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Rosetta To Play Orbital Mechanics To Reach Churyumov-Gerasimenko

Simply getting to comet Churyumov-Gerasimenko will require a major redesign of its flight plan -- and part of this is trying to find new replacements for the two asteroids Rosetta was supposed to rapidly fly by for additional science observations during its previously planned circuitous 9-year trip to Wirtanen.
by Bruce Moomaw
Sacramento - Mar 23, 2003
"SpaceDaily" has now acquired additional information on the favored new mission plan for Europe's Rosetta comet-rendezvous spacecraft, whose planned January launch to comet Wirtanen had to be cancelled due to the disastrous failure of the immediately preceding launch of its Ariane 5 booster.

While a delayed launch to Wirtanen next January cannot be completely ruled out, the most probably replacement mission for the craft is a launch next February . Since this comet's nucleus is thought to be considerably bigger than Wirtanen's, this will require considerable replanning of the landing procedure for its small ejectable comet-nucleus lander (as described in SpaceDaily's March 20 article).

But simply getting to the comet also requires major redesign of its flight plan -- and part of this is trying to find new replacements for the two asteroids Rosetta was supposed to rapidly fly by for additional science observations during its circuitous 9-year trip to Wirtanen.

The first of those two asteroids was 4979 Otawara -- only a few kilometers wide, which may actually be a small chunk of the third-biggest asteroid Vesta broken loose by an ancient impact. (Vesta is the only big asteroid with actual flows of volcanic basalt on its surface; America's "Dawn" spacecraft is scheduled to visit it in 2010 and spend almost a year orbiting it for detailed study.)

The second would have been 140 Siwa -- a big "C-type" asteroid, thought to be made of the same darker "carbonaceous chondrite" rock that makes up most rocky bodies in the outer Solar System, which condensed out of the original solar-orbiting nebula out of which the Solar System formed at lower temperatures than the silicate rock bodies of the inner System, and so is much richer in water and even in organic compounds. (Siwa, at 110 km, would have been the biggest asteroid yet visited by a spacecraft.)

Those, however, are now out of reach. Rosetta's new flight plan calls for it to match orbits with comet Churyumov with an even more complex set of loops around the Sun than its original flight plan to Wirtanen did. It will still make a gravity-assist flyby of Mars -- but if the Feb. 2004 launch to Churyumov is chosen, it will also make three gravity-asist flybys of Earth instead of only two.

The new plan would involve Rosetta being initially launched into a near-Earth orbit with a period of exactly one year, allowing it to return to and fly by Earth at that time to get a boost into a more elongated orbit that will take it to Mars. (If it misses this launch window, it can be launched a year later directly from Earth to Mars -- but since Mars will be farther from the Sun in its mildly elliptical orbit than it would have been for a launch last January, such a direct flight to Mars will require a more powerful booster: either a Russian Proton, or the improved "Ariane 5 ECA" which failed so disastrously in December and might not be ready for this mission even by early 2005.)

However it gets to Mars, Rosetta will fly by that planet in Feb. 2007 (making some science observations as it does so) and getting a gravity-assist boost to further elongate its orbit. It will then return to Earth to make its second gravity-assist flyby of our home planet that November, putting itself into a still more elongated orbit with a period exactly two Earth years long -- so that it will return once again to make its third and last gravity-assist flyby of Earth in Nov. 2009, putting itself into a still more elongated orbit taking it almost as far from the Sun as Jupiter.

As it sails away from the Sun on that orbit, it will fire a burn on its main engine in mid-2011, moderately adjusting its path to help match orbits with Churyumov. Then -- three years later, as it starts to approach the Sun again -- it will close in on the comet's nucleus and carry out a months-long string of finer maneuvers to rendezvous with it after a total journey of about 10 1/2 years (two years longer than the originally planned flight).

Rosetta's planners have already carried out an extensive hunt for any asteroids it could fly past during this revised series of loops around the Sun, and have indeed found two. The first is 437 Rhodia, which it would fly past in Sept. 2008 at a speed of about 41,000 km/hour.

Rhodia -- only about 25 km wide -- may be a particularly unusual asteroid. It is thought to have an albedo higher than that of any other known asteroid, reflecting fully 56% of the light hitting it -- which would imply that it is made of some mineral as white as chalk (possibly a chance extrusion of some white rock like anorthosite, which formed on a bigger asteroid and was later broken loose by a collision).

The second asteroid target would be 21 Lutetia, a big asteroid about 100 km wide which Rosetta would fly past at about 55,000 km per hour in July 2010. While Lutetia is about the same size as Siwa, it is definitely odder -- it's one of the biggest of the so-called "M-class" asteroids, which until recently have been thought to be made largely of metallic nickel-iron alloy of the type that makes up many recovered meteorites.

M-class asteroids (tagged, like the other declared classes of asteroids, by the near-infrared spectra of their rocks as seen from Earth) are rather uncommon -- they make up only about 4 percent of asteroids. They have been thought to be pieces of the metal cores that formed at the centers of the dozen or so large "planetesimals", several hundred km wide, that orginally formed in the Asteroid Belt, before most of them were gradually shattered into smaller fragments by repeated collisions over the eons.

However, more detailed near-IR spectra recently show some signs that most of the bigger M-class asteroids -- including Lutetia -- may not be metallic at all. Instead, they may be made of silicate rocks that were exposed to some water during their early history. Many of the smaller M asteroids -- as well as 16 Psyche, the biggest of all -- don't seem to show such evidence, and may be the real thing. If Rosetta does visit Lutetia, its color photos, close-up IR spectra and magnetic field measurements will likely settle this question.

There, is, however, a catch. Matching orbits with Churyumov will require more maneuvering fuel than Wirtanen would have. And so, in order to take the orbital paths needed to intercept the asteroids, Rosetta would have to rendezvous with its main comet target when the comet is closer to the Sun than Wirtanen would have been -- only 540 million km from the Sun, as opposed to the 600 million km planned for Wirtanen.

Since, as a comet approaches the Sun, the "coma" of gas and dust boiling off it dramatically increases, as Churyumov reaches its perihelion it will get much harder for the comet-orbiting spacecraft and its lander to make their observations. (Rosetta's design specifications only guarantee its full operation beyond doubt until the comet approaches within 490 million km of the Sun.) Scientists therefore would very much like to rendezvous with the comet when it's still 600 million km from the Sun as originally planned, to prolong their detailed observation time there. They will thus have to decide which they prefer: those two asteroid flybys, or as much as six extra months of time studying Churyumov itself in detail.

They have plenty of time to make that decision, however -- it can actually delayed until after launch. At any rate, given the initial alarming indications immediately after the cancellation of last January's launch that they might be able to find a workable replacement target for Rosetta at all, ESA scientists are quite happy even to have such a choice.

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