Comet Coma Rendezvous Sample Return (CCRSR) Mission Concept

This blog entry continues both at looking at white papers on missions to the solar system's small, primitive bodies and to continue the occasional series of mission concepts in development to use ASRG plutonium power supplies. In the former series, you'll remember that the number one priority mission for comet exploration in the coming decade is a warm sample return (more on the warm part, later). In the latter series, NASA has developed a new electrical power supply system that uses much less plutonium that the old RTGs or MMRTGs. The agency is ea gar to test the ASRGs on a flight mission, and has funded 12 mission concept studies to explore what types of Discovery missions ($450M) these power supplies would enable.

Comets are believed to be the least altered bodies in the solar system, preserving both the mineral (in dust form) and volatiles present at the birth of our solar system. Returning samples to be studied in terrestrial laboratories has been a high priority for the scientific community. The Stardust mission partially fulfilled that goal by collecting dust particles during a high velocity flyby of comet Wild 2. The brief nature of the encounter and the high velocity limited the number of dust particles collected and the types of particles that could survive the high speed impact with the sample collectors.

Ultimately, the goal is to land on a comet, collect a sample containing both dust and volatiles, and return the frozen sample to year. Frozen is the key. Volatiles that melt will undergo various chemical reactions that will alter the samples. Unfortunately, such a mission is bedeviled by questions of both how to collect the samples (what is the surface a comet like and once we land on one, is that a good guide to all comets?) and of how to keep samples frozen well below the freezing point of water within a small re-entry capsule (even during the descent through Earth's atmosphere). This class of mission has been put off to the following decade with a goal of advancing the technology in the coming decade.

An alternative mission would collect a sample and allow the volatiles to warm during the return voyage and plummet through Earth's atmosphere. The dust particles would be unaffected by the warming, and the melted and altered volatiles would still provide clues. Such a mission might be possible in the New Frontiers program ($650M).

CCRSR takes a different approach. It would not land on a comet, but would instead make multiple passes through a comet's coma and jets. The encounter with dust particles would be at low speeds, preserving fragile samples. No volatiles would be collected, but a mass spectrometer would measure their composition in real time. By sampling different jets, the mission may be able to sample different portions of the comet's interior. Multiple collectors will be used so samples from specific jets can be identified. In addition to the mass spectrometer, adust detector (to estimate amounts of samples collected) and wide and narrow angle cameras would be flown.

Editorial Thoughts: This proposal gets around a key problem of sampling any small body: given the wide range of surface materials and surface densities possible, how do you intelligently design a collection system? It falls short of the hoped for goal for this decade to return both dust and volatiles (even if warmed over). Discovery mission opportunities, however, are more frequent than New Frontiers opportunities, so this mission could meet much of the goal at a lower cost and with better chance of selection.

A key goal of the next Discovery mission selection may be to test the ASRG system. Because CCRSR returns to Earth, it would have to jettison its ASRG's in deep space and make the return voyage using solar panels. The white paper suggests that the mission might be possible with only solar panels -- a possible knock in the coming selection.

Link: The Comet Coma Rendezvous Sample Return (CCRSMission Concept - The Next Step Beyond Stardust

Trojan Asteroid Rendezvous

In the last post, I listed the highest priority missions for asteroid and comet missions. The highest priority New Frontiers mission for main belt and Trojan asteroids was listed as a mission to the Trojan asteroids. (Trojan asteroids share Jupiter's orbit and are found in the L4 or L5 points leading or trailing Jupiter.) An entire white paper is devoted to justifying the high priority given to this mission.

Asteroids, like comets, are believed to be remnants left from the formation of the solar system. For bodies that underwent little heating, they probably contain relatively pristine samples of the materials from which the planets formed. For bodies that underwent significant heating (such as the main belt asteroid Vesta) they may preserve the record of processes by which the early planets formed.

Scientists would like to examine asteroids from a variety of locations in the solar system as a way to probe the gradient of conditions and materials believed to have been present during planet formation. Two theories exist as to the original location of the Trojan asteroids. The simplest would have that they formed where they are now, in which case they record conditions where Jupiter and its moons formed. A new theory, however, suggests that the four giant outer planets migrated from the locations at which they originally formed. In this model, Uranus and Neptune migrated outward into the cometary realm. Most comets would have been ejected from the solar system or pushed out into the Kuiper belt. A small fraction (hundreds of thousands) were thrown inward to become the Trojan asteroids. In this case, the Trojans are easily assessable Kuiper belts worlds.

