After drifting apart due to an unknown glitch, the two US satellites that are part of the experimental Orbital Express mission rejoined as they flew nearly 500 kilometres above the Earth on Saturday.
During a test on 11 May, the mission鈥檚 two satellites, ASTRO and NextSat, undocked and separated to a distance of 10 metres as planned. Then, ASTRO experienced a serious problem with its sensor flight computer. In response, its emergency system kicked in and ASTRO flew 120 metres away to avoid an inadvertent collision with NextSat (see Paired satellites drift dangerously apart).
But ASTRO (Autonomous Space Transfer and Robotic Orbiter) then coasted up to 6 kilometres away from NextSat. Because of the failure, ASTRO was unable to keep track of NextSat鈥檚 location on its own, so ground observers had to tell the two spacecraft where they were in relation to each other.
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One of the goals of this mission, led by the US Defense Advance Research Projects Agency (DARPA), which manages the mission, is to demonstrate that spacecraft can work somewhat independently. This could eventually be used for a satellite that could fix and refuel ailing satellites in Earth鈥檚 orbit.
In this case, mission managers had to come to the aid of ASTRO. Ground controllers fired ASTRO鈥檚 thrusters to bring it within 3 kilometres of NextSat. From that distance, ASTRO鈥檚 infrared camera and laser rangefinder were able to spot NextSat. On Friday, Boeing, the company that built ASTRO, sent its satellite new guidance information.
鈥淎fter that, it was all ASTRO鈥檚 show,鈥� says Lt. Col. Fred Kennedy, DARPA鈥檚 Orbital Express programme manager. 鈥淲e watched in the mission control centre as ASTRO performed a series of small manoeuvres, bringing it first to within 1 kilometre of NextSat, then 320 metres, 120 metres and finally into the approach corridor.鈥�
ASTRO rejoined NextSat, which serves as an experimental target satellite and depot station, as the pair flew over an area where it was not possible to send real-time images to the ground. So mission controllers had to rely on bits of data to assure them that the docking occurred as planned.
DARPA is analysing what caused ASTRO鈥檚 initial computer problem.
