Exploring the Interior of a Comet

Note: While I'm traveling this month with only occasional short access to the internet, I'm reading through some of the back log of proposed mission concepts.  I'll post short summaries of the more interesting ideas.  Unfortunately, I'm unlikely to have time to search down internet sites to provide hot links to the abstracts and presentations I'm reading.  I'll try to provide sufficient information on each that you hopefully can easily do a search to find the original documents.

In this post, I'll a mission concept for the Discovery program (~$425M PI cost, ~$800M fully burdened cost) that would explore the interior of a comet.

The proposal was described in an extended abstract (2 pages) for this year's 41st Lunar and Planetary Science Conference.

Deep Interior Radar Imaging of Comets
LPSC abstract 2670.pdf

One surprise of our exploration of asteroids and the tiny moons of Mars is that many small bodies appear to be rubble piles loosely held together by gravity.  We know little about the interior of comets (fast, distant flybys produce poor gravity measurements).  However, as the authors of the abstract state, the early reconnaissance of comets "has so far yielded the discovery of an unanticipated range of diversity in geomorphic forms: multiplicities of pits, craters with vertical overhangs, global scale layering, mesas and plains.  It has also revealed new geological processes that are revolutionizing our concepts of the cometary interior -- the discovery of repetitive mini-outburts, of patches of enhanced H2O ice, and of caldera-like depressions and smooth flows... It is time to capitalize on these discoveries by moving into a new, detailed exploratory phase where we learn how comets work."

The authors propose to use ice penetrating radar to produce images of the interior of a comet with 10 m resolution that would be comparable to a "medical ultrasonographic brightness scan."  Ice and ground penetrating radars have a long heritage both from airplanes for Earth studies and at Mars where they have successfully explored the upper few kilometers of the Martian crust.  For a small comet -- the proposed target is the 3 km diameter 79P/duToit-Hartley comet -- the radar could study the entire interior as the spacecraft orbits the spinning comet.

The authors note that the Rosetta mission will conduct an inital probe of the interior of a comet as it tracks radio waves from its Philae lander through the body of a comet.  They imply that radar would offer superior interior imaging, but acknowledge the contribution the Rosetta mission will make.

The authors do not state whether or not they have proposed this mission for the current Discovery mission selection.

Editorial thoughts: I don't know if the radar system required for this mission is so costly that a dedicated mission would be needed.  Ice penetrating radars come in different flavors.  The radar proposed for NASA's Jupiter Europa Mission, for example, is a more capable and massive (and mass tends to be strongly correlated with instrument cost) than the radar proposed for ESA's Jupiter Ganymede Mission.  It would seem to me that a spacecraft capable of additional studies such as surface imaging and compositional studies of emitted gases and dust in addition to the radar instrument would be a very attractive mission.  However, the radar unit may be too costly for a Discovery-class mission to fly additional instruments.  The abstract does not provide any information on this.

I have also been struck by the diversity of comet surfaces we have seen from the few flyby spacecraft to date.  Their interiors may be equally varied.  It may be that rather than one mission providing a definitive answer, ice penetrating radar may be needed on a number of comet missions before we begin to understand that variety and the processes that created it.

Hayabusa 2 Mission Approved

Source: JAXA

Correction: A reader corrected the budget situation: So far, the Ministry of Edcuation, Science ... approved the mission. However the budget allocations are decided by the Ministry of Finance and the parlament. Usually at Dec. or January we know Jaxa´s next budget.

The Japanese government has approved the Hayabusa 2 mission that will attempt a second near Earth asteroid sample return.  The mission will launch in 2014 to visit the carbon rich (and believed to be a sample of the early primitive solar system) 1999 JU3 asteroid, which is believed to be one kilometer in diameter.  The samples would be collected in 2018 and returned to Earth in 2018.  A number of engineering modifications are planned to the Hayabusa 1 design to take advantages of lessons learned from that mission.

This mission has similar goals to the OSIRIS-REX proposal in the current NASA New Frontiers competition, especially in the goal to sample a body believed to be carbon rich.  The spacecraft would visit different a different asteroid (1999 RQ36).  If both missions fly, we would get an idea of the diversity of these primitive asteroid fragments.

For more information, check out the article at Spaceflightnow.com or the official Hayabusa 2 website.

