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/

Io White Papers

Jason Perry has posted an excellent summary of the two Io Decadal Survey white papers. Check them out at his blog, The Gish Bar Times.

247 White Papers - A Treasure Chest!

The planetary science community poured its time and heart into producing a treasure trove of white papers. These papers cover almost every aspect of future planetary exploration from the scientific justification, to specific mission proposals, to the enhancements needed to the Deep Space Network that enables it all.

I've read a fair number of these papers (50?). Most are excellent, a few represent ideas that still need work, and a couple seemed silly. If you want a serious read on almost any topic (future Titan exploration, for example) just search through these for an excellent background on the science and possible missions.

There's one downside, though. The papers are listed at the Decadal Survey website across 10 web pages in the order in which they are submitted, making a search time consuming. (Google searches weren't working well as of today. Mostly, you get hits on entries from this blog.)

So, two alternatives. I will continue to summarize and comment on the papers I find most interesting. Alternately, you can go to the websites of the various analysis groups to see the white papers for their subject areas. Here are links to the analysis groups (listed in the random order I first found and bookmarked the pages):

All papers (see links at bottom of page to other pages with papers)

Small bodies (SBAG) - asteroids, comets, trans-Neptunian objects, small moons

Mars (MEPAG)

Outer Planets (OPAG)

Venus (VEXAG)

Astrobiology

There are a number of white papers that didn't fit into these categories (e.g., Deep Space Network upgrades that could increase data return from missions several fold). For them, I've compiled the ten web pages hosting all the input into a single Excel spreadsheet. The papers are still listed in the order published, but you can search on a single page. E-mail me at vkane56[at]hotmail.com if you are interested.

Oh, and it may not be exactly 247 papers. Some papers were posted multiple times as they were revised, and I don't know if the drafts are still on the web pages. (I have a life so I didn't check.) The true number is still over 200, which represents a treasure chest for anyone interested in future planetary exploration.

Example of white paper listing on the Decadal Survey web pages.

Tight NASA Budgets

A report by Space News supports the budget outlook I've been expecting. NASA's purchasing power has been flat over the last 15 years. Budget increases are not expected under the Obama administration.

Editorial Thoughts: This budget outlook comes at a time when NASA's manned spaceflight and Earth observation programs are both requiring more funding. The result likely will be flat budgets at best for planetary programs or lower budgets. I believe that a key measurement of the Decadal Survey in progress will be whether or not its proposals remain meaningful if planetary budgets decrease. A plan that holds together only with full funding may prove to be less than useful.

Phobos-Grunt Delay Confirmed

Launch will be moved back from this fall to 2011: http://www.google.com/hostednews/afp/article/ALeqM5ikw3CuSiSD3XpmbaTlUeMumyXZdg

Dive! Dive! -- Titan Submersible

In the past couple of weeks, I've posted three entries on the proposed Titan Mare Explorer that would float a probe for months on the surface of a Titan lake.

I learned yesterday from a Decadal Survey white paper that Titan lake floaters are small dreams. A white paper that apparently is based on a SwRI study proposes a Titan lake submersible. Details on the implementation are sketchy, but it appears that the probe would first float on the surface and then fill ballast tanks (or alternatively detach from a flotation device) to sink to the lake bottom. It's not clear whether the ballast tanks could be blown to resurface. There's also no information on how the craft would communicate to Earth once below the surface. Would the lake liquids be transparent to radio waves or would the craft periodically resurface?

The white paper includes the most succinct summary of why chemical measurements of Titan's lakes are a high priority: "the mixing ratios of minor constituents (hydrocarbons, nitriles, noble gases) dissolved in the ethane-methane fluid of Titan’s lakes are expected to be higher than in the atmosphere, enabling spectrometric measurements of significantly higher sensitivity than were achievable with the Huygens GCMS or the Cassini INMS. Measurements of the dissolved species will provide information needed to better constrain models of the formation and evolution of Titan and its atmosphere." In addition, "The relative deficiency of Titan’s atmosphere in oxygen gives rise to the question whether prebiotic organic chemistry at Titan (1) is terrestrial in nature, occurring in cryovolcanic ammonia-water flows or in melt pools resulting from impacts, or (2) represents an altogether different chemistry, “where ammonia substitutes for water, and Nchemical groups substitute for O-chemical groups” [Raulin and Owen, 2002, p.383; Raulin, 2008b]. With its hydrocarbon lakes and ammonia-water cryomagma, Titan affords a unique laboratory for the investigation of alternative—“weird”—biochemical processes involving nonwater polar solvents (ammonia) or nonpolar solvents (hydrocarbons) [cf. NRC, 2007]."

