Houston — Scientists in charge of experiments aboard Columbia generally are ecstatic. With most of the spacecraft's third mission completed they say the value of the reusable shuttle as a platform for scientific investigations has been amply demonstrated.
Now they look forward to future missions that will include such ambitious projects as manned laboratories (Spacelab) or auxiliary satellites tethered to the shuttle craft. If that tether is made of a material that is an electrical conductor, the shuttle-satellite combination could become an orbiting electrical generator dozens of kilometers long as it cuts across Earth's magnetic field. This possibility now is under study as a way to power shuttle equipment.
Astronauts Jack Lousma and Gordon Fullerton were well into their seventh and last day in orbit at this writing. In spite of some loss of backup capability in Columbia's ''downlink'' communications to ground stations, flight controllers say they think the test flight is going well. They still are aiming for a landing at White Sands, N.M., at 2:34 p.m. Eastern Standard Time Monday.
Flight controllers showed their high spirits by waking up the crew Sunday morning with the song ''Those Magnificent Men in Their Flying Machines.'' The astronauts responded with a rendition of ''We're Sitting on Top of the World.''
Meanwhile, on Saturday the astronauts had turned Columbia to face its open equipment bay toward the sun. This allowed sun-monitoring instruments to gather data, something they had been unable to do earlier when the equipment bay was facing toward the Earth or outer space.
If the solar physicists gather the data they want -- including hoped for X-ray observations of solar flares, should they occur -- the scientific phase of this mission will be a complete success. At this writing, one such flare had already been well recorded. Already, the other experimenters report they are very pleased with what their projects have accomplished.
These experts are working with the main group of scientific experiments, the so-called OSS-1 Pathfinder Mission managed by the Goddard Spaceflight Center of the Office of Space Science and Applications of the National Aeronautics and Space Administration.
Experiments include a study of plant growth under weightless conditions. This cannot be evaluated until the plants are returned to Earth. However, the astronauts report they are growing well. The experiment -- a forerunner of botanical studies to be made with Spacelab -- is carried in a locker in the forward part of the shuttle.
The other eight Pathfinder experiments are in Columbia's equipment bay. They are mounted on an instrument pallet supplied by the European Space Agency, which will be a standard shuttle fixture. It connects instruments with the power and control wiring of the spacecraft.
Several of the instruments are measuring the shuttle's environment. They probe electromagnetic fields, monitor dust, and trap particles coming in from interplanetary space. Their data will help future experimenters anticipate the working environment with which their equipment must cope.
Already, to judge from the experience of this mission, it should be possible to make sensitive astronomical observations without undue interference from the shuttle itself. Geophysicists have wondered if the shuttle's own electromagnetic effects would interfere with studies of Earth's magnetic field. Again, it seems that such interference is minimal.
This is one of the findings from the Plasma Diagnostic Package (PDP) of the University of Iowa. Grasped by the Canadian-built mechanical arm, the PDP was taken out of the equipment bay and waved around both sides of the shuttle and over its back. There it measured electric and magnetic forces and sampled the electrically charged particles of Earth's outer atmosphere -- the ionosphere, whose gas of charged particles is called a plasma.
An electron gun in the equipment bay shot out beams of electrons. These perturbed the plasma, as beams of particles from the sun perturb it naturally, causing magnetic storms and aurorae. Astronaut Fullerton maneuvered the arm so the PDP could measure the effects. This was in addition to predetermined patterns made under computer control.
The experiment is a small beginning of what are expected to become wide-ranging experiments with the outer atmosphere's magnetic environment and radiation belts. Future work could involve observations made simultaneously by shuttle-carried equipment and satellites in other orbits. Considering what has been learned about Columbia's capability for doing science, Goddard Mission Scientist Werener Neupert asks, ''How else could you carry out experiments with the ionosphere, cosmic dust, solar X-rays, and biological material all on one spacecraft at the same time?''