Stardust and Deep Impact Close in on Their Second Comet Encounters

 

Deep Impact ejected formation at Tempel 1.  From http://deepimpact.umd.edu/gallery/T1_Ejecta_Devel.html

NASA's Stardust spacecraft just performed a trajectory correction maneuver to fine tune the timing of its encounter with the comet Tempel 1.  Stardust, you'll recall, returned dust samples from the comet Wild 2 and now is in an extended mission to encounter Tempel 1.  That latter comet was impacted by a projectile from the Deep Impact spacecraft.  The primary goal of the Stardust encounter will be to image the crater produced by the Deep Impact projectile.  The depth and form of that crater will tell us a lot about the surface properties of Temple 1.  Imaging the crater was a key goal of the Deep Impact mission, but the crater could not be seen through the ejected cloud that resulted from the impact.

 

Expected views of the Deep Impact Crater from the Feb. 14, 2011 Stardust encounter.

There is a nice two page fact sheet on the Stardust Tempel 1 encounter at http://stardustnext.jpl.nasa.gov/mission/pdfs/SD_NEXT_Fctsht.pdf.  (Thanks to Emily Lakdawalla at the Planetary Society for the link.)

Meanwhile, the Deep Impact craft is on it way to its own second comet encounter with Hartley 2 on Nov. 4 of this year (http://epoxi.umd.edu/2science/objectives.shtml).  Look about 2/3 of the way down this page for a nice explanation of the geology of Tempel 1, under the heading, Talps and Layers.  Essentially, the surface of Tempel 1 is composed of layered piles (talps) that may have formed from low speed impacts.  If you are interested in comets, this is an interesting read on the science that flyby missions can do.

Why a Comet Sample Return is So Hard

The small body science community has ranked a comet sample return as its top priority for the last decade or so.  This mission was included (optimistically, for reasons explained below) in the first set of mission candidates for the New Frontiers (~$650M) missions and remains on that list.  In the current Decadal Survey, the goal of the community appears to secure funding for serious technology development to enable fhe mission for the following decade.

A recent presentation to the Decadal Survey Small Bodies panel (http://www.spacepolicyonline.com/pages/images/stories/PSDS%20PB2%20Weaver.pdf) illustrates why this mission is so hard.  The first requirement, to keep the ices collected frozen, may be the hardest.  It is the ices and their record  of the early solar system's volatile inventory that makes this mission so valuable.  The samples, however, have to be kept frozen during the collection, then within the sample return capsule for the long flight home, during the atmospheric entry, and finally during the capsule retrieval.

(The freezing temperature of water at sea level is 273 K).

Another challenge is design the mechanism(s) that will collect the sample from potentially meters inside the comet.  A recent examination of technology obsticals for sampling near Earth asteroids (http://futureplanets.blogspot.com/2009/10/parting-look-at-primitive-body-sample.html) put it bluntly: it is not clear how to design a reliable sampling mechanism for near Earth asteroids.  This is probably even more true when your goal is not to sample the surface of a rocky asteroid, but to sample at depth a mixture of ice and rock.

A third challenge is that we know little about the surface of comets.  We know only that they exhibit a wide range of geomorphologies.  Whether that diversity extends a variety of surface and near surface types at the scale at which a lander would sample is unknown.  This simply makes designing the sampling mechanism that much harder.  The Rosetta mission's lander will tell us a great deal about the surface of one comet, but even if we can rely on its data for all comets, the answers could not be incorporated into a mission that would fly before the end of the coming decade at the earliest.

It now seems to be generally accepted that returning a frozen sample of a comet is beyond the scope of a New Frontiers class mission and is probably is a $1-2B mission.  In recognition of this, NASA is now willing to allow a mission that returns a dust sample plus the thawed remains of any volatiles:

"Scientific community interest in a Comet Surface Sample Return (CSSR) mission has been very high for many years. The advantages of such a mission have been stated in many documents including the decadal survey. Flyby missions to comets are fairly simple, and the Deep Space-1, Stardust, and Deep Impact missions have produced remarkable data. Rendezvous missions such as the ESA’s Rosetta mission (Figure 2.3) are more challenging, and a sample return mission can take twice as long as a rendezvous mission, thereby increasing cost and risk. The decadal survey concluded that bringing back a warm (i.e., non-cryogenic) sample was within a New Frontiers mission budget. While cometary science goals make the return of a cryogenic core sample highly desirable, such a mission may not fit within the fiscal limits and programmatic timescale of the New Frontiers Program. The science yield from a warm sample return mission will have to be strongly defended by proposers."

From Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity (http://www.nap.edu/openbook.php?isbn=NI000529)

 One Discovery mission proposal would collect only dust samples at low velocity from the comet's dust streams during a rendezvous (http://futureplanets.blogspot.com/2009/09/comet-coma-rendezvous-sample-return.html).

Small Body Priorities

Last month, I posted a blog entry on a survey of the scientific community on priorities for small body planetary missions (http://futureplanets.blogspot.com/2009/10/decadal-survey-democracy-in-action.html).  The results of the poll were recently posted.  You can read all the details at  http://www.lpi.usra.edu/decadal/sbag/poll_report.pdf, but I'll post a few highlight here copied from the report.  Clicking on the images will bring up larger versions.

A Parting Look at Primitive Body Sample Return

A number of the last posts have looked at mission priorities for primitive bodies being formulated for the U.S. Decadal Survey. This post will close out that series by looking at a series of presentations from last May at an ESA sponsored meeting.

Sample returns from comets and asteroids rank high in priorities for missions to primitive bodies in the priorities being set by the scientific community. ESA and Japan (JAXA) are both considering missions to return samples from a near Earth asteroid. For ESA, this mission is a candidate for selection as the next medium class science mission, but the proposed Marco Polo mission doesn't fit within the ESA budget. For JAXA, this would be a follow on mission to the troubled HAYABUSA mission. The two agencies are investigating options for combining these missions.

ESA held a conference last May on near Earth asteroid sample return. Quite a large number of presentations are posted on the meeting website (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=43784). I won't try to summarize all the presentations, but will instead provide highlights.

The ESA defined mission appears to still be in definition, with a several spacecraft configurations under consideration from true landers to touch-and-go sampling.

Samples would be returned to Earth in the mid-2020s after a year and a half remotely studying the asteroid and collecting samples.
Less information is available on the website for the JAXA mission. In fact, the overview presentations for both the ESA and JAXA missions were not posted. However, some ideas for the potential science compliment of a JAXA mission can be found at http://sci.esa.int/science-e/www/object/doc.cfm?fobjectid=45164.

Two presentations were made for possible NASA New Frontiers near Earth sample return missions. Neither gave out many details on the missions -- this is a competitive field. However, the OSIRIS-REx presentation shows the dedication needed to win selection. The team prepared proposals for three sequential Discovery (~$450M) mission competitions. In the last competition, the team was a semifinalist, but wasn't selected because the review board felt that the mission was too risky for the mission budget. The team planned to submit a New Frontiers (~$650M) mission proposal for the competition in progress.

A summary presentation laid out the key technical risk and political challenges facing the Marco Polo mission. Based on what I've read, the technical problem highlighted applies to sample return from any asteroid or comet -- we know too little about the surface properties to be sure what technique(s) might work. (But this issue is being worked on; see Small Body Sampling Techniques being Developed at JHU/APL.)

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

Priorities for Asteroids and Comets - Part 2

The last blog entry looked at overall priorities recommended for asteroid and comet missions by the white papers sponsored by the Small Bodies Analysis Group (SBAG), and scientific group that advises NASA. The three separate white papers recommended Discovery class missions ($425M) as their highest mission priorities so that the diversity of these worlds could be explored.

This blog entry quotes from each of the relevant white papers to recommended list the mission priorities within each mission class.

Discovery Missions

For main belt and Trojan asteroids, the top two priorities for Discovery missions are:

"D/P-type Asteroid Rendezvous: Spectroscopically, these unusual asteroids are very similar to many outer solar system objects (e.g., comets, Trojans, irregular satellites, Kuiper belt objects). According to recent dynamical models, perhaps all of these objects came from a disk of comet-like objects originally located beyond the Jovian planets. D/P-types may be transplanted Kuiper belt objects now located within relatively easy reach of our spacecraft to understand in detail the mineralogy and processes of these primitive objects.
Themis Family and the MB Comets Rendezvous: The Themis family, produced by one of the largest disruption events in MB history, is filled with primitive objects. Family members display C-, D-, and B- type surfaces that could provide a window into the compositional stratification in the parent body and the aqueous alteration processes that may have been common in the early evolution of primitive bodies."

For near-Earth asteroids, the top Discovery mission priorities are:

"Reconnaissance of the population of NEOs: Asteroid rendezvous (which can include a flyby component) missions with imaging and surface-modification capabilities could study the several NEOs in detail, and should be able to connect asteroid taxonomic types with meteorite classes, as did the NEAR mission.
NEO Search from a space-based platform: A space-based discovery platform has clear performance advantages over the same-size ground-based telescope due to the lack of atmospheric absorption (particularly in the IR), weather, continuous operation, and diffraction-limited optical performance. Space-based platforms also have more limited lifetimes, higher costs, and more restricted operating modes than do ground-based systems."

