Centauri Dreams
Imagining and Planning Interstellar Exploration
Operations Throughout the Solar System
A reminder of how challenging it is to operate with solar power beyond the inner system is the fact that Juno carries 18,698 individual solar cells. Because it is five times further from the Sun than the Earth, the sunlight that reaches Juno is 25 times less powerful, a reflection of the fact that the intensity of light is inversely proportional to the square of the distance from the source.
In other words, if you’re going to use solar power this far out from the Sun, you’d better have plenty of surface area. Juno carries three 9-meter solar arrays that could, at Earth’s distance of 1 AU, generate as much as 14 kilowatts of electricity. But at Jupiter’s distance, controllers are expecting a realistic output of about 500 watts. Making solar power operations possible here is improved solar cell performance and a mission plan that avoids Jupiter’s shadow.
Image: This is the final view taken by the JunoCam instrument on NASA’s Juno spacecraft before Juno’s instruments were powered down in preparation for orbit insertion. Juno obtained this color view on June 29, 2016, at a distance of 5.3 million kilometers from Jupiter. Credit: NASA/JPL-Caltech/SwRI/MSSS.
Planning Rosetta’s Finale
The European Space Agency’s Rosetta mission is coping with the same issue. Rosetta will end its mission to Comet 67P/Churyumov-Gerasimenko on 30 September with a controlled descent to the surface. The increasing distance between the Sun and the comet alongside which Rosetta travels means that its own solar power will be insufficient to operate its instruments or downlink data. Thus Rosetta is destined to join the Philae lander on the comet’s surface.
“We’re trying to squeeze as many observations in as possible before we run out of solar power,” says Matt Taylor, ESA Rosetta project scientist. “30 September will mark the end of spacecraft operations, but the beginning of the phase where the full focus of the teams will be on science. That is what the Rosetta mission was launched for and we have years of work ahead of us, thoroughly analysing its data.”
Controllers will use much of August to adjust Rosetta’s trajectory, inducing a series of elliptical orbits that will progressively close on the comet. A trajectory change about twelve hours before impact will put the spacecraft on course for final descent. Rosetta will touch down at about half the speed of Philae, but there will be no possibility of communications from the orbiter once it reaches the surface because the high gain antenna will probably not be pointing toward Earth. Even so, we should get some spectacular images at high resolution during the descent.
This ESA news release has more, including mention of the fact that Rosetta entered safe mode last month when about five kilometers from the comet due to dust-related navigation system issues. While the spacecraft recovered, the glitch bears witness to how challenging operations this close to a comet can be. Bringing Rosetta down to the surface may create similar problems.
Image: During Rosetta’s final descent, the spacecraft will image the comet’s surface in high resolution from just a few hundred metres. This OSIRIS narrow-angle camera image was taken on 28 May 2016, when the spacecraft was about 5 km from the surface of Comet 67P/Churyumov-Gerasimenko. The scale is 0.13 m/pixel. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.
Dawn at Ceres
The Dawn spacecraft has been slotted to remain at Ceres rather than proceed on to the main belt asteroid Adeona. What this means is that we’ll be able to stay close to Ceres while the dwarf planet approaches perihelion, an interesting place to be given the many unanswered questions about this world and its unusual bright spots. A new study published in Nature finds that Ceres’ Occator Crater has the highest concentration of carbonate minerals ever seen outside the Earth.
The primary mineral in the brightest area of Occator is found to be sodium carbonate, the upwelling of which is suggestive of warmer conditions inside Ceres than previously believed. Thus we have an intriguing hint of liquid water in comparatively recent geological time, with the salts as possible remnants of a large internal body of water. Says Maria Cristina De Sanctis (National Institute of Astrophysics, Rome), lead author of the paper on this work:
“The minerals we have found at the Occator central bright area require alteration by water. Carbonates support the idea that Ceres had interior hydrothermal activity, which pushed these materials to the surface within Occator.”
Image: The center of Ceres’ mysterious Occator Crater is the brightest area on the dwarf planet. The inset perspective view is overlaid with data concerning the composition of this feature: Red signifies a high abundance of carbonates, while gray indicates a low carbonate abundance. Dawn’s visible and infrared mapping spectrometer (VIR) was used to examine the composition of the bright material in the center of Occator. Using VIR data, researchers found that the dominant constituent of this bright area is sodium carbonate, a kind of salt found on Earth in hydrothermal environments. Scientists determined that Occator represents the highest concentration of carbonate minerals ever seen outside Earth. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
The paper is De Sanctis et al., “Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres,” published online by Nature 29 June 2016 (abstract).
On to the Kuiper Belt
New Horizons, has been given the go-ahead for an extended mission, which includes a flyby of the Kuiper Belt object 2014 MU69. Although many of us have been taking an extended mission for granted given New Horizons’ unprecedented success, the confirmation brings a sense of relief. The flyby is to take place on January 1, 2019, offering us the chance to look at the kind of ancient object considered to be a building block of the Solar System. As we look ahead, we still have a wealth of data from the Pluto/Charon encounter to receive and analyze.
