Centauri Dreams
Imagining and Planning Interstellar Exploration
Life from a Passing Star
Remember ‘Nemesis’? The idea was that mass extinctions on Earth recur on a timescale of between 20 and 40 million years, and that this recurrence could be accounted for by the existence of a faint star in a highly elliptical orbit of the Sun. Put this object on a 26 million year orbit and it would, so the theory ran, destabilize Oort cloud comets, causing some to fall into the inner system at a rate matching the record of extinctions. Thus a cometary bombardment was to be expected on a regular basis, as were the mass extinctions that were its consequence.
No one has found Nemesis, though other theories about recurring mass extinctions are in play, including recent work from Lisa Randall and Matthew Reece that explores dark matter as the trigger, with the Sun periodically passing through a disk of the stuff. Of course, finding dark matter itself continues to be a problem. Moreover, the wide range in the proposed recurrences gives rise to the possibility that these events are not periodic at all but simply random.
Robert Zubrin now offers a paper arguing that random encounters between our Solar System and passing stars can account for the Oort Cloud disruptions leading to extinctions without the need for Nemesis. Appearing in the International Journal of Astrobiology, the paper weighs other discussions of periodicity and goes on to propose a model for calculating the frequency of these encounters. The model rides on the treatment of the galaxy as a gas, with stars as particles at a density of approximately 0.003 stars per cubic light year.
These stars, argues Zubrin, are clearly not in synchronized motion but have random velocities with respect to each other on the order of 10 kilometers per second. Much rides on the effective encounter distance — when do stars pass closely enough to disrupt the outer cometary shell? One encounter every 26 million years occurs in Zubrin’s calculations if the distance of effective encounter is taken to be 22,000 AU, which would send a passing star through the Sun’s Oort Cloud, while at the same time exposing the Sun to the cometary cloud around the passing star.
Image: The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA.
We also have to take into account that the Sun is among the larger stars, the most common type of encounter being with far less massive M dwarfs. Zubrin assumes such stars have Oort Cloud analogs of their own, though we have as yet no observational evidence for this. He makes the case that comet bombardment of Earth will more likely occur from disrupted objects in the passing star’s Oort cloud than through objects native to the Sun’s Oort Cloud.
From the paper:
If the Sun were to travel through the alien star’s Oort Cloud at a range of 20,000 AU, it would probably be in the cloud for about that distance. Assuming a disruption range of 10 AU, it would sweep out a path with a volume of π(102) 20,000=6.3 million cubic AU. Assuming 4 Oort cloud objects per 1000 cubic AU, this implies that approximately 25,000 alien cloud objects could potentially be captured per pass, providing a significant chance of impact events to follow.
Indeed, disrupted Oort objects can in Zubrin’s calculations fall into their target stellar systems quickly, creating a cloud of short period comets and potential planet impacts while the other star is still relatively nearby. In the case where an M-dwarf passes through the Sun’s Oort Cloud while the Sun remains outside the smaller cloud of the dwarf, the dwarf star’s planetary system would be bombarded by objects from the Sun’s Oort Cloud while our own Solar System remained unaffected. By the calculations of this paper, while we might expect bombardments on the order of 30 million years or so in our system, our Oort Cloud would be delivering bombardments to a passing dwarf star every 7.5 million years.
Crucially, these events, transferring objects from one system to another, could happen fairly swiftly. If an object from an M-dwarf’s comet cloud were destabilized as it passed through our Solar System at a range of 10 AU, for example, it would have the possibility of reaching perihelion swiftly, in a matter of years. Note that Zubrin derives the figure of 10 AU for the distance a visiting star needs to come to an Oort Cloud object to turn it into a comet; i.e., the Sun can capture a visiting star’s Oort cloud objects if it passes within 10 AU of the object.
Here, then, is the hook for astrobiology:
If we estimate that each Oort Cloud object disrupted has an average mass of 1 billion tons, then an encounter [with a star] at 20,000 AU would appear to have the potential to import about 25 trillion tons of mass from another solar system into our own. Of course, only a tiny fraction if it would hit the Earth. But even so, the potential to transfer biological material is evident.
Most of these bombardments of our own system would occur from M-dwarf comet clouds, given the high percentage of M-dwarfs in the galaxy. The table below shows the distribution.
Table 1. Comparative responsibility of star types for cometary bombardment of our Solar System. Credit: Robert Zubrin.
