Asteroids are not exciting objects when compared to planets, which have a riot of color, varied surfaces, swirling storms, and exciting moons. The few spacecraft that have encountered asteroids have found them to be battered and cratered worlds, but slowly a new understanding is emerging.
When the Solar System was forming about 4.6 billion years ago, a vast cloud of gas and dust collapsed to form the Sun and planets. Jupiter grew rapidly and eventually was so massive that its gravity prevented anything very large from forming between it and the orbit of Mars. The debris that was prevented from forming a planet became known as the asteroid belt. Two asteroids formed at nearly the same distance from the Sun under the same conditions, but these two, Ceres and Vesta, could hardly be more different. Both are the target of a new spacecraft mission that will hopefully unlock the mysteries of both and perhaps revolutionize our thinking on asteroids.
Ceres is the largest asteroid with a diameter of approximately 605 miles and is nearly circular. It orbits around 250 million miles from the Sun and takes about 4.6 years to orbit. Guiseppe Piazzi discovered this first asteroid on January 1, 1801. Ceres contains about one-quarter of the asteroid belt’s total mass. It can shine as bright as magnitude +7.0 but can only be resolved into a disk by the Hubble Space Telescope. Unfortunately, Ceres is so small that only the largest surface features can be seen. Among the most interesting is a bright white spot that may be an icy impact crater. There is also a dark circular area of unknown origin. It rotates quite swiftly in about 9 hours. Ceres may have a rocky core overlain with an icy mantle. The mantle might be as thick as 70 miles and may contain more fresh water than Earth. There may actually be a thin atmosphere, a surface covered with frost and it may be as warm as 0º F.
Vesta was discovered by Heinrich Olbers on March 29, 1807 and is the second largest asteroid approximately 360 miles diameter along its widest axis. It orbits about 220 million miles from the Sun and takes about 3.6 years to orbit. It rotates even quicker than Ceres in about 5.3 hours. It is an oblate spheroid, squashed at the poles in shape, but is deformed further at the south pole by a huge impact crater about 285 miles across. This is the most obvious feature seen by the Hubble Space Telescope and is up to seven miles deep with a rim that rises around seven miles above the surrounding terrain. A central peak towers up to 11 miles from the crater floor. It is estimated that about 1% of Vesta was excavated by the impact and may be responsible for the meteorites found on Earth that are identified as being from Vesta.
Vesta is unique among asteroids because it has completely separated (differentiated) into a metallic core with a molten mantle and rocky crust. The rising molten material from within Vesta appears to have breached the crust and covered portions of the surface with a magma ocean that quickly froze into dark lava plains or mare. No other asteroid has this characteristic and the meteorites that match Vesta’s composition are clearly of volcanic origin.
Vesta may look like our Moon with dark mare, bright impact craters, and a huge impact basin that never filled with magma. As exciting as Vesta may appear, it is nothing more than a star in the sky shining no brighter than magnitude +5.3 at best. This makes Vesta the brightest asteroid and through a telescope as small as a 3” refractor it reveals a warm yellowish hue.
New rocket propulsion technology is finally available that will make exploring the asteroid belt easier by not only allowing a spacecraft to fly from one asteroid to another, but also to orbit around each one for months to make a detailed analysis. Dawn is just such a spacecraft and will explore both Ceres and Vesta to determine why they evolved so differently. Dawn is equipped with an ion engine that shoots out charged ions of xenon gas that accelerate it up to 68,000 miles per hour. It will allow Dawn to maneuver more easily and therefore is capable of going into orbit around the asteroids and then breaking free of the orbit.
Dawn bristles with instruments, which include a camera, two spectrometers, an altimeter, and a magnetometer. It will be able to thoroughly map both asteroids along with determining the minerals on their surfaces and will be able to detect any atmosphere and magnetic field. It will also be able to measure the elevation and depth of the varying landforms. Dawn may be able to determine the history and evolution of Ceres and Vesta and why they are so different. It will be able to orbit both asteroids hardly more than one hundred miles above their surface, which will provide details as small as a few feet across. Ceres could reveal fractures and craters where fresh water ice has either been excavated or oozed out from beneath the surface. Vesta’s huge impact crater may be deep enough to look deep into the heart of the asteroid to potentially reveal where the magma may have come from or how it was created in the first place for a world so small that it seems there couldn’t possibly be enough internal heat to generate volcanic activity.
Dawn is scheduled to launch on June 20, 2007 and will fly past Mars in March 2009 for a gravity assist into the asteroid belt. It will arrive at Vesta first during October 2011 and will go into orbit for seven months thoroughly exploring and mapping it. It will leave Vesta during April 2012 and arrive at Ceres during February 2015 and will likewise thoroughly explore it. This will be a first for a spacecraft by orbiting two different worlds. It will orbit Ceres until July 2015 and if the spacecraft is in good health, it may be possible to explore one or more additional asteroids.
The cost of the Dawn mission has soared to $446 million and this nearly killed the mission, but it actually ended up costing more to scrap it than what was needed to resolve the technical issues with the ion propulsion engine. This fact, along with the promise of discovery and the mystery of the asteroids, has restored funding and the mission is set to launch. At long last there is a mission that will be able to peer back into the dawn of the Solar System. Ceres and Vesta will provide plenty of surprises on how the Solar System was created.