mission Architecture for the Solar Gravity Lens

Over the past several years we’ve looked at two missions that are being designed to go beyond the heliosphere, much farther than the two Voyagers that are our only operational spacecraft in what we can call the Local Interstellar Medium. Actually, we can be more precise. That part of the Local Interstellar Medium where the Voyagers operate is referred to as the Very Local Interstellar Medium, the region where the LISM is directly affected by the presence of the heliosphere. The Interstellar Probe design from Johns Hopkins Applied Physics Laboratory and the Jet Propulsion Laboratory’s Solar Gravity Lens (SGL) mission would pass through both regions as they conduct their science operations.

Both probes have ultimate targets beyond the VLISM, with Interstellar Probe capable of looking back at the heliosphere as a whole and reaching distances are far as 1000 AU still operational and returning data to Earth. The SGL mission begins its primary science mission at the Sun’s gravitational lens distance on the order of 550 AU, using the powerful effects of gravity’s curvature of spacetime to build what the most recent paper on the mission calls “a ‘telescope’ of truly gigantic proportions, with a diameter of that of the sun.” The vast amplification of light would allow a planet on the other side of the Sun to be imaged at stunning levels of detail.

Image: This is Figure 1 from the just released paper on the SGL mission. Caption: A visualization of the key primary optical axes (POA) and the projected image plane of the exoplanet. The imaging spacecraft is the tiny element in front of the exoplanet image plane. Credit: Helvajian et al.

Let’s poke around a bit in “Mission Architecture to Reach and Operate at the Focal Region of the Solar Gravitational Lens,” just out in the Journal of Spacecraft and Rockets, which sets out the basics of how such a mission could be flown. Remember that this work has proceeded through the NASA Innovative Advanced Concepts (NIAC) office, with Phase I, II and now III studies resulting in the refinement of a design that can satisfy the requirements of the heliophysics decadal survey. JHU/APL’s Interstellar Probe takes aim at the same decadal, with both missions designed to return data relevant to our own star and, in SGL’s case, a more distant one.

Given that it has taken Voyager 1 well over 40 years to reach 159 AU, getting a payload to the gravitational lens region for operations there and beyond as the craft departs the Sun is a challenge. But the rewards would be great if it can be made to happen. The JPL work and a great deal of theoretical study prior to it have revealed that an optical telescope of no more than meter-class equipped with an internal coronagraph for blocking the Sun’s light would see light from the target exoplanet appearing in the form of an ‘Einstein ring’ surrounding the solar disk. High-resolution imagery of an exoplanet can be extracted from this data. We can also trade spatial for spectral resolution. From the paper:

The direct high-resolution images of an exoplanet obtained with the SGL could lead to insight on the on-going biological processes on the target exoplanet and find signs of habitability. By combining spatially resolved imaging with spectrally resolved spectroscopy, scientific questions such as the presence of atmospheric gases and its circulation could be addressed. With sufficient SNR and visible to mid-infrared (IR) sensing [26], the inspection of weak biosignatures in the form of secondary metabolic molecules like dimethyl-sulfide, isoprene, and solid-state transitions could also be probed in the atmosphere. Finally, the addition of polarimetry to the spatially and spectrally resolved signals could provide further insight such as atmospheric aerosols, dust, and, on the ground, properties of the regolith (i.e., minerals) and bacteria and fauna (i.e., homochirality)…

I won’t labor the issue, as we’ve discussed gravity lens imaging on many an occasion in these pages, but I did want to make the point about spectroscopy as a way of underlining the huge reward obtainable from a mission that can collect data at these distances. The paper is rich in detailing the progress of our thinking on this, but I turn to the mission architecture for today, offering as it does a remarkable new way to conceive of deep space missions both in terms of configuration and propulsion. For we’re dealing here with spacecraft that are modular, reconfigurable and highly adaptable using clusters of spacecraft that practice self-assembly during cruise.