Arrival: Juno in Orbit
People in the space business always joke about the stress levels at any launch, but if you’re keeping tabs on a billion dollar spacecraft like Juno, I’d say the arrival can create just as many, if not more, gray hairs. Plenty of people are breathing easier this morning after Juno’s successful 35-minute engine burn and entry into orbit around Jupiter, confirmation of which came in just before midnight Eastern US time (03:53 UTC on July 5). Congratulations to the entire team.
All of this was part of a sequence of arrival events — Juno’s orbit-insertion phase (JOI) — that included spinning up the spacecraft from 2 to 5 revolutions per minute as an aid to stability, along with attitude changes in anticipation of the main engine burn, which began at 23:18 EDT. The latter decreased the spacecraft’s velocity by 542 meters per second to make orbital capture possible. Juno has already been turned again to allow its solar cells to work at full capacity.
Image: The Juno team celebrates at NASA’s Jet Propulsion Laboratory in Pasadena, California, after receiving data indicating that NASA’s Juno mission entered orbit around Jupiter. Rick Nybakken, Juno project manager at JPL, is seen at the center hugging JPL’s acting director for solar system exploration, Richard Cook. Credit: NASA/JPL-Caltech.
I always love control room photos when things are going well, remembering especially the New Horizons team last summer, images of which mingled joy with astonishment at what the doughty spacecraft was seeing. Juno is now in a 53.5 day orbit in preparation for an eventual 14-day orbit that will be achieved after a final engine burn on October 19. It’s at that point that the mission’s primary science collection period begins.
In this Cornell University news release, Jonathan Lunine, a member of the university’s Carl Sagan Institute, likens Juno’s work at Jupiter to an older discipline here on Earth, one that can help us understand the earliest days of the Solar System. What sort of materials, for example, did Jupiter take in as it grew into the gravitational behemoth it is today?:
[Jupiter is] a unique record for the outer solar system of what these protoplanets might have been like. We’re doing the astronomical equivalent of ‘broken pottery’ archaeology, trying to piece back together the original molecules and ice grains that got evaporated and dissociated inside Jupiter billions of years ago.”
We’ll have the opportunity to look at Jupiter in a number of new ways. The Galileo probe was unable, for example, to measure the water abundance in Jupiter beneath the clouds, but Juno’s microwave radiometer should be able to provide that measurement. Water abundance, in turn, tells us something about the materials that Jupiter absorbed early in its life, And by extension, we can apply this knowledge to the numerous gas giants we’re finding around other stars.
But it’s also going to be fascinating to learn whether or not the giant planet has a solid core. Juno’s gravity experiment will bring the spacecraft to within a few thousand kilometers of the cloud tops, allowing a measurement of the gravity field accurate enough to make the call. Bear in mind that the Cassini probe will burn up in Saturn’s atmosphere in 2017. During its final close flybys inside the rings, the same gravity experiment can be run. Says Lunine:
“[Cassini] will be passing just above the cloud tops like Juno does at Jupiter, underneath the rings of the planet, which will be pretty spectacular. The chance to be able to measure the core for both Jupiter and Saturn is really a tremendous opportunity.”
Bear in mind that even before Juno got to Jupiter, a number of science operations were already in progress, including work with the Jupiter Energetic Particle Detector Instrument (JEDI) to investigate the interplanetary medium as the spacecraft approached. Based at the Johns Hopkins University Applied Physics Laboratory, the JEDI team has been looking at ‘upstream ions,’ as explained by Dennis Haggerty, APL’s instrument scientist for the JEDI investigation:
“Jupiter is a very leaky planet. It has a unique particle identity, especially in terms of sulfur — which is not found in high numbers in the solar wind — and we’ve seen particles from Jupiter ‘upstream’ of the planet from missions including Voyager, Galileo and New Horizons.”
More in this APL news release, which describes the JEDI instrument and its upcoming work on Jupiter’s aurorae, which have a power density ten times greater than Earth’s, and an overall power that is greater by a factor of 100. JEDI will help explain how this system is energized.
Image: This artist’s concept depicts NASA’s Juno spacecraft above Jupiter’s north pole. Launched in 2011, the Juno spacecraft will arrive at Jupiter on July 4, 2016. As part of its instrument suites, it carries three Johns Hopkins APL-built Jupiter Energetic Particle Detector Instrument (JEDI) units to study the giant planet from an elliptical, polar orbit. Juno will repeatedly dive between the planet and its intense belts of charged particle radiation, coming only 5,000 kilometers (about 3,000 miles) from the cloud tops at closest approach. Juno’s primary goal is to improve our understanding of Jupiter’s formation and evolution. Credit: NASA/JPL-Caltech.
