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SNAP-1 (2000)

First nanosatellite to demonstrate in-orbit inspection of another spacecraft and orbit autonomous attempted rendezvous, SNAP-1 (2000)

The SNAP-1 and Tsinghua-1 satellites were both released from the Cosmos launcher in different directions specifically to avoid the possibility of accidental re-contact. SNAP-1 ended up in an orbit about 2 km below that of Tsinghua-1 and, being relatively light (6.5kg), suffered more from the effects of atmospheric drag than the much heavier (50 kg) Tsinghua-1 microsatellite. This meant that, relative to Tsinghua-1, SNAP-1 dropped in altitude more quickly. This was exacerbated by a very active sun (at solar maximum), causing the atmospheric density at 700 km to be higher than normal. SSTL measurements showed that on average, SNAP-1 was falling about 10 m per day with respect to Tsinghua-1.

The Micro Propulsion System on board SNAP-1 was first used to maintain altitude by overcoming the relative atmospheric drag effects, and then also to climb back up to an altitude about 1 km higher than that of Tsinghua-1. Most of the propellant was used for this climb in December 2000. In this period Tsinghua-1 separated from SNAP-1 by more than 10,000 km along their orbital tracks. A long sequence of  thruster firings was initiated under the automatic control of the On Board Computer, and the GPS navigation system was used to keep track of the orbital changes. Over a period of 30 days, the thruster was fired about four times per day, giving a change in velocity (delta v) of about 10 cm/s per day. Once SNAP-1 was higher than Tsinghua-1, the along-track gap began to close. In January 2001, SNAP-1 was about 300 m higher than Tsinghua-1 with the gap closing. In total, the propulsion system raised the altitude of SNAP-1 by about 4 km (corresponding to a total delta v of 2.1 m/s), all done with just 32.6 grams of butane propellant.

At maximum separation, Tsinghua-1 and SNAP-1 were about 15,000 km apart. But by means of the propulsion manoeuvres, SNAP-1 brought itself to within 2000 km of its target. Thus, while a true rendezvous was not achieved, the agility and manoeuvrability of SNAP-1 under automatic control was amply demonstrated, meeting its objective.
 
SNAP-1 propulsion system
 
CAD image of SNAP-1 and Tsinghua-1

SNAP-1