Saturday, September 26, 2009

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."


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