The SGL mission is based on a constellation of identical craft, the primary components being what the authors call ‘proto-mission capable’ (pMC) spacecraft, with final ‘mission capable’ (MC) craft being built as the mission proceeds. Smaller pMC nanosats, in other words, dock during cruise to build an MC; five or perhaps six of the latter are assumed in the mission description in this paper to allow full capability during the observational period within the focal region of the gravity lens. The pMC craft use solar sails for a close pass by the Sun, all of them launched into a parking orbit before deployment toward the Sun. The sailcraft fly in formation following perihelion, dispose of their thermal shielding, then their sails, and begin assembly into MC spacecraft.

How to separate a final, fully functional MC craft into the constituent units from which it will be assembled in flight is no small issue, and bear in mind the need for extreme adaptability, especially as the craft reach the gravitational lensing region. Near-autonomous operations are demanded. The SGL study used simulations based on current engineering methodology (CEM) tools, modifying them as needed. The need for in-flight assembly stood out from the alternative. From the paper;

Two types of distributed functionality were explored: a fractionated spacecraft system that operates as an “organism” of free-flying units that distribute function (i.e., virtual vehicle) or a configuration that requires reassembly of the apportioned masses. Given that the science phase is the strong driver for power and propellant mass, the trade study also explored both a 7.5-year (to ∼800 AU) and 12.5-year (to ∼900 AU) science phase using a 20 AU/year exit velocity as the baseline. The distributed functionality approach that produced the lowest functional mass unit is a cluster of free-flying nanosatellites (i.e., pMC) each propelled by a solar sail but then assembled to form an MC spacecraft.

Out of all this what emerges is a pMC design with the capability of a 6U CubeSat nanosatellite, self-contained and three-axis stabilized, each of these units to carry a critical part of the larger MC spacecraft. Power and data are shared as the pMCs dock. The current design for the pMC is a round disk approximately 1 meter in diameter and 10 cm thick, with the assembled MC spacecraft visualized as stacked pMC units. One pMC would carry the primary and secondary mirrors, a second the science package, optical communications package and star tracker sensors, and so on. In-space assembly need not be rushed. The paper mentions a time period of several months as needed to complete the operation.

The 28-year cruise phase ends in the region of 550 AU, with two of the five or six MC spacecraft now maneuvering to track the primary optical axis of the exoplanet host star, which is the line connecting the center of the star to the center of the Sun. The host star is thus a key navigational resource which will be used to determine the precise position of the exoplanet under study. Interestingly, motion in the image plane has to be accounted for – this is due to the effect of the wobble of the Sun caused by gas giants in our Solar System. Such wobbles are hugely helpful for those using radial velocity methods to study planets around other stars. Here they become a complicating factor in extracting the data the mission will need to construct its exoplanet imagery.

The disposition of the spacecraft at 550 AU is likewise interesting. All of the MC spacecraft are, as the acronym makes clear, capable of conducting the mission. It now becomes necessary to subtract the Sun’s coronal light from the incoming data, which is accomplished by having one of the spacecraft follow an inertial path down the center of the spiral trajectory the other craft will follow (the other craft all move in a noninertial frame to make it possible to acquire the SGL photons). Having one craft on an inertial path means it sees no exoplanet photons, and thus its coronal image can be subtracted from the data gathered by the other four craft. The inertial path spacecraft also acts as a local reference frame that can be used for navigation.

Image: A meter-class telescope with a coronagraph to block solar light, placed in the strong interference region of the solar gravitational lens (SGL), is capable of imaging an exoplanet at a distance of up to 30 parsecs with a few 10 km-scale resolution on its surface. The picture shows results of a simulation of the effects of the SGL on an Earth-like exoplanet image. Left: original RGB color image with (1024×1024) pixels; center: image blurred by the SGL, sampled at an SNR of ~103 per color channel, or overall SNR of 3×103; right: the result of image deconvolution. Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II.

The spacecraft are moving at more than 20 AU per year and have up to five years between 550 and 650 AU to lock onto the primary optical axis of the exoplanet host star. As the craft reach 650 AU, the optical axis of the host star becomes what the authors call a ‘navigational steppingstone’ toward locating the image of the exoplanet, which once acquired begins a science phase lasting in the area of ten years.