What Lies Beneath
Juno invariably calls up memories of Arthur C. Clarke’s A Meeting with Medusa, a 1971 novella that Greg Benford mentioned in a conversation this past weekend. It’s been many years since I’ve read it, but I may have to revisit the work now that Alastair Reynolds and Stephen Baxter have produced a sequel, a timely arrival given the Juno activities. The Medusa Chronicles evidently presents Clarke’s character Howard Falcon with a host of new challenges, but I’ll want to refresh my memory of the Clarke before tackling it.
In Clarke’s tale, Falcon skippers a balloon craft making a slow descent through Jupiter’s upper atmosphere, where he runs into enormous life-forms, one the Medusa of the title. And here we go back to another Cornell physicist who changed our view of Jupiter, Edwin Salpeter. I’ll send you to Larry Klaes’ fine 2009 essay Edwin Salpeter and the Gasbags of Jupiter for a more detailed look, but I do want to at least mention Salpeter’s work with Carl Sagan on life in the atmospheres of gas giants on the morning of the Juno arrival.
The duo produced a paper titled “Particles, Environments, and Possible Ecologies in the Jovian Atmosphere” that appeared in The Astrophysical Journal in late 1975. Sagan would go on to discuss such life-forms in the popular TV series Cosmos. Let me quote Larry on the kinds of life Sagan and Salpeter imagined:
Sagan and Salpeter envisioned three main types of Jovian creatures. There were sinkers, small organisms which were constantly falling towards the deadly deep, dense, and hot layers of the planet but always managed to survive long enough to produce offspring that would stay up in the more habitable air layers to repeat their cycle of life. The other aerial residents of Jupiter were known as floaters, which Sagan would later describe as being “kilometers across, enormously larger than the greatest whale that ever was, beings the size of cities.” Floaters were seen as drifting across the vast alien sky in great herds, looking like a collection of immense balloons, which in essence there were, using the lighter elements of Jupiter’s atmosphere to stay aloft.
Image: Physicist Edwin Salpeter, whose work with Carl Sagan brought the idea of airborne life in Jupiter’s clouds to a wide audience.
Throw a class of hunter species into the mix and you have a vibrant and violent ecology, one that, the paper pointed out, could have been detected by the Voyager probes’ cameras if a floater of this sort existed and were high enough up in the cloud deck to be seen. Presumably Juno’s views will be spectacularly better, so perhaps some scrutiny of its imagery in search of unusual moving objects could be advocated. In any case, it’s a shame that Clarke, Sagan and Salpeter couldn’t be here to witness Juno’s arrival and the abundant imagery to follow of a realm they once imagined so vividly.
Interstellar Comparisons
No one thinks big better than Adam Crowl, a Centauri Dreams regular and mainstay of the Icarus Interstellar attempt to reconfigure the Project Daedalus starship design of the 1970’s. If you’re looking for ideas for science fiction stories, you’ll find them in the essay below, where Adam considers the uses to which we might put the abundant energies of the Sun. Starships are a given, but what about terraforming not just one but many Solar System objects? Can we imagine a distant future when our own Moon is awash with seas, and snow is falling on a Venus in the process of transformation? To keep up with Adam, be sure to check his Crowlspace site regularly. It’s where I found an earlier version of this now updated and revised essay.
By Adam Crowl
By 2025 Elon Musk believes SpaceX can get us to Mars – a journey of about 500 million kilometres, needing a speed of over 100,000 km/h. By comparison travelling to the stars within a human lifetime via the known laws of physics requires energies millions of times more potent than that budget-price trip to Mars. In our energy hungry modern world the prospect seems fanciful, yet we are surrounded by energies and forces of comparable scale. By taming those forces we will be able to launch forth towards the stars, save our civilization and extend the reach of our biosphere.
How so? Consider the sunlight received every second by planet Earth, from the Sun. About 1.4 kilowatts of energy for every square metre directly facing the Sun – all 128 trillion of them – means a total power supply of 175,000 trillion watts (175 petawatts). That’s 8,750 times more than the mere 20 terawatts human beings presently use. Earth itself receives a tiny fraction of the total available – the Sun radiates about 2.2 billion times more, a colossal 385 trillion trillion watts (385 yottawatts).
Just how much does a starship need?
Project Daedalus proposed a fusion propelled star-probe able to fly to nearby stars in 50 years. To do so it would fuse 50,000 tonnes of deuterium and helium-3, expelling them as a rocket exhaust with an effective jet speed of 10,000 km/s. A total useful energy of 2500 million trillion joules (2.5 zettajoules) – the actual fusion energy available in the fuel was about 10 times this, due to the inefficiency of the fusion rocket motor. However that gives us a useful benchmark. Though vast, this is dwarfed by the energy from the Sun. A full Daedalus fuel-tank is equivalent to just 4 hours of sunlight received by planet Earth.
Image: The Daedalus starship, as envisioned by Adrian Mann.