Submersion through the lake depth to the bottom would allow the examination of changes in pressure, temperature, and composition along the descent profile. The paper mentions possible sampling of the lake bottom sediments. The paper also talks about the measurement of lake tides from the bottom (using upward viewing sonar).

In addition to studies below the surface, the white paper emphasizes the importance of studies at the surface to study Titan's hydrological (methane-ological?) cycle. Apparently the craft would spend considerable time on the surface.

The paper briefly mentions that it might be possible to build a dual probe that includes a submersible and a floater. It doesn't say which or both might include the RTG power source. (Retaining it on the floater for long-term meteorological and hydrological studies would seem more important to me; the submersible might be battery powered and relay its findings through the floater.)

Click on image for summary of SwRI concept study.

The paper emphasizes that this would be a New Frontiers (~$650M) class mission.

Editorial Thoughts: An intriguing idea. I suspect that the design problems are greater than suggested by the paper ("...no major technical drivers that must be overcome..."). Just the problem of keeping the craft warm in a dense, extremely cool fluid would seem a challenge to me. And might the craft's temperature (it has to be kept warm inside and that heat will leak out) cause problems?

Whatever the technical challenges, this is an intriguing idea that seems to be exploration at its best. Imagine having the descent camera return images from the bottom of an alien sea (okay, it will probably just be dull mud flats, but still...).

Note: There's also been a lively discussion of Titan lake probes here at Unmanned Spaceflight that you might want to check out.

Resources:

Titan Lake Probe white paper

SwRI Concept for a Titan Submersible

Abstract for a the Titan Mare Explorer

Results of Poll

I've had a poll that closed a couple of days ago on the blog. The results are:

The in-depth exploration of Mars over the last dozen plus years has lead to a revolution in our understanding of that world. Which world(s) would you like to see as the next target of a focus exploration program?

Venus	38 (17%)
The moon	18 (8%)
Asteroids	15 (6%)
Comets	2 (0%)
Jovian system	49 (22%)
Titan/Enceladus	84 (37%)
Other	16 (7%)
None	0 (0%)

Votes so far: 222

Titan and Enceladus have been long favorites in this poll, as have Venus and the Jovian System. It is easy to envision multiple missions to any of these destinations. It will be interesting to see if the Decadal Survey picks a clear winner for focused attention (and if it does, it may well continue to be Mars).

Another poll is still open at the blog:

Which New Frontiers Mission do you favor?

South Pole -- Aitken Basin Sample Return	17 (10%)
Venus in Situ Explorer	71 (42%)
Comet Surface Sample Return	12 (7%)
Network Science	4 (2%)
Trojan/Centaur Reconnaissance	11 (6%)
Asteroid Rover/Sample Return	12 (7%)
Io Observer	24 (14%)
Ganymede Observer	15 (9%)

Vote on this poll Votes so far: 166

Phobos-Grunt to be Delayed Two Years?

The BBC reports that this mission, with its Chinese orbiter, is likely to be delayed two years to 2011. See http://news.bbc.co.uk/2/hi/science/nature/8254820.stm

DPS Future Mission Abstracts

The upcoming Division for Planetary Sciences meeting will have a number of posters on future planetary missions. Unfortunately, the server that hosts the abstracts provides temporary url's. So, I am breaking with a tradition of quoting only small sections of published (even if only on the web) material and reproducing three abstracts that I thought were interesting. I'm also listing all the titles of relevant abstracts so that you can decide if you want to take the time to find the abstracts on your own by following links at http://dps09.naic.edu/program.shtml