The details of image acquisition are themselves fascinating and as you would imagine, complex – I send you to the paper for more. My focus today is the novelty of the architecture here. If we can assemble a mission capable spacecraft (and indeed a small fleet of these) out of the smaller pMC units, we reduce the size of sail needed for the perihelion acceleration phase and make it possible to achieve payload sizes for missions far beyond the heliosphere that would not otherwise be possible. We build this out of a known base; in-space assembly and autonomous docking have been demonstrated, and technologies for assembly operations continue to be refined. NASA’s On-Orbit Autonomous Assembly from Nanosatellites and CubeSat Proximity Operations Demonstration mission are examples of this ongoing research.

What a long and winding path it is to extend the human presence via robotic probe ever further from our planet. This paper examines technologies needed to advance this movement, and again I point to the ongoing Interstellar Probe study at JHU/APL as another rich source for current and projected thinking about the needed technologies. In the case of the SGL mission, what is being proposed could have a major impact on the search for life elsewhere in the universe. Imagine a green and blue exoplanet seen with weather patterns, oceans, continents and rich spectral data on its atmosphere.

But I come back to that mission architecture and the idea of self-assembly. As the authors write:

We realize that this architecture fundamentally changes how space exploration could be conducted. One can imagine small- to medium-scale spacecraft on fast-traveling scouting missions on quick cadence cycles that are then followed by flagship-class space vehicles. The proposed mission architecture leverages a global technology base driven by miniaturization and integration, and other technologies that are coming into fruition, including composite materials based on hierarchical structures, edge-computing platforms, small-scale power generation, and storage. These advances have had an effect on the small spacecraft industry with the development of a worldwide CubeSat and nanosat ecosystem that have continually demonstrated increasing functionality in missions…

We’ll continue to track robotic self-assembly and autonomy issues with great interest. I’m convinced the concept opens up mission possibilities we’ve yet to imagine.

The paper is Helvajian, “Mission Architecture to Reach and Operate at the Focal Region of the Solar Gravitational Lens,” Journal of Spacecraft and Rockets. Published online 1 February 2023 (full text). For earlier Centauri Dreams articles on the SGL mission, see JPL Work on a Gravitational Lensing Mission, Good News for a Gravitational Focus Mission and
Solar Gravitational Lens: Sailcraft and In-Flight Assembly.


Into the Maelström

“‘This,’ said I at length, to the old man — ‘this can be nothing else than the great whirlpool of the Maelström’… The ordinary accounts of this vortex had by no means prepared me for what I saw. That of Jonas Ramus, which is perhaps the most circumstantial of any, cannot impart the faintest conception either of the magnificence, or of the horror of the scene — or of the wild bewildering sense of the novel which confounds the beholder.” So wrote Edgar Allen Poe in 1841 in a short story called “A Descent into The Maelström,” reckoned by some to be an early instance of science fiction. In today’s essay, Adam Crowl explores another kind of whirlpool, armed with the tools of mathematics to take the deepest plunge imaginable, into the maw of a supermassive black hole. Adam’s always fascinating musings can be followed on his Crowlspace site.

by Adam Crowl

The European Southern Observatory’s (ESO) GRAVITY instrument is a beam combiner in the infra-red K-band that operates as a part of the Very Large Telescope Interferometer, combining infra-red light received by four different telescopes, out of the eight operated (four 8.2 metre fixed telescopes and four 1.8 metre movable telescopes).

The latest measurements of the stars orbiting the Milky Way’s Galactic Core Super-Massive Black Hole (SMBH), otherwise known as Sagittarius A* (pronounced as ‘Sagittarius A Star’), by the GRAVITY instrument have determined its mass and distance to new levels of accuracy:

Ro = 8,275 parsecs (+\-) 9.3 parsecs and a mass of (in 106 Msol) 4.297 +\- 0.013.

Image: The galactic centre in infrared. Credit: NASA.

In round figures, that’s 27,000 light-years and 4.3 million Solar masses. The closest that light can approach a Black Hole and still escape is the Event Horizon, which is the spherical boundary at the distance of the Schwarzschild Radius, which is a radius of 2.95325 kilometres per solar mass. Thus 4.3 million solar masses is wrapped in an Event Horizon 12.7 million kilometres in radius. In aeons to come, when the Milky Way and M31 have collided and their black holes have coalesced, the combined Super Massive Black Hole (SMBH) will mass 100 million solar masses with an event horizon almost 300 million kilometres in radius.

