by Paul Gilster | Aug 12, 2016 | Exoplanetary Science |
I often work out my thoughts on the topics we discuss here while taking long walks. I try to get in five miles a day but more often it’s about three. In any case, these long, reflective walks identify me as the neighborhood eccentric, an identity that is confirmed by the things I write about. What’s interesting about that is that so many people have a genuine interest in the stars and how we might get there. Some of the best questions I’ve ever had have been from people whose interest is casual but persistent, and one good question usually leads to another.
Hence I wasn’t surprised on yesterday’s walk to find myself talking with a neighbor about exomoons and why we study them. After all, we have a Solar System in which moons are commonplace. Isn’t it perfectly obvious that different solar systems would have planets with moons?
The answer is yes, but it also follows that things that seem perfectly obvious still have to be confirmed. But let’s unpack it a bit more than that. We’re familiar with our own system’s configuration, in which moons of astrobiological interest are orbiting gas giants a long way from the inner system. But we know from our exoplanet work that large planets like these can exist in warmer places. Thus the notion of habitable moons around gas giants, or perhaps double planets in the habitable zone, something like a larger version of Pluto and Charon in a comfortable orbit.
Popular films like Avatar keep the exomoon theme in front of the public, whose interest is understandable. After all, could anything be more exotic than a warm gas giant orbited by something a bit like the Earth? From an astrobiological perspective, the thought of Europa or Titan analogs in warm orbits is thrilling, a reminder that life may have gained many footholds in the galaxy. The Hunt for Exomoons with Kepler project is all about figuring out the occurrence rate of large moons so we can learn whether such moons are common.
The other aspect of exomoon detection has to do with increasing our expertise. It wasn’t so long ago that we had yet to detect our first exoplanet. Now we’re delving into planetary atmospheres and working out the orbital dynamics of multi-planet systems. An exomoon detection would be a major proof of concept, demonstrating the growth of our skills. It would also begin to build an exomoon catalog that will help us understand how important exomoons may be to planetary habitability. How big a role does our own moon play in keeping our planet habitable?
There’s also plenty to learn about how planetary systems form in the first place. We now think our Moon formed in a massive collision (the Big Whack) with a Mars-sized object in the early days of our planet’s history. How likely an event is this, and how often does it happen in other Solar Systems? We still have a lot to learn about how the satellite systems around various planets emerge, especially when we consider the wild variety of moons we see in our Solar System. Building the exomoon catalog will help answer these questions.
The Joys of Beta Pictoris b
I hadn’t planned to get into exomoons today, but serendipity struck. After yesterday’s conversation I ran across Phil Plait’s latest essay for Slate. The popular astronomer and science popularizer (author of Death from the Skies! and, of course, Bad Astronomy), now explains that because of an unusual alignment beginning in 2017, we may be able to detect an exomoon, if there is one, around the planet Beta Pictoris b.
We’re dealing with a system far different from our own. Some 60 light years away, the star Beta Pictoris is more massive than the Sun and a mere infant, at 25 million years old, compared to our own star (around 4.5 billion years). This is a solar system in formation. Moreover, it has been under intensive study since scientists realized it was surrounded by a large circumstellar disk. The planet Beta Pictoris b was first imaged in 2003, a world more massive than Jupiter that orbits its host every 20 years. You can see its movement in the time-spaced images below.
Image: Infrared images of the planet ? Pictoris b obtained in 2003 (a), 2009 (b) and 2010 (c), showing the planet’s movement in an orbital plane that is nearly edge-on as seen from Earth. The host star is in the central part, but its light has been suppressed to show the fainter planet. The white dots in b and c denote previous positions of the planet. Faint blobs are optical effects. It is not possible to tell from these images whether the planet is orbiting towards or away from us, but {Ignas] Snellen and colleagues’ spectroscopic observations clearly indicate that the planet is currently in a part of its orbit where it is moving towards us. Credit: ESO.
Note in the description above that the planet’s orbital plane is close to edge-on from our perspective. It’s not close enough to make a transit possible, but what Plait talks about is
the next best thing. Drawing on a paper by Jason Wang (UC-Berkeley) and colleagues, Plait explains that the region around the planet called its Hill Sphere will pass in front of the star from our perspective. The Hill Sphere is the area around an astronomical body in which its gravity dominates. In other words, within the Hill Sphere, a moon could be retained by the planet.