Telescope studies shed little light on this question because the spectra are featureless, as are the spectra of C-, P-, D-type asteroids and cometary nuclei. Either theory of their formation would suggest that these should be volatile-rich worlds, but the spectral are enigmatic. A spacecraft mission is needed -- preferably to visit a number of bodies -- to resolve these questions.

The Trojan white paper lists two overarching questions for a mission to the Trojan asteroids:

"1. Did the Trojan asteroids originate near Jupiter’s orbit or farther out in the solar system?

2. What do compositions of these primitive bodies tell us about the region(s) of the solar nebula in which they formed?"

These questions would be answered by focusing on a set of specific questions for the body (or preferably, bodies) visited:

"1. How much and what types of ice and organics are present on and within Trojan asteroids?

2. What is the mineralogy of the silicates present on and within Trojans?

3. How do the geological processes that have occurred on the Trojans compare to those that have affected other small bodies?

4. What is the relationship between Trojan asteroids and comets, TNOs, outer planet satellites, and main belt asteroids?

5. Are densities and bulk compositions of Trojans diverse or homogeneous?

6. How are the spectral and physical properties of Trojan surfaces modified over time by the space environment?"

At least one mission concept is being actively developed, a Discovery-class mission that would make use of NASA's new plutonium ASRG power sources to allow flyby, orbital, and landed phases. The summary that follows is from a post done last January.

Ilion Mission Concept

While spacecraft have orbited and landed on near Earth asteroids and flown by main belt asteroids (and the Dawn spacecraft will orbit 2 of the 3 largest main belt asteroids in the next decade), no spacecraft has visited a Jovian Trojan asteroid. Ilion would do that by:

"The Ilion mission will flyby several Trojans and rendezvous and land on one of them. It carries remote sensing instruments to characterize the asteroid’s structure and landed instruments to measure its surface composition. Preliminary orbit calculations have shown that several of the Trojans can be reached by Discovery-class missions with reasonable travel times... Approximately the final 2 years of the cruise will be spent within the L5 Trojan cloud... After [orbit insertion], Ilion will observe the target asteroid for several months and a landing site will be identified. After landing, a variety of compositional and physical measurements can be made."

Resources

The Trojan Asteroids: Keys to Many Locks

Asteroids Community White Paper

ILION: A JOVIAN TROJAN ASTEROID ORBITER AND LANDER MISSION CONCEPT

Thoughts on Titan Mare Explorer

John R. passed along these thoughts (posted with his permission) in response to the blog posts about a proposed Titan Mare Explorer (TIME) (http://futureplanets.blogspot.com/2009/09/titan-mare-explorer-abstract.html and http://futureplanets.blogspot.com/2009/09/titan-mare-explorer-time.html).

Here's a link to a large image of a watery horizon on a cloudy day on Earth:

http://retiredeagle.files.wordpress.com/2009/01/dsc02585.jpg

Makes me think we might want to pass on the imagery.

The chemical analysis is a must, and sounding for depth is too easy not to do, but I seriously wonder the value of a single depth measurement. As I understand it, the weight of water ice under liquid methane would be fairly low in Titan's gravity, so I wonder if the lake bottom might be very heterogeneous, making a point measurement like that sort of arbitrary. A single altimetry track provided by radar, if some wavelength could penetrate the liquid, would be infinitely more useful.

Even the chemical analysis will leave us wondering about anisotropies. The Earth's oceans vary in salinity by a factor of about 1.5 from one location in open water to another. And here's an interesting map of salinity for Lake Pontchartrain.

http://pubs.usgs.gov/of/2002/of02-206/env-issues/images/pg160fig2.gif

All of which is just to flag, mindful of the Galileo Probe's experience at Jupiter, the risk of anisotropies and the impact that has on the value of collecting data. Clearly, the value is still there, and we'd love to have it in hand, but it undermines the meaningfulness of the data to some extent, as long as we're comparison-shopping billion-dollar missions.

Editorial Thoughts: These issues emphasize, in my mind, the value of TIME being able to make measurements over months, which its plutonium power source would allow. That would allow it to examine the surface conditions under varying weather conditions (assuming they vary meaningfully over a few months). If the winds or currents can push the lander (raft? boat?) over a meaningful transect of the surface, then there would be more chances to sample compositional heterogeneity. This would also be useful for depth sounding. I wonder if the probe could be designed so that the structure above the surface would be more likely to catch the wind?

