WHILE astronomers generally look forward to the launch of the Hubble Space Telescope next year, those who study the childhood of the universe have an important launch of their own this fall. It involves a satellite that will give them their first comprehensive view of cosmic ``fossils'' left over from the birth of our universe. These ``fossils'' are the photons of the low-energy microwave radiation that appears to permeate the cosmos. They started out as photons of white light from the universe's ``Big Bang'' explosive birth. In the 10 to 20 billion years since that origin, the radiation's wavelength has lengthened a thousand-fold. Today it is only a faint hiss in sensitive radiotelescopes.
But that hiss - that cosmic background radiation, to use its formal name - holds clues to what happened to our universe at times far earlier than even the space telescope will be able to explore. Thus the Cosmic Background Explorer (COBE) satellite NASA plans to orbit this fall is, in its way, as important an astronomical advance as the more highly publicized Hubble facility.
Free of the atmospheric distortions that limit the vision of ground-based telescopes, the Hubble instrument will be able to see farther into space than astronomers have ever seen before. The farther away an astronomical object is, the longer it has taken the radiation, by which we see it, to reach us. To put it another way, we see distant objects at earlier stages in their development than objects closer to us. The Hubble telescope should see some galaxies and other objects in their earliest stages of evolution. The COBE satellite, by contrast, won't look deep into space. It will search for subtle variations in background radiation that could reflect conditions before galaxies began to form.
The radiation is highly uniform in all directions. This reflects conditions in the early universe when things were so hot that particles of matter and photons (particles of radiation) freely exchanged energy. By the time the universe was 300,000 to 500,000 years old, however, it had cooled to 3,000 Kelvin (3,000 K. or 4,999 degrees F.) and matter and radiation no longer interacted freely. The radiation now has cooled to a mere 2.8 K. (-455 degrees F).
While the radiation's high degree of uniformity is due to very early conditions, any tiny variations should reflect what happened to the radiation after the first few hundred thousand years. Variations, for example, may hint at the early clumping of matter before galaxies formed - a clumpiness that could subtly distort the radiation.
Earth's atmosphere has frustrated scientists seeking such information even more than it has annoyed optical astronomers. It doesn't just distort observations of the background radiation, it absorbs many of its most interesting frequencies. Instruments sent above the atmosphere for a few minutes on rockets have helped these scientists glimpse what they are missing. These fleeting observations have suggested there are some telltale variations in the cosmic background.
Like the Hubble telescope, COBE was to have been launched by the space shuttle. But unlike Hubble, which is still to ride the shuttle, the satellite was adapted for launch by a Delta rocket after the Challenger accident. Traveling 900 kilometers (560 miles) above Earth in a circular, near-polar orbit, it should be able to make a thorough survey of the background radiation over the entire sky twice during its expected one-year observing life.
The data this satellite returns will be esoteric. But the information scientists can extract from it should interest anyone who puzzles over the ancient questions - where have we come from, how has our universe evolved?