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spectrometer for monitoring gasses in the cargo bay and around Challenger's jet thrusters; and an experiment for calibrating solar cells.

SPAS also figured in a milestone series of tests during which the pallet was released by the remote manipulator system arm to fly free in space, then retrieved by the arm and re-stowed in the cargo bay. This was the first time the space shuttle had truly interacted with another spacecraft in orbit, and was a key demonstration of its value and its versatility. As astronaut Crippen put it, "We pick up and deliver."

The free-flight tests began with the arm-controlled during these maneuvers by Ride and Fabian-picking up the SPAS from its berth in the cargo bay then releasing the pallet while holding it up and away from the vehicle. It was the fourth time the remote manipulator arm had flown on a shuttle, but the first time it had released a payload. The crew then fired Challenger's small reaction control jets to move the orbiter below and ahead of SPAS as they both circled the Earth. Finally, Challenger maneuvered up to a point 300 meters (1,000 feet) ahead of SPAS, then slowly moved back to the pallet, closed in and grabbed it with the manipulator arm. Later in the same day, Challenger was sent on another series of maneuvers around the satellite, followed by another approach and arm grapple.

The point of these more than nine hours of proximity operations was to establish that the shuttle could effectively use new techniques and devices for monitoring another spacecraft with radar, radio contact and optical sighting, then could approach a payload and safely grapple it. These demonstrations were an unqualified success, and are encouraging for the planned STS-13 arm retrieval rescue of the Solar Maximum Mission satellite now tumbling out of control in orbit.

SPAS Flies Alongside Orbiter

During these proximity operations, the SPAS pallet itself was by no means passive-it proved a steady platform and effective power supply for its instruments, several of which required micro-gravity conditions outside of Challenger's cargo bay. One of SPAS's tasks during free flight, completed as planned, was to monitor gaseous contaminants in and around the shuttle from a distance. Another accomplishment was to take data on the effect of orbiter jet firings on the SPAS spacecraft at close quarters. A third job was to photograph Challenger during the time when the two spacecraft were separated, with cameras provided by NASA.

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