YES2, a 6-kilogram capsule (pictured), was hung from a satellite by a tether before falling to Earth. Credit: ESA

As Nature went to press, scientists were scrambling to locate a small capsule from space now believed to be somewhere in Kazakhstan. The capsule is part of an ambitious experimental space-mail delivery system that aimed to use a 30-kilometre-long satellite tether — the longest yet. In the early hours of 25 September, the ribbon from the satellite orbiting 300 kilometres above Earth was cut and the capsule parachuted to Earth, although off-target.

The test forms part of the European Space Agency's Foton-M3 mission, which set out on its 12-day Earth-orbiting science mission on 14 September. Foton-M3 is carrying more than 40 science experiments, including the tether test, called Young Engineers' Satellite 2 (YES2), a €2.7-million (US$3.8-million) project involving more than 450 students. The idea was to demonstrate the feasibility of a cheap way of returning small objects to Earth from distances similar to that of the International Space Station some 330 kilometres away.

Delivering small payloads from low Earth orbits is not as easy as simply dropping them. “You don't want this object to spin around the Earth,” says Francesco Emma, head of the European Space Agency's education office. The capsule, or any other payload, must slow down so that it can come out of its orbit. This can be done using a retro-rocket, but tethers are another way. “You can lower the capsule,” says Emma. “When it is released it doesn't have enough energy to spin.”

The tether, just 0.6 millimetres thick, was made from a super-strong polyethylene fibre called Dyneema and weighed 5.5 kilograms. YES2 consisted of three parts: the 6-kilogram capsule, together with heat shield, that returned to Earth; a carrier that held the capsule in place; and a deployer, bolted to Foton-M3, that held both capsule and carrier and a spool of tether.

The capsule was deployed in two stages — 3.5 kilometres of tether was released at first. After the landing location was targeted, the idea was to reel out the rest of the tether. Once the pendulum-like tether and capsule had swung to the vertical, the tether would be cut, and the capsule would parachute to Earth in about 30 minutes. But the rate of cable uncoiling was 5 metres per second, not 12 metres per second as planned, so the cable unwound only 8.5 kilometres, Russian reports suggest — although this was still the first successful tether mission to Earth. A transmitter on the satellite should allow mission controllers to locate the capsule's landing site.