Title LIFE, Life Investigation For Enceladus
Author Block Peter Tsou1, I. Kanic1, C. Lane1, C. Sotin1, L. Spilker1, T. Spilker1, N. Strange1
1JPL.
Abstract Enceladus, a small icy moon of Saturn, is one of NASA outer planet life search targets and unique in its current active jets. As with comets, this enables a low-cost flyby sample return mission like STARDUST. Samples from Enceladus will expand our in-depth knowledge of “life” and allow us to effectively plan for future missions.
Cassini found Enceladus’ jets composed of fine icy particules and hydrocarbons. Saturn’s E ring is sustained by these jets for at least the last 300 years. Clearly there is a subsurface heat source generating such jets. Several theories for the origin of life on Earth would also apply to Enceladus; thus, obtaining the samples from the plume will provide breakthrough understandings of the nature of current or past life markers.
The highly detailed analyses of Apollo and STARDUST samples revolutionized our knowledge of the Moon and comets and provided fundamental insights into remarkable processes that occur early in the formation of the Solar System. These in-depth analyses are not possible with astronomical remote sensing or in-situ instrumentations. Since the duration of these plumes is unknown, it is imperative to capture these samples by the earliest flight opportunity- the Discovery AO by the fall of 2009.
For LIFE, we have a trajectory to encounter the plume at less than 4 km/s ensuring a more gentle capture of organics than STARDUST at 6 km/s. With less than 14-year mission duration, the samples can be returned to Earth before 2029. By capitalizing on the STARDUST heritage of design-to-cost mindset, the mission cost can be controlled. For cost reduction, the upcoming Discovery AO offers unique free ASRGs and allows the use of Jupiter for gravity assist.

Title The Case for Uranus and Neptune
Author Block Mark D. Hofstadter1, C. Sotin1, S. Brooks1, L. Fletcher1, A. Friedson1, R. Moeller1, N. Murphy1, G. Orton1, T. Spilker1, D. Wenkert1
1JPL.
Abstract Uranus and Neptune are composed mostly of ices, such as H2O, making them fundamentally different from Jupiter or Saturn. These ice giants, and their unique satellites and rings, have an important story to tell us about the formation, evolution, and structure of planets in our Solar System and beyond. To understand that story, we must learn the basic properties of their interiors. We do not know if they have extensive solid- or liquid-water layers (making them almost overgrown icy satellites) or if the H2O-H2 phase diagram allows structures unlike any other planet in our solar system. How internal heat is transported through the interior and atmosphere is also important to learn. We wish to know the nature of atmospheric convection and circulation and how they relate to internal and solar forcing. We also wish to know the composition and temperature of the atmosphere as a function of latitude, altitude, and time. One of the great surprises of the Voyager encounters was the discovery of strongly tilted dipole magnetic fields, offset from the planet's centers. How and where is the field generated? How does its unique geometry affect the transfer of energy from the solar wind to the magnetosphere? A mission to Uranus or Neptune, supported by healthy ground-based observing and laboratory campaigns, should be a priority for the next decade. Either planet can serve as the archetypal ice giant, but cross-disciplinary priorities can be used to choose one over the other. A recent JPL study identified trajectories that could deliver significant science payloads into orbit around either planet, and found that it may be possible to do so at Uranus for under the New Frontiers cost cap and using solar-power. This research was carried out at JPL/Caltech under contract with NASA.


Title Exploration Strategy for the Dwarf Planets 2013-2022
Author Block William M. Grundy1, W. B. McKinnon2, E. Ammannito3, J. C. Castillo-Rogez4, W. J. Merline5, K. S. Noll6, A. S. Rivkin7, J. A. Stansberry8, M. V. Sykes9, A. J. Verbiscer10
1Lowell Obs., 2Washington University, 3INAF-IFSI, Italy, 4JPL/Caltech, 5Southwest Research Institute, 6Space Telescope Science Institute, 7JHU/APL, 8Steward Observatory / Univ. of Arizona, 9Planetary Science Institute, 10University of Virginia.
Abstract Dwarf planets are now recognized as a third class of planets, along with terrestrial and giant planets. In terms of physical attributes (hydrostatic shape, presence of atmospheres, satellites), there is no clear dividing line between dwarf planets on one hand and terrestrial planets and large icy satellites on the other. Five dwarf planets are presently recognized - Eris, Pluto, Haumea, Makemake, and Ceres - and this list will only grow with time. All five are icy or at least water-rich. As of 2009, New Horizons (a New Frontiers mission) is en route to the first encounter with the Pluto-Charon system in 2015, and Dawn (a Discovery mission) is in flight and slated to orbit Ceres, also in 2015. Given the newness of this field of study, many scientific questions about the dwarf planets remain to be addressed, which impacts our understanding of the Solar System as a whole. We will summarize both the critical science questions for and scientific importance of dwarf planet exploration, and for the 2013-2022 timeframe of the Planetary Science Decadal Survey, the most important mission targets and other efforts necessary to understand these worlds.



