Sweden's Prisma satellite mission is nearly halfway through a 10-month mission testing European technologies that could enable future formation-flying and robotic servicing projects.

The mission reached a new first last week, when the satellites spent several days at close distances, eventually accomplishing an approach to a range of about 7 meters, or 23 feet, on Wednesday.

Project officials posted a video of the operation on the Prisma mission's blog.

The Mango satellite captured these views of the smaller Tango target spacecraft last week. Credit: Swedish Space Corp.
 
It was the most ambitious test of the Prisma mission so far.

Managed by Swedish Space Corp., the two Prisma satellites blasted off together in June and separated Aug. 11 to begin orbital trials.

The satellites, nicknamed Mango and Tango, are testing cutting edge rendezvous and autonomy technologies from France, Germany, Denmark, Sweden and other European nations.

Tango, the smaller of the two satellites, is about the size of a microwave. With a mass of 88 pounds, Tango has fixed solar panels and acts as the target during Prisma's demonstrations.

The active spacecraft is Mango, a 331-pound satellite with the dimensions of a typical kitchen stove. Mango is outfitted with conventional hydrazine and exotic green propellant maneuvering systems.

Last Wednesday's demo leaned on a proximity Global Positioning System experiment from DLR, the German space agency.

"We are very busy in the control room operating the two satellites at very close distances most of the time," mission officials posted Thursday on the Prisma website. "All the time the SSC developed safety modules are making sure that the constellation is collision free and safe."

Since Mango and Tango separated in August, the larger spacecraft successfully tested a new non-toxic green propellant thruster with greater efficiency than conventional hydrazine, a caustic substance that introduces challenges during pre-launch processing.

Based on ammonium dinitramide, the new propellant technology was developed by a subsidiary of Swedish Space Corp. focusing on ecological propulsion systems. The company is also designing more potent green propellant engines for larger satellites.

Controllers also started up and tested a radio frequency navigation system developed by CNES, the French space agency.

Upcoming operations include an approach of Mango within 3 feet of Tango using a Swedish vision-based sensor.

Managers will gradually loosen the reigns on the satellites, giving the spacecraft more authority to make autonomous decisions and test the range limits of tracking and navigation instruments.

"The priorities are to demonstate autonomous formation-flying, meaning that we regard these two satellites as one entity," said Staffan Persson, Prisma project manager at Swedish Space Corp. "So they are supposed to keep a fixed position relative to each other without ground control in the loop."