Spaceflight Now: Breaking News

British and Chinese satellites head for space rendezvous

Posted: January 15, 2001

Engineers look over the SNAP-1 spacecraft in a clean room prior to launch. Photo: SSTL
A tiny British-built spacecraft is achieving a variety of firsts in the nanosatellite technology field. SNAP-1 will finish off this series of ground-breaking accomplishments in the next few months as it approaches a rendezvous with another satellite.

Both that satellite -- the Chinese-owned Tsinghua-1 -- and SNAP-1 were built by Surrey Satellite Technology, Limited, based at the University of Surrey in the United Kingdom.

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 for all attitude control maneuvers.

SNAP-1 is 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 and to successfully use the American GPS system for orbital navigation.

These feats will culminate in February or March when SNAP-1 approaches Tsinghua-1 for the first orbital rendezvous carried out by a nanosatellite. However, the tag-up will only be a passing glance. Project officials say that once the two cross orbital paths, they will once again begin to separate from one another.

Tsinghua-1 and SNAP-1 spacecraft in a launch site clean room. Photo: SSTL
The events leading up to the upcoming encounter go all the way back to the two craft's launch in June 2000 aboard a Cosmos-3M rocket. After riding into space inside the cramped quarters of the Cosmos nose cone along with a Russian military satellite, the two Surrey-built craft were released in different directions "specifically to avoid the possibility of accidental re-contact," the SNAP-1 project team told Spaceflight Now.

After separating from the spent Cosmos-3M upper stage, SNAP-1 was left in an orbit around two kilometers below that of Tsinghua-1. SSTL officials say that, due to atmospheric drag, SNAP-1 continued dropping approximately 10 meters per day when compared to Tsinghua-1.

To overcome this loss in altitude, ground controllers had to fire up SNAP-1's butane-gas thrusters in order to sustain altitude. After stabilizing the effects of atmospheric drag, engineers then had to order the nanosatellite to climb three kilometers to a point one kilometer above the orbit of Tsinghua-1.

"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. It is currently unclear if this will have any influence on the future mission of SNAP-1.

A close-up image of SNAP-1. Photo: SSTL
Under the laws of orbital mechanics, any object in an orbit lower than its target travels faster and will begin to pull away from its objective. Thus, SNAP-1 eventually ended up over 10,000 kilometers ahead of Tsinghua-1 at one time in December.

When SNAP-1's orbital altitude exceeded that of Tsinghua-1, the process mentioned above began to reverse. With SNAP-1 now in a higher orbit, it is travelling slower than its counterpart in the lower orbit, which allows Tsinghua-1 to play catch-up until SNAP-1 once again decays to an orbit lower than Tsinghua-1's. When this occurs some time in March, the closest approach between the pair will take place.

"How close will they be in terms of along-track distance when this happens is highly dependent on fluctuations in the atmospheric density over the next month or so," a project official told Spaceflight Now.

Currently, project officials say that SNAP-1's orbit is around 300 meters higher than Tsinghua-1's, with the difference still closing.

"Once we have observed how close SNAP-1 approaches Tsinghua-1, all primary R&D mission objectives will have been met. Thereafter, SNAP-1 will continue to operate in orbit carrying our communications experiments," SSTL's SNAP-1 team concluded.