by Paul Gilster | Jul 17, 2009 | Outer Solar System |
Other than Monday, the week here has been devoted to the outer planets, and before I leave that subject, I want to work in the findings of a team of astronomers looking at the early history of the asteroid belt. Recent numerical simulations suggest that many of the objects found in the ‘main belt’ between the orbits of Mars and Jupiter actually formed far out in the Solar System, moving inward during a violent spasm of planetary evolution.
That points to an early system that, at particular times, underwent upheaval caused by a rearrangement of the gas giant planets. This is the so-called Nice model, so named because much of the work on it was performed at the Observatoire de la Côte d’Azur in Nice. The model proposes that the gas giant planets migrated to their present positions long after the protoplanetary gas disk had dissipated, playing a role in the Late Heavy Bombardment of the inner planets some 3.9 billion years ago, and producing many other effects, including the formation of the Oort Cloud and the Kuiper Belt.
What’s interesting for our view of the asteroid belt is that the ‘main belt’ asteroids between the orbits of Mars and Jupiter range widely in their composition, from igneous rocks to mixtures of rock and ice. And while it’s long been assumed that these asteroids formed in place out of a primordial disk that experienced chemical changes, the new simulations suggest that many asteroids formed in the outer system and, at the time of the Late Heavy Bombardment moved inward to their present positions.
Image: The asteroid belt lies in the region between Mars and Jupiter. The Trojan asteroids lie in Jupiter’s orbit, in two distinct regions in front of and behind the planet. Credit: Lunar and Planetary Institute.
The LHB was clearly not limited to the Earth, but devastated the Moon and other planets as well. Kleomenis Tsiganis (Aristotle University, Thessaloniki) notes the evidence for this idea in the asteroid belt, which the team used in its modeling:
“Some of the meteorites that once resided in the asteroid belt show signs they were hit by 3.5 to 3.9 billion years ago. Our model allows us to make the case they were hit by captured comets or perhaps their fragments. If so, they are telling us the same intriguing story as the lunar samples, namely that the solar system apparently went berserk and reconfigured itself about 4 billion years ago.”
This is a new view of the asteroid belt, one that will need follow-up through studies of meteorites, asteroids and the moon. Needless to say, data we can also gain from missions to the Kuiper Belt, like the Haumea orbiter we’ve been discussing, would materially benefit this analysis. The paper is Levison et al., “The Contamination of the Asteroid Belt by Primordial Trans-Neptunian Objects,” Nature 460 (16 July 2009), pp. 364-366 (abstract).
by Paul Gilster | Jul 16, 2009 | Outer Solar System |
Gentry Lee (Jet Propulsion Laboratory) discusses the question of extraterrestrial life on a program called Are We Alone, which airs this evening on the Discovery Channel at 2100 EDT (0100 UTC on the 17th). Lee is chief engineer for the Solar System Exploration Directorate at the Jet Propulsion Laboratory, a veteran of Viking and Galileo, and a co-author of Arthur C. Clarke. He was also involved with Carl Sagan on COSMOS, so he knows something about video productions.
Pushing Back Astrobiology
As I’ve noted in the two previous posts, we’re moving into an era of re-examination of the Solar System. It’s one that leads inevitably to a new understanding of the concept of habitable zones, with life now being considered a possibility on places that were once thought off-limits. Europa is unusual enough, but the evidence for that ocean beneath the ice is persuasive. Can we extend the paradigm all the way out to the Kuiper Belt? If so, missions like the Haumea orbiter or probes to other trans-Neptunian objects become imperative.
At the Aosta conference, I had the chance to talk to Joel Poncy after his presentation on the Haumea project, a conversation that led to astrobiology. Poncy referred me to a paper by Steve Desch (Arizona State) and colleagues discussing cryovolcanism on distant objects like Charon, and making some startling statements about the possibility of liquid water 40 AU and more from the Sun.
Liquid Water in Cold Objects
Desch looks at the evolution of Kuiper Belt Objects using a model his team constructed that shows how they might retain enough heat to keep subsurface water in a liquid state. Listen to what these people are saying about Pluto’s large moon Charon:
We predict that Charon contains a rocky core… of radius 330 km, surrounded by a slushy layer about 30 km thick containing a mix of ADH [ammonia dihydrate] and liquid water and ammonia. Above this layer is a layer of solid water ice… from 360 to 470 km, surrounded by an undifferentiated crust of rock, water ice and ADH, about 130 km thick. Only about half of the mass of Charon ever experiences differentiation. Our thermal evolution models suggest that within a few x 108 yr, the subsurface liquid will freeze entirely.
