by Paul Gilster | Dec 22, 2004 | Exoplanetary Science |
David J. Stevenson, who is George Van Osdol Professor of Planetary Science at CalTech, has an entertaining way with titles. The average scientific paper has a title whose tone is dry, direct and frequently off-putting. Stevenson gives us these, as any scientist must, but give him the chance and he produces the Swiftian “A Modest Proposal: Mission to Earth’s Core,” which ran in Nature in 2003 (available here). He has also written, fascinatingly, on the possibilities of oceans on worlds other than Europa; thus his 1999 essay “An Ocean in Callisto?” (The Planetary Report Vol. 19 No 3, pp. 7-11), and “An Ocean in Uranus?” (The Planetary Report Vol. 6 No 16, 1986).
At the recent American Geophysical Union meeting, Stevenson produced the ultimate in what we writers call the ‘omniscient viewpoint’ by presenting “How to Build a Planetary System.”
So how do you build a planetary system? It turns out that we know surprisingly few answers. Stevenson argues that while such systems are common (at least based on current observational data), the question of Earthlike planets is still unresolved. “In all likelihood, the diversity of outcomes is large and non-deterministic and our particular outcome is correspondingly uncommon despite the abundance of systems,” Stevenson writes in the abstract to his AGU presentation.
So don’t expect the average solar system to look like ours. What we do believe at this juncture is that the formation of a planetary disk provides the raw materials for planet building, and seems unavoidable. But the aggregation of this matter into small planetesimals is poorly understood. If they do form, they should produce planetary bodies, but even so, the subsequent formation of gas giants is mysterious, though it is clear they are common. So given the gaps in our knowledge, we should expect to find more and more extrasolar systems that diverge widely from our Solar System.
Stevenson’s interest in oddball planet formation was evident in his 1999 paper on interstellar planets in Nature called “Possibility of Life-Sustaining Planets in Interstellar Space.” Here he argues that planets capable of sustaining life may exist far from any star; their presence will be correspondingly hard to detect. From the paper:
We thus see that bodies with water oceans are possible in interstellar space. The “just right” conditions are plausibly at an earth mass or slightly less, fortuitously similar to the expected masses of ejected embryos during giant planet formation. For a 50/50 ice-rock body, the ocean is very deep and may be underlain by high pressure phases of water ice with a rock core at still greater depths, but bodies with earthlike water reservoirs may have an ocean underlain with a rock core. Either way, these bodies are expected to have volcanism in the rocky component and a dynamo-generated magnetic field leading to a well-developed (very large) magnetosphere. Despite thermal radiation at microwave frequencies that corresponds to the temperatures deep within their atmospheres (analogous to Uranus) and despite the possibility of non-thermal radio emission, they will be very difficult to detect. If, as many have suggested, life can develop and be sustained without sunlight (but with other energy sources, plausibly volcanism or lightning in this instance) then these bodies may provide a long-lived stable environment for that life (albeit one where the temperatures slowly decline on a billion year timescale). It is even conceivable that these are the most common sites of life in the Universe.
If that doesn’t give science fiction writers something to work with, what does? The complete text of this Nature paper can be found here (PDF warning). You’ll find some descriptions of Stevenson’s recent research, including more on the possible Callisto ocean, at his Caltech page, which also includes a complete list of his publications.
by Paul Gilster | Dec 21, 2004 | Outer Solar System
Cassini’s most interesting view of Titan’s atmosphere to date is shown here, highlighting what appears to be layer after layer of haze. The image is in the ultraviolet and taken from Titan’s night side; the haze layers extend several hundred kilometers above the surface. The region shown here is in Titan’s equatorial region, about 10 degrees south latitude.
A nice wrap-up of Cassini data from both Titan and Dione, as presented at the American Geophysical Union’s fall meeting in San Francisco, can be found on this JPL page. Especially noteworthy is the peculiar, braided character of the fractures on Dione’s surface, where the terrain consists of ice cliffs apparently created by tectonic forces. “This is one of the most surprising results so far. It just wasn’t what we expected,” said Dr. Carolyn Porco, Cassini imaging team leader (Space Science Institute, Boulder, CO).
We are now just three days away from the separation of the Huygens Titan probe from the Cassini orbiter, with descent to Titan’s surface planned for January 14.
Image Credit: NASA/JPL/Space Science Institute.
by Paul Gilster | Dec 20, 2004 | Antimatter
Other than high-energy particle accelerators, where on Earth would you look for antimatter? The answer seems to be high above Antarctica. A team of scientists led by Akira Yamamoto of Japan’s High Energy Accelerator Research Organization (KEK) is actively hunting antimatter that may be striking the Earth from space. The detector: an instrument carried by a 40-million cubic foot balloon that is circling the South Pole at an altitude of 39 kilometers (24 miles). If successful, the team will learn a good deal about antimatter and may prove the existence of so-called Hawking radiation, low-energy antiprotons created by ancient black holes from the Big Bang era.
Called BESS-Polar (Balloon-borne Experiment with a Superconducting
Spectrometer), the experiment is a collaborative effort that unites KEK and various Japanese agencies including the Japan Aerospace Exploration Agency with NASA. BESS has been tested in Canada and used to study cosmic rays; it also collected a small number of low-energy antiprotons, but the new venue in Antarctica may yield a much greater windfall. With the continent now in constant daylight, there are no day-night temperature fluctuations to contend with, allowing the balloon to remain at altitude longer. “We journeyed to the bottom of the world so that we could get a nice, long flight,” said Project Manager Prof. Tetsuya Yoshida of KEK. “Longer flights mean better statistics.”