Another design, the Laser-Sail, masses 2,500 metric tons and requires a laser power of 5 petawatts, to accelerate the Laser-Sail starship at 1 gee for 190 days to a cruise speed of half light-speed or 150,000 km/s. A laser-power equal to what Earth intercepts from the Sun, 175 petawatts, could launch ~67 Laser-Sail starships per year. Total energy required per sail is 8.24 yottajoules, equal to 5.45 days of Earth-sunlight.
What else could we do with power supplies that can launch starships? Power on the scale of Worlds (i.e. One Earth = 175 petawatts) allows the remaking of Worlds. Terraforming is the shaping of the dead worlds of the Solar System into more life-friendly environments. Mars, for example, is considered to be the most life-friendly nearby planet other than Earth, yet it lacks an oxygen atmosphere, a significant magnetic field, and is colder than Antarctica. To release Earth-levels of oxygen from its rocks, power an artificial magnetosphere to deflect away the potentially harmful solar-wind, add nitrogen to reduce the fire risk, and keep the planet warm, the energies required are similar to those required to launch starships.
Releasing oxygen from Martian rocks requires melting the rock, usually composed of about 30% oxygen, and breaking the chemical bonds – a process called pyrolysis. What results is a melt of mixed metals, like iron, and semi-metals, like silicon, and oxygen gas, plus some unmelted refractory compounds like aluminum oxide. For every kilogram of oxygen released, about 30 megajoules of energy is needed. Earth-normal oxygen levels require a partial pressure of 20 kilopascals (20 kPa), which means a mass of 5.4 tons of oxygen for every square metre of Martian surface – 775 trillion tons in total. The total energy required is 10 yottajoules.
Adding 80 kPa of nitrogen, like Earth’s atmosphere, requires mining the frozen nitrogen of Neptune’s moon Triton, doubling the total energy required. Pluto’s vast plains of convecting nitrogen ice may provide another possible source, though without the handy proximity of a big planet’s gravity well for getting a boost towards the Sun it might prove uneconomical in energy terms. Shipping it from Saturn’s moon Titan, as Kim Stanley Robinson imagines in his ‘Mars Trilogy’, requires 8 times the energy of using Triton as a source, due to Saturn’s less favourable gravity conditions.
Warming Mars to Earth-like levels, via collecting more solar energy with a vast solar mirror array, means collecting and directing about 50 petawatts of solar energy (equal to about 10 Laser-Sail starships). Before we use that energy to gently warm Mars, it can be concentrated via a “lens” into a solar-torch able to pyrolyse oxygen out of Mars’s rocks. With 50 petawatts of useful energy the lens can liberate sufficient oxygen for breathing in a bit over 6 years.
Image: A blue and green Mars emerges from terraforming. Credit: Daein Ballard – The original image was uploaded on en.wikipedia as en:Image:MarsTransitionV.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=939978
The final task, creating an artificial magnetosphere, is puny by comparison. A superconducting magnetic loop, wrapped around the Martian equator, can be used, powered up to a magnetic field energy of ~620,000 trillion joules (620 petajoules), by about 12.4 seconds of energy from the solar-mirrors. This is sufficient to create a magnetosphere about 8 times the size of Mars, much like Earth’s.
Total one-time energy budget is 20 yottajoules – 8,000 “Daedalus” starprobes, or 243 laser-sail starships equivalent. The ongoing power-supply of 50 petawatts is enough to propel 10 Laser-Sail starships at a time.
To terraform the other suitable planets and moons of the Solar System requires similar energy and power levels. For example, if we used a solar-torch to break up the surface ice of Jupiter’s moon, Europa, into hydrogen and oxygen, then used it to ‘encourage’ the excess hydrogen to escape into space, the total energy would be about 8 yottajoules, surprisingly similar to what Mars requires. The nitrogen delivery cost is about 6 yottajoules, again similar to Mars. Ongoing energy supply would be 10 petawatts – two starships worth.
A less exotic location to terraform would be Earth’s Moon. One advantage, as well as proximity to Earth, is no extra input of energy from the Sun is needed to stay warm. However, unlike Europa or Mars, water as well as atmosphere needs to be delivered, multiplying the energy required. If shallow seas are sufficient – an average of 100 metres of water over the whole surface – the energy to deliver ice and nitrogen from Triton, then make oxygen from lunar rocks, is 27 yottajoules.
The only solid planet with close to Earth gravity is Venus. To remake Venus is a vastly more challenging task, as it has three main features that make it un-Earthly: too much atmosphere, too much day-time and not enough water. Take away the atmosphere and the planet would cool rapidly, so while it is often likened to Hell, the comparison is temporary. The energy required to remove 1 kilogram from Venus to infinity is 53.7 megajoules. Venus has over a thousand tons of atmosphere for every square metre of surface – some 467,000 trillion tons of which is carbon dioxide. To remove it all requires 25,600 yottajoules, thus removal is far from being an economical option, even in a future age when yottajoule energy budgets are commonplace.