Image: From Tales of Mystery and Imagination, by Edgar Allan Poe, with illustrations by Arthur Rackham (1935).

Into the Maelström

Mass-energy, so General Relativity tells us, puts dents into Space-time. Most concentrations of mass-energy, like stars, planets and galaxies, form shallow dents. Black Holes – like the future SMBH – go deeper, forming an inescapable waterfall of space-time inwards to their centres, the edge of which is the Event Horizon.

Light follows the curvature of space-time, traveling the shortest pathways (geodesics). At the Event Horizon the available geodesics all point towards the “middle” of the Black Hole. For particles with rest mass, like atoms, dust and space-ships, geodesics can’t be followed, merely approached, so they follow different pathways just as inexorably towards the centre.

Instead of flying radially inwards towards the future SMBH’s Centre, let’s ponder orbiting it. For most orbital distances from any Black Hole any small mass in orbit will experience nothing different to orbiting around any other large mass. Too close and you’ll experience extreme tidal forces if the black hole is small, so to avoid being torn to shreds when approaching really close a really big Black Hole is needed. The future SMBH massing 100 million solar masses with a Schwarzschild radius of 300 million kilometres has very mild Tidal Forces at the Event Horizon, though potentially significant for things as big as stars and planets.

We have multi-year ‘movies’ of stars orbiting around our SMBH, though none as close as we will explore. Close orbits get measured in multiples of M – which is half the Schwarzschild radius. At a radius of r = 3M space-time is so curved that the geodesics form a circle around the Black Hole. Light can thus orbit indefinitely, building up to potentially extraordinary energy densities if nothing else gets in its way, forming a so-called Photon Sphere. But the centre of the Galaxy is full of dust and gas, so something is always getting in the way. Eventually even photons get so energetic they perturb each other out of the Sphere.

For particles that don’t follow geodesics, merely approximate them, the Innermost Stable Circular Orbit (ISCO) is further out, at r = 6M. Objects here travel at half the speed of light. Other shapes of orbits can dip a bit closer in, down to r = 4M. Deeper in and motion near the Black Hole is no longer “orbital”. You must point away from the Black Hole and apply thrust or in-fall is inevitable.

The equation of orbital motion from the ISCO radius (rI) all the way to the centre was only recently worked out in closed form for rotating and non-rotating (stationary) Black Holes. Previously numerical Relativity methods were used, complicating modelling of Accretion Disks around astrophysical Black Holes. The Equation of Motion of a test particle (i.e. very small mass) around a non-rotating Black Hole, which our future SMBH might approximate, is straightforward:

Φ is the angular distance traveled, with a range from negative infinity to zero, by convention. I’ve plotted r against Φ here:

The Red Circle at r = 3 M is the Photon Sphere and the Yellow Circle at r = 2M is the Schwarzschild Radius aka Event Horizon. In this case the plot starts at r = 5.95M with the test particle circling the Black Hole 6 times before hitting the central point. The proper time experienced by an observer spiralling into the Centre is a bit more complicated. We can parameterise x as follows to make the mathematics easier:

with ψ running from an angle π to 0. Then the Proper-Time τ of the inspiral trajectory is:

The above equation is true for any black-hole, spinning or stationary. For a stationary Black Hole, rI = 6M, so the equation simplifies to:

But what is M? It’s the “geometrised” mass of the Black Hole, which is derived by muliplying the mass by G/c2. Similarly the proper time is in units of “geometrised” time, so it needs to be divided by the speed of light, c, to convert to seconds.

In the case of the fall from r = 5.95M to r = 0, thus ψ = (5.95/6) ∗ (π) to ψ = 0, the total time is τ = 1291.14M. In the case of our Galaxy’s SMBH a proper time of M is 493 seconds. So the inspiral time is 176.6 hours and the Event Horizon is reached with 1.32 hours to go.