We have the prospect, then, of material from one stellar system impacting a planet in the other, or at least, being captured in that system’s Oort cloud and stored until another encounter with a passing star causes it to be disrupted and fall inward. Notice that Zubrin is talking about microbes in the transferred material that would have to survive a journey far less than the multiple light years assumed necessary for interstellar panspermia, though they would have to survive Oort-like conditions, having traveled from their inner system to the comet cloud.
It may also be noted that with a typical time between incoming encounters of 25 million years, it is probable that our Solar System has had about 140 incoming-delivery encounters with other stars since life first appeared on Earth some 3.6 billion years ago… If each encounter with a dwarf star typically releases 1000 solar system Oort cloud objects, then our Solar System has been responsible for releasing some 140,000 objects into others over this period. But, as a large G star, the sun probably delivered at least three times as many bombardments on other stellar systems as it received.
Our star keeps orbiting galactic center somewhere in the range of every 225 to 250 million years. A lot of material could be exchanged in this way:
…while we have only travelled through the Oort clouds of other, mostly dwarf, stars 140 times, dwarf stars have probably travelled through our own Oort Cloud about 420 times. If only 10% of encounters actually result in the transfer of microbial life from the Earth to another solar system, then we have been responsible for seeding 42 other solar systems with life. If each of these were then to act as a similar microbial transmitter, the result would be billions of inhabited worlds seeded by Earth.
Here it’s interesting to speculate, as Zubrin goes on to do, about whether there is an optimal impact rate for the evolution of advanced life. More frequent impacts might actually be a useful evolutionary driver — the author notes that the biosphere recovered from the K-T impact within 5 million years, offering up mammals and birds that proved long-term survivors. But too frequent an impact rate would not allow sufficient recovery time. Thus it is conceivable that areas of the galaxy with perhaps double our population of stars might be those more likely to feature advanced species and civilizations.
So we have no ‘Nemesis’ to fall back on — a cometary impact of roughly 1 every 25 million years has no specific driver within our own system, but results from the movement of stars, a random motion as the Sun moves through the Milky Way. Harvesting objects from passing stars, most of them red dwarfs, we collect them on timescales of years or decades rather than millions of years, the result of their relatively close disruption. We wind up with a mechanism for exchanging materials with other stellar systems that could have implications for life.
A key question: Can life survive Oort-like conditions to allow such transfers? We’re also hampered by our lack of knowledge about the Oort Cloud itself and, indeed, the local interstellar medium beyond the Kuiper Belt. Zubrin draws his best estimates from the current peer-reviewed papers, as he must, but it’s clear that to tighten this kind of argument, we’re going to need data from future explorations of the Oort. In such ways does a speculative astrobiology sharpen its focus, within a process of scientific inquiry that is by necessity multi-generational.
The paper is Zubrin, “Exchange of material between solar systems by random stellar encounters,” published online by the International Journal of Astrobiology 18 June 2019 (abstract).
Study Sees ‘Oumuamua as a Natural Object
A paper called “The Natural History of ‘Oumuamua,” just out in Nature Astronomy, puts the emphasis on the word ‘natural.’ We know how much of a stir in the media the interstellar visitor has made given its peculiarities, and the hypothesis put forward by Harvard’s Avi Loeb that it could be a technological object. Now we have a group of 14 astronomers, European as well as American, who have assessed the available data from all angles.
This is a worthwhile effort, assembling a team at the International Space Science Institute (ISSI) in Bern, Switzerland that intends to meet once again later in the year. It considers the question of whether the extraterrestrial spacecraft hypothesis is supported by examination of all the peer-reviewed work that has thus far appeared. Matthew Knight (University of Maryland), working with Alan Fitzsimmons (Queen’s University Belfast) assembled the team. Knight believes that natural phenomena can explain ‘Oumuamua:
“We put together a strong team of experts in various different areas of work on ?Oumuamua. This cross-pollination led to the first comprehensive analysis and the best big-picture summary to date of what we know about the object. We tend to assume that the physical processes we observe here, close to home, are universal. And we haven’t yet seen anything like ?Oumuamua in our solar system. This thing is weird and admittedly hard to explain, but that doesn’t exclude other natural phenomena that could explain it.”
Image: This artist’s impression shows the first interstellar object discovered in the Solar System, ?Oumuamua. Observations made with the NASA/ESA Hubble Space Telescope, CFHT, and others, show that the object is moving faster than predicted while leaving the Solar System. The inset shows a color composite produced by combining 192 images obtained through three visible and two near-infrared filters totaling 1.6 hours of integration on October 27, 2017, at the Gemini South telescope. Credit: ESA/Hubble, NASA, ESO/M. Kornmesser, Gemini Observatory/AURA/NSF.
