Air-launched rocket orbits two satellites.

Air-launched rocket orbits two satellites

April 5 marked the first time engineers have sent satellites into orbit without using a launch pad to start the trip. Instead, the rocket carrying the two small orbiters left Earth on a B-52 jet, which took off from the NASA Dryden Flight Research Center in California's Mojave desert and flew to an altitude of about 42,000 feet over the Pacific Ocean. The jet then released the three-stage rocket, which fell for about 5 seconds before the motor of its winged first stage ("It looks like an X-15," said one official) ignited and separated, carrying the payload into a pole-crossing orbit 508 to 687 kilometers above Earth.

The air-launced rocket, called Pegasus, was developed by Orbital Sciences Corp. (OSC) of Fairfax, Va., and funded by the Defense Advanced Research Projects Agency (DARPA) in Washington, D.C.

Attached to Pegasus was the satellite Pegsat, which in turn carried a small Navy communications-relay satellite that it released the same day for a variety of meteorological and oceanographic purposes. Pegsat itself holds two canisters of barium, which will release their contents later this month or early in June for studies of Earth's magnetic field. As solar ultraviolet radiation ionizes the neutral barium particles, Earth's magnetic field lines will capture the glowing particles, allowing ground-based cameras to photograph the magnetic field's structure, says Robert Pincus of the NASA Goddard Space Flight Center in Greenbelt, Md.

The Pegasus rocket was instrumented to measure the temperatures, pressures, structural loads, vibrations and other phenomena of the system's maiden orbital flight. The next Pegasus mission, due this summer, is to launch seven small, experimental communications satellites, called Microsats, for DARPA.

OSC officials say Pegasus offers several advantages over a conventional, vertically launched system. One is the cost savings of assembling and testing everything with the rocket lying flat on the ground rather than having to build special structures to hold it upright. Starting with a jet also offers freedom from most of the weather problems that threaten a fixed launch pad. Concern about a possible buildup of electrostatic discharges during the ascent through the clouds led planners to postpone the flight for a day despite clear weather over the part of the Pacific where the plane would release the rocket, but OSC officials say such constraints will diminish once they amass more experience with the system. Other advantages include greater ease of launching missions on short notice and the ability to launch the rocket at a wide range of angles.