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The Dawn spacecraft and visits to Vesta and Ceres 
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The robot spacecraft Dawn launched in 2007 toward the asteroid belt, on a mission to fly past two of the largest non-planetary bodies in our solar system: it will go by the large asteroid Vesta in 2011, and the dwarf planet Ceres in 2015.

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image by NASA (source)

Dawn slingshot (ie performed a "gravity assist flyby") off Mars about a year ago to get the speed necessary to get out to the asteroid belt--it got within just 549 kilometers of the planet. Dawn is the NASA's first exploratory mission to be powered exclusively by efficient ion drive thrusters; "ion drive" means the three thrusters achieve acceleration by shooting out charged particles--in this case charged xenon gas particles--instead of chemical exhaust. An ion thruster was recently used by a successful European moon probe whose name I now forget. ;) Acceleration by ions is very slow compared with chemical rockets, but very efficient: Dawn only needs 425 kg of Xenon to get all the way out to Ceres, which orbits at 2.7 times the distance of Earth (414 million km).

This is a cool mission because we don't even have good photos of Ceres and Vesta yet. This is the best Hubble has managed of Ceres

Image
image by NASA (source)

and of Vesta

Image
image by NASA (source)

Ceres was the first asteroid discovered, in 1801; Vesta was the fourth, in 1807. Along with the second, Pallas, and the third, Juno, they were considered planets for about 50 years, until astronomers started finding many other asteroids out in the belt, and realized they couldn't call them all planets. Ceres is so big--487 km radius--and nicely round that it's now classified as a dwarf planet, like Pluto (which is still much larger: 1,153 km radius); Ceres contains about a third of the total mass of the entire asteroid belt!

Oh, also, Ceres was lost after discovery, when it moved too close to the Sun to be seen. Fortunately, that sharp German mathematician Carl Friedrick Gauss was on the scene--only 24 years old at the time--and worked out a slick method of determining a body's Keplerian motion from only three observations, which succeeded in predicting Ceres' position once it got clear of the Sun so as to be visible again.

Ceres is thought to be a protoplanet that formed about 4.57 billion years ago, and would have cooled rather quickly without a heavy metal core. Ceres is very icy, and because of its fairly large size, it's possible that it had a liquid water interior "ocean" at one point--or even still!

The closer, smaller (529 km average diameter) Vesta on the other hand is a nice hard rocky metally thing, possibly another protoplanet; radioactive decay of aluminium isotopes could have heated and remelted the protoplanet, giving it a magma ocean for a while; it's the only remaining intact body in the system that's gone through that process.

However, Vesta almost didn't survive: less than a billion years ago, something big smashed into it, leaving a crater covering 80% of its diameter--the crater is 460 km across, 13 km deep, with a 4-12 km high rim around it, and an 18 km central peak. This impact is thought to have knocked away 1% of Vesta's total mass; this was handy for studying Vesta, though, because some of that formed asteroids ("V-Type") which have become meteors and hit the Earth, so we've actually recovered little meteoric pieces of Vesta! (Currently we have physical pieces of only 5 bodies in our solar system: the Earth, the Moon, Mars, Vesta, and the comet Wild 2.) The crater may go all the way down into Vesta's mantle, and should make for some great photos next year when Dawn pays it a visit!

The visit may also help solidify Vesta's candidacy as a dwarf planet: a dozen bodies were submitted for consideration in 2006

Image
image by NASA (source)

including Vesta; the large Kuiper belt objects 2003 EL61 (Haumea) and 2005 FY9 (Makemake) have already been accepted as dwarf planets. To be a dwarf planet you have to be rounded by your own gravity, but not so gravitationally independent that you've cleared out other similar bodies from your own orbit; Vesta is thought to be sufficiently round, except for that massive crater, which would not be held against it if that is indeed found to be the cause of its irregular shape. (The fantastically oblong Haumea isn't a sphere, but still qualifies because it has been rounded by its own gravity; its length is due to its very rapid rotation: once every 3.9 hours!)


Fri Mar 12, 2010 3:55 pm
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Joined: Fri Aug 30, 2013 11:53 am
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BC wrote:
Ceres is thought to be a protoplanet that formed about 4.57 billion years ago, and would have cooled rather quickly without a heavy metal core. Ceres is very icy, and because of its fairly large size, it's possible that it had a liquid water interior "ocean" at one point--or even still!

It may have more fresh water than Earth. Along with low gravity (IIRC space elevators are easily feasible there) it could be a pretty awesome location.


Fri Aug 30, 2013 12:13 pm
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It sounds pretty awesome no matter what! : D


Fri Aug 30, 2013 11:43 pm
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