As we have no new observations of ‘Oumuamua, the paper produced by this team is an analysis of existing data, including a December 2017 paper on the object’s shape and spin co-authored by Knight. When the scientist calls the object ‘weird,’ he’s at least partially referring to its apparent acceleration along its trajectory, which suggests a comet even if astronomers could find no evidence of the kind of gaseous outflow that would propel even a small acceleration. We see no coma of ice, dust and gas, no evidence for gas jets, no cometary ‘tail.’
Karen Meech (University of Hawaii), who was lead author on the research paper that reported the discovery of ‘Oumuamua not long after it was identified at the Pan-STARRS observatory, had noted the object’s red color and elongated shape, apparent in changes in its reflectivity as it rotated. But on the matter of cometary behavior, Meech sees no need for anything beyond natural processes at work, saying “…while it is disappointing that we could not confirm the cometary activity with telescopic observations it is consistent with the fact that ?Oumuamua’s acceleration is very small and must therefore be due to the ejection of just a small amount of gas and dust.”
To see full text of the paper, see this link (thanks Alex Tolley for an alternate link!) On the specific question of alien technologies, the paper has this to say:
The key argument against the solar-sail hypothesis is ‘Oumuamua’s light-curve amplitude. For a solar sail to cause the observed non-gravitational acceleration, it needs to remain properly oriented towards the Sun. However, to yield the observed brightness variations, its orientation would need to be varying as viewed from Earth. Furthermore, since the actual dimensions of the solar sail would be >10:1, the orientation as viewed from Earth would need to be very nearly edge on, and remain so throughout the observations despite viewing geometry changes. It has not been shown that an orientation exists that can achieve all of these constraints imposed by the observational data. Furthermore, as discussed earlier, the shape of ‘Oumuamua’s light curve, with broad maxima and narrow minima, is consistent with an elongated ellipsoid.
We also find this on albedo:
The claim that ‘Oumuamua must be at least ten times ‘shinier’ than all Solar System asteroids to make the Spitzer Space Telescope data consistent with the ground-based observations is incorrect. The Spitzer observations are consistent with geometric albedos 0.01 ? pv ? 0.5…, with a most likely albedo of pv ~ 0.1. Comets have geometric albedos of pv = 0.02–0.07, carbonaceous and silicate asteroids have pv = 0.05–0.21, and the most reflective asteroids have pv ~ 0.5… Thus ‘Oumuamua’s measured reflectivity of about 0.1 is entirely consistent with normal Solar System small bodies.
And on the argument that the kinematics of the object are unusual:
While provocative, this argument is baseless. First, ‘Oumuamua’s trajectory is consistent with predictions for detectable inactive interstellar objects. Second, the measured number density cannot be claimed to be at odds with expectations because of our ignorance of the size distribution of interstellar objects.
‘Oumuamua is destined to remain enigmatic, for our dataset represents all that could be collected before the interstellar visitor had traveled beyond the view of our telescopes. With only a few weeks in play to observe the object, the ISSI astronomers acknowledge its rarity (“We have never seen anything like ?Oumuamua in our solar system,” says Knight) while finding its movement explicable through natural means. All report anticipating results from the Large Synoptic Survey Satellite (LSST), which comes online in 2022 and may give us more interstellar objects of the same kind, allowing a deeper and perhaps less controversial analysis.
“In the next 10 years, we expect to begin seeing more objects like ‘Oumuamua. The LSST will be leaps and bounds beyond any other survey we have in terms of capability to find small interstellar visitors,” says Knight. “We may start seeing a new object every year. That’s when we’ll start to know whether ‘Oumuamua is weird, or common. If we find 10-20 of these things and ‘Oumuamua still looks unusual, we’ll have to reexamine our explanations.”
The paper is Bannister et al. (The ‘Oumuamua ISSI Team), “The Natural History of ‘Oumuamua,” Nature Astronomy 19 July 1, 2019 (abstract). Knight’s 2017 paper is “On the Rotation Period and Shape of the Hyperbolic Asteroid 1I/’Oumuamua (2017 U1) from Its Lightcurve,” Astrophysical Journal Letters Vol. 851, No. 2 (12 December 2017). Abstract.
