51动漫

As if engaged in a slow-motion dance, two mated satellites used a robot arm to draw apart, then come together again several hours later on Monday. It is the first step towards the mission鈥檚 ultimate goal of separating completely and docking with each other autonomously from a distance of 7 kilometres away.

The $300-million Orbital Express mission 鈥� run by the US Defense Advanced Research Projects Agency (DARPA) 鈥� uses two satellites. One, called ASTRO (Autonomous Space Transport Robotic Operations), is designed to dock with and test repairs on another, called NextSat.

The 90-day mission is meant to test autonomous operations in a spacecraft so that one day, a fix-it satellite might be deployed to fuel, inspect or even repair a wounded satellite without help from human controllers.

The pair have been attached to one another end-to-end since they launched on 8 March.

On Monday, ASTRO鈥檚 robotic arm grabbed hold of NextSat. It lifted the satellite away from a 鈥榣aunch ring鈥� that had kept the pair from being damaged from vibrations during lift-off, then ejected the ring in order to expose the claw-like docking mechanisms for later tests.

Watch the 鈥榰nmating鈥� operation as , or as it would have looked to a hypothetical observer .

Gas stations in space

Ejecting the launch ring was also important because it covered up a key set of cameras, says Colonel Fred Kennedy, mission programme manager at DARPA. 鈥淚f the ring hadn鈥檛 come off, we would have been blind,鈥� Kennedy told 51动漫.

Watch a . Kennedy says he expects the launch ring to stay in orbit for several years before it re-enters Earth鈥檚 atmosphere.

This was not Orbital Express鈥檚 first test. ASTRO has already used its robotic arm to give a spare battery to NextSat, and the satellite duo has swapped hydrazine fuel, another in-space first.

If future satellites are built to be refuelled in space, they 鈥渃ould carry less fuel to start with鈥�, says Max Meerman, an engineer in Vancouver, Canada, at the aerospace firm MDA, which built ASTRO鈥檚 robotic arm. 鈥淭his way more payload can be launched.鈥�

Helper satellites

Soon the pair will separate completely, without being connected by the robotic arm. Their first free-flying test on 5 May will have the pair separate by 10 metres and then dock. Eventually, mission managers hope to have ASTRO do the approach and docking from 7 kilometres away.

The success of the recent test is especially gratifying for mission managers. That is because shortly after launch, ASTRO experienced pointing problems because some of its hardware was installed upside down (see 鈥楳echanic鈥� satellite suffers guidance system glitch).

鈥淭he first few days were touch and go,鈥� Kennedy told 51动漫. 鈥淲e were frankly pretty frightened. We didn鈥檛 know whether we鈥檇 be able to continue.鈥�

The spacecraft was saved, however, by having NextSat there to help. NextSat took over control and pointed the pair鈥檚 solar arrays towards the Sun to keep the batteries charged.

鈥淚n any other instance, that would have been a fatal problem for a spacecraft,鈥� Kennedy says. 鈥淚t seems to me that anybody who goes up with a new spacecraft ought to strongly consider the idea of an on-orbit host.鈥� He says several little 鈥榟elper鈥� satellites could patrol the skies and come to the aid of satellites that experience trouble right after lift-off.

Will Knight contributed reporting to this article.