45.Missions, Past and Future Presentations:
45.01. MARCO POLO: A Near Earth Object Sample Return Mission in the ESA program Cosmic Vision 2015-2025

45.02. Solar System Science with WISE

45.03. Current Status of the International Lunar Network (ILN) Anchor Nodes Mission

45.04. Titan Mare Explorer (time): A Discovery Mission To A Titan Sea

45.05. Solar System Science with the Wide-field Infrared Survey Explorer

45.06. In Situ Assessment of Habitability with the SAM Suite Investigation on the 2011 Mars Science Laboratory

45.07. Mission Architecture Options for Enceladus Exploration

45.08. Lunar Science Below the Surface - the MoonLITE Low Cost Penetrator Mission

45.09. New Solar System Researches expected by a New Telescope Project at Mt. Haleakala, Hawaii

45.10. Search for Indices of a Prebiotic or Biotic Organic Activity on Mars with the Gas Chromatograph-Mass Spectrometer of the Sample Analysis at Mars



16.Decadal Survey White Papers Presentations:

16.01. Argo: The Next Step in the Exploration of the Outer Solar System

16.02. LIFE, Life Investigation For Enceladus

16.03. Saturn Atmospheric Science in the Next Decade

16.04. Entry Probe Missions to the Giant Planets

16.05. Cassini Solstice Mission

16.06. The Case for Uranus and Neptune

16.07. The Value of Landed Meteorological Investigations on Mars: The Next Advance for Climate Science

16.08. Space Weathering Impact on Solar System Surfaces - Community White Paper for Planetary Science Decadal Survey 2009 - 2011

16.09. A Dedicated Space Observatory For Time-domain Solar System Science

16.10. Laboratory Studies in Support of Planetary Science

16.11. Exploration of Europa

16.12. Ganymede Science Questions and Future Exploration

16.13. Uniqueness Of The IRTF For NASA Missions And Planetary Astronomy

16.14. Europa Jupiter System Mission (EJSM): Exploration Of The Jovian System And Its Icy Satellites

16.15. Exploration Strategy for the Dwarf Planets 2013-2022

16.16. The Case for Enceladus Science

16.17. A JPL Planetary Science Summer School Trojan and Centaur Reconnaissance Mission: Science

16.18. Science Performance of the Pupil-mapping Exoplanet Coronagraphic Observer (PECO)

16.19. Community Consensus White Paper on Goals and Priorities for the Study of Centaurs and Small Trans-Neptunian Objects in the 2010s

16.20. The Ionosphere Of Mars: A Community White Paper For The Planetary Decadal Survey

16.21. Recommended Exploration Strategy for the Outer Planets 2013-2022

16.22. The Case for Ceres: Report to the Planetary Science Decadal Survey Committee

16.23. Technologies Required to Support the Outer Planets Exploration Strategy for 2013-2022

16.24. The Future of Io Exploration, 2013-2023: A White Paper Submitted for the 2011 Planetary Decadal Survey

16.25. The Irregular Satellites

16.26. A JPL Planetary Science Summer School Trojan and Centaur Reconnaissance Mission: Mission Design

16.27. Jupiter Atmospheric Science in the Next Decade

16.28. Larger Icy Satellites

16.29. Titan’s Atmosphere and Surface Explored by Future in Situ Balloon Investigations

Venus New Frontiers Mission Concepts

The Venus Exploration Analysis Group (VEXAG), which provides inputs into NASA's Venus exploration plans, has posted a presentation on possible New Frontiers ($650M) missions to Venus. The same group has also proposed a >$3B flagship mission to the same world. That mission would be composed of an orbiter/data relay that would image small portions of the planet with radar at ~5 m resolution. (The strategy is similar to the HiRise camera used at Mars, which photographs a small fraction of that globe at high resolution.) Two balloons would conduct long term atmospheric measurements while two landers would survive at least a day to make detailed chemical measurements of the surface. This presentation will serve as input into setting priorities for planetary exploration in the Decadal Survey.

The first part of the presentation lays out the case for exploring Venus. It's a long list, but key argument is that Venus is the third example of a terrestrial planet with an atmosphere. While Mars has received considerable attention in the last 15 years, only the Venus Express mission has conducted in-depth studies of that world in the same time frame. Studying Venus provides a contrast that can help us understand the role of early planet evolution, plate tectonics, and atmospheric change on the evolution of terrestrial worlds. The three primary questions that underlie the goals for Venus exploration are:

• "How did Venus originate and evolve, and what are the implications for the characteristic lifetimes and conditions of habitable environments on Venus and similar extrasolar systems?"
• "What are the processes that have shaped and still shape the planet?"
• "What does Venus tell us about the fate of Earth's environment?"