For comets, the priorities are less specific: "Discovery class missions are a vital component of any strategy to understand the nature of comets, as they can address the diversity of comets and their activity mechanisms through flybys of multiple targets and low-cost rendezvous investigations of specific targets, including the characterization and reconnaissance of the best CNSR targets."

New Frontiers Priorities

For main belt and Trojan asteroids, the top two priorities are:

"Trojan Rendezvous: The Trojan asteroids are important targets because they i) represent primitive mineralogies that are not in the meteorite collection; ii) have potential to provide constraints on dynamical models of the early solar system; and iii) preserve evidence of early solar system volatile-rich processes... A Trojan rendezvous with spacecraft equipped for geochemical remote sensing and in-situ surface probes/landers can significantly advance our knowledge of this link between the terrestrial planets and the outer solar system.
Multiple Flybys of MB: Some of the science goals of a rendezvous mission such as shape, surface processes, and mineralogy, and evolution of these objects may be at least partially achieved via a fly-by mission that targets multiple objects representing a range of poorly understood spectral types. While the spacecraft may be relatively simple, scope, length and complexity of the operations would put it into the NF class."

Near-Earth asteroids

Sample return: "The highest priority is a sample return from a volatile-rich object not known to be represented in meteorite collections. A P-, D-, or W-class object would be optimal, followed by the other “wet” classes such as B, C and G. For targets that are well-characterized by ground-based spectroscopy and imaging, as well as in-orbit characterization before sampling, existing technologies will be able to return samples."
Grand Tour: "The second priority is a Grand Tour mission to rendezvous with a number of NEOs of a variety of classes. Only a spacecraft mission is capable of elucidating the distinctions between the various compositional classes and providing the imaging detail needed to understand the details of formation of difference physical types. Such a spacecraft could have several (perhaps three or four) penetrators, microlanders, or similar low-cost easily-deployed surface exploration modules. This mission could be at a somewhat lower cost than the sample return mission."

Comets

"We reiterate the finding of the previous Decadal Survey that the return to Earth for analysis of a sample from the surface of a comet’s nucleus remains a critical component of NASA’s systematic investigation of comets. The complexity of such a mission pushes it into the New Frontiers class, but the potential scientific return justifies this larger investment. In particular, a Comet Surface Sample Return (CSSR) mission will reveal the complexity of cometary organics, and whether comets could have provided pre-biological material to the Earth and other planets." [In this mission concept, the samples are not kept frozen, so any ices will melt and possibly undergo chemical change during the return to Earth.]

Flagship Mission

Only one flagship (>$1B) mission is a priority, a comet sample return that keeps the collected ices frozen on their return to Earth. "The holy grail of cometary spacecraft missions is the return to Earth of a cryogenic sample extracted from deep (> 1 m; the deeper, the better) within a nucleus, which is referred to generically as the Cryogenic Nucleus Sample Return (CNSR) mission. Owing to the complexity and technical challenges associated with CNSR, it is commonly assumed that such a mission will fall into the NASA Flagship class...We strongly recommend that NASA invest in a detailed study of the technical feasibility and cost of a CNSR mission during the next decade (2011-2020), with a goal of enabling such a mission in the following decade (2021-2030)."

You can find all the SBAG white papers and white papers on these bodies submitted by the rest of the science community at http://www.psi.edu/decadal/

Note: The main belt-Trojan asteroids white paper listed a number of additional lower priority missions for both the Discovery and New Frontiers class missions. Please check out that white paper if you are interested.

Priorities for Asteroids and Comets - Part 1

For the next several postings, I'll be exploring the priorities set for planetary exploration in Decadal Survey White Papers by each of the Analysis Groups that advise NASA. There are four groups that I know of: MEPAG (Mars), VEXAG (Venus), OPAG (outer planets), and SBAG (small bodies). These groups are composed of scientists active within their disciplines (although it is common for scientists to be active in two or more groups).

The goal of the Decadal Survey is recommend a set of planetary science priorities that translates into a set of mission and research goals to be funded by NASA. In a previous job, I developed product roadmaps for a high technology company. Ultimately, a roadmap distills a set of options into the mix of mission (or products) that fit within budget constraints to produce the highest return (in science or profit). The Decadal Survey is looking to each of the Analysis Groups to recommend the priorities within its own discipline. (Mechanisms have been put in place to seek input from other sources in the planetary science community, too.)