Another option is to freeze the atmosphere by shading the planet totally. To do so would require placing a vast shade in an orbit between Venus and the Sun, about a million kilometres closer. In this position, the gravity of the Sun and Venus are balanced, allowing the shade to stay fixed in the sky of Venus. With a diameter about twice Venus’s 12,100 kilometres, an endless night produced by the shade would allow Venus to cool down over a period of decades. Eventually the carbon dioxide would rain, then snow, covering the planet in dry-ice. Some form of insulation (foamed rock?) would then be spread over the carbon dioxide to keep it from bursting forth as gas again.
Alternatively it might be pumped into natural cavities, once the sub-surface of Venus is better mapped. The energy cost of assembling such a vast shade, which would mass thousands of tonnes at least, would be far less than the cost of removing the carbon dioxide. So close to the Sun, the shade would intercept the equivalent of 8 times what Earth receives from the Sun – 1,400 petawatts in total, sufficient to propel 280 Laser-Sail starships, or power the terraforming of the other planets. Or both.
The next desirable for Venus is the addition of water. If 100 metres depth is required the total energy to ship it from Triton is 144 yottajoules. Using 50 petawatts of power, the time to export the water is about 122 years, with a 30 year travel time for ice falling Sunwards from Neptune. The total energy of creating an artificial magnetosphere similar in size to Earth’s would be 6 exajoules (6 million trillion joules) – a tiny fraction of the energy budget.
Image: Venus as seen by the Japanese Akatsuki orbiter. The planet was captured in infrared light, showing a surprising amount of atmospheric structure on its night side. The vertical orange terminator stripe between night and day is so wide because of light diffused by Venus’ thick atmosphere. Can we use the Sun’s abundant energies one day to transform this world into a home for life? Credit: ISAS/JAXA.
Further afield than the Inner System and the Outer Planets (including IX, X, XI…) is the Oort Cloud, a spherical swarm of comets thousand to ten thousand times the Earth-Sun distance. According to current planet formation theories there were once thousands of objects, ranging in size from Pluto to Earth’s Moon, which formed out of the primordial disk of gas and dust surrounding the infant Sun. Most coalesced via collisions to form the cores of the big planets, but a significant fraction were slung outwards by gravitational interactions with their bigger siblings, into orbits far from the Sun. One estimate by astronomer Louis Strigari and colleagues hints at 100,000 such objects for every star.
The technology to send a laser beam to a starship accelerating to half light-speed over thousands of Earth-Sun distances opens up that vast new territory we’re only just beginning to discover. A laser able to send 5 petawatts to a Laser-Sail at 1,000 times the Earth-Sun distance, would be able to warm a Pluto-sized planet to Earth-like temperatures at a distance of a light-year. Powering starships will thus enable the spread of the Earth’s biosphere to thousands of worlds which would otherwise remain lifeless. Life on Earth spread out in abundance, aeons ago, once it learnt the trick of harnessing the Sun’s energy via photosynthesis to make food from lifeless chemicals. Bare new volcanic islands are quickly colonised by living things, thanks to the power of the Sun. Humankind can do the same, but on a vastly greater scale – it’s the natural thing to do.
Calibrating Distances to Low Mass Stars
Accurate distances are critical for understanding the physical properties of brown dwarfs and low-mass stars. We need to know the intrinsic brightness of these objects to proceed, but we can’t know that until we have an idea of their distance. After all, a relatively faint star can seem much brighter if nearer to us, while a distant bright star can appear deceptively dim. Intrinsic brightness is a measure of how stars would appear if observed at a common distance.
Enter an exoplanet search that began at the Carnegie Institution for Science in 2007, using the Carnegie Astrometric Planet Search Camera (CAPSCam) to look for gas giant planets and brown dwarfs orbiting nearby low-mass stars. A new report from the program tells us that it has measured the distance to 134 low-mass stars and brown dwarfs, 38 of which have had no previously measured trigonometric parallax.
These are all stars considered too faint for inclusion in the European Space Agency’s Hipparcos survey, but as the report explains, they are considered “…the best templates for studies of radii, atmospheric composition, metallicity, and other spectroscopic properties. In addition, low mass stars with excellent distances provide the templates for spectrophotometric distances to more distant stars.”
The Carnegie survey has focused on M, L and T dwarfs in the southern sky (CAPSCam operates on the 2.5-m du Pont instrument at the Las Campanas Observatory
in Chile). All the objects surveyed were believed to be within 20 parsecs (about 65 light years). The CAPSCam work is primarily about searching for exoplanets using astrometry, which attempts to detect the wobble of a host star around the center of mass of the system. The wobble is a telling indicator that a planetary object is affecting the host star’s motion.