Benchmarks for a ‘Second Venus’
The latest find from TESS, the Transiting Exoplanet Survey Satellite, is a reminder of how interesting, and useful, a planetary system can be even if we find no Earth-like worlds there. This seems obvious, but so much of the public attention to exoplanets has to do with finding a clone of our own world that we can forget the power of a ‘second Jupiter’ or, in this case, a ‘second Venus.’ For at L 98-59 we have not one but three planets that may fit this description.
One Venus, hellish as it is, would seem to be enough. But learning about planets with varying kinds of atmospheres that are in orbits that produce runaway greenhouse effects can help us place our own system’s evolution in context. To be sure, we don’t yet know what kind of atmospheres these planets have, or if they have atmospheres at all, but the encouraging thing is that tight orbits around relatively bright stars are what we need as we look toward future tools like the James Webb Space Telescope.
Astrophysicist Joshua Schlieder (NASA GSFC) is a co-author of the paper on this work, which was led by colleague Veselin Kostov:
“If we viewed the Sun from L 98-59, transits by Earth and Venus would lead us to think the planets are almost identical, but we know they’re not. We still have many questions about why Earth became habitable and Venus did not. If we can find and study similar examples around other stars, like L 98-59, we can potentially unlock some of those secrets.”
Image: The three planets discovered in the L98-59 system by NASA’s Transiting Exoplanet Survey Satellite (TESS) are compared to Mars and Earth in order of increasing size in this illustration. Credit: NASA’s Goddard Space Flight Center.
What we have at L 98-59 could conceivably become a primer in atmospheric transformation. The primary is an M-dwarf about a third the mass of the Sun, found 35 light years away in the constellation Volans. The planets include L 98-59b, about 80 percent the size of the Earth, and the smallest planet yet discovered by TESS. Here we have a 2.25 day orbit receiving 22 times the amount of insolation as the Earth. Moving outward, we find L 98-59c, about 1.4 times the size of Earth, in a 3.7 day orbit with 11 times the amount of energy Earth receives. The furthest planet found so far is L 98-59d, about 1.6 times Earth’s size, in a 7.5 day orbit with 4 times Earth’s radiant energy, possibly Venus-like or conceivably a hot Neptune.
This may not exhaust the possibilities, for there is the prospect of further discovery here, says GSFC’s Jonathan Brande, likewise a co-author of the paper:
“If you have more than one planet orbiting in a system, they can gravitationally interact with each other. TESS will observe L 98-59 in enough sectors that it may be able to detect planets with orbits around 100 days. But if we get really lucky, we might see the gravitational effects of undiscovered planets on the ones we currently know.”
The paper points out that these worlds are too small to retain atmospheres rich in hydrogen, so the focus will be on secondary atmospheres that are the result of volcanic activity, and infalling volatiles from the rest of the system via comets. The authors calculate that all three planets are in range for JWST to produce a transmission spectrum showing atmospheric features. The expected signal-to-noise ratio compares to another nearby red dwarf planet, GJ 1132b.
Understanding why Earth is habitable and Venus is not will depend upon our analysis of planets that have evolved through the greenhouse phase. In this regard, the L 98-59 planets stand out, particularly since other Venus analogs thus far discovered orbit fainter stars. From the paper:
The L 98-59 planets receive significantly more energy than the Earth receives from the Sun (a factor of between 4-22 more than Earth’s insolation) and fall into the region that Kane et al. (2014) dubbed the Venus Zone. This is a region where the atmosphere of a planet like Earth would likely have been forced into a runaway greenhouse, producing conditions similar to those found on Venus. The range of incident fluxes within the Venus Zone corresponds to insolations of between 1-25 times that received by the Earth. Planets in the Venus Zone that can be spectroscopically characterized will become increasingly important in the realm of comparative planetology that aims to characterize the conditions for planetary habitability. In that respect, and considering the potential for atmospheric characterization…, L 98-59 could become a benchmark system.
What we know of these worlds will be refined by future TESS observations, possibly uncovering other planets here and monitoring activity on the host star. The paper is Kostov et al., “The L 98-59 System: Three Transiting, Terrestrial-size Planets Orbiting a Nearby M Dwarf,” The Astronomical Journal Vol. 158, No. 1 (27 July 2019). Abstract / Preprint.
