[meteorite-list] Dawn Spacecraft Begins Approach to Dwarf Planet Ceres

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=4425

Dawn Spacecraft Begins Approach to Dwarf Planet Ceres
Jet Propulsion Laboratory
December 29, 2014

* Dawn has entered its approach phase toward Ceres
* The spacecraft will arrive at Ceres on March 6, 2015

NASA's Dawn spacecraft has entered an approach phase in which it will 
continue to close in on Ceres, a Texas-sized dwarf planet never before 
visited by a spacecraft. Dawn launched in 2007 and is scheduled to enter 
Ceres orbit in March 2015.

Dawn recently emerged from solar conjunction, in which the spacecraft 
is on the opposite side of the sun, limiting communication with antennas 
on Earth. Now that Dawn can reliably communicate with Earth again, mission 
controllers have programmed the maneuvers necessary for the next stage 
of the rendezvous, which they label the Ceres approach phase. Dawn is 
currently 400,000 miles (640,000 kilometers) from Ceres, approaching it 
at around 450 miles per hour (725 kilometers per hour).

The spacecraft's arrival at Ceres will mark the first time that a spacecraft 
has ever orbited two solar system targets. Dawn previously explored the 
protoplanet Vesta for 14 months, from 2011 to 2012, capturing detailed 
images and data about that body.

"Ceres is almost a complete mystery to us," said Christopher Russell, 
principal investigator for the Dawn mission, based at the University of 
California, Los Angeles. "Ceres, unlike Vesta, has no meteorites linked 
to it to help reveal its secrets. All we can predict with confidence is 
that we will be surprised."

The two planetary bodies are thought to be different in a few important 
ways. Ceres may have formed later than Vesta, and with a cooler interior. 
Current evidence suggests that Vesta only retained a small amount of water 
because it formed earlier, when radioactive material was more abundant, 
which would have produced more heat. Ceres, in contrast, has a thick ice 
mantle and may even have an ocean beneath its icy crust.

Ceres, with an average diameter of 590 miles (950 kilometers), is also 
the largest body in the asteroid belt, the strip of solar system real 
estate between Mars and Jupiter. By comparison, Vesta has an average diameter 
of 326 miles (525 kilometers), and is the second most massive body in 
the belt.

The spacecraft uses ion propulsion to traverse space far more efficiently 
than if it used chemical propulsion. In an ion propulsion engine, an electrical 
charge is applied to xenon gas, and charged metal grids accelerate the 
xenon particles out of the thruster. These particles push back on the 
thruster as they exit, creating a reaction force that propels the spacecraft. 
Dawn has now completed five years of accumulated thrust time, far more 
than any other spacecraft.

"Orbiting both Vesta and Ceres would be truly impossible with conventional 
propulsion. Thanks to ion propulsion, we're about to make history as the 
first spaceship ever to orbit two unexplored alien worlds," said Marc 
Rayman, Dawn's chief engineer and mission director, based at NASA's Jet 
Propulsion Laboratory in Pasadena, California.

The next couple of months promise continually improving views of Ceres, 
prior to Dawn's arrival. By the end of January, the spacecraft's images 
and other data will be the best ever taken of the dwarf planet.

The Dawn mission to Vesta and Ceres is managed by JPL, a division of the 
California Institute of Technology in Pasadena, for NASA's Science Mission 
Directorate, Washington. UCLA is responsible for overall Dawn mission 
science.

More information about Dawn:

http://dawn.jpl.nasa.gov

Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
Elizabeth.Landau at jpl.nasa.gov

2014-443