Usefully for distance studies, CAPSCam is also capable of collecting parallax information. Here a target’s position in the sky is measured against a far more distant background from different points in the Earth’s orbit, allowing us to calculate the distance of the closer object. Stellar parallax was the method first used by Friedrich Bessel in measuring the distance to the star 61 Cygni at Königsberg Observatory. Accurate parallax computations from Earth’s surface are limited to about 300 light years because the more distance a star is, the smaller its parallax.
Image: A brown dwarf in relation to Earth, Jupiter, a low-mass star and the Sun. Credit: NASA.
Among the more interesting finds is GJ 3470, an M-dwarf with no previously published parallax. The Carnegie team comes up with a distance of roughly 29 parsecs, or 95 light years. GJ 3470 is significant because it is one of the few low-mass stars known to host a large planet, in this case a Neptune analog. The distance measurement allowed the researchers to refine earlier radius and density estimates, finding the planet is about half as dense as Neptune.
Also interesting is 2MASS J01392170?3936088, previously computed at about 15 parsecs, but now found to be slightly less than 9 parsecs (29 light years). Another brown dwarf, 2MASS J01365662+0933473, had no previously computed parallax. The new work finds it at a distance of 6.4 parsecs (20 light years). A number of earlier parallax distance computations are revisited in the paper, in some cases as much as doubling the previously calculated number.
In general, the CAPSCom work meshes with earlier parallax studies, with a few exceptions. From the paper:
For 79 of the 96 stars with previously published parallaxes, our measurements have lower uncertainty. In general there is very good agreement between ours and previous measurements; only 12 of the 96 disagree by more than 3 ? (of the less accurate measurement), and for 8 of these 12, the difference in parallax is <5%.
And as mentioned earlier, 38 of the CAPSCam targets had no previously measured parallax. Measurements like these can also help us learn a given star’s age, as witness G 161-71, whose parallax combined with the star’s radial velocity allows the team to peg it as part of the 30-50 million year old Argus association, a so-called ‘moving group’ of stars of similar age. The M-dwarf LP 870-65 is similarly found to be part of another moving group, the AB Doradus association, allowing the researchers to peg its age as approximately 100 million years.
Any fine-tuning of distance measurements is useful in determining stellar properties, but we’re especially interested in brown dwarfs because of the clues they may offer about star formation. After all, they can be found in mass ranges between gas giant planet and small star. M-dwarfs, meanwhile, are already known to host numerous planets, whose transit signatures will be studied by upcoming missions like TESS (Transiting Exoplanet Survey Satellite) and PLATO (PLAnetary Transits and Oscillations of stars). All this is part of populating our catalog of planets around nearby brown dwarfs, M-dwarfs, K-dwarfs and G-class stars like the Sun.
The paper is Weinberger et al., “Trigonometric Parallaxes and Proper Motions of 134 Southern Late M, L, and T Dwarfs from the Carnegie Astrometric Planet Search Program,” The Astronomical Journal Vol. 152, No. 1 (2016). Abstract / Preprint.
Deep Stare into a Dusty Universe
It’s not often that I get the chance to back up and take a broad look at the universe, the kind of thing that reinforces my interest in cosmology and structure at the grandest scale. But today I’ll take my cue from the Royal Astronomical Society’s annual meeting, now underway in Nottingham UK, which gives me the chance to look at a new catalog from the European Space Agency’s Herschel Space Observatory. On offer is a guide to hidden sources of energy in the universe, on a scale at which the Milky Way itself is but a bit of froth on a cosmic wave.
As presented by Haley Gomez (Cardiff University) at the National Astronomy Meeting, the project known as the Herschel Astrophysical Terahertz Large Area Survey (Herschel-ATLAS) is offering a deep look at galaxies through time. Because about half the light emitted by stars and galaxies is absorbed by interstellar dust grains, Herschel’s ability to work in the far-infrared can reveal that light re-emitted, showing us the sources of energy that would be hidden at visible wavelengths, and at a level of detail that has previously been beyond our reach.
The work highlights half a million far-infrared sources near and far, including galaxies twelve billion light years away, less than two billion years after the Big Bang. The latter are dusty and difficult to observe with conventional telescopes, and as the image below reveals, they are often gravitationally lensed by closer galaxies in the same field of view. Gravitational lensing occurs when an intervening massive object distorts spacetime so that light, following the curved path, is ‘bent’ and magnified. The effect is now exploited in a wide range of astronomical studies.
Image (click to enlarge): An illustration of the time reach of the Herschel ATLAS and the kinds of objects it has discovered. The Big Bang occurred 13.7 billion years ago. The points in the diagram (approximately 40,000) show some of the Herschel ATLAS objects. The survey discovered nearby galaxies fairly similar to our own (see galaxies to the top left), and also galaxies that are so far away that we see them as they were only two billion years after the Big Bang. The figure to the bottom right shows a detailed image made with the Atacama Large Millimetre Array of one of these early galaxies. The spectacular ‘Einstein ring’ shows that the far-infrared emission from this source has been gravitationally lensed by the gravitational field of an intervening galaxy. These ultra-distant galaxies are forming stars at a rate 1000 times greater than in the Milky Way and are shrouded by dust from the view of optical telescopes. These violently star-forming and dusty galaxies are the ancestors of the galaxies around us in the Universe today. Credit: The Herschel ATLAS team, the European Space Agency, ALMA and NRAO.
