British craft finishes mission after missed rendezvous

Posted: June 4, 2001

Tsinghua-1 and SNAP-1 spacecraft in a launch site clean room. Photo: SSTL
The United Kingdom's first nanosatellite failed in its daring rendezvous attempt with a Chinese craft earlier this year, but scientists say the mission's other tests and demonstrations of nanotechnology were successfully completed.

The nanosatellite -- called SNAP-1 -- was launched on June 28, 2000, aboard a Cosmos-3M rocket along with the Chinese microsatellite Tsinghua-1 and a Russian spacecraft. Both SNAP-1 and Tsinghua-1 were built by Surrey Satellite Technology, Limited, based at the University of Surrey in the UK.

SNAP-1's mission was to culminate in March with the first orbital rendezvous ever carried out by a nanosatellite. This final mission goal failed because of the timing of the mission in respect to the solar cycle and because the spacecraft was placed into an orbit that was around two kilometers lower than the orbit of its rendezvous counterpart, Tsinghua-1.

Once at this lowered altitude, SNAP-1 continued dropping up to 10 meters per day due to the increased solar activity at the time. After leveling the descent of SNAP-1, the ground team then commanded the craft to began to fire its butane-fueled cold-gas thruster to raise the orbit of the satellite to a point one kilometer above that of Tsinghua-1, setting up the rendezvous attempt.

"We were not expecting to have to carry out this three km 'climb' and so we have had to use virtually all our reserve propellant in carrying out this maneuver," the project team explained. This unexpected extra use of propellant would spell the end of the rendezvous attempt.

Differential drag profiles caused by fluctuating solar activity created the need for even more thruster firings to complete the rendezvous, but with less than four-and-a-half grams remaining of the original 32 grams of propellant, the craft ran out of fuel.

The closest approach between SNAP-1 and Tsinghua-1 occurred on March 18 as the two craft passed at a distance of around 2,000 kilometers.

Engineers look over the SNAP-1 spacecraft in a clean room prior to launch. Photo: SSTL
"Thus, whilst rendezvous was not achieved, the agility and maneuverability of SNAP-1 under automatic control was amply demonstrated," SSTL officials wrote in a paper highlighting the accomplishments of SNAP-1.

Advances made by SNAP-1 in nanosatellite technology include it being the first ever nanosatellite to ever take advantage of three-axis stabilization instead of the more orthodox attitude control method -- at least for smaller spacecraft -- of spin-stabilization. Full use of three-axis stabilization employs the use of small attitude control thrusters or momentum wheels for all orientation-changing maneuvers.

SNAP-1 was also the first nanosatellite to utilize a propulsion system to perform orbit-changing engine firings. The miniature craft was also the first satellite weighing under 10 kilograms to take images of another spacecraft in orbit and to successfully use the American GPS system for orbital navigation.

SNAP-1 continues to operate in Earth orbit, demonstrating new attitude control methods using its momentum wheel, studying the effects of atmospheric drag on nanosatellites, and using its camera to image Earth.