Dawn breaks for asteroid Vesta

NASA’s Dawn spacecraft has been captured into orbit around the asteroid Vesta to begin a year long investigation to unravel some of the mysteries of the early Solar System.

Vesta as seen by Dawn on 9 July 9, from a distance of about 41,000 km (26,000 miles) Credit: NASA

Launched in 2007, Dawn homed in right on target to rendezvous with Vesta on Saturday 16 July. It was expected to begin orbiting Vesta at approximately 06:00 GMT, but the exact time of capture is uncertain as it depended on the asteroid’s mass and strength of its gravitational pull.

Space scientists back on Earth had a nail-biting wait of around a day while the the unmanned probe realigned its radio antenna to beam signals back to them. They are now looking forward to seeing detailed close-up pictures of Vesta.

A recent technical problem was quickly resolved and will not impede the mission. “The problem was in an electrical circuit that controls valves used to deliver xenon propellant from the main tank to the thruster”, Dawn chief engineer and mission manager Marc Raymann told Skymania News as the probe was making its final approach. “As we have another controller onboard, we knew right away that we could switch to it and resume thrusting. That worked perfectly, and Dawn remains on target and on schedule for Vesta.”

Over the last few weeks, Dawn has been in the approach phase of its mission. While the main focus was on use of the ion thruster for propulsion, it also began imaging Vesta in never before seen detail. As Dawn crept closer to Vesta and the asteroid came into view, it no longer had to rely solely on radio signals for navigation. Adjustments could be made to the trajectory based on visual cues. The approach phase will also have allowed the mass and gravity of Vesta to be more accurately determined, as the asteroid tugged on the approaching spacecraft. The orbit of Dawn will be affected slightly depending on the mass of Vesta, which in turn will give information about the interior of the rocky body.

From early August, Dawn will orbit Vesta many times to perform a general survey. Then it will slowly spiral inwards to a lower orbit for closer inspection. Dawn will view the surface features at multiple angles, mapping them in both visible and infra-red light. The resulting topographic maps will enable the height of mountains and the depth of craters to be measured. Cosmic rays, which are high energy particles from outer space, that strike the asteroid will cause particles to be emitted which can be observed by the gamma ray and neutron detector.

Vesta boasts a massive crater on its south pole which is 460 kilometres wide and 13 kilometres deep. The collision that caused this massive crater would have sent rocks hurtling out into the Solar System, and it is believed that some of the meteorites found on Earth originated from this impact. Analysis by Dawn will be able to confirm if this is indeed the case. Dawn will also discern if Vesta has any natural satellites. According to Dawn deputy principal investigator Carol Raymond, the search for moons orbiting Vesta is on-going, but so far none have been found.

In July 2012 Dawn will depart from Vesta and continue its journey towards the dwarf planet Ceres, arriving there in February 2015. While Vesta is a rocky body similar to the inner planets, Ceres contains icy material and is more like the moons which orbit the gas giants of the outer Solar System. Ceres and Vesta are both remnants of a failed planet and comparing the different evolutionary paths of these two bodies will help to discover some of the secrets of planet formation.

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