Dawn Experienced Loss of Thrust on June 27

Mission managers at the NASA Jet Propulsion Laboratory (JPL) are now struggling to discover the source of a glitch that lead to the Dawn spacecraft experiencing a temporary loss of thrust on June 27.

The orbiter is currently heading for a July 16 rendezvous with Vesta, the largest asteroid/protoplanet in our entire solar system. Due to the fact that it sports an ion propulsion system, the probe will not carry out orbital insertion maneuvers.

Rather, the smooth thrust that these engines provide ensures a smooth insertion, without the need for last-minute maneuvers. However, in order for this to happen, Dawn needs to maintain a steady trajectory, and its ion engines need to remain reliable all the way.

The loss of thrust they experienced on June 27 made JPL mission controllers uneasy, since the error appears to have originated in the propulsion system's brain. The digital control and interface unit is housing the computer and circuits that control this system, and the glitch originated within.

A single electronic circuit from this subsystem is believed to be responsible for the abnormal event. Early analyses reveal that this component appears to be constantly losing an electronic signal, which means that the valves controlling the flow of xenon fuel did not open properly.

As this happened, the spacecraft's computers took a series of actions that were programmed before the probe left Earth. Dawn turned its high-gain antenna towards our planet, in order to enable a “safe-communications” mode.

By June 30, JPL mission engineers were able to address the glitch, and send a new computer routine to the probe. A second digital control and interface unit with equivalent capabilities was turned on, while the troublesome equipment were shut down to prevent energy loss.

“Dawn is still on track to get into orbit around Vesta, and thanks to the flexibility provided by our use of ion propulsion, the time of orbit capture actually will move earlier by a little less than a day,” explains JPL mission manager and chief engineer Marc Rayman.

“More importantly, the rest of Dawn's schedule is unaffected, and science collection is expected to begin as scheduled in early August,” he goes on to say. At this point, the propulsion system is operating within nominal parameters.

At Vesta, the probe will begin the second leg of its mission. It will remain here for about a year, before beginning the trip to the dwarf planet Ceres.

Both Vesta and Ceres are located in the Inner Asteroid Belt, which can be found between the orbits of Mars and Jupiter. The 330-mile (530-kilometer) wide asteroid captured the imagination of astronomers because it may very well be a protoplanet.

Studies have thus far revealed that Vesta may have a core, mantle and crust, just like the rocky planets of the inner solar system do. Its development may have been halted by some mysterious mechanism just a few million years after the Sun formed.

“Bodies like Vesta are building blocks. So we're going back and doing some sort of investigation into our roots, the roots of the solar system,” explained University of California in Los Angeles (UCLA) expert Christopher Russell, the principal investigator of the mission.

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