Surviving the Plunge

Falling into a Black Hole is probably fatal. However, like any fall, it’s not gravity that’ll kill you, but the sudden stop at the end. The final destination is the concentration of mass at the very centre. As the Centre is approached the first derivative of gravitational acceleration with respect to the radial distance vectors – the tidal forces – that will be experienced will become extreme.

Black holes are the pointy end of a spectrum of astrophysical objects. Stars exist due to their dynamic balance between the outward pressure from their fusion energy production and the inward pressure from their self gravity. When fusion energy production ends, the cores of stars begin collapsing, held above the Abyss of gravitational collapse by successive fusion energy reactions, then electron degeneracy pressure from squeezing free electrons too close together (via the Pauli Exclusion Principle), and when that isn’t enough, neutron degeneracy pressure and beyond.

Pressure is a measure of the ‘expansive’ energy packed in a volume. Dimensionally we can see that F / m2 (Pressure) = E / m3 (Energy per unit volume), so that as the mutual gravitational squeeze pushes inwards on a mass of particles which are pushing back against each other thanks to the Pauli Exclusion Principle for fermions (electrons, protons, neutrons etc) that pressure increases and increases, in a feed-back loop. Too much and equilibrium is never achieved. Thanks to Special Relativity we know that energy has mass, so that Pressure adds to the inward squeeze of gravity as particles are squeezed harder together. When the Gravity Squeeze – Push-Back Pressure process self-amplifies and runs away, the mass collapses ‘infinitely’ inwards forming a Singularity. Such a Singularity cuts itself of from the rest of the Universe when it squeezes inwards past the mass’s Schwarzschild Radius:

The resulting Event Horizon defines a Black Hole, by being a ‘surface of no return’ for everything, including light. Nothing escapes from within the Event Horizon. The minimum mass to cause such an inwards collapse and form a Black Hole for a mass of fermions (i.e. the same particles that make Stars, humans and space-ships) in the present day Universe is about 3 solar masses, squished into a volume smaller than 18 kilometres across.

Before we get to that point there are White Dwarfs and Neutron Stars – objects supported against collapse by Electron Degeneracy Pressure and Neutron Degeneracy Pressure, respectively. White Dwarfs are typically composed of carbon and oxygen – the ashes of helium fusion – and have observed masses anywhere between 0.1 and 1.3 Solar masses. Their radius is proportional to the inverse 1/3 power of their mass:

R* is a reference radius. For a cool white dwarf of 1 solar mass, the radius is about 0.8 Earth’s – 5,600 km. A space vehicle falling from infinity, on a flyby very close to such a star’s surface will rush past the lowest point of its orbit at 6,900 km/s, experiencing over 430,000 gee acceleration. In free-fall however it feels only the first derivative of that acceleration:

Which in this example is 0.15 gee per metre of radial stretching directed outwards and inwards along the direction of the radial distance to the white dwarf and a squeezing force half that directed laterally inwards from the sides. Easily resisted by small structures, like bodies and space-ships.

Neutron stars are smaller again – typically 20 km wide for a 1.3 solar mass neutron star. A near surface flyby isn’t recommended, since the tidal forces are thus almost a million times stronger. Close proximity to a magnetic neutron star is probably lethal anyway due to the intense magnetic fields long before the tidal forces rip you to shreds. Heavier neutron stars get smaller – just like white dwarfs – until they totally collapse as a black hole.

Black holes reverse the trend. The Event Horizon gets bigger linearly with their mass and there’s no upper limit to their mass. Our future Galactic SMBH’s Event Horizon will be 295.325 million kilometres in radius, give or take. Substituting the Schwarzschild Radius equation into the Tidal force equation gives us:

So the tidal force at the Event Horizon is 0.1 microgee per metre. The Moon could almost enter the Event Horizon peacefully…

How far into the SMBH can we, as Observers, then fall? If we can brace ourselves against 1,000 gees per metre of squeezing and stretching, then quite a long way…

Which gives a distance of 139,430 kilometres from the centre. In other words 99.953% of the way to the central Singularity.