‘Dragonfly’ Chosen to Explore Titan
We’ve looked at a number of concepts for exploring Titan over the years, from aircraft capable of staying aloft for a year or more to balloons and boats that would float on the moon’s seas. Dragonfly, the work of a team based at Johns Hopkins University’s Applied Physics Laboratory in Laurel, MD, is a rotorcraft with the capability of exploiting Titan’s thick atmosphere to stop, sample, and move on, shaping its investigations along the way as it explores an environment rich in targets.
These are the advantages of a rover, though here we’re in a landscape so exotic that it enables different tools than the ones we use on Mars. And with the success of the Martian rovers in mind, what good news that NASA has chosen Dragonfly as the next mission in its New Frontiers program. We can anticipate launch in 2026 and arrival at Titan in 2034, with a craft that will sample surface organics and examine prebiotic chemistry and potential habitability.
Image: This illustration shows NASA’s Dragonfly rotorcraft-lander approaching a site on Saturn’s exotic moon, Titan. Taking advantage of Titan’s dense atmosphere and low gravity, Dragonfly will explore dozens of locations across the icy world, sampling and measuring the compositions of Titan’s organic surface materials to characterize the habitability of Titan’s environment and investigate the progression of prebiotic chemistry. Credit: NASA/JHU APL.
APL’s Elizabeth ‘Zibi’ Tuttle is chief investigator for Dragonfly:
“Titan is such an amazing, complex destination. We don’t know the steps that were taken on Earth to get from chemistry to biology, but we do know that a lot of that prebiotic chemistry is actually happening on Titan today. We are beyond excited for the chance to explore and see what awaits us on this exotic world.”
No more so than all of us who lamented the end of Cassini’s mission and wondered whether the dynamic concepts for Titan exploration being offered would ever see the light of day. But think about how much Cassini itself played into Dragonfly. We have 13 years of data from the Saturn orbiter, which makes target selection a matter of choosing among abundance. The plan is for Dragonfly to land on the moon’s equator, in the dune fields now known as Shangri-La.
Dragonfly can then move on in flights of about 8 kilometers each, sampling the wide-range of Titan’s geography, while heading for an impact crater called Selk, which appears to be rich in complex molecules — carbon united with hydrogen, oxygen and nitrogen. Along the way, the mission takes advantage of Titan’s calm, dense atmosphere (four times denser than Earth’s), along with its low gravity, to travel widely. The plan is for a journey of more than 175 kilometers, twice the distance of all the Mars rovers thus far deployed.
Dragonfly itself has eight rotors, giving it the aspect of a large drone. APL talks about one hop per full Titan day, each of these being 16 Earth days, within the context of a two-year mission. Although Dragonfly in flight is a dazzling prospect, the craft will spend most of its time on the surface conducting science measurements with the help of power from a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), with recharging occurring at night.
In flight, Dragonfly will provide images of surface geology (imagine the views we are going to see, although bear in mind that during flight, the high gain antenna will be stowed — no live streaming), while scouting for ground sites to study and building up a profile of Titan’s atmosphere. On the surface, the craft will use a mass spectrometer to examine the moon’s chemistry and the production of biologically interesting compounds. A neutron-activated gamma-ray spectrometer will offer readings on surface composition, while meteorology sensors will track atmospheric variation as imaging and seismic sensors probe geologic features.
Image: This artist’s concept shows a possible model of Titan’s internal structure that incorporates data from NASA’s Cassini spacecraft. In this model, Titan is fully differentiated, which means the denser core of the moon has separated from its outer parts. This model proposes a core consisting entirely of water-bearing rocks and a subsurface ocean of liquid water. The mantle, in this image, is made of icy layers, one that is a layer of high-pressure ice closer to the core and an outer ice shell on top of the sub-surface ocean. A model of Cassini is shown making a targeted flyby over Titan’s cloudtops, with Saturn and Enceladus appearing at upper right. The model, developed by Dominic Fortes of University College London, England, incorporates data from Cassini’s radio science experiment. Credit: A. D. Fortes/UCL/STFC.
