by Paul Gilster | Feb 25, 2007 | Missions |
Centauri Dreams usually confines itself to the outer planets and beyond, but this photo of Mars taken by the Rosetta spacecraft’s Philae lander is just too unusual to pass up. You can see one of Rosetta’s solar arrays in the foreground, with the Syrtis region on the Martian surface some 1000 kilometers below. The lander is scheduled for a 2014 touchdown on comet 67P Churyumov-Gerasimenko. Numerous system checks are ahead as Rosetta prepares for a near-Earth swingby in November of this year.
Image: Stunning image taken by the CIVA imaging instrument on Rosetta’s Philae lander just 4 minutes before closest approach to Mars. Credit: CIVA/Philae/ESA Rosetta.
by Paul Gilster | Feb 24, 2007 | Outer Solar System |
Most speculation about finding life on Europa revolves around drilling through the perhaps kilometers-deep ice to sample the ocean beneath. But paleobiologist Jere H. Lipps (University of California, Berkeley) envisions a different exercise. Lipps, who has studied polar environments for twelve years in Antarctica, notes that turnover of ice on that continent often brings organisms to the surface that would otherwise be hidden.
Is ice shifting similarly on Europa? Absolutely. Looking at images of that fractured surface, we see a dynamic environment where water from beneath seems to have welled up and re-frozen. The surface is strewn with domes, ridges and tilted ice rafts. Evidence of life might be found in places where blocks of ice have pushed up to form ridges and rills.
Lipps puts it this way:
“This is a paleontological search strategy, which is what I do. If I want to collect fossils in Nevada, I get a map and look for likely spots, like rock outcroppings, where fossils will be found. Ice turned on its edge is just a geologic outcrop to me let’s go there and see if we can find evidence of past or present life.”
What about radiation in the hostile Jovian environment? Lipps believes it is unlikely to penetrate more than one or two meters. That would leave even near-surface ice environments — cracks, overhangs, and so on — that could support life forms. And just as the paleontologist in New Mexico can see an evolutionary history of life over time by studying layers of sedimentary rock, ices of different ages could provide the same perspective to a rover or — let’s hope one day — a human on Europa.
Image: The reddish ovals in the center of this image may be areas where water from Europa’s underground ocean upwelled and froze on the surface. Credit: Galileo Project, NASA.
Lipps spoke on a panel at the American Association for the Advancement of Science meeting that included William McKinnon (Washington University), whose advocacy of a Europa mission we discussed not long ago in the first part of this post. A central point about the search for life on Europa is not to get too doctrinaire about just how and where we may find it, for Earth’s poles show life’s unique adaptability. Lipps again:
“Based on analogy with Earth’s polar seas, Europan life may occur in many habitats: on soft and rocky bottoms at the ocean’s floor, associated with hydrothermal vents on the floor of the oceans, at different levels in the water column as plankton and nekton, and in and on the ice cover itself. Some of these might contain complex associations of life forms, including both micro- and macroscopic forms and consumers and predators.”
Lipps and Sarah Rieboldt proposed likely habitats for Europan life in “Habitats and taphonomy of life on Europa,” Icarus 177 (2005), pp. 515-527. Taphonomy refers to the processes that occur during fossilization, a necessary study if paleontologists are to extract accurate information about the organisms involved. Some day let’s hope we have boots on the ground on Europa to pursue these investigations, but a sampling strategy could be created earlier for properly equipped rovers.
by Paul Gilster | Feb 23, 2007 | Advanced Rocketry |
A nice upgrade to existing satellite engine technology comes out of Georgia Tech, where researchers have developed a design that allows the engine to optimize available power, much like the transmission of a car. Thus the engine can burn at full throttle in ‘first gear,’ maximizing acceleration, while dropping into a much more economical gear for long-term space operations. “You can really tailor the exhaust velocity to what you need from the ground,” says team leader Mitchell Walker.
The engine at work here is known as a Hall effect thruster, a plasma-based propulsion system that operates with xenon, a gas that is injected into a discharge chamber where its atoms become ionized. The electrons that are stripped from the outer shell are trapped in a magnetic field, while the heavier xenon ions are accelerated out into space by an electric field. What Georgia Tech has introduced is better control over the exhaust stream through an enhanced electric and magnetic field design.
Image: Georgia Tech’s enhanced engine uses a novel electric and magnetic field design that helps better control the exhaust particles. Ground control units can then exercise this control remotely to conserve fuel. Credit: Georgia Institute of Technology.