In this blog entry, I'll start with the priorities recommended by SBAG for exploring asteroids (near Earth to the Trojan that share their orbit with Jupiter) and comets. I start here because the White Papers from this community most clearly set out (so far as I've found in my reading to date) priorities within each of their sub-disciplines. It's perhaps easier for SBAG to have clear priorities at this point. While single SBAG White Papers cover literally thousands of bodies, White Papers for other disciplines focus on single aspects of a single world (for example, atmospheric studies at Venus) or a single body (a Uranus orbiter).

White Papers tend to follow a common outline. They start by showing why the study of the object or phenomenon is scientifically compelling, then discuss the key outstanding questions, and then establish measurement priorites to answer those questions. For comets and asteroids, the scientific rational is essentially the same. These bodies represent the most unaltered worlds left from the formation of the solar system. Studying them will reveal the conditions and processes of the early solar system.

For asteroids, "The sheer diversity of asteroids is their most compelling feature. They are a reservoir of information on a huge range of solar system history, chemistry, physical processes, and evolution. Because they often are collisional fragments, asteroids are windows into processes that are hidden on the terrestrial planets by time, geochemical evolution, or simply deep burial."

For comets, "Comets represent the most unaltered (i.e., primitive) samples of the early Solar System and, even though new results have shown that the surfaces of short period comets have undergone major evolutionary modifications, carefully selected samples can still be expected to provide key information on the processes of planetary formation during the first few hundred million years of Solar System history."

The priorities for asteroids are set out in three papers, one for main belt and Trojan asteroids, another for near-Earth asteroids, and a third for comets. The priorities for each are similar:

1. Maintain active programs of telescopic research and laboratory studies. The sheer diversity of theses bodies allows telescopic observations to lead to new understandings of these worlds while laboratory studies allow scientists to interpret findings from telescopic and spacecraft missions.
2. Make Discovery missions ($425M per mission) the highest priority for exploring these worlds. The diversity of these worlds means that a mission to almost any of them can make a significant contribution to the field. The comet White Paper goes so far as to recommend increasing the frequency of Discovery missions from to every 18 to 24 months (from the current approximately every 3.3 year frequency).
3. Fly a New Frontiers mission ($650M per mission) to address a high priority target.
4. In the case of cometary exploration, perform technology development to enable a future Flagship mission (>$1B) to return frozen samples from a comet.

The next blog entry will list priorities for specific missions.

You can find all the SBAG white papers and white papers on these bodies submitted by the rest of the science community at http://www.psi.edu/decadal/

Next Generation Small Bodies Missions

Comet Wild 2

Comets and asteroids have been an on-going priority for planetary exploration for some time because of the clues they hold for the formation of the solar system. A number of missions have flown past or orbited these worlds and returned dust samples from a comet. Two sophisticated missions are on their way to rendezvous and land on a comet nucleus (Rosetta) and orbit two of the three largest asteroids (Dawn). In addition, two spacecraft are on their way to fly past comets. The Japanese Hayabusa craft is returning to Earth and may bring with it samples from a comet. The Russians are preparing their Phobos-Grunt spacecraft to return samples from that small moonlet of Mars.

So, what is next for the exploration of these small bodies? NASA's advisory boards set mission priorities only for the largest mission classes. (The smallest class missions in the Discovery program are selected from proposals submitted for each competition without prior prioritization of missions.) There are no plans that I am aware of for a large mission ($1-3B range). For the medium mission class, the scientific community has included three small bodies missions in its list of eight mission priorities:

Comet surface sample return to return unaltered dust and ice to Earth for thorough chemical analysis and expected breakthroughs in our understanding of the early history of our solar system

A Trojan/Centaur asteroid reconnaissance mission that would flyby or orbit one of these outer solar system classes of asteroids and provide our first look at these classes of bodies

An asteroid sample return mission to provide samples of primative materials that likely have not survived entry into the Earth's atmosphere to be included in our collection of meteors. (The Marco Polo mission to return samples from an asteroid is in competition for selection ESA's next medium class (300 euro) and would be implemented jointly with the Japanese space agency (JAXA). )

Marco Polo Mission

The multitude of missions to these bodies in the past and in flight raises the bar for future missions. Simple flybys seem unlikely (unless to a very distant object such a Centaur or en route to another destination). Simply orbiting or rendezvousing with a body also seems to be a repeat of past missions and hence would seem unlikely to receive funding. (As a scientist, I like repeating measurements in different places because that allows us to learn about the processes that create similarity and variance. However, doing this with multi-hundred dollar (or euro or rubble or equivalent in your favorite currency) doesn't seem to get your proposal funded.)