And so we begin preparations for exploring a place of liquid methane seas and the potential for liquid water beneath a surface where water ice stands in for bedrock. We’ll be studying up close an atmosphere composed primarily of nitrogen with about 5 percent methane which, when exposed to sunlight, forms complex organic compounds. All this within a hydrological cycle of methane clouds and flowing liquid methane that fills lakes and seas. Congratulations to Elizabeth ‘Zibi’ Tuttle and the entire team. What prospects await…
PI Elizabeth "Zibi" Turtle of JHUAPL showing off a 1/4 scale model of #Dragonfly on NASA Science Live. pic.twitter.com/LR2UsCWbHD
— Marcia Smith (@SpcPlcyOnline) June 27, 2019
Addendum: Paul Voosen’s article on Dragonfly on the Science site adds some details about the equipment aboard:
Dragonfly won’t be equipped with a robotic arm, like the recent Mars rovers. Its exploration will first be guided by an instrument on its belly that will bombard the ground with neutron radiation, using the gamma rays this attack releases to differentiate between basic terrain types, such as ammonia-rich ice or carbon-rich sand dunes. Its two landing skids will also each carry a rotary-percussive drill capable of taking samples and feeding them through a pneumatic tube to a mass spectrometer that can analyze their composition. The sampling system represented a risk for the mission; NASA scientists were concerned Titan’s hydrocarbon-rich atmosphere could clog it, Zurbuchen says. “It’s the oil spill version of an atmosphere.” Over the past 2 years, after extensive testing with “pathological” materials and a redesign, Turtle says, the agency’s fears were allayed.
Voosen also points out that the Dragonfly seismometer could use vibrations induced by Titan’s tidal lock with Saturn to give us some indication of the size of the ocean beneath the crust. Ultimately, the nuclear power source could last for 8 years, meaning extended missions are in the cards.
Progress on Asteroid Discovery, Impact Mitigation
We have two stories with good news on the asteroid impact front this morning. The first, out of the University of Hawaii’s Institute for Astronomy, is the announcement of the detection of a small asteroid prior to its entering the Earth’s atmosphere. That many not sound unusual, but this is the first time an object could be detected in time to move people away from a impact site, even though asteroid 2019 MO was only about 4 meters across and burned up in the atmosphere. The key is warning time, and here that time would have been half a day.
An impactor like the 20-meter object that exploded over Chelyabinsk, Russia in 2013 could, with these same methods, be detected by the ATLAS facility at Maunaloa (Hawaii) several days before impact. ATLAS is made up of two telescopes, one on Hawai?i Island, the other 160 kilometers away at Haleakal?, Maui, providing whole-sky scans every two nights. About 100 asteroids larger than 30 meters in diameter are discovered by the facility every year.
In the case of 2019 MO, the ATLAS Maunaloa site picked up the object on the morning of June 22 when it was about 500,000 kilometers from Earth. Meanwhile, the Pan-STARRS 2 survey telescope at Haleakal? had imaged the same part of the sky about two hours earlier than ATLAS, revealing the incoming asteroid. The combination of data from the two sites allowed the object’s entry path to be refined, showing a likely impact that matched later Nexrad weather data in Puerto Rico, showing an entry over the ocean about 380 kilometers south of San Juan.
Image: A map of the predicted trajectory and final impact location for asteroid 2019MO. The predicted path is based on observations from the University of Hawai?i’s ATLAS and Pan-STARRS survey telescopes. Credit: Larry Denneau (IfA/ATLAS), Brooks Bays (SOEST).
So we’re getting some lead time on impactors even at this small scale, an indication that surprises from larger objects will be less likely in the future. Likewise cheering is news from research at NASA Ames which has just appeared in a special issue of Icarus, growing out of a workshop sponsored at Ames by the NASA Planetary Defense Coordination Office. Its theme: A new look at Tunguska, the 1908 impact that wreaked havoc in Siberia.
500,000 acres of uninhabited forest were flattened by the event, which was seen by few but left enough damage to alert scientists that this had not been a volcanic explosion or a mining accident, even though the first serious investigations didn’t occur until the 1920s. Today we look back at Tunguska as a kind of signature event, says Ames scientist David Morrison:
“Tunguska is the largest cosmic impact witnessed by modern humans. It also is characteristic of the sort of impact we are likely to have to protect against in the future.”
And the good news is that computer modeling discussed in the new papers shows that the interval between impacts like Tunguska is not, as has been previously estimated, on a timescale of centuries but rather millennia. 50 million combinations of asteroid and entry properties were analyzed by the computer models deployed here, some of them trying to match atmospheric pressure waves with the seismic signals recorded on the ground at the time, while others homed in on the kind of event that could produce the Tunguska treefall pattern and soil burn distribution. Four different computer modeling codes arrived at similar conclusions.
Thus we tighten our understanding of how rocks break apart in the atmosphere. The results show a stony Tunguska impactor between 50 and 80 meters in diameter that entered the atmosphere at about 15 kilometers per second, producing the equivalent of a 10 to 30 megaton explosion at between 9.5 and 14.5 kilometers altitude. The researchers used the latest estimates of the asteroid population to make the calculation of millennia between such impacts, correcting the earlier estimate that had been based on a smaller impactor.
