COBE Maps Milky Way's Direction And Raises a Tantalizing Question

THIS Sunday, the Cosmic Background Explorer (COBE) satellite will be just half-way through its two-year mission to survey the afterglow of this universe's birth. But it is already a landmark in astronomical history. Its findings have resolved decades of uncertainty about the nature of the microwave radiation that bathes the cosmos. As University of California (Berkeley) astrophysicist Joseph Silk has said, they show ``the unambiguous signature of the primordial fireball,'' from which the universe emerged 10 billion to 20 billion years ago.

COBE has measured the motion of our Milky Way galaxy through that background radiation. It moves at 300 meters a second (670 miles an hour) toward a point in the southern part of the constellation Leo.

Taking advantage of Earth's position near the Milky Way's edge, the satellite has looked toward the galactic center to take the clearest pictures ever made of this galaxy. Interstellar dust blocks this view for optical telescopes. COBE's infrared sensors see right through it. They show a classic spiral galaxy seen edge on, looking just as it would to the crew of an incoming intergalactic spacecraft.

COBE also tantalizes astronomers with a continuing puzzle. Its data so far show the background radiation to be very smooth. Astronomers expect to see small irregularities that would reflect disturbances imposed on the radiation when matter gathered into galaxies. None have shown up. But data analysis has barely begun.

Actually, the 2,300-kilogram (5,000-pound) $400 million COBE finished its first mission phase Sept. 21. Circling Earth in its polar orbit 893 kilometers (555 miles) high, COBE is still fully operational in terms of the capabilities planned for the second phase of its mission. This includes refining its background radiation data by taking repeated measurements across the sky.

COBE's biggest expected achievement, however, is behind it - accurately measuring the background radiation spectrum over wavelengths from 0.5 to 10 millimeters. The radiation's intensity at different frequencies is the same as that of a black body, meaning a perfect emitter and absorber of microwave radiation. COBE measured its temperature at about 2.735 degrees C above absolute zero (minus 454.7 degrees F). That's just what scientists expect for fossil radiation left over from the primordial explosion.

Goddard Space Flight Center in Greenbelt, Md., which manages COBE, reports that, when the first COBE scientific paper was reviewed for publication, referees called COBE's survey ``one of the most important cosmological experiments of the century.''