The missions proposed in the presentation are pieces of the Flagship proposal. They are:

Venus In-Situ Explorer (VISE) that would be a combination atmospheric probe to measure atmospheric composition to the from the upper atmosphere to the surface and then measure surface composition. The focus of this mission would be to understand the accretion of terrestrial planets including whether Venus had an ocean, how the atmosphere has evolved, and the type of volcanism. Instruments would include, "cameras, spectrometers, NMS/GS [neutral mass spectrometer/gas chromatograph], meteorology package," and instruments to, "determine mineralogy, elemental composition, and surface texture." While the presentation is silent on the topic, this would presumably be a short-lived lander that would function for only one to a few hours on the surface.

Long-lived balloons with drop sondes and a data relay orbiter to address the question of how the planet's atmosphere works. Apparently multiple balloons would be used that would float at different altitudes and each carry a gas chromatograph/mass spectrometer, radio tracking, an atmospheric structure instrument, a nephelometer to measure cloud particles, a lightening detector, and a TLS (tunable laser spectrometer?). Drop sondes would be released from the balloons to drop to the surface and would carry one or more instruments for trace gas measurements, imaging the surface, and measuring pressure and temperature. (No details on how many drop sondes per balloon or whether they would function all the way down to impact on the surface.) The orbiter would carry a near infrared imager and topographic radar (which presumably would measure only surface elevations and not image the planet).

Venus Global Surveyor orbiter that would focus on the evolution of the crust, shallow interior, and search for active volcanism from a circular orbit. The instrument list includes a radar altimeter, imaging radar, and a near infrared imaging spectrometer. No information is given on the potential resolution of the radar images, although there would presumably be more detailed than Magellan's.

Venus atmospheric sample return that would focus on the origin and evolution of the atmosphere. The atmospheric sample would be collected from an altitude of 110 km during a brief run through the atmosphere. (A similar mission was proposed for Mars several years ago.) Earth-based instruments would be able to make much more detailed measurements of atmospheric composition than could be done from an in-situ instrument that must survive the rigors of launch and atmospheric entry while meeting stringent weight, size, and power limitations.

Next Generation Geochemical Lander would focus on surface composition and gasses trapped in the surface material to study how the crust formed and the chemical interaction of the atmosphere and surface. The lander would be targeted to a site expected to have vesicular basalt (so trapped gasses can be examined) and would survive for 24 hours to allow detailed chemical analysis. The choice of sampling site(s) near the lander would be made by operators on Earth, so this wouldn't be a blind sample grab. Instruments would include a 10 cm depth drill, mass spectrometer, x-ray diffraction, x-ray fluorescence, cameras for descent and surface panoramas, and a microscopic camera.

Geophysical Lander to study the interior structure and thermal evolution. Details on this idea are limited (and it is new to me, so perhaps it is a recent concept still being fleshed out). Instruments would include a magnetometer, a corner cube reflector (to allow precise location determination by radar, although I don't know if this would be from an orbiter or from Earth), electromagnetic sounding (presumably to detect subsurface layering), heat flow (I was under the impression that heat flow measurements are both hard (they've only been done for the Earth and moon) and require long times, so I'm not sure what approach would be used), and panoramic camera, and a gamma ray spectrometer (presumably for bulk surface composition measurements).



Editorial Thoughts: Two immediate issues come to mind for me. The first is how many of these missions would really fit within a $650M New Frontiers budget. The VISE and long-lived balloons have both been proposed for previous New Frontiers competitions, so they seem like the most likely candidates. I sat in on a VEXAG-sponsored meeting last December and the comments there were that the Flagship orbiter with high resolution imaging and the long-lived (24 hours (this is Venus, so long-lived is relative!)) landers would cost approximately a $1B each. Perhaps the orbiter listed here is less capable and flying just a single lander (instead of the two planned for the Flagship mission) reduces costs sufficiently. I have no idea how difficult the other two missions would be to conduct.

The second issue is that this is a long list of missions. Realistically, only one -- and at the outside, two -- would likely fly in the next decade. What is the priority for these missions? From the last Decadal Survey (completed in 2003), VISE was the highest priority (and it still hasn't flown). What is the priority now? I hope that VEXAG will make this clear as this Decadal Survey progresses over the next few months. One factor that could change the priorities is Russia's plans to send Venera-D to Venus in 2016 with an orbiter to conduct radar imaging at higher resolutions than Magellan did and to send a short-lived lander to the surface. Apparently this will be the first in a series of new Venus missions for Russia.