Of course, a few hundred million years is well down the road. The news is that Charon may contain liquid water now. The paper continues:
We conclude that objects with a densities similar to Pluto and Triton, 2.0 g/cm3, as small as 500 km in radius, could retain liquid to the present day. Our time-dependent thermal models of KBOs show that it is possible for Charon and Quaoar and many other small KBOs to retain liquid water to the present day.
Cryovolcanism on Other Worlds
Thus the interesting reflectivity of Haumea, which may well indicate cryovolcanism, a mechanism that should also be at work on objects like Charon as liquid is delivered to the surface. Another world with evidence for water ice is Quaoar. We could nail this process down, Desch notes, if we find evidence of cryovolcanism when New Horizons flies past the Pluto/Charon system in 2015. It’s hard to imagine these coldest objects in the Solar System maintaining sub-surface water, but even the Uranian moons Miranda and Ariel show recent resurfacing, and we all remember Triton.
It’s a long way from possible cryovolcanism to astrobiology, but finding liquid water in the Kuiper Belt would at least open a long-shot window for life in the most unexpected places. The paper is Desch et al., “Cryovolcanism on Charon and Other Kuiper Belt Objects,” 38th Lunar and Planetary Science Conference (2007), available online.
by Paul Gilster | Jul 15, 2009 | Outer Solar System |
Yesterday’s look at a fast orbiter mission to Haumea raises useful questions. The mission, developed conceptually by Thales Alenia Space and presented at Aosta by Joel Poncy, is particularly demanding because this outer system object has no atmosphere. You can make the case for a Neptune orbiter with associated study of Triton, as several readers have already done, but if you want to orbit Haumea, no aerobraking is possible to ease orbital insertion.
The Haumea mission, in other words, deliberately pushes the state of the art in both propulsion and power generation. Poncy noted in his talk at the Hotel Europe that his team had adapted an in-house software model to optimize the propulsion possibilities. The team considered only electric or magneto-plasma technologies (for the latter, think VASIMR — Variable Specific Impulse Magnetoplasma Rocket). They assume a direct trajectory to Haumea with arrival around 2035, when the object is at 49 AU, and they weigh the benefits of a gravity assist by Jupiter to help shorten the journey.
Image: The orbits of Hi`iaka (outer satellite) and Namaka (inner satellite). Namaka’s orbit is nearly edge on as viewed from Earth. Every nine days Namaka passes directly in front of or behind Haumea as seen from Earth. Credit: University of Hawaii/D. Ragozzine.
Taking the hardware to its limits, Poncy and colleagues come up with what looks to be a feasible mission concept, one with a specific impulse of 10000 seconds, a launch mass of 3000 kilograms and a flight time of about 21 years. Says Poncy:
This set of parameters corresponds to what should be achievable in a near future, provided that VASIMR and beta-voltaic technologies are implemented into respectively propulsion and power units by adapting their design and operating point to this class of spacecraft.
A lot is in play here. For one thing, VASIMR is more promise than reality at these levels — will it perform as advertised? For another, Poncy himself notes the problem with generating the power needed to fly this mission. Here’s his thought on that:
…the current technologies would fall short of the needs. The beta-voltaic technology looks promising, as values up to 24W/kg might be within reach in the coming years providing that the packaging design and the battery assembly are adapted to the production of several kW… If we want to go beneath 20 years [flight time], then the industry and the agencies will have either to undertake even more ambitious developments for the power generation, so as to reach about 50W/kg in terms of power density, or find new disruptive technologies.
Yesterday I talked about the scientific value of a Haumea mission, but the second motivation for going to this distant object is to use the mission as a technology driver. Poncy was exactly right when he told the audience at Aosta that if we’re going to get serious about Solar System colonization and, ultimately, interstellar travel, we need to develop the near-term technology to reach and orbit objects in the outer system with less than ten to twenty years of cruise. And don’t forget, these trans-Neptunian objects (TNOs) are rich in volatiles and organics, interesting places for future robotic or even human outposts.
Image: Haumea and its moons. Credit: NASA GSFC.