Nobody explains such concepts as well as Phil Plait, so I’ll give him the floor here, drawing directly from his essay:
The size of the sphere depends on the mass of the planet, the mass of the star, and the distance between them. For example, the Earth’s Hill sphere reaches out to about 1.5 million kilometers. The Moon, orbiting 380,000 km away, is well inside that, so its motion is mostly influenced by the Earth (some people like to say the Moon orbits the Sun more than it does the Earth, but those people are wrong). Weirdly, Pluto’s Hill sphere is much larger than Earth’s, but that’s because it’s so far from the Sun that an object can orbit Pluto from farther away and still be heavily influenced by it.
What emerges with regard to Beta Pictoris b is that its Hill Sphere is 160 million kilometers in radius. We get no transit of the star by the planet itself, but by August of 2017, the planet will be at its closest approach to the star and the Hill Sphere region will transit. We’ll be able to look for debris or exomoons. A large moon passing in front of the star would be the first entry in the exomoon catalog.
But even if we get no exomoon detection, bear in mind that we may make other interesting observations. This young planet is still being born, and it may well contain a circumplanetary disk of its own, or even a ring system that is the residue of planet formation. “The transit of ? Pic b’s Hill sphere,” Wang et al. write, “should be our best chance in the near future to investigate young circumplanetary material.” We’ll also learn a lot more about how Beta Pictoris b perturbs the circumstellar disk, a window into early solar system formation.
All this is good material for my next walk and the conversations sure to follow. The paper is Wang et al., “The Orbit and Transit Prospects for ? Pictoris b constrained with One Milliarcsecond Astrometry,” accepted at the Astrophysical Journal (preprint).
by Paul Gilster | Aug 5, 2016 | Deep Sky Astronomy & Telescopes |
Those of you who have been following the controversy over the dimming of KIC 8462852 (Tabby’s Star) may remember an interesting note at the end of Bradley Schaefer’s last post on Centauri Dreams. Schaefer (Louisiana State University) had gone through his reasoning for finding a long-term dimming of the star in the DASCH (Digital Access to a Sky Century@Harvard) database. His third point about the star had to do with the work of Ben Montet (Caltech) and Joshua Simon (Carnegie Observatories).
Montet and Simon’s work relied on an interesting premise. Tabby’s Star had been discovered because it was in the Kepler field, and thus we had high-quality data on its behavior, the unusual light curves that the Planet Hunters team brought to the attention of Tabetha Boyajian. As the researchers note in a new paper, Kepler found ten significant dips in the light curve over the timespan of the Kepler mission, dips that were not only aperiodic but irregular in shape, and that varied enormously, from fractions of one percent up to 20% of the total flux of KIC 8462852.
Image: Montage of flux time series for KIC 8462852 showing different portions of the 4-year Kepler observations with different vertical scalings. Panel ‘(c)’ is a blowup of the dip near day 793, (D800). The remaining three panels, ‘(d)’, ‘(e)’, and ‘(f)’, explore the dips which occur during the 90-day interval from day 1490 to day 1580 (D1500). Credit: Boyajian et al., 2015.
Schaefer noted in his Centauri Dreams post (see Further Thoughts on the Dimming of KIC 8462852) that if Tabby’s Star were actually fading at a rate of 0.164 mag/cen, then it should have undergone fading during the period it was under observation by Kepler (in fact, it should have faded by 0.0073 mag over the Kepler lifetime on the main Cygnus field). Montet and Simon have now presented us with their analysis in a paper just up on the arXiv server.
A fading of the kind Schaefer described would be well above the photometric precision of the Kepler instrument. Montet and Simon realized they could search for long-term trends by using the full-frame images (FFI) collected during the Kepler mission. Eight of these were recorded at the beginning of the mission, with another FFI recorded each month throughout the mission. Given that the mission lasted four years, a star dimming at the rate Schaefer suggests should decrease in brightness by 0.6% over the Kepler baseline. And as the authors point out, using FFI data avoids the removal of the dimming trend by the data processing pipeline.