If Russia is successful and shares the data with the world-wide scientific community, then the urgency for NASA to prioritize Venus might be less. (Or perhaps there are still key holes that NASA could fill, hopefully in cooperation with Russia.) The solar system is a big place, and it would be easy to argue for making Mars, Venus, Jupiter/Europa, or Titan/Enceladus the focus of any single space agency's priorities. Perhaps Russia could focus on Venus and NASA and ESA on Mars and possibly Jupiter/Europa.

Resources:

VEXAG presentation on New Frontiers mission goals and concepts: http://www.lpi.usra.edu/decadal/vexag/InnerPlanetsPanel08272009_v3.pdf

Summary of Venara D: http://en.wikipedia.org/wiki/Venera-D

VEXAG Flagship proposal: http://vfm.jpl.nasa.gov/

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?

Thoughts on Scary Messages

Proposed spending in FY10 Budget Proposal

The last blog entry, Scary Messages, talked about the gap between NASA's planetary budget and its ambitions. The Decadal Survey has the job of deciding which missions and programs get funded. The current budget forecasts funds the Mars program at ~60% of historic levels and doesn't have the funds to fly the Jupiter Europa Orbiter (JEO).

In this blog entry, I want to explore some ideas to give a feeling for the types of solutions that might be available. I'm not trying to sell anyone on any particular plan -- my opinion counts for less than squat in this process. This is an exercise to explore the option space for educational purposes.

We'll start with the current projected budget. In the following chart, I average the projected spending on mission programs as presented in the FY10 budget proposal and then multiplied the figures by ten to get an estimated decadal spending amount (all figures are in millions):

At the rate of three Discovery missions per decade, each Discovery mission costs ~$817M. The two expected New Frontiers missions cost around $1.3B apiece. This figures are higher than the normally quoted $450M and $650 million per mission for these two programs. Those lower figures are the amounts that the Principle Investigators receive. On top of that, NASA has to buy a launch and pay for various kinds of overhead.

In all my options, I make three assumptions. First, the Discovery and New Frontiers programs will continue with target of three and two launches per decade, respectively. These are extremely popular programs with the scientific community, and I don't expect them to go away. Second, I assume that the Lunar Quest program does go away as a line item. I expect that all lunar programs at NASA will get de-emphasized now that it appears that NASA astronauts will not be returning to the moon anytime soon. In doing this, I make no statement about the worth of the missions in this line item. They may eventually be funded as Discovery or New Frontiers missions. The third assumption is that the budget available for the entire decade can be projected from the FY10 proposed funding plan.

The planned Mars funding buys what the current head of NASA's science program calls 60% of a Mars program (compared to funding levels of the previous decade). That comes at a time when the minimum cost of meaningful Mars missions seems to be moving to the $1B to $1.5B range, each. NASA plans to make up for this shortfall by pooling funds with ESA to create an international Mars program.

Option 1: Full Jupiter Europa Orbiter Funding

In this option, the Outer Planets line item, Lunar Quest line item, and part of the Mars line item go to fund JEO. (I'm ignoring the fact that Cassini operations are funded out of the Outer Planets line item. I presume it will still be funded, but lack the detailed budget numbers to subtract it. These are back of the envelope estimates!) The Mars program takes a 16% cut from its currently planned levels. there's no changes to the Discovery or New Frontiers programs. (Percentages in all cases refer to the percent of funding based on a run out of the current budget plan.)


Option 2: Enhance Discovery and New Frontiers Missions

I am sure that there are many interesting planetary missions that can be flown for $450M (the current PI funding for Discovery) or $650M (ditto for New Frontiers). However, many of the interesting planetary questions seem to require more money than then program can afford. This is especially true for the the New Frontiers program. A great many interesting missions seem to come in around the $1B mark: Venus landers, comet sample returns, Ganymede orbiter, high resolution mapper for Venus, to name but a few. In this option, Discovery missions (three per decade) are raised to $650M for PIs and New Frontiers missions (two per decade) become small flagship missions for $1B for PIs. (Although at $1B, these missions may be out of the class of what a PI can lead and may revert to a laboratory such as JPL or APL managing the missions.) Mars funding is held to the currently planned levels.

