Scientists can make some reasonable guesses about what the universe is like. But its current state depends on what it was in its beginnings. So scientists want to improve their fuzzy understanding of those early times.

They believe that the planned launch next April of a satellite, which will analyze what some call the footprints of the early universe, will dramatically clarify that picture. The NASA craft, called the Microwave Anisotropy Probe or MAP, will record the cosmic microwave background radiation -- a steady, low-key energy glow that permeates the cosmos. This is the heat left over from the Big Bang, a hearty explosion that scientists think gave birth to the universe.

In the early universe, this background radiation, which takes the form of photons -- particles of light -- wandered randomly in a thick fog of hot material.

"This will serve in many ways as a test of our standard model of the evolution of the universe."

That changed about 300,000 years after the Big Bang. The universe was a baby compared to its present age, estimated at between 10 billion and 20 billion years old. At that time, the photons separated from matter.

Matter, along with energy, makes up all the stuff we know around us. (Matter and energy are actually aspects of the same thing.)

Anyway, this separation freed the photons to go relatively straight. They have been doing so ever since, leading physicists to think they reflect the universe at that early time.

This "tells you the global starting positions for all structure formation," said Lyman Page, another physics professor at Princeton and one of the project's originators along with Wilkinson.

The MAP mission won't be the first to scan these rays. Another NASA satellite, the Cosmic Background Explorer or COBE, collected this type of data in 1992, for example.

But MAP -- a joint project of NASA's Goddard Space Flight Center and Princeton University -- would be the most detailed and comprehensive look at the background radiation by far.

During its two-year mission, it would map the temperature of the background radiation at points across the sky to an accuracy of a millionth of one degree, with resolution some 30 times higher than that of the COBE craft.

Launched from Kennedy Space Center, the satellite would orbit Earth about five times further away than the moon, for at least two years.

The resulting data should help clarify several key questions, including how galaxies formed.

The background radiation varies very slightly in temperature depending on where in the sky it comes from.

The hotter radiation, physicists think, comes from denser areas of the early universe. These had more gravity, which sucked in matter, causing it to accelerate and heat up. Scientists believe the material eventually condensed into galaxies. The hotter radiation reflects the size and density of the regions that gave birth to galaxies.

"It wouldn't be seen in the same place as the galaxies themselves," explained Page. "The galaxies only formed much later." Nevertheless, the patterns of the radiation would help elucidate how the structures formed.

Knowing the size of the dense regions that gave birth to galaxies would, in turn, tell us whether the universe is "open," "closed" or "flat."

Open means it would last forever, expanding, as it has done since the Big Bang. Closed means it would eventually re-collapse under its own weight, billions of years from now. Flat means it is balanced between those two extremes, expanding just fast enough to fend off collapse.

In a feat of cosmic surveying, MAP scientists plan to draw a theoretical triangle between us on Earth and two points at opposite edges of one of these hot regions in the young universe. They then plan to measure the angles in that triangle.

If the angles' combined width adds up to more than a certain amount, Wilkinson explained, that indicates the universe is closed and will shrink into oblivion.

That's because calculations show a closed universe is spherical, and a triangle on the face of a sphere has wider angles than one on a flat sheet. Conversely, narrower angles suggest an ever-expanding "open" universe.

It's important to realize that the triangles and shapes under discussion are not in ordinary space but in four-dimensional space-time.

Four dimensions are impossible to picture, but Albert Einstein proposed, and physicists believe, that four reflect the universe's true structure much better than do three. Space and time can stretch and change depending on the observer's viewpoint. Only space-time, a mathematical construct including both, is consistent everywhere.

One of MAP's most important goals is to help explain the puzzle of what scientists call dark matter.

By studying the motion of galaxies, scientists have inferred that the universe contains some 10 times more stuff per given amount of space than we can detect. Scientists wonder what this undetected "dark matter" is.

MAP could elucidate some of its properties because its mass, and its interactions with itself and with ordinary matter, would all influence the background radiation.

The probe would also allow astronomers to obtain a much better estimate of the age of the universe, based on a number of other variables such as the expansion rate.

That estimate can be used to cross-check other theories.

If the estimated age turns out to be younger than that of the oldest stars, there is something wrong with either the Big Bang theory, or physicists' understanding of star formation. Either way, such an observation would overturn some scientists' most accepted beliefs about the universe.