Mission managers of NASA's Dawn asteroid probe had a long Saturday, waiting for news from the asteroid belt. Eventually they got the news they were hoping for: Dawn had entered Vesta orbit. This is the first time in history that an object in the asteroid belt has been orbited by an artificial satellite.
At 10 p.m. PT on Friday (1 a.m. ET, Saturday), the ion thruster-propelled spacecraft was due to arrive in orbit around Vesta, one of the largest asteroids living in the asteroid belt between Mars and Jupiter.
However, the probe had been firing its thrusters for several days straight. Until the thrusters had switched off, the probe wasn't able to orientate its antenna to communicate with Earth.
At 11:30 p.m. PT on Saturday (2:30 a.m. ET, Sunday), Dawn ceased thrusting, orientated its antenna and transmitted the signal. Although the exact time of orbital capture is not known at this time, it is confirmed that Dawn has been captured by Vesta's gravitational field and that it is now orbiting the large, 330 mile- (530 kilometers) wide asteroid.
"Today, we celebrate an incredible exploration milestone as a spacecraft enters orbit around an object in the main asteroid belt for the first time," NASA Administrator Charles Bolden said in a release. "Dawn's study of the asteroid Vesta marks a major scientific accomplishment and also points the way to the future destinations where people will travel in the coming years. President Obama has directed NASA to send astronauts to an asteroid by 2025, and Dawn is gathering crucial data that will inform that mission."
Using ion thrusters to propel it through interplanetary space, the spacecraft has been sneaking up on Vesta, rather than speeding up and rapidly slowing down. The latter method used by rocket-propeled spacecraft can have catastrophic consequences if the target should be missed. With Dawn's ion drive, if the target is missed, there would have been enough fuel to take another shot at entering orbit.
Interestingly, as the exact mass (and therefore gravity) of Vesta is not known, the exact time of orbital capture cannot be calculated. If the asteroid is more massive, Dawn would have been captured sooner by a stronger gravitational field; if Vesta is less massive, the spacecraft would be captured later by a weaker gravitational field. Until more data is relayed from Dawn, we won't know the precise time of capture.
Vesta is often considered to be a planetary embryo that had its growth stifled early in the history of the solar system. Vesta is one of only three known remnant "protoplanets," the others being Pallas, a main belt asteroid, and Ceres, the only main belt dwarf planet.
Dawn will remain in orbit around Vesta for a year, before gently boosting away to begin the trip to Ceres, the second half of its asteroid belt adventure.