Image: Trees flattened by the intense shock wave created in the atmosphere as the space rock exploded above Tunguska on June 30, 1908. The photograph was taken by the Soviet Academy of Science 1929 expedition led by Leonid Kulik. 500,000 acres, the size of a large metropolitan city, were flattened. Flattening trees requires an immense shock wave. Credit: Wikimedia Commons.
The new Tunguska work also draws upon extensive video observations and maps of the Chelyabinsk event. The advances in computer models that result show that the Chelyabinsk impactor was likely a stony asteroid that broke up about 24 kilometers above the ground, producing a shock wave like that of a 550 kiloton explosion. Chelyabinsk-class objects are thought to impact the Earth every 10 to 100 years, so learning how to provide sufficient warning, as the ATLAS and Pan-STARRS work shows us is now possible, can potentially save lives.
“Because there are so few observed cases, a lot of uncertainty remains about how large asteroids break up in the atmosphere and how much damage they could cause on the ground,” says Lorien Wheeler, a researcher from Ames, working on NASA’s Asteroid Threat Assessment Project. “However, recent advancements in computational models, along with analyses of the Chelyabinsk and other meteor events, are helping to improve our understanding of these factors so that we can better evaluate potential asteroid threats in the future.”
The seven papers on Tunguska and computational modeling of impact events growing out of the Ames workshop appear in a special issue of Icarus that can be found here.
Inside ESA’s Advanced Concept Team Interstellar Workshop
It’s always good to have eyes and ears on the ground at events I can’t get to, so I was pleased when Aleksandar Shulevski contacted me with the offer to send back notes from the European Space Agency’s Advanced Concepts Team Interstellar Workshop in Noordwijk in the Netherlands. Born and raised in Bitola, Republic of Macedonia, Aleksandar is a science fiction reader and amateur astronomer who followed up electrical engineering studies in Skopje with an MSc in astronomy at Leiden University (Netherlands), dealing with calibration issues on the LOFAR radio telescope. He received a PhD from the University of Groningen, doing research on active galactic nuclei radio remnants observed with LOFAR. After working at the Netherlands Institute for Radio Astronomy (ASTRON) as junior telescope scientist, he is now a research scientist at the Anton Pannekoek Institute for Astronomy at the University of Amsterdam, specializing in low-frequency radio transients and pursuing his interest in SETI. Aleksandar could only attend half of the workshop, and unfortunately no video is available, but I’m told a fall issue of Acta Futura should include papers on many of the topics covered in Noordwijk.
by Aleksandar Shulevski
I have attended only the first day of the Interstellar Exploration workshop, organized by the Advanced Concepts Team (ACT) at the ESTEC ESA site in Noordwijk. Even though I follow the goings on in the community, I must admit that I almost missed the workshop; maybe the organizers should have advertised it more widely. Obviously this is just a personal opinion.
The program indicated potentially an interesting gathering, and I am pleased to say that I was not disappointed. The auditorium was packed, I estimate that more that the total number of participants was about 120. The technical director of ESA opened the workshop, remarking that interstellar topics are more and more in the limelight, especially since Breakthrough Initiatives launched Breakthrough Starshot: “It’s never too early to start discussing interstellar matters”.
Michael Hippke started the morning session with a popular talk on interstellar communication. He gave a thorough overview on the history of thought dealing with the problem of communicating with other worlds. The radio window was de-emphasised on account of more exotic techniques like X-ray lasers, neutrinos etc. Michael thus established the “out of the box” thinking needed for such a meeting.
Rob Swinney (BIS) gave a historical overview of fusion propulsion concepts, covering the design of the Daedalus craft, as well as the successor design effort (Icarus). His remarks were in line with his enthusiasm: “We tend to underestimate what we can do on long timescales”.
The ongoing effort funded by NASA to outline and design a mission concept for a probe to 1000 AU was the topic of the presentation by Pontus Brandt. The tentative launch date is to be sometime in the 2030s. Pontus went over past missions which are on their way out of the Solar System (Pioneer, Voyager, New Horizons), tracing their origins to a strategy for Solar System research outlined in the Simpson committee report of 1960. The committee was chaired by John A. Simpson and James A. Van Allen as part of the contribution of the Space Studies Board, which was established in 1958 to focus on space research for the National Academies of Sciences, Engineering, and Medicine.