Titan Mare Explorer (TiME)

As I've mentioned in past blog entries, NASA is funding a series of studies of Discovery missions that could make use of the newly developed ASRG plutonium power supplies. One of the most intriguing has been the Titan Mare Explorer (led by Ellen Stofan), which would put a probe on the surface of one of the lakes of Titan. Public information on the proposed mission has been scanty, but an abstract for the upcoming Division of Planetary Sciences meeting provides a few hints (and I'll add a couple of tidbits I've picked up).

From the abstract, "The scientific objectives of the mission are to: determine the chemistry of a Titan sea to constrain Titan’s methane cycle; determine the depth of a Titan sea; characterize physical properties of liquids; determine how the local meteorology over the seas ties to the global cycling of methane; and analyze the morphology of sea surfaces, and if possible, shorelines, in order to constrain the kinetics of liquids and better understand the origin and evolution of Titan lakes and seas."

I've heard through the grapevine that the instrument suite would be limited (as befits a ~$450M mission) to a mass spectrometer, a meteorology and physical properties experiment (probably several instruments in a package), and a descent and surface camera. This may compare favorably with the instrument package that was proposed for the lake lander in the Titan Saturn System Mission flagship proposal -- it's hard to tell without detailed listings of the proposed instruments. One key instrument that I haven't heard of for the proposed Discovery mission would be a gas chromatograph, which would enable detection of complex molecules. The TSSM lake lander had a combined mass spectrometer/gas chromatograph while I've heard of only a mass spectrometer for the proposed Discovery mission. It isn't clear if the gas chromatograph has been dropped, or just isn't listed.

The Discovery proposal calls for, as I understand it, six months of observations as the probe floats on the lake surface with a possible mission extension of several more months. Depending on how far the probe travels on the surface of the lake, this might allow depth measurements along a significant transect and might even bring the probe to a shore.

Editorial Thoughts: This is an exciting mission proposal. I'd love to see images from the surface of an alien ocean. The measurements of the lake composition would greatly advance our understanding of Titan chemistry.

However, there are some caveats to keep in mind. Data relay would be direct to Earth. Think of tens to hundreds of bits per second, most likely. We are unlikely to get great panoramas of photos. Think postage stamp images. Secondly, Titan is a cold place place (to put it mildly). Designing a probe that can reliably survive months on a Discovery budget may prove to be optimistic. Remember that one of the areas of technology development proposed to enable future Titan landers is technology to survive and operate in the frigid climate.

Still, I like this proposal, and hope that it is feasible in a Discovery budget.

Resources: Titan Mare Explorer abstract

ASRG Mission Concepts: CHopper

This is the third in a series of discussions of missions that would be enabled by NASA's new plutonium power source, the ASRG. These power systems use a quarter the plutonium of previous designs, allowing NASA to consider using is small supply of plutonium for small missions. Previous entries summarized the Io Volcano Observer and the Venus VALOR balloon mission.

The Comet Hopper (CHopper) is a study proposal led by Dr. Jessica Sunshine of the University of Maryland. (Perhaps not the best name for an investigator proposing to use a plutonium power supply instead of panels that utilize sunshine. :> ). This proposal addresses two key issues regarding comets: (1) the nature of their surface changes across the comet nucleus and (2) the rates (and perhaps, types) of activity they display changes as they move to different positions in their orbit around the sun. The ESA Rosetta mission will partially address these issues. It, however, will place a lander only at a single location on the nucleus and will follow its comet for only a portion of its solar orbit (1.5 years out of a 6.5 year orbit).

Because it uses plutonium as its power source, CHopper would have two advantages over Rosetta. First, a single craft can be both a lander and an orbital craft. If the mission used solar panels, it would be virtually impossible for it to do both. (Think about managing repeated landings on a comet with Rosetta's 14 m solar panels extending from either side of the craft!) Second, because it doesn't need solar power, it can operate when the craft and comet are far from the sun.

The CHopper team has published a short summary of their mission. I'm reproducing the introduction here (using their words minimizes my mistakes), but I encourage you to read the entire one page summary.