The results: The study, which worked with KIC 8462852 and seven nearby comparison stars, found that in the first three years of the Kepler mission, Tabby’s Star dimmed at a rate of 0.341%±0.041% per year. Over the next six months, it decreased in brightness by 2.5%, and then stayed at that level during the duration of the primary Kepler mission. The paper continues:
We then compare this result to a similar analysis of other stars of similar brightness on the same detector, as well as stars with similar stellar properties, as listed in the KIC, in the Kepler field. We find that 0.5% of stars on the same detector and 0.7% of stars with similar stellar properties exhibit a long-term trend consistent with that observed for KIC 8462852 during the first three years of the Kepler mission. However, in no cases do we observe a flux decrement as extreme as the 2.5% dip observed in Quarters 12-14 of the mission. The total brightness change of KIC 8462852 is also larger than that of any other star we have identified in the Kepler images.
Image: Photometry of KIC 8462852 as measured from the FFI data. The four colors and shapes (green squares, black circles, red diamonds, and blue triangles) represent measurements from the four separate channels the starlight reaches as the telescope rolls. The four subpanels show flux from each particular detector individually. The main figure combines all observations together; we apply three linear offsets to the data from different channels to minimize the scatter to a linear fit to the first 1100 days of data. In all four channels, the photometry is consistent with a linear decrease in flux for the first three years of the mission, followed by a rapid decrease in flux of ≈ 2.5% over the next six months. The light gray curve represents one possible Kepler long cadence light curve consistent with the FFI photometry created by fitting a spline to the FFI photometry as described in Section 4. The large dips observed by Boyajian et al. (2016) are visible but narrow relative to the cadence of FFI observations. The long cadence data behind this figure are available online. Credit: Montet & Simon.
M. A. Thompson (University of Hertfordshire) and colleagues published a recent study in Monthly Notices of the Royal Astronomical Society reporting their findings using millimetre and sub-millimetre photometry. The paper finds that a dust cloud orbiting Tabby’s Star would have to be no larger than 7.7 Earth masses of material within a radius of 200 AU, adding “Such low limits for the inner system make the catastrophic planetary disruption hypothesis unlikely.”
Montet and Simon don’t necessarily agree, but in any case there are other problems. The authors think the light curve is “…consistent with the transit of a cloud of optically thick material orbiting the star,” and that such a cloud could be small enough to meet Thompson and team’s requirements. The breakup of a small body or a recent collision producing a large dust cloud could also produce a cometary family that transited the host star as a single group. But we’re still not out of the woods:
To explain the transit ingress timescale, the cloud would need to be at impossibly large distances from the star or be slowly increasing in surface density. The flat bottom of the transit would then suggest a rapid transition into a region of uniform density in the cloud, which then continues to transit the star for at least the next year of the Kepler mission. Moreover, such a model does not naturally account for the long-term dimming in the light curve observed in both DASCH and the Kepler FFI data, suggesting that this idea is, at best, incomplete.
A deeply mysterious star, our KIC 846285. Montet and Simon call for alternative hypotheses and new data to help us explain existing observations, and we can be glad to have Tabetha Boyajian’s team on the case thanks to the success of the recent Kickstarter campaign. Observations are already in progress at the Las Cumbres Observatory Global Telescope Network, and the Kickstarter funds will take us deep into 2017. For more on the Las Cumbres work, see Corey Powell’s recent interview with Boyajian for Discover Magazine, from which this:
From our new observations, we’ll be able to tell a lot about the material that’s passing in front of the star: if it’s some kind of dusty thing, some kind of solid thing. [Boyajian’s working hypothesis is that the dimming is caused by a huge swarm of comets, set loose perhaps by some cataclysmic event around the star.] What’s also important is that we will also get a baseline of spectral observations so we can look at if there’s any radial velocity shift or if there’s any variable emission of the lines, things we’d expect comets to have.
The paper is Montet and Simon, “KIC 8462852 Faded Throughout the Kepler Mission,” submitted to the AAS Journals and available as a preprint. The Thompson paper on circumstellar dust in this system is “Constraints on the circumstellar dust around KIC 8462852,” published online by Monthly Notices of the Royal Astronomical Society 25 February 2016.
by Paul Gilster | Aug 3, 2016 | Antimatter |
Talking about antimatter, as we’ve done in the past two posts, leads to the question of why the stuff is so hard to find. When we make it on Earth, we do so by smashing protons into targets inside particle accelerators of the kind found at the Fermi National Accelerator Laboratory in Batavia, IL and CERN (the European Organization for Nuclear Research). It’s not exactly an efficient process from the antimatter production standpoint, as it produces a zoo of particles, anti-particles, x-rays and gamma rays, but it does give us enough antimatter to study.