In this scenario, there would be funds for a small flagship mission to an outer planet destination, perhaps a Jovian icy moons observer or a Titan/Enceladus observer and relay for in situ Titan probes.

Option 3: Focus on Flagships

This option retains the enhanced funding for New Frontiers and the outer planets. The New Frontiers line funds two small Flagship missions ($1B equivalent PI funding to each) and funds the JEO mission (or two small flagship outer planet missions). These increases are paid for by reducing Mars funding to the equivalent of two small Flagship missions (with ESA as a partner, probably the equivalent of three small Flagships can be flown). The Discovery program is held to currently planned funding levels ($450M to PI per mission.)


Option 4: De-emphasize Mars

This option increases Discovery mission funding to $650M PI funding per mission, New Frontiers to $1B PI funding per mission, and funds the equivalent of JEO or to two small flagship missions to the outer planets. This option reduces Mars funding, with ESA contributions, to the equivalent of two small flagship missions.

Editorial Thoughts: Picking a favorite among these options is a personal choice of which solar system targets you favor. I'm personally torn between options two and four. I believe that we need to up the spending caps for Discovery and New Frontiers missions to address high priority science questions that just can't be answered within their current budgets. Whether or not you prefer option two or four depends on whether you find Mars or the outer planets more interesting. I switch preferences about every day.

I hope that you will submit your preferred alternatives either to the comments or e-mail them to me at vkane56[at]hotmail.com

Scary Messages

I finally found the time to listen to Ed Weiler's talk in early July on the state of NASA's planetary budget (although the messages are similar for other parts of NASA's science budget except Earth monitoring). The key takeaways:

• NASA's planetary budget has shrunk to approximately half of what it was at the end of the last Decadal Survey (~2003) that set mission priorities
• NASA can now fund about 60% of its Mars program -- hence the joint program in definition with the European Space Agency
• NASA cannot afford an outer planets flagship mission without cannibalizing some or all of the Mars program, the New Frontiers program, and/or the Discovery program
• The loss of the low cost, small Delta 2 launcher will raise the cost of small missions by tens of millions of dollars and potentially a $100M

As a result of these pressures (and if anything, I see the planetary budgt shrinking in the future to fund the manned spaceflight program or to hold down deficits), the program that comes out of the current Decadal Survey may be quite different than the current program.

Resources: Ed Weiler's presentation and Q&A before the Decadal Survey committee

Titan Mare Explorer Abstract

Since the links to the abstract for this poster keep breaking, I'm posting the abstract here.

Title	Titan Mare Explorer (time): A Discovery Mission To A Titan Sea
Author Block	Ellen R. Stofan1, J. Lunine2, R. Lorenz3, O. Aharonson4, E. Bierhaus5, B. Clark6, R. Kirk7, B. Kantsiper3, B. Morse3 1Proxemy Research, 2University of Arizona, 3APL, 4California Institute of Technology, 5Lockheed Martin, 6Space Science Institute, 7USGS.
Abstract	The discovery of lakes and seas in Titan’s high latitudes confirmed the expectation that liquid hydrocarbons exist on the surface of the haze-shrouded moon. The lakes and seas fill through drainage of subsurface runoff and/or intersection with the subsurface alkanofer, providing the first evidence for an active condensable-liquid hydrological cycle on another planetary body. The unique nature of Titan’s methane cycle, along with the prebiotic chemistry and implications for habitability, make the lakes and seas of the highest scientific priority for in situ investigation. The Titan Mare Explorer mission is an ASRG (Advanced Stirling Radioisotope Generator)-powered mission to a sea on Titan. The mission would be the first exploration of a planetary sea beyond Earth, would demonstrate the ASRG both in deep space and a non-terrestrial atmosphere environment, and pioneer low-cost outer planet missions. 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. The focused scientific goals, combined with the new ASRG technology and the unique mission design, allows for a new class of mission at much lower cost than previous outer planet exploration has required.

Titan Mare Explorer

The link to the abstract in my previous post appears to have broken. I've republished the entry with links that work correctly on my computer. If the active links don't work, you can try pasting the following in:

http://www.abstractsonline.com/plan/ViewAbstract.aspx?mID=2337&sKey=f5a63250-0cc1-4d95-9940-1a071b9728c8&cKey=e963b6e7-aa12-43f8-88c4-3a4504e092f4

Please let me know if the links break again either in comments or in e-mail to vkane56[at]hotmail.com.

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