 |
 |
||
 |
|
||
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
||||||||
 |
Planetary News: Asteroids and Comets (2007)Dawn Takes Off for the Asteroid BeltBy Emily LakdawallaSeptember 27, 2007
Dawn lifted off today at 11:34 UTC aboard a Delta II rocket to begin an eight-year mission to the largest worlds of the asteroid belt, Vesta and Ceres. Never yet visited by spacecraft, Vesta and Ceres are part of a rock-and-dust-rich region of the solar system that was prevented from coalescing into a planet by the gravitational influence of Jupiter. Thus these two worlds, the largest in the asteroid belt, represent fossilized remnants of the formation of the solar system. Shortly after launch, Dawn unfolded its enormous solar panels,
each eight meters long, to begin to provide power to its ion engines. With
the power system operating properly, it radioed its health to Earth at 13:44
UTC. Ion
engine propulsion was proved on NASA's Deep Space 1 mission to asteroid Braille
and comet Borrelly and later used successfully by ESA's SMART-1 mission
to the Moon and JAXA's Hayabusa mission
to asteroid Itokawa. Dawn is NASA's first science mission to depend wholly
on ion propulsion. The propulsion system consists of three 30-centimeter-diameter ion thrusters, which use an electrical charge to ionize xenon atoms to speeds ten times faster than the speeds achieved by chemical thrusters. The tradeoff is that the ion engines expel much smaller volumes of propellant than chemical thrusters can throw, which in turn means that Dawn's ion engines provide very low amounts of thrust. Dawn carries a total of 425 kilograms (937 pounds) of xenon propellant, which it will use over more than 2,100 total days of thruster operation time. A mission powered by ion engines has a very different profile compared to a traditional chemically powered spacecraft. Chemically powered spacecraft coast for long periods, with rare, short bursts of thruster activity (usually measured in seconds or minutes) to change course. Ion engines operate almost continuously, achieving course changes through hundreds of hours of thrusting. These missions are more challenging to navigate, and in fact the Dawn team won't know precisely when they will rendezvous with their various destinations until they test and observe the performance of their spacecraft in flight.
However, with the challenges come benefits. The ion propulsion system
will enable Dawn to be the first planetary mission ever to attempt orbiting
two different bodies. Dawn is now on its way to a gravity-assist flyby
of Mars. Because its eventual goal is to go into orbit around the relatively
lightweight worlds of Vesta and Ceres (each
having only about a ten millionth the mass of Earth), and also because the
low-thrust ion engine system does not provide a lot of braking power, Dawn must
rendezvous with each world at a low relative speed. That, in turn, means
that Dawn's journey will be long. Its Mars flyby will take place in
February 2009, and its arrival at Vesta in August 2011. After nine months
at Vesta, it will depart in mid-2012, to meet Ceres in February 2015. The
exact timeline may vary depending upon the performance of Dawn's ion engines.
Going in to orbit at two different bodies will enable Dawn to perform in-depth studies of two completely different worlds with the same suite of instruments. Ceres is currently classified as a "dwarf planet,"a designation it shares with Pluto and Eris. It is a round, smooth world displaying signs of water ice on its surface and may have similarities to the icy satellites of the outer solar system, including the possible presence of an underground ocean. The water at Ceres may have prevented it from developing internal volcanic activity. Smaller Vesta is noticeably non-spherical, with a surface consisting of rock and metal; the composition of its rocks suggest that it was at least partially molten at one time. The successful launch marks a change in fortune for Dawn, which has met numerous hurdles on its way into space. During development, cost overruns and concerns about technical issues with the xenon propellant tank resulted in a NASA review, then a "stand down" order, and finally the mission's outright cancellation on March 2, 2006. Following an appeal by the Jet Propulsion Laboratory, which developed the mission, to the Office of the Administrator, NASA reversed Dawn's cancellation just weeks later. It was poised for launch in June, but a series of weather-related problems forced it to delay launch until after the Phoenix lander launched to Mars in August. The launch was the 76th consecutive success for the Delta II rocket, the longest record for any launch vehicle. |
|||||||||
 |
 |
|||||||||
[ what is this? ]