Long term thinking is key, along with lessons learned from past experience. In a set of stunning visuals, Brandt outlined current plans. Most likely the probe will form its trajectory by a Jupiter gravity assist, burning its final fuel supply there to achieve final escape velocity considerably greater than that of Voyager 2, reaching interstellar space in 15 years. Mission goals are observations of the infrared background as well as taking the first comprehensive “outside view” of the Solar system.
Michael Waltemathe discussed the philosophical and religious aspects of interstellar travel. What if any ETIs in existence are morally superior? Will humanity export the concept of original sin among the stars? Issues like planetary protection in the interstellar context were raised, and obviously, there were few definite answers.
Phil Lubin’s presentation dealt with directed energy propulsion, discussing detailed concept designs on Breakthrough Starshot, specifically the emitter array. Excellent beam handling was reported, and no issues (apart from funding) are show-stoppers on the path of scaling up to the operational laser system. However, an issue that warrants serious research effort is the communication subsystem. Laser comms will suffer from unattainable pointing accuracy requirements. The lack of deceleration in the target system may be problematic, but potentially overcome by launching a huge number of StarChips in succession, thus replacing the need for orbiting craft. The beamer system has tangential benefits as well, like Solar System exploration applications, and planetary defense. Long term R&D commitment is needed to realize the full potential of this effort.
[PG: I notice that MIT Technology Review just posted an article looking at some of the issues involved in getting this kind of laser array operational, while reviewing other issues re Starshot that Aleksandar mentions].
The morning session was followed by a less structured period for the duration of which multiple discussion groups were formed upon the suggestion of interested attendees who proposed topics. The organizers suggested most of the questions that needed to be addressed at these sessions. I proposed to discuss the propagation effects ISM plasma would have on the radio link used for communication with a Breakthrough Starshot style StarChip. There are multiple papers on the topic and we went over David Messerschmitt’s concepts of signal conditioning, discussed at length in his book featured on Centauri Dreams in the past [for more on Messerschmitt, see for example, Is Energy a Key to Interstellar Communication?]
The discussion period was followed by a panel on which each discussion topic had a representative, and the audience had a chance to ask questions. The afternoon plenary session began with a talk by Andreas Hein on worldships, in which he presented results from his latest paper in Acta Futura. Everything else being equal, and assuming economic growth continues unabated, Hein believes Earth will be able to build a worldship in a few hundred years, although the concept of a worldship may be by then outpaced by more practical developments.
Angelo Vermeulen presented the research done by him and collaborators at TU Delft and elsewhere on evolving spacecraft which borrow from biology to convert asteroids to interstellar craft. The study focused on the life support aspects of the problem and went on to model at length various constraints like (for example) the impact of the mineral content of the asteroid being used on mission viability.
Jeffrey Punske focused on language development on generation ships. Illustrating the various (and not always obvious) reasons for language drift, he concluded that it is inevitable that the language of a generation ship will evolve so far from Earth standard in a matter of a few hundred years as to render communication between the two unintelligible, something which mission planners should take into account. He dismissed the notion that a universal translator device is a viable development. If communication becomes a ritual performed by a priestly caste on the ship using the archaic language of the predecessors, maybe communication will still be possible in some form even after the divergence occurs.
Elke Hemminger addressed the sociology of interstellar exploration. Using an interactive approach, she engaged the audience in discussing philosophical topics. What if the values we hold most dear in our current societal organization are incompatible with the mission requirements of a colony ship? What are we willing to sacrifice to make the mission a successful one? Individual freedom? Is it worth it to go to the stars if we make such sacrifices?
These are more than abstract topics and may have more immediate concern on the human condition. It can easily turn out that dealing with climate change will require modification of our values. It may well be that totalitarian societies are more suited to dealing with emergencies than democracies. How do we balance our social values against the imperative of survival?
ESA’s Advanced Concepts Team closed the session by outlining the results of the latest Global Trajectory Optimisation Competition, this time dealing with Galaxy colonization, in which three types of interstellar generational vessels are imagined to be sent around the Milky Way in a quest 90 million years long. This was the 10th edition of the GTOC competition, hosted this year by the Mission Design and Navigation section at the Jet Propulsion Laboratory. China’s solution won, in a very interesting and difficult problem posed by JPL. An August workshop will allow top teams from the competition to present papers at the Astrodynamics Specialist Conference in Maine.
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