"The Comet Hopper (CHopper) mission explores the compositional and morphologic heterogeneity of a comet. Recent cometary flybys (Deep Impact at P/Tempel 1, Stardust at 81P/Wild 2, and Deep Space 1 at 19/P Borrelly) have revealed great diversity among comets as well as significant variation within individual bodies. Understanding the inherentdiversity of a comet nucleus and the origins thereof are now a clear objective of future cometary exploration. CHopper is an instrumented lander that will build upon the results of these recent missions. With a 2012-2013 launch Chopper will examine in detail the inner coma and surface of comet P/McNaught 2 (P/2004 R1).

"CHopper observes the comet while formation flying over one full orbital period obtaining measurements during the descent to, and on the surface of, the nucleus. CHopper takes advantage of the low cometary gravity field to take off and land (“hop”) multiple times during each descent to the surface. ... six “sorties” to the surface are envisaged to investigate changes with heliocentric distance."

My take on this proposal is that it is a clever design that is enabled by an ASGR power source. I would like to see this mission fly. How well it does in the proposal competition (if NASA opens the next Discovery competition to ASRG designs) will depend on the details of the design: What is the design risk? Do the instruments address the key science questions? Can the mission be implemented within a Discovery program budget?

ASRG Missions: Venus Balloon

In a previous post, I discussed the a new generation of plutonium-powered power sources for future planetary missions. NASA is funding mission studies to determine whether or not lightweight radioisotope power sources would enable one or more low cost (Discovery class, or ~$450M) missions.

Kevin Baines of JPL has proposed a number of Venus Discovery missions including an orbiter to study the atmosphere (whose goals were more than fulfilled by the Venus Express mission), and two balloon missions. The earlier balloon proposal, the VEVA (Venus Exploration of Volcanoes and Atmosphere) would have had been the most complex with a number of elements, while the second would have had just two balloon platforms. At least twice, to my recolleciton, Baines has been a finalist in the Discovery selection process, and both times other missions were selected.

The previous proposals suffered the limiation of being battery powered (solar cells are of questionable to no use within Venus' clouded atmosphere). Baines' current proposal would use an ASRG power source to enable much longer operation (a month instead of hours to days) and over ten times the data return. Baines and his collaborator, Tibor Balint of JPL, were kind enough to send me a copy of a poster on the mission they presented at a recent conference, and I've reproduced the abstract here:

"In situ exploration of Venus is expected to answer high priority
science questions about the planet’s origin, evolution,
chemistry, and dynamics as identified in the NRC Decadal
Survey and in the VEXAG White Paper. Furthermore,
exploration of the polar regions of Venus is key to
understanding its climate and global circulation, as well as
providing insight into the circulation, chemistry, and
climatological processes on Earth. In this paper we discuss
our proposed Nuclear Polar VALOR mission, which would
target one of the polar regions of Venus, while building on
design heritage from the Discovery class VALOR concept,
proposed in 2004 and 2006. Riding the strong zonal winds at
55 km altitude and drifting poleward from mid-latitude this
balloon-borne aerial science station (aerostat) would
circumnavigate the planet multiple times over its one-month
operation, extensively investigating polar dynamics,
meteorology, and chemistry. Rising and descending over 1
km altitude in planetary waves – similar to the two VEGA
balloons in 1985 – onboard instrumentation would accurately
and constantly sample and measure other meteorological and
chemical parameters, such as atmospheric temperature and
pressure, cloud particle sizes and their local column
abundances, the vertical wind component, and the chemical
composition of cloud-forming trace gases. As well, when
viewed with terrestrial radio telescopes on the Earth-facing
side of Venus, both zonal and meridional winds would be
measured to high accuracy (better than 10 cm/sec averaged
over an hour). Due to three factors: the lack of sunlight near
the poles; severe limitations on the floating mass-fraction
available for a power source; and the science requirements for
intensive and continuous measurements of the balloon’s
environment and movement, a long-duration polar balloon
mission would require a long-lived internal power source in a
relatively lightweight package. For our concept we assumed
an Advanced Stirling Radioisotope Generator (ASRG). In
return, this mission would provide two orders of magnitude
more science data than expected from the original batterypowered
VALOR concept, and could reduce measurement
uncertainties by a factor of five. In addition to the science
return, the secondary objective of this proposed mission would
be to space qualify ASRGs through all mission phases and in
various operating environments. Lifetime testing would be
demonstrated using a second ASRG on the carrier that would
keep operating after the in-situ element is delivered. Based on
the results of this and another eight ongoing NASA funded
studies, NASA will make a decision about the inclusion of
ASRGs in the next Discovery AO, due in the summer of 2009."