While the bar has been raised, there doesn't seem to be any shortage of ideas for future missions to small bodies. The concepts that I have seen discussed include:

Small missions (Discovery class). Proposals and concepts here include missions that would orbit and then land on small bodies (often with several landings). Examples include the Ilion mission to a Trojan asteroid and the CHopper comet redezvous/lander craft. The Deep Interior mission would not land on a comet but would instead orbit it and image its interior in high resolution with surface penetrating radar. A proposal to return samples from several near Earth asteroids was a finalist for the last Discovery selection. Why it wasn't selected isn't known: possible reasons include technical issues, cost concerns, potential science return, or something else. There is also a Discovery mission concept in the works that would collect dust samples at low velocity while flying in formation with a comet and then returning the samples to Earth.

Piecing together hints from various places, it appears that craft that orbit and then land on small bodies could fit within a Discovery budget. A key issue for Discovery class missions is affording a large set of instruments. If you have a craft that observes the body at a distance and on the surface, generally two sets of instruments would seem to be needed. For an asteroid with a rocky surface, one could imagine that the surface instruments could consist of a simple camera, a contact spectrometer for composition, and a robotic arm to place the spectrometer. For a body with significant organics, however, the instrument list for the landing phase becomes more extensive. The proposed instrument list for CHopper, "In addition to high resolution imaging and multi-spectral mappers, the in situ scientific payload instruments under consideration include a dust flux monitor, organic analyzer, elemental analyzer, microscopic imager, and gas mass spectrometer," bears out this argument. As few as two remote sensing instruments and four surface instruments (with the dust flux monitor probably working in both modes). In this case, it might be better to think of the craft as a lander that can also carry out some remote sensing.
Medium missions (New Frontiers class). Several studies have suggested that a surface sample could be collected and returned from an asteroid or comet within the budget of a New Frontiers mission (see, for example http://futureplanets.blogspot.com/2009/02/asteroid-sample-return-missions.html and links within that blog post). The Marco Polo mission, once the ESA and JAXA contributions are added together, appears to be in this price range. With sample return missions, we would be get some remote sensing of a body from orbit or rendezvous (the instrument list would probably be short to afford the sample return equipment) and actual samples for detailed analysis in Earth labs.

In general, near Earth asteroids probably would most easily fit within this price range. A study of a comet sample return mission suggested that such a mission might fall into the price range, but with a lot of cavaets (more on that in a moment) See http://www.lpi.usra.edu/opag/nov_2007_meeting/presentations/cssr.pdf for the mission analysis and http://www.lpi.usra.edu/opag/nov_2007_meeting/presentations/cssr_science.pdf for the science rational and goals.
Large missions (Flagship). Comets pose a particular challenge for sample return because the desired material to return includes solids (dust, pebbles, etc.) and ices (especially important because we don't have volatile samples from the formation of the solar system). Comet sample return requires that the sample be kept frozen both to preserve water ice (and other ices ideally, but that requires even colder temperatures) and to prevent melting water from chemically altering the solid sample. This cooling requirement apparently pushes comet sample return missions out of the New Frontiers budget at closer to $1B. (See slide 11 in this presentation for a discussion of the budget dilemna: http://www.lpi.usra.edu/sbag/meetings/jan2009/presentations/AHearn_SBAG-EPOXI.pdf). NASA's advisory committees have said that a proposal to return non-cooled samples should be allowed to be submitted for the New Frontiers competitions, but that the proposer will have to justify why the mission can still fulfill the science goals with a warm sample. That sounds like a potential deal breaker that would move comet sample return missions into the large mission class.

My take on all this is that we can expect to continue to see proposals for small body missions in the Discovery program with missions moving to more ambitious goals of landing or carrying out other new types of measurements. I would be surprised if there isn't a proposal for an asteroid sample return mission in the current New Frontiers competition (although we generally learn only of the finalists, not of the proposals that didn't get that far). However, the holy grail of small bodies missions for some time has been to return a large sample from the surface of a comet. It appears that that mission may not fly unless the Decadal Survey in progress makes that mission a high priority for selection as a large mission in the next decade.

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?