But there is another way to find antimatter, for it occurs naturally in the outer Solar System and even closer to home. James Bickford (Draper Laboratory, Cambridge MA) has looked at how we might trap antimatter that occurs in the Earth’s radiation belts. In a report for NIAC back in 2006 (available here), Bickford laid out a strategy for using high temperature superconductors to form two pairs of RF coils with a radius of 100 meters, to be powered by nuclear or solar power. The idea is that the magnetic field created through the RF coils will concentrate and trap the incoming antiproton stream.
Now the model changes from production on Earth to harvesting natural antimatter in space. We get antimatter in the Solar System because high-energy galactic cosmic rays (GCR) bombard the upper atmosphere of the planets, causing ‘pair production,’ which is the creation of an elementary particle and its antiparticle. The kinetic energy of the cosmic ray particle is converted into mass when it collides with another particle. According to Bickford’s calculations, about a kilogram of antiprotons enter our Solar System every second, and any planet with a strong magnetic field is fair game for collection.
As the planet’s magnetic field holds the antimatter particles, they spiral along the magnetic field lines. This is a process that continually replenishes itself both for matter and antimatter. Jupiter is a source, but Saturn is even better, for a larger flux enters its atmosphere. Saturn is, in fact, the place where the largest total supply of antiprotons appears, with reactions in its rings injecting 250 micrograms per year into the planet’s magnetosphere. But we can start with the Earth, for the antimatter production process was confirmed here in 2011.
These results came from the PAMELA (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) satellite, a joint mission among scientists from Italy, Germany, Russia, and Sweden (see Antimatter Source Near the Earth). The most abundant source of antiprotons near us is found to be in a thin belt that extends from a few hundred to about 2000 kilometers above Earth, moving along Earth’s magnetic field lines and bouncing between the north and south magnetic poles.
Image: An antimatter reservoir near our planet in the form of a belt of antiprotons that lies within the innermost portion (pink) of Earth’s magnetosphere, the large bubble-like region interior to the blue arc that is controlled by the planet’s magnetic field. Credit: Aaron Kaase/NASA GSFC.
Compared to harvesting antimatter on Earth, space harvesting is five orders of magnitude more cost effective, and Bickford’s report suggests we could be collecting 25 nanograms of antimatter per day near our planet. And here’s a spectacular mission concept that can grow out of this, also drawn from the Bickford report:
The baseline concept of operations calls for a magnetic scoop to be placed in a low-inclination orbit, which cuts through the heart of the inner radiation belt where most antiprotons are trapped. Placing the vehicle in an orbit with an apogee of 3500 km and a perigee of 1500 km will enable it to intersect nearly the entire flux of the Earth’s antiproton belt. The baseline mission calls for a fraction of the total supply to be trapped over a period of days to weeks and then used to propel the vehicle to Saturn or other solar system body where there is a more plentiful supply. The vehicle then fully fills its antiproton trap and propels itself on a mission outside of our solar system.
We can imagine fuel depots in the Solar System that could support our growing infrastructure with missions to Mars and the asteroids. There is even the possibility, tantalizingly referenced in the report, of using the galactic cosmic ray flux enroute to a destination to further bulk up the fuel supply. It’s bracing stuff, and a reminder that when we talk about gathering antimatter for a mission, we aren’t necessarily limited to the sparse production from today’s colliders.
Symmetry Violations
But back to the original question. Why is antimatter so hard to find? If it is truly ‘mirror matter,’ as the title of Robert Forward’s book suggests, shouldn’t there be equal amounts of it, and shouldn’t that equality have prevailed from the beginning of the universe? It seems logical to think so, but of course if that had occurred, we would not be here to contemplate the problem.
Now we’re entering the realm of charge-parity (CP) symmetry, which asserts that physics should be unchanged if we plug in antiparticles where particles currently are. Most particle interactions show this charge-parity symmetry to hold, and it carries the implication that the universe should have begun with equal amounts of matter and antimatter. Why and where CP symmetry does fail is a serious question, one that has us looking for any observable violation of the principle.