A key factor is specific impulse (ISP), which measures how much thrust is produced per unit of fuel in each second of an engine’s burn. Looked at another way, ISP is a measure of how many seconds one pound of propellant can produce one pound of thrust. As specific impulse (stated in seconds) rises, it takes less fuel to produce a given amount of thrust, and the amount of payload compared to propellant can also rise.
In a telephone interview, Walker told me that the Georgia Tech work is not about creating a new engine but pushing existing engines into regimes in which they normally don’t function well:
We took an engine that normally runs at 2500 seconds and we backed it down to 1000 seconds. In other words, we traded exhaust velocity for more thrust. When we did that the engine efficiency dropped from 65 percent down to about 25 percent — the engine did not like to run there. What we’ve been able to do is to focus ions that would otherwise crash into the chamber wall to create huge efficiency losses. That drives the efficiency of the engine back up while running at 1000 seconds.
All of which is good news for various space missions, since the design — modified from a donated Pratt & Whitney satellite engine — can reduce onboard fuel needs by 40 percent, thus freeing up space for payload. An already efficient engine thereby becomes more stingy still with its fuel using proven technologies. Remember that Deep Space 1, launched in 1998, tested out ion propulsion using xenon, producing only one-fiftieth a pound of thrust at full throttle, but with high specific impulse.
For that matter, the European Space Agency’s SMART-1 lunar mission used solar-electric methods, with the electricity generated from its solar panels being used to accelerate xenon ions. Today’s low thrust ion engines are so efficient that they can run for months or years. Researchers at the Jet Propulsion Laboratory operated an NSTAR thruster for a continuous 30,352 hours — these engines are workhorses — and both power and specific impulse are being improved in ion thruster designs like NEXT, the NASA Evolutionary Xenon Thruster.
A good backgrounder on ion propulsion from New Scientist is here, with details on NEXT. The classic book on the subject is Robert Jahn’s The Physics of Electric Propulsion (McGraw-Hill, 1968). A 2006 paperback edition from Dover brings this core text back onto bookstore shelves.
by Paul Gilster | Feb 22, 2007 | Exotic Physics |
A device called a Photonic Laser Thruster is making news since a December demonstration of the technology by its inventor, Young Bae. The founder of the Bae Institute in Tustin CA, Bae has pursued antimatter and fusion research for twenty years at places like SRI International and Brookhaven National Laboratory. His current work on photon thrust is raising some eyebrows, as noted in this news release from the Institute, which quotes the Air Force Research Laboratory’s Franklin Mead:
“I attended Dr. Bae’s presentation about his PLT demonstration and measurement of photon thrust here at AFRL. It was pretty incredible stuff and to my knowledge, I don’t think anyone has done this before. It has generated a lot of interest around here.”
In one form or another, something called a ‘photon drive’ has been in the back of inventors’ minds since the days of the German researcher Eugen Sänger, who published a designed he called a ‘photon rocket’ that would use gamma rays produced by the annihilation of electrons and positrons for thrust. The problem with that one is that you can’t control the exhaust stream, since highly energetic gamma rays penetrate any materials that would be used to contain them. And in any case, Sänger’s concept is a variation on an antimatter engine, which is not what Dr. Bae is up to.
Photons themselves, having no mass or charge, present obvious problems to anyone trying to get thrust out of them. Fortunately, they do impart momentum, which is how solar sails work. What Bae’s system does is to bounce photons using “…a photonic laser and a sophisticated photon beam amplification system.” Bouncing photons between spacecraft thousands of times is said to do the trick, producing measurable thrust that can be used in space applications.
In fact, according to the Institute’s Web site, “A small scale PLP, Photonic Laser Thruster (PLT) has recently been successfully demonstrated by Bae Institute, and the result has demonstrated the principle of PLP [Photonic Laser Propulsion] by amplifying photon thrust by 3,000 times.”
Keeping multiple spacecraft in precise formation (down to nanometer lengths) may be one outgrowth of this technology, and if so, it’s an important one. NASA’s MAXIM (Micro-Arcsecond X-ray Imaging Mission), for example, would use numerous spacecraft flying in tight formation in order to study black holes. The need for precise adjustment between independent craft has resulted in Bae’s Phase II study for NASA’s Institute for Advanced Concepts. Think space-borne interferometry, with the spacecraft kept in configuration through PLT and space tethers. The resultant formation, says the Bae Institute, will prove 100,000 times more precise than existing methods of flying spacecraft in formation.
