Loss of nitrogen gas or hydrazine propellant is blamed for the premature end of NASA's much-delayed Demonstration of Autonomous Rendezvous Technology (DART) mission. The cause of the higher-than-expected fuel consumption is under investigation.

The Orbital Sciences-built DART satellite was placed into a 545km (340 miles) orbit by the company's Pegasus XL air-launched booster on 15 April. But 12h into the 24h mission the autonomous control system retired the satellite to a safe orbit after detecting low fuel levels.

DART had successfully completed the location and rendezvous phases of its mission, closing to within 92m (300ft) of the Mublcom target satellite. But the plan to make several approaches to within 5m and fly around the satellite was aborted due to fuel depletion. DART fired its thrusters to lower its orbit for eventual re-entry.

DART experienced numerous navigation errors beyond what was expected, but the mission was a partial success despite the loss of 40 planned manoeuvres, says project manager Jim Snoddy.

DART's autopilot flew accurately to the final approach. During the GPS-guided precision approach, with the spacecraft's video guidance sensor locked on to Mublcom, controllers noticed excessive propellant consumption.

The control system commanded a safe retreat from Mublcom after its 16 nitrogen cold-gas thrusters ran out of fuel. Snoddy will not speculate on the root cause of the failure, and whether nitrogen gas depletion caused excessive propellant use or vice versa. There was enough hydrazine left to perform the final planned manoeuvre to a safe orbit, allowing relatively early decay and re-entry.

Source: Flight International