Moreover, we’re intent on exploring the moons of the gas giants, which is going to demand developing next generations orbiters, landers and deep-drilling capabilities. A fast journey to an object like Haumea thus becomes a way to extend our science to planetary objects within 100 AU that can at the same time increase our capabilities for reaching Jupiter or Saturn space with the kind of heavy payloads we want to see in operation there. Poncy sees Haumea as a targeting goal for developing the next tools we need as we expand our studies of closer worlds like Europa and Titan.
The paper is Poncy et al., “A Preliminary Assessment of an Orbiter in the Haumean System: How Quickly Can a Planetary Orbiter Reach Such a Distant Target?” It’s in Proceedings of the Sixth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, and should therefore pop up in the near future in Acta Astronautica.
by Paul Gilster | Jul 14, 2009 | Outer Solar System |
One of the surprises of the Aosta conference was Joel Poncy’s presentation on a fast orbiter mission to Haumea. Poncy (Thales Alenia Space, France) and colleagues have been developing ideas for the extraordinarily difficult challenge of not just sending a probe to the outer system, but slowing it down for orbital capture. It’s one thing to do this, say, for Neptune, where a thick atmosphere can be used for aerobraking, but it’s quite another to contemplate doing the same for an airless trans-Neptunian object (TNO) like Haumea.
Nonetheless, there are solid reasons for thinking about such a mission. The first is purely scientific. As Poncy did, I’ll use outer planet specialist Mike Brown’s illustrations of what has happened to our Solar System in the last few decades. The first illustration shows the Solar System most of us grew up with, a system with nine planets that were more or less clearly defined, with what was assumed to be a certain amount of debris and cometary material further out.
Now, of course, we see a new Solar System. Depending on how we define planets, we can declare that we have numerous such objects in the outer system — call them ‘dwarf planets’ — along with, much further out, the enormous, spherical system known as the Oort Cloud. Think about this: The number of objects with a diameter beyond 500 kilometers has doubled in just ten years from thirty-five to more than seventy as we’ve continued our investigation of trans-Neptunian objects. It is fully assumed that within another decade or two, we’ll know of hundreds more of these objects.
Let me quote Poncy on this:
If we now recap all sizable Solar System planetary objects larger than 500 km, we get 19 objects closer than the orbit of Uranus, orbit-able after a decade or so of cruise with current technologies. Uranus itself can be flown by but not orbited for a decent travel time. We have already more than 40 objects at Uranus and beyond and this number will grow considerably by 2020. This is even starting to change the appellation ‘Outer System,’ which was previously used to name the part beyond the frost line at 4 AU, and is now sometimes used to designate the part beyond 30 AU.
Image: A new Solar System emerges. Credit: Mike Brown/Caltech.
Consider, too, that we once thought of the the Solar System as being enclosed in a well defined heliosphere that separated it from true interstellar space. Now we have objects like Sedna, with an aphelion (942 AU) that is well beyond the heliosphere. In moving to its perihelion at 76 AU, Sedna moves from interstellar space into the heliosphere and then gradually works its way back out again. The new Solar System is packed with objects that defy all the definitions we once brought to the term.
As for Haumea itself (once known as 2003EL61), we’re looking at a most unusual little world. You can study its light-curve to determine that it’s spinning quickly, about once every 3.9 hours. More unusually, rather than being spherical, it appears to be a flattened ellipsoid with its largest axis in the range of 2000 kilometers. One of the thirty largest objects in the Solar System, it is orbited by two moons, the largest — Hi’iaka — being rather large in itself, with a diameter estimated to be 300 kilometers.
Image: Relative sizes of the larger outer system objects. Note the ellipsoidal shape of 2003EL61, now known as Haumea. Credit: NASA/ESA/A. Feild (STSci).
The current thinking is that Haumea is the result of a major collision, one that produced the entire system of TNO and two moons. Adding to the interest is that Haumea is quite reflective, its surface covered with crystalline water ice. This could be interpreted as evidence for cryovolcanism (think Triton), and brings home the usefulness of an orbiter. After all, a flyby mission is not going to be able to track transient phenomena over time. Moreover, the orbital dynamics of the system, including the interactions of its moons and the tidal effects of Haumea itself, are complex and will need long study to understand.