Here’s a snippet from the Phase I study that preceded Dr. Bae’s current work for NIAC:
In addition to redefining and simplifying the existing NASA mission concepts, such as SPECS and MAXIM, PTFF [Photon Tether Formation Flight] enables other emerging revolutionary mission concepts, such as New World Imager Freeway Mission proposed by Prof. Cash, which searches for advanced civilization and in exo-planets Fourier Transform X-Ray Spectrometer proposed by Dr. Schnopper. As the present concept is more publicized, many other exciting concepts are expected to be inspired by PTFF. One of such possible NASA missions is the construction of ultralarge space telescope with diameters up to several km for observing and monitoring space and earth-bound activities.
If it will get New Worlds Imager working, I’m all for it. But will it? Here’s a link to David Livingston’s interview with Bae, in which he discusses PTFF as well as antimatter and fusion concepts. This New Scientist story on Bae’s work is also helpful. Less helpful, perhaps, is the claim on the Bae Institute’s Web site, describing photonic laser propulsion as “Our patent-pending, innovative, yet highly realistic, photon propulsion concept capable of accelerating spacecraft to near light speed without propellant.”
Perhaps, but first things first. Let’s get those space interferometry missions working, and on that score, NIAC’s Robert Cassanova reminds us in New Scientist that alternative concepts are still in play.
by Paul Gilster | Feb 22, 2007 | Exoplanetary Science |
Our first ‘sniffs of air from an alien world,’ as David Charbonneau calls them, have brought with them a bit of a surprise. Charbonneau (Harvard-Smithsonian Center for Astrophysics) is one of a team of astronomers who have measured the spectrum from the atmosphere of a transiting exoplanet. What the team expected to find was evidence of common molecules like water, methane and carbon dioxide. Yet the scientists found none of these. The spectrum they acquired was flat.
HD 189733b is the world in question, orbiting a star about sixty light years from Earth in the constellation Vulpecula. The transiting planet is a ‘hot Jupiter,’ slightly larger and more massive than Jupiter itself, orbiting once every two days about three million miles from its star. This remarkable work consisted of studying the so-called ‘secondary eclipse’ that occurs when the planet disappears behind the star, thus extracting the planetary data from the much brighter stellar signature.
Here’s the method, as described by Charbonneau colleague Carl Grillmair (Caltech):
“Normally, trying to see a planet next to a star is like trying to see a firefly next to an airport searchlight several miles away. But in the case of our planet and the one being reported by the other teams, you can take the combined spectrum of the star and planet, and then when the planet passes behind the star, take another spectrum. By subtracting the second spectrum of just the star from the first, you can divine the spectrum of the planet itself.”
What remarkable work with the Spitzer Space Telescope, which seems more capable than anyone had imagined (and recall that transiting exoplanets had yet to be discovered when Spitzer was designed). The key to these studies is the use of infrared. At infrared wavelengths, a planet is much brighter in comparison to its star than in visible light. The method worked perhaps beyond expectation, but the result was still unusual: Looking for water vapor in the data and a prominent methane signature, the team found its results indicative of something else, something that is most likely blocking these molecules from detection.
Image: This artist’s concept shows a cloudy Jupiter-like planet that orbits very close to its fiery hot star. NASA’s Spitzer Space Telescope was recently used to capture spectra, or molecular fingerprints, of two “hot Jupiter” worlds like the one depicted here. This is the first time a spectrum has ever been obtained for an exoplanet, or a planet beyond our solar system. Credit: NASA/JPL-Caltech/T. Pyle (SSC).
And what could that something be? Helping to untangle the puzzle is the spectrum of a different planet, HD 209458b, which is under investigation both by Jeremy Richardson (GSFC) and colleagues and a JPL team led by Mark Swain. Their spectra show silicates — molecules containing silicon and oxygen — which may exist on this planet as dust grains from which clouds can form. So what’s going on in these separate spectral studies may be the marker of an interesting cloud structure. “We think that both planets may be cloaked in dark silicate clouds,” said Charbonneau. “These worlds are blacker than any planet in our solar system.”
Additional data (Grillmair and Charbonneau, for example, have only been able to observe their planet for 12 hours during two eclipses) should help clarify the situation. The Grillmair and Charbonneau paper “A Spitzer Spectrum of the Exoplanet HD 189733b” is to appear in the Astrophysical Journal Letters, and is available here as a preprint. The GSFC study is “A spectrum of an extrasolar planet”, appearing in Nature 445 (22 February 2007), pp. 892-895, abstract here. A separate paper on HD 209458b by Mark Swain’s team at JPL is to appear in Astrophysical Journal Letters (no preprint yet available).