We have no definitive answer, but we do have interesting results from the T2K experiment in Japan, as reported in New Scientist following their discussion at Neutrino 2016 (the XXVII International Conference on Neutrino Physics and Astrophysics), held in London in early July. The researchers at T2K have been monitoring the oscillations that occur when neutrinos spontaneously change ‘flavors’, from electron to muon to tau. Neutrinos as well as antineutrinos each come in these three types, and all three types can undergo such oscillations.
Image: The inside of the Super-Kamiokande detector in Japan. Credit: T2K.
Observing that 32 muon neutrinos that traveled between the J-PARC accelerator in Tokai to the Super-Kamiokande neutrino detector in Kamioka had turned into electron neutrinos, the team ran the same experiment with muon antineutrinos. Charge-parity symmetry says that the rates of change should be the same, but the researchers report just four muon antineutrinos have changed into the anti-electron neutrino. The numbers are small but the possible violation of CP symmetry is provocative. Results from NoVA, a similar experiment sending neutrinos between Illinois and Minnesota, are showing roughly similar values for apparent CP violation.
More data are needed to reach any firm conclusions, but these results point to the direction of future work at both installations. Some process that violates CP symmetry has to be in place to explain the overwhelming difference between the amount of matter and antimatter in the universe. Thus we can expect any results showing deviations from this symmetry will make news. Meanwhile, from a propulsion standpoint, we have to reckon with the paucity of antimatter by imagining creative ways of creating or finding enough to use in our future experiments. Space-based antimatter harvesting may prove to be the most cost effective way to proceed.
I’ll close by quoting James Bickford in a 2014 interview, where I think he strikes just the right note about the need for small scale experiments as well as avoiding antimatter hype:
For the most part, propelling spacecraft to near the speed of light with antimatter lives in the realm of Star Trek. The technical obstacles are non-trivial and probably won’t be solved in the near future, if ever. From this perspective, the potential for antimatter probably has been overhyped. However, the small scale experiments are just the first baby steps that could help us down the long path. More importantly, research and development in this area is part of a broader framework that could help fundamental science and our understanding of the universe. Antimatter plays a central role in some of the Holy Grail problems of physics, such as the nature of dark matter and why matter dominates over antimatter.
by Paul Gilster | Aug 1, 2016 | Antimatter |
I spent this past weekend poking into antimatter propulsion concepts and in particular looking back at how the idea developed. Two scientists — Les Shepherd and Eugen Sänger — immediately came to mind. I don’t know when Sänger, an Austrian rocket designer who did most of his work in Germany, conceived the idea he would refer to as a ‘photon rocket,’ but he was writing about it by the early 1950s, just as Shepherd was discussing interstellar flight in the pages of the Journal of the British Interplanetary Society. A few thoughts:
Sänger talked about antimatter propulsion at the 4th International Astronautical Congress, which took place in Zurich in 1953. I don’t have a copy of this presentation, though I know it’s available in a book called Space-Flight Problems (1953), which was published by the Swiss Astronautical Society and bills itself as a complete collection of all the lectures delivered that year in Zurich. If you like to track ideas as much as I do, you’ll possibly be interested in an English-language popularization of the idea in a 1965 book from McGraw Hill, Space Flight: Countdown for the Future, which Sänger wrote and Karl Frucht translated.
Greg Matloff has speculated that what may have drawn Sänger to antimatter is specific impulse, which reaches surreal heights if you can produce an exhaust velocity equal to the speed of light (see The Starflight Handbook for more on Matloff’s thinking). The speed of light being about 3 X 108 m/sec, Matloff worked out a specific impulse of 3 X 107 seconds. Recall that specific impulse measures engine efficiency. In other words, a higher specific impulse produces more thrust for the same amount of propellant.
Sänger must have been dazzled by this ultimate specific impulse, which he conceived possible only through the mutual annihilation of matter with antimatter. But recall that when Sänger was developing these ideas, the only form of antimatter known was the positron, or positively charged electron, which had been discovered by Carl Anderson in 1932 (he would win the Nobel for the work in 1936). When you bring positrons and electrons together, you produce gamma rays, an energetic form of electromagnetic radiation that moves at the speed of light.
Antimatter propulsion solved? Hardly. What the Sänger photon rocket had to do was to create a beam of gamma rays which could be channeled into an exhaust, somehow overcoming the problem that the gamma rays produced by the matter/antimatter annihilation emerge in random directions. They are highly energetic and would penetrate all known materials, a lethal problem for the crew and a showstopper for directed thrust unless Sänger could develop a kind of ‘electron-gas mirror’ to direct the gamma rays. Sänger never solved this problem.