What wonders might we see? Quantum Gravity is yet to give a clear answer. Traditionally an imploding mass ends in the Singularity, which is a geometrical point. But quantum particles can’t be reduced to a singular point and retain quantum information. A possibility, due to the massively distorted space-time around the collapsing mass, is that ultimately the quantum particles all “bounce” after hitting Planck density and explode back outwards. To external Observers this is seen, in time-dilated fashion, as the slow-leak from the Event Horizon that is Hawking Radiation. Or, if the particles “twist” in a higher dimension, so they bounce as a new Big Bang forming another Universe. This can be seen as an emergence from a White Hole, as White Holes must keep expanding else they collapse into another Black Hole.

None of those options are ‘healthy’ to be around as flesh-and-blood Observer, so presently surviving the plunge is in doubt.

As we conclude, let’s check back in with Edgar Allan Poe, who knew a few things about terrifying plunges himself. In “Descent into the Maelstrom,” he gives us a look into what might be considered a 19th Century conception of a black hole and the journey into its bizarre interior:

“Looking about me upon the wide waste of liquid ebony on which we were thus borne, I perceived that our boat was not the only object in the embrace of the whirl. Both above and below us were visible fragments of vessels, large masses of building timber and trunks of trees, with many smaller articles, such as pieces of house furniture, broken boxes, barrels and staves. I have already described the unnatural curiosity which had taken the place of my original terrors. It appeared to grow upon me as I drew nearer and nearer to my dreadful doom. I now began to watch, with a strange interest, the numerous things that floated in our company. I must have been delirious — for I even sought amusement in speculating upon the relative velocities of their several descents toward the foam below. ‘This fir tree,’ I found myself at one time saying, ‘will certainly be the next thing that takes the awful plunge and disappears,’ — and then I was disappointed to find that the wreck of a Dutch merchant ship overtook it and went down before. At length, after making several guesses of this nature, and being deceived in all — this fact — the fact of my invariable miscalculation — set me upon a train of reflection that made my limbs again tremble, and my heart beat heavily once more.”


Mummery, A. & Balbus, S. “Inspirals from the innermost stable circular orbit of Kerr black holes: Exact solutions and universal radial flow,” Physical Review Letters 129, 161101 (12 October 2022).

Fragione, G. and Loeb, A., “An Upper Limit on the Spin of SgrA* Based on Stellar Orbits in Its Vicinity” (2020) ApJL 901 L32
DOI 10.3847/2041-8213/abb9b4


Alex Tolley follows up his analysis of agriculture on Mars with a closer look at the Interstellar Research Group’s MaRMIE project – the Martian Regolith Microbiome Inoculation Experiment. Growing out of discussions on methods beyond hydroponics to make the Red Planet fertile, the project is developing an experimental framework, as described below, to test our assumptions about Martian regolith here on Earth. A path forward through simulation and experiment could help us narrow the options for what may be possible for future colonists. Fertile regolith, achieved through perchlorate removal, would open up possibilities far beyond what is achievable through hydroponics.

by Alex Tolley

Successful settlement of distant locations requires living off the land, which requires resourcing food. Failure can lead to disaster, as experienced by some of the early American colonies. While near Earth space settlements could be supplied with packaged food, this would be too costly for an expanding Mars base over the long term. Food and air must be supplied from local sources, a point that has been emphasized by the Mars Society’s president, Robert Zubrin (Zubrin, 2011).

In the mid-20th century, it was assumed that agriculture on Mars would be like that on Earth, with crops growing in the Martian soil, but under clear domes to maintain air pressure, and light for photosynthesis. As a result, the focus for settlement was on the shiny technologies of transport and the design of Martian bases and cities.

Image: The Martian Base: Painting for The Exploration of Space by Arthur C Clarke. Credit: Leslie Carr.

This rosy picture of farming on Mars was disturbed after the Apollo missions when it became apparent that plants did not grow well in lunar regolith samples. The Phoenix lander’s discovery of perchlorates on the surface of Mars meant that the Martian regolith would be toxic to plant growth without remediation. Perchlorates are found on Earth, for example in the Atacama desert, but in far lower concentrations than the 0.5-1.0% concentrations found on the surface of Mars. Perchlorates are used in industry, and the US EPA regulates perchlorate contamination because of its toxicity.

