The five astronauts aboard the space shuttle Challenger started, speeded up, slowed down and stopped 28 scientific experiments and instruments today as they began working toward the day when the shuttle could be a factory and laboratory in space.

Moving about the spacecraft as rapidly as they could on the third day of the shuttle's seventh flight, the astronauts handled almost three times as many experiments as any previous shuttle crew.

At the same time, Challenger today powered up the first satellite ever designed to be recaptured by the shuttle's mechanical arm.

"The SPAS Shuttle Pallet Satellite is up and activated and looks like the champ we know it is," Astronaut John M. Fabian said as Challenger flew across Africa.

The 14-foot-long cylinder will be picked up by the shuttle's robot arm on Wednesday, lifted high out of the cargo bay and released into space by astronauts Fabian and Sally K. Ride. Fabian and Ride will be operating it from computer consoles.

Once the satellite is perched safely above the shuttle and out of range of its tail, astronauts Robert L. Crippen and Frederick H. Hauck will fly the shuttle to a spot about 1,000 feet away. The two spacecraft will stay separated, in identical orbits, for eight hours.

Crippen and Hauck will fly the shuttle to a rendezvous with the satellite, which Ride and Fabian will retrieve with the mechanical arm. After the satellite is captured, it will be stowed in the cargo bay, fastened down and brought back to earth when the shuttle lands at the Kennedy Space Center on Friday morning.

Built by the West German aerospace giant of Messerschmitt-Bolkow-Blohm, the 3,200 pound satellite carries eight instruments designed by German scientists and financed by the West German government. It also carries three cameras built for NASA that will be used to take pictures of the shuttle in space.

Messerschmitt paid NASA less than $8 million for its satellite's ride. NASA gave the company a discount so it could use the satellite to exercise the shuttle's robot arm.

For a time today, the astronauts struggled to turn on some of the eight instruments on the German-built satellite. Most balky was a mass spectrometer, designed to measure how much the exhaust gases of the shuttle's engines pollute the cargo bay environment as they are being fired in space to maintain or move the shuttle's position.

The mass spectrometer was finally cranked up late this afternoon, about six hours after the other instruments were turned on. No one seemed to know why it took so long but by the end of the day no one seemed to care.

"We had a very successful afternoon," Flight Director John Cox said late today from the Johnson Space Center in Houston. "We did everything that was called for in the flight plan, although some of the things we did might have been done out of order."

In addition to the eight experiments on the West German satellite, Challenger is carrying more than 20 other experiments in the cargo bay and in its mid-deck. Just below the shuttle cockpit a device is conducting an experiment to separate out the protein components in human blood.

Among the other experiments and instruments are a new type of solar cell, which is being tested to see how well it holds up in the extreme heat and cold of outer space, a telescope looking at distant planets and stars, and an electronically controlled camera that is observing the Earth in the infrared, picking up changes in heat that can't be seen with the naked eye.

Possibly the most interesting experiment of all is the cheapest one. It was devised and built by students of predominantly black Camden and Wilson High Schools in Camden, N.J., and involves a colony of 150 carpenter ants, a small computer, a photoelectric light and a pair of cameras that will be used to photograph the ant colony's reaction to weightlessness in space.

The two schools' experiment grew out of an idea discussed at a dinner by an RCA Corp. vice president and the principal of Camden High School, Riletta Cream. The $10,000 fee for the experiment was paid for by RCA.

Once known only for its basketball and football teams, Camden High School organized a chess team a few years ago that has won the South Jersey conference championship four years in a row and came in second last year in the state.

This year, the same students who have been playing on the chess team launched an experiment in space. They chose carpenter ants because these ants have a social structure that can be observed. The students wanted to see if the ants would stay organized under the stresses of lift-off, weightlessness and return to Earth.

The ants have several other qualities that make them good space subjects. They have hard external skeletons to take the jostling and G-forces of liftoff, they live in confined spaces requiring little support, they have feet that can cling to smooth surfaces without gravity and they tend to keep to themselves and not wander off like regular ants.

Several of the students who devised and built the experiments came to the Kennedy Space Center to see their experiment launched. Said one, Carlton Jackson, "This whole thing . . . shows we're good at more than just playing games. We do have brains to put to work."