The Radiator Problem
Writing in 1952, Les Shepherd went to work on antimatter equally limited by the fact that only the positron was then known — the antiproton would not be confirmed until 1955 (by Emilio Segrè and Owen Chamberlain — Nobel in 1959). Shepherd was a nuclear fission specialist who helped to found the International Academy of Astronautics and served as president of the International Astronautical Federation (see my obituary for Shepherd from 2012 for more). And his 1952 paper “Interstellar Flight” remains a landmark in the field.
Even without the antiproton, Shepherd would have known about Paul Dirac’s prediction of its existence and doubtless speculated on the possibilities it might afford. As Giovanni Vulpetti told me just after Shepherd’s death:
Dr. Shepherd realized that the matter-antimatter annihilation might have the capability to give a spaceship a high enough speed to reach nearby stars. In other words, the concept of interstellar flight (by/for human beings) may go out from pure fantasy and (slowly) come into Science, simply because the Laws of Physics would, in principle, allow it! This fundamental concept of Astronautics was accepted by investigators in the subsequent three decades, and extended/generalized just before the end of the 2nd millennium.
Vulpetti himself has been a major figure in that extension of the concept, with papers like “Maximum terminal velocity of relativistic rocket” (Acta Astronautica, Vol. 12, No. 2, 1985, pp. 81-90); and “Antimatter Propulsion for Space Exploration” (JBIS Vol. 39, 1986, pp. 391-409). Many back issues of JBIS are available for a fee on the journal’s website (http://www.jbis.org.uk/), though I haven’t yet checked for this one. But be aware that Dr. Vulpetti is also making his papers available on his website (http://www.giovannivulpetti.eu/).
Looking back at Shepherd’s “Interstellar Flight” paper is a fascinating exercise. Assuming that we could solve the Sänger problem, Shepherd saw that there were other issues that made antimatter extremely problematic. Obviously, producing antimatter in the necessary amounts would be a factor, as would the key problem of storing it safely, but Shepherd had something else in mind when he wrote “The most serious factor restricting journeys to the stars, indeed, is not likely to be the limitation on velocity but rather limitation on acceleration.”
The paper then moves to examine what happens as we unleash the power of matter/antimatter annihilation. Have a look at this:
We see that a photon rocket accelerating at 1 g would require to dissipate power in the exhaust beam at the fantastic rate of 3 million Megawatts/tonne. If we suppose that the photons take the form of black-body radiation and that there is 1 sq metre of radiating surface available per tonne of vehicle mass then we can obtain the necessary surface temperature from the Stefan-Boltzmann law…
The result is an emitting surface that would reach temperatures of about 100,000 K. We need, in other words, to dispose of waste heat in the form of thermal radiation. Even assuming a way of channeling the gamma rays of positron/electron annihilation (or looking ahead to other forms of antimatter and their uses), Shepherd could see that accelerations high enough to shorten interstellar flight times drastically would have to solve the thermal dissipation problem.
The real difficulty, always assuming that we can find suitable energy sources for the job, lies in the unfavourable ratio of power dissipation to acceleration as soon as we become involved with high relative velocities. The problem is fundamental to any form of propulsion which involves non-conservative forces (e.g., the thrust of a rocket jet) to produce the necessary acceleration. The only method of acceleration which one can conceive that would not be subject to this difficulty, would be that caused by an external field of force.
So can we produce radiators that can handle temperatures of 100,000 K? Perhaps there are ways, but Shepherd could only note that the matter was so far beyond existing technologies as to make the speculation pointless. Sänger’s photon rocket — or any vehicle somehow creating an exhaust velocity near the speed of light, has to reckon with the radiator problem.
Remarkably ahead of their time, both Les Shepherd and Eugen Sänger helped define the problems of antimatter propulsion even before we had found the antiproton, a form of antimatter that offers new possibilities that would be explored by Robert Forward and many others. But more on that tomorrow.
The Sänger references are given above. Les Shepherd’s ground-breaking paper on interstellar propulsion is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. For more background on this issue, see Adam Crowl’s Re-thinking the Antimatter Rocket, published here in 2012.
by Paul Gilster | Jul 6, 2016 | Outer 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.