Because of the adverse nature of regolith on plant growth, the focus shifted to soilless agriculture using hydroponics or aquaponics, but as we saw in the previous post, there are limitations on the use of hydroponics. Plants with extensive root systems needed for support, especially trees, can’t be grown this way, eliminating the availability of tree fruits and nuts. Most of our grains cannot be grown using current hydroponic methods either. It really would be useful if the regolith could be altered to make it suitable for traditional agriculture, perhaps more like the farms in arid areas, such as the Middle East.

In 2022, after participating in a panel discussion on establishing a sustainable human presence on Mars at a science fiction convention (LibertyCon) in Tennessee, members of the Interstellar Research Group (IRG) including Doug Loss, Joe Meany, and Jeff Greason, considered how some experiments could be done to test how best the regolith might be treated to remove the perchlorates with bioremediation using bacteria, and convert the sterile regolith into soil suitable for agriculture. Some species of bacteria metabolize chlorates and perchlorates for energy, and therefore could be used to remediate the regolith. Relatively small, low mass cultures could be brought from Earth and exponentially cultured to meet the requirements for the volumes of regolith to be treated.

Bioremediation of perchlorate contaminated soils is established practice (Hatzinger 2005), suggesting that if it could be adapted to Martian conditions, this may be a viable solution to remove the perchlorates and solve the toxicity issue.

This use of bacteria, a low mass approach to remediate the regolith was the inspiration for core IRG members to propose a project, the Martian Regolith Microbiome Inoculation Experiment.(MaRMIE).

Mars is almost certainly too dry and cold to just irrigate the regolith on the exposed Martian surface with an inoculant of perchlorate metabolizing organisms. Knowledge about the required conditions for successful large scale regolith bioremediation, especially of temperature and pressure, was required, as well as the issue of UV and ionizing radiation.

Simulating the Martian Surface

The initial idea was to run experiments in a sealed chamber that mimicked the Martian surface environment to determine whether a terrestrial type soil might be created in which agriculture could be practiced. This Mars simulation chamber would contain a Martian regolith simulant (MRS) with added perchlorate, and inoculated with suitable bacteria. If the bacteria could break down the perchlorate, it would indicate that this approach could, in principle, be used to remediate the regolith from the surrounding area, which would then be used inside a greenhouse to grow the food crops. By doing so, the mass, complexity, and likely equipment failures of a hydroponic system could be avoided, and a more traditional agricultural approach could be practiced. This was a far more scalable solution than a technical one, allowing food production anywhere it would be needed, and was in much closer alignment with ISRU.

Early thinking was to design the experiment and have an outside PI with expertise and funding to refine the design and run the experiments. The IRG would publish a review article, and at some point participate in writing a paper on the results.

At this point, the initial group decided to invite others who might be interested in providing input and expertise to investigate the biological remediation of regolith. Of particular importance was the need to design experiments that could be done at suitable facilities. IRG hopes the guidelines that develop out of this work may be of use to anyone pursuing research into agriculture for future use on Mars, and offers them to any organization that chooses to draw on them.

Prior work had identified various bacteria that had the genes that encoded the enzymes to reduce the [per]chlorate and extract energy from it (Balk 2008, Bender 2005, Coates 2004). As the genes coding for the various enzymes for perchlorate metabolism were known it has been suggested that by just liberating the oxygen from the perchlorate the regolith could be a useful source of life support and rocket fuel oxidant (Davil 2013), therefore offering another avenue of ISRU using an engineered bacterium.

Will bioremediation need to be taken inside the base, and if so, can or should it be done as close to Martian conditions as possible, or should it be done as close to the living or working conditions, and plant growing conditions in the agricultural greenhouse? Would it be better to grow the bacteria in a bioreactor rather than in situ, or even extract the enzymes to treat the regolith, thus controlling the bacteria growth and both reducing the perchlorates and liberating the oxygen as a useful side product?

These questions can only be answered with experiments testing the various bacterial inoculants under varying conditions from terrestrial to Martian, as well as applying economic and other analyses to determine the more effective way to use bioremediation on the regolith as an initial step to making it a proactive soil for farming.

