Spacelab 3, now orbiting on board Challenger, is helping scientists overcome a historic limitation -- the impossibility of experimenting with a planet's atmosphere in an earthbound laboratory. Now the atmosphere of a planet or star can be modeled with unprecedented realism in the gravity-free Spacelab environment.
This is the object of the University of Colorado's Geophysical Fluid Flow Cell experiment. Its drab technical name masks the fact that it represents a significant breakthrough in experimental technique.
If GFFC works, it will be the first time that planetary scientists have been able to simulate the swirling winds and turbulent convection of the spherical shell of atmosphere by manipulating a spherical shell of fluid in the laboratory. In this case, it's a hemispherical shell of silicon oil.
This cannot be done on Earth. Our planet's gravity swamps any attempt to model physically an atmosphere in miniature. There is no way to isolate a scale model so that it can have its own miniature gravitational field, unaffected by outside forces. The best experimenters have been able to do is to study fluid flows in flat rotating ``dishpan'' experiments. They can also set up mathematical models on computers. But these cannot cope with the full complexity of real atmospheric circulations.
There is virtually no external gravity to interfere with atmospheric modeling in the orbiting Spacelab, however. As the University of Colorado's John Hart explains, this makes it possible to study a miniaturized spherical or hemispherical atmosphere realistically.
The gravity of a planet or star can be simulated by an electric field. The silicon oil Dr. Hart uses responds to this force just as a real atmosphere responds to gravity. ``We can model such gravitational affects as buoyancy and the rising and sinking motions of convection,'' he says.
For an experiment that simulates the atmospheres of stars and planets, the equipment seems incongruously tiny. The planetary surface or interior is represented by a steel hemispherical shell 2 centimeters in radius. This is surrounded by an outer translucent sapphire shell. The silicon oil ``atmosphere'' fills a 1-centimeter gap between the two shells. Gravity is mimicked by an electric field generated by 10,000 volts AC across the oil-filled gap.
The baseball-sized assembly can rotate at different speeds scaled to those of different planetary or stellar bodies. The inner steel shell can be warmed to simulate heating from within, as in the case of the sun, or from without, as in the case of Earth. And it can be set up to mimic the combination of internal and external heating that powers the Jovian atmosphere.
On this flight, only solar and Jovian circulations will be modeled. To simulate Earth's atmospheric circulation, barriers to the flow, such as continents, would have to be included. Later experiments may have such barriers. But, on this flight, the surface of the inner hemisphere is smooth.
Dr. Hart explains that he and his co-experimenter, Juri Toomre, ``hope to learn what sets of parameters lead to different circulation patterns -- what [combinations of rotation, heating, and `gravity'] give rise to banded atmospheric circulation, such as that of Jupiter, or a north/south differentiated circulation, such as that of the sun. The solar atmosphere circulates faster at the equator than at higher north and south latitudes.
With the help of mission specialists in Spacelab, Hart and Toomre expect to get about 50,000 photographs of various circulation patterns during 80 to 90 hours of on-orbit experiment time. These are to be analyzed by computer. Somewhere among them, there may be new clues as to what creates and maintains Jupiter's red spot and drives the circulation of the sun's outer layer.