Image: BESS shortly before launch in Antarctica, December 13, 2004. Credit: Dr. Robert Streitmatter of the Laboratory for High Energy Astrophysics (LHEA) at NASA’s GSFC.
One problem BESS-Polar may help us gain insight into is where antimatter went after the Big Bang. Theoretically, equal amounts of matter and antimatter should have been created, but the universe seems dominated by ordinary matter. Physicist John Cramer theorizes that the early universe had 100,000,001 protons for every 100,000,000 antiprotons; he calls the electrons and protons of the visible universe, in a wonderful phrase, “…the few ragged survivors of the ‘antimatter wars’ of 16 billion years ago.” BESS-Polar will be attempting to discover whether where are any large areas of antimatter that survived those primordial wars. (And if you aren’t familiar with Cramer’s ‘Alternate View’ columns in Analog, be sure to check his online venue).
Centauri Dreams‘ take: the more we learn about antimatter, whether from primeval black holes or accelerators, the closer we’ll be to powerful new forms of propulsion. Steve Howe’s wonderful antimatter sail concept seems a practical way to design a mission to the Kuiper Belt, but even it would require more antimatter than we can currently produce, and a Centauri mission would demand vast amounts by today’s puny production standards. Nonetheless, we’re learning how antimatter could be used to create fusion reactions in concepts like Magnetically Insulated Inertial Confinement Fusion, which University of Michigan professor Terry Kammash continues to develop. Clearly, antimatter has a key role to play in getting us out of the Solar System.
The BESS home page is here. To track BESS via GPS, go to the Antarctic Operations site. The KEK press release on BESS is available here.
by Paul Gilster | Dec 18, 2004 | Culture and Society
“We hesitate about where to go from here in space. Yet our delay in exploiting this window of opportunity could close off choices for our descendants if the no-growth paradigm–or a failure of nerve–should come to dominate the industrial nations… Because of our technologies, and the scales of our political and economic organizations, we now have the option of taking a conscious evolutionary step, expanding the presence and influence of humanity beyond the biosphere that evolved us–and possibly beyond the limits that otherwise would constrain our future… Our generation is the first to have this choice. It may be up to us to prove that intelligence armed with technology has long-term survival value.”
— Michael Michaud in Life in the Universe (AAAS Selected Symposium 31, 1979). Found in the online repository Space Quotes to Ponder.
by Paul Gilster | Dec 17, 2004 | Missions
The nuclear-electric mission to Neptune discussed here on the 14th is one of two now being studied by NASA. The other is powered by chemical rockets and, like Cassini, would use gravity assists to reach Neptune in considerably less time. Its team, led by Andrew Ingersoll of the California Institute of Technology, is working on a design that, like University of Idaho professor David Atkinson’s nuclear-electric mission, will be submitted to NASA in mid-2005.
A faster mission has many advantages, but a major question arises: how do you stop when you get there? Unlike Voyager, the Neptune missions are to be capable of orbiting the planet and dispatching probes to both it and its largest moon, Triton. One answer Ingersoll’s team is studying is aerocapture, which uses the destination planet’s atmosphere to alter the spacecraft’s trajectory, putting it into orbit after a single pass.
If this sounds familiar, you may recall the aerocapture maneuver in the film 2010, a spectacular, flaming arrival at Jupiter that used a heat shield and pinpoint positioning to brake the spacecraft into a circular orbit. NASA has been studying aerocapture for a long time, though at present its experience with actual missions has been limited to a milder variant called aerobraking; the broader term for all these maneuvers is aeroassist. The Mars Climate Orbiter has already proven aerobraking; the spacecraft circularized the elliptical orbit its chemical rocket put it in around Mars by using drag from the atmosphere on its solar array. Doing this for hundreds of orbits resulted in a circular path around the planet.
When I talked to NASA’s Les Johnson at Marshall Space Flight Center in Huntsville about aerocapture, he called the procedure ‘aeroassist on steroids.’ Johnson is manager of NASA’s In-Space Propulsion Program. More than most technologies, aerocapture is destination-dependent; Johnson told me that both Neptune and Titan are possible candidates for its use because of their interest and their great distance. For the outer planets, the beauty of aerocapture is that you need carry no fuel to get yourself into orbit at destination. Another key advantage: it allows a much faster trip time.
From the interview:
If you look at the systems studies, you see that if you don’t have to carry all that fuel with you, you can can add science payload. And your launch vehicle can be smaller because it doesn’t have as much weight to throw. Finally, your trip time is reduced because when you’re traveling interplanetary distances and have to slow down to capture, you spend half your time thrusting and half your time slowing down. Forget that with aerocapture; you do all your slowing down in a thirty minute maneuver at destination.
Of course, it’s a spectacular, nail-biting arrival. But when I asked Johnson how tight the parameters were for the maneuver, his answer surprised me:
Our people have some convincing data that the entrance corridors are wide so that if you miss your mark in the atmosphere you have a lot of flexibility for correcting as you proceed. That the integrity of the ablaters and the materials — the thermal protection we’ll need — is going to be tolerant and will be able to do the capture.
So armed with aerocapture and gravity assists, the Ingersoll team may have an edge when it comes to speed. On the other hand, the nuclear-electric technologies Atkinson envisions would allow for bigger payloads and more power resources available during the spacecraft’s primary mission around Neptune; this is what NASA’s Project Prometheus is all about.
We’ll be tracking both these designs as they evolve into final reports. In the meantime, a good backgrounder on all the aeroassist methods can be found at this NASA page. Image credit (above): NASA.