While bioremediation is one approach to removing perchlorates, the fact that they are readily water-soluble suggests that if free water is available, the regolith could be simply washed to flush out the perchlorates. This would require more plant to wash the regolith and then remove the salts to recycle the water. This method would work more effectively on regolith than soil and would not require the controlled conditions of bacterial growth and the time to build the culture.

The Phoenix lander detected the perchlorates as they deliquesced on exposure. Experiments have shown that the perchlorates will deliquesce under Martian conditions (Slank 2022).

Removal of the toxic perchlorates is just the start of the process to make the regolith fertile. There have been a number of experiments with regolith simulant to grow a variety of plants and crops under terrestrial conditions of temperature and pressure, the sort of conditions that might be expected in a Mars greenhouse that has humans managing the farm.

By far the best results have been achieved by increasing the illumination to terrestrial levels and adding carbon-rich soils to the regolith, which now will also include the many soil organisms that improve the soils. A partial solution that has also worked is to grow cover crops like alfalfa grass or reuse the waste from prior crops to be added into the regolith to improve its water retention and nutrient supply. (Kasiviswanathan 2022).

So far none of these crop growing experiments have been attempted at pressures and temperatures that differ from optimal terrestrial conditions. There is considerable space to repeat these experiments under different conditions, especially if it proves important to build structurally lighter greenhouses, or even use artificial illumination in below-ground farms, much like container farming today. While oxygen can be extracted from Martian air, water and rocks, nitrogen is less readily available, as is phosphorus. These macronutrients and the other micronutrients will have to be found and extracted to support plant growth whatever farming method is used.

As a result of all these questions, the MaRMIE project has expanded in scope beyond bioremediation, to include crop growth experiments under non-terrestrial conditions.

An Experimental Framework

The project has generated an outline of the experiments that might be done, starting with bioremediation, and extending out into the more general issue of agriculture under conditions that differ from terrestrial ones. Even this is the tip of the iceberg as gene engineered organisms might well be better adapted to conditions on Mars, reducing containment costs, nutrients, and allowing faster scale-up to support an expanding settlement.

The experimental framework encompassing the ideas to date has 4 phases:

1. Remediating perchlorates in the regolith, and any problematic chemicals produced as a result of the remediation. This requires acquiring Martian regolith simulants (MRS) and the addition of perchlorates, testing a number of bacterial and microfungal agents to remediate the MRS under terrestrial conditions, and then in stages of pressure and temperature modified towards Martian conditions.

2. Developing a microbiome tailored to Martian conditions with which to inoculate the regolith. The microbiome should lessen or remove tendencies toward cementation of the regolith as well as gradually convert it into actual soil, if possible. “Actual soil” implies the provision of required nutrients for plant growth. This includes testing microbiomes to add to the MRS along with testing pioneer plant species to condition the regolith to become more like soil.

3. Testing plant growth in microbiome-inoculated regolith under Martian lighting levels and atmospheric conditions, gradually increasing the atmospheric pressure until plant growth is acceptable.

4. Continuing plant growth testing per #3, but gradually lowering ambient temperatures toward Martian levels until plant growth diminishes unacceptably.

5. Developing agricultural structures to provide appropriate conditions, with inoculated regolith, lighting levels, atmospheric pressure, and temperature levels previously determined, and with shielding from ionizing radiation.

As for output, the initial idea to publish some sort of review paper on the known issues and prior work, indicating the direction of experimental work needed, is still in process.

As noted at the outset, the IRG cannot execute these experiments and offers this work as a contribution to the field of planetary studies. IRG hopes that this framework will be seen and used as a structure for designing experiments and building on the results of previous experiments, by any researchers interested in the ultimate goal of viable large-scale agriculture on Mars.


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Slank, R. et al. (2022) “Experimental Constraints on Deliquescence of Calcium Perchlorate Mixed with a Mars Regolith Analog” The Planetary Science Journal, 3:154 (11pp), 2022 July

Kasiviswanathan P, Swanner ED, Halverson LJ, Vijayapalani P (2022) Farming on Mars: Treatment of basaltic regolith soil and briny water simulants sustains plant growth. PLoS ONE. 17(8): e0272209.