It’s intriguing to note that some galactic change can still be traced in the relatively recent (in astronomical terms) past. Even one billion years ago, the Herschel ATLAS catalog demonstrates, galaxies were undergoing faster star formation and were more dusty than galaxies today. Nathan Bourne (University of Edinburgh) describes the result:
“We were surprised to find that we didn’t need to look far in the past to see signs of galaxy evolution. Our results show that the reason for this evolution is that galaxies used to contain more dust and gas in the past, and the universe is gradually becoming cleaner as the dust is used up.”
Thus we have a look at cosmic history from the violent star-formation periods in a roiling early universe to a far more sedate cosmos today. Bear in mind that only a few hundred dusty sources from the early universe have been previously studied, whereas the Herschel results offer half a million galaxies as cosmic context, a potential game-changer in our study of early star formation. Says Göran Pilbratt (Herschel Project Scientist): “Although Herschel made its last observation in 2013, current and future generations of astronomers will find the H-ATLAS maps and catalogues essential for finding their way around the hidden universe.”
From the paper (internal references omitted for brevity):
The chief original goal of the survey was to provide a relatively shallow Herschel survey over a very large area of sky, with the specific aims of providing measurements of the dust masses and dust-obscured star formation rate for tens of thousands of nearby galaxies…, complementing the large optical spectroscopic surveys of the nearby Universe, such as the Sloan Digital Sky Survey (SDSS)… and the Galaxy and Mass Assembly project (GAMA). However, the exceptional sensitivity of Herschel, aided by the large and negative k?correction at submillimeter wavelengths…, has meant that a significant fraction of the sources in H-ATLAS actually lie at high redshift…. The H-ATLAS survey is therefore useful both to astronomers studying the nearby Universe and to those studying the early (z > 1) Universe.
And a note of exoplanetary interest that I wouldn’t have expected:
The large area of the survey means that there are also potential uses for it in Galactic astronomy…, with one practical example being a search for debris disks around stars…
Image (click to enlarge): A small glimpse of one region, a tenth of the full area of the Herschel ATLAS images. Everything in this image, apart from the picture of the Moon, which has just been placed there to show the area of sky covered by the survey and the small square that shows the area covered by the Hubble Deep Field, consists of far-infrared emission from cosmic dust. The faint wisps are far-infrared emission from dust grains in the Milky Way but everything else in the image is a dusty galaxy. There are approximately 6000 dusty galaxies detected in this image, while the entire survey contains roughly half a million dusty galaxies, from galaxies similar to our own, to violently star-forming and very dusty galaxies that are being seen as they were over ten billion years ago. This image also shows how the field of hidden astronomy has evolved. The Hubble Deep Field was the first area surveyed by a dust sensitive camera called SCUBA almost 20 years ago. Five galaxies were found and the observations took 50 hrs, meaning it took 10 hours observing time to detect a galaxy. The Herschel-ATLAS maps released today cover an area 100,000 times larger and it took Herschel only 5 seconds on average to detect a galaxy in these images. Credit: The Herschel ATLAS team and the European Space Agency.
Two papers are in press at Monthly Notices of the Royal Astronomical Society. The only one I’ve been able to study so far is Valiante et al., “The Herschel-ATLAS Data Release 1 Paper I: Maps, Catalogues and Number Counts,” not yet available on arXiv, but I’ll get the links to it and a companion paper up as soon as I have them.
Spacecoach: Toward a Deep Space Infrastructure
With manned missions to Mars in our thinking, both in government space agencies and the commercial sector, the challenge of providing adequate life support emerges as a key factor. We’re talking about a mission lasting about two years, as opposed to the relatively swift Apollo missions to the Moon (about two weeks). Discussing the matter in a new essay, Brian McConnell extends that to 800 days — after all, we need a margin in reserve.
Figure 5 kilograms per day per person for water, oxygen and food, assuming a crew of six. What you wind up with is 24,000 kilograms just for consumables. In terms of mass, we’re in the range of the International Space Station because of our need to keep these astronauts alive. McConnell, a software/electrical engineer based in San Francisco, has been working with Alex Tolley on the question of how we could turn most of these consumables into propellant. The idea is to deploy electric engines that use reclaimed water and waste gases to do the job.
With a nod to the transportation technologies that opened the American West, McConnell and Tolley have dubbed the idea a ‘Spacecoach.’ Centauri Dreams readers will remember Tolley’s Spaceward Ho! and McConnell’s A Stagecoach to the Stars, and the duo have also produced a book on the matter for Springer called A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach. The new essay is a welcome addition to the literature on what appears to be a practical concept.