Can we get to this unusual object, which is now close to aphelion at 51 AU from the Sun? It’s certainly within range of our probes, but orbiting it would not be easy. We’ll look at the why and how of such a mission tomorrow.
by Paul Gilster | Jul 13, 2009 | Advanced Rocketry |
One of the things that makes travel both entertaining and exasperating is the assurance that the best laid plans will come up against events beyond your control. Thus I arrived at Milan’s Malpensa airport in plenty of time for my Delta flight, only to be told that the flight had been canceled. But Delta moved quickly on setting me up with an Alitalia flight to New York that left only thirty minutes after the first had been scheduled to leave, and after two more connections (and a twenty-three hour day on the move) I arrived back home. The photo is a shot of the streets of Aosta one early morning, from a walk I took on the last day to remember it by.
I returned with a satchel full of notes, the conference proceedings, numerous business cards and the world’s worst back-ache, a consequence of trying to move too fast in crowded airports with laptop and luggage. While the latter heals, I’ve also decided not to try to move through the Aosta material in one go — there’s too much of it, and too many papers I want to tell you about. So I’ll fold in the Aosta talks with a gradual return to other news items. Tomorrow, for example, I want to discuss a potential mission to Haumea, the curious object that may have much to tell us about the composition of the outer Solar System. So let me sort my notes and we’ll discuss it.
For today, a return to the news suggests a look at an article on the msnbc site discussing spacecraft at the nanotech level. This one caught my eye because of its reference to ‘needle-sized spacecraft.’ Robert Freitas was the one who told me about the ‘needle’ starship, a tiny vessel packed with nanotechnological assemblers that, arriving in a destination system, could use raw materials from asteroids there to build a scientific station for analysis and data return. I mentioned this idea in a recent Washington Post story, and in the current article, work at the University of Michigan using a new kind of thruster comes into play, with possible interstellar ramifications.
From the story:
The technology is called a “nano-particle field extraction thruster,” or nanoFET. The tiny thrusters that work much like miniaturized versions of massive particle accelerators. The device uses a series of stacked, micron-thick “gates” that alternate between conductive and insulating layers to create electric fields. These small but powerful electric fields charge and accelerate a reservoir of conductive nanoparticles, shooting them out into space and creating thrust.
The engine is evidently etched onto silicon via micro-electromechanical systems technologies, with tens of thousands of accelerators fitting into a tiny area. Translate very small effects into constant acceleration over years of time and you could theoretically achieve something like the needle probe idea of Freitas, though Gilchrist does not talk about nanotechnology inside, but rather the propulsive force that will send the probe, whatever its payload.
Image: nanoFET characteristic size scales. Credit: University of Michigan Department of Aerospace Engineering.
Gilchrist’s Web site at the University of Michigan shows a bibliography with papers related to this topic, but most are at least six years old — I assume the site just needs updating. But a university page on NanoFET propulsion offers more, describing the new electric propulsion system as a way to “….utilize electrostatically charged and accelerated nanoparticles as propellant. Millions of micron-sized nanoparticle thrusters would fit on one square centimeter, allowing the fabrication of highly scaleable thruster arrays.” Obviously not suitable for launch to orbit, these field emission thrusters are intended for acceleration and attitude control.
The page offers a look at how the system works:
Conductive nanoparticles would be transported to a small liquid-filled reservoir by a micro-fluidic flow transport system. Particles that come into contact with the bottom conducting plate would become charged and pulled to the liquid surface by the imposed electric field. If the electrostatic force near the surface can cause charged nanoparticles to break through the surface tension, field focusing would quickly accelerate the particles through the surface. Once extracted, the charged nanoparticles would be accelerated by the vacuum electric field and ejected, thus generating thrust.
So we now know to keep an eye on the Plasmadynamics & Electric Propulsion Laboratory at Michigan, whose concept would offer a highly efficient engine — and one with variable specific impulse — that should be scalable for a wide range of future space missions. nanoFETS are said to be able to adjust their specific impulse over a range from 100s to 10,000s with a high efficiency range throughout the entire specific impulse regime. A useful paper for learning still more about them is Liu et al., “Nanoparticle Electric Propulsion for Space Exploration,” American Institute of Physics 978-0-7354-0386-4/07, available online.
One more thing: The Plasmadynamics & Electric Propulsion Laboratory at Michigan offers an updated list of publications here, a number of which (particularly the conference references) relate to the nanoFETS concept.