What fascinates me about the Spacecoach is that it enables us to begin building a space infrastructure that can extend past Mars to include the main asteroid belt. Using electric propulsion driven by a solar photovoltaic array, it achieves higher exhaust velocity than chemical rockets by a factor or ten, pulling much greater delta v from the same amount of propellant. Use water as propellant and you reduce the mass of the system by what McConnell estimates to be a factor of between 10 and 20. Huge reductions in cost follow.
Water as propellant? McConnell comments:
Electric propulsion is not a new technology, and has been used on many unmanned spacecraft. The idea is to use an external power source, typically a solar photovoltaic array, to drive an engine that uses an electrical or magnetic field to heat and accelerate a gas stream to great speed (tens of kilometers per second). Because these engines can achieve much higher exhaust velocity than chemical rockets, 10x or better, they can achieve greater change of velocity (delta v) using the same amount of propellant. This means they can venture to more ambitious destinations, carry more payload, or a combination of both. It also turns out these engines can also use a wide range of materials for propellant, including water.
Image: Rendering of the “kite” design pattern for a Spacecoach, with a person shown to the right for scale. This is but one possible configuration, but McConnell notes that the pattern minimizes the materials required even as it provides a sizeable habitable area. Credit: Rudiger Klaen.
We can imagine such ships as interplanetary vessels that never enter an atmosphere. They’re also completely reusable, allowing costs to be amortized, and their habitable areas are large inflatable structures that can be assembled in space. Thus we travel within a modular spacecraft using external landers and whatever other modules are required by the mission at hand. They’re also, compared to today’s chemical rocket payloads, a good deal safer:
The use of water and waste gases as propellant, besides reducing the mass of the system by a factor of ten or more, has enormous safety implications. 90% oxygen by mass, water can be used to generate oxygen via electrolysis, a simple process. By weight, it is comparable to lead as a radiation shielding material, so simply by placing water reservoirs around crew rest areas, the ship can reduce the crew’s radiation exposure several fold over the course of a mission. It is an excellent heat sink and can be used to regulate the temperature of the ship environment. The abundance of water also allows the life support system to be based on a one-pass or open loop design. Open loop systems will be much more reliable and basically maintenance free compared to a closed loop system such as what is used on the ISS. The abundance of water will also make the ships much more comfortable on a long journey.
Having just watched “To the Ends of the Earth,” a superb BBC story about a ship making a passage from Britain to Australia in the age of sail, the word ‘comfortable’ catches my eye. A Spacecoach is a large craft with huge solar arrays and the capability of being spun to generate artificial gravity, thus alleviating another major health hazard. Conditions are more Earth-like, and the abundance of water makes for what would otherwise seem absurd scenarios. Imagine taking a shower on a flight to Mars! The Spacecoach’s water management makes it possible.
McConnell believes that much of the mission architecture can be validated on Earth without the need to build a full-scale spacecraft, with the major emphasis on tuning up the electric propulsion technology that drives the concept. Using water, carbon dioxide and waste gases to test the engines can be the subject of an engineering competition, after which the engines could be tested in small satellites. Ultimately, manned Spacecoaches could be tested in cislunar space before their eventual deployment deeper into the Solar System.
Image: An artist’s concept of two Bigelow BA 330 inflatable modules configured into a space station. Modules like these could provide habitable areas for a Spacecoach. Credit: Bigelow Aerospace (http://www.bigelowaerospace.com).
McConnell calls the Spacecoach the basis of a ‘real world Starfleet,’ and adds this:
These ships will not be destination specific. They will be able to travel to destinations throughout the inner solar system, including cislunar space, Venus, Mars and with a large enough solar photovoltaic sail, to the Asteroid Belt and the dwarf planets Ceres and Vesta. They’ll be more like the Clipper ships of the past than the throwaway rocket + capsule design pattern we’ve all grown up with, and their component technologies can be upgraded with each outbound flight.
So if you haven’t acquainted yourself with McConnell and Tolley’s earlier work on the Spacecoach in these pages, have a look at Traveling to Mars? Just Add Water!, which recaps the basics of the design and outlines surface exploration strategies from orbiting Spacecoaches by telepresence. The key, though, is to mitigate the propellant issue by making consumables into propellant. Get that right and much else will follow, including the prospect of reliable, safe interplanetary transport of the kind needed to build a truly space-going civilization.
And after that? I’ve always believed that after sending instrumented interstellar probes, we’ll expand into regions outside our Solar System slowly, building space habitats as we go, mining local objects for needed materials. A functioning, space-going civilization builds out that infrastructure from within. It’s the ‘slow boat to Centauri’ scenario — our machines, enabled by artificial intelligence, get there first — but it’s a deep future that includes a human presence around other stars. When I see something as evidently practical as the Spacecoach, I get a renewed jolt of confidence that we at least know how to begin such a journey.
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