Princeton physicists help design new NASA satellite

A million miles from Earth, the brainchild of a few Princeton physicists is now collecting data on the very nature of the universe: How old is the universe, is it finite, was there a beginning, will it have an end? Never before could these questions be tackled with such numeric precision.

Since it arrived at its destination on Oct. 1, NASA's Microwave Anisotropy Probe has been forming a map of the cosmic microwave background.

Physics professor David Wilkinson explained, "It turns out that depending on the cosmology of the theoretical model for our universe, you get different patterns of these fluctuations. We know what causes them and we know when they were laid down, and now it's a matter of doing a careful measurement."

Wilkinson is part of the MAP team of scientists from Princeton, UCLA, University of Chicago, University of British Columbia, Brown University and NASA's Goddard Space Flight Center in Maryland. Though each institution brought to the team experience in a particular area of the mission, the expertise in microwave anisotropy — "how it varies as you scan across the sky" — was here at Princeton, Wilkinson said.

Astrophysics professor David Spergel leads the team in interpreting the data collected by MAP. In just the last month and a half of up-time MAP has not sent back enough readings for the team to accurately understand the data, but "We've seen enough to know that the instruments are performing very well," Spergel said.

After enough measurements are taken, the data will reveal with unprecedented accuracy the age of the universe, the density of atoms in the universe and the overall shape of the universe.

Spergel said scientists now estimate that the universe is five percent known matter, 25 percent unknown matter and the remaining 70 percent energy and that the universe is 13 billion years old — plus or minus four billion years.

MAP's data will be 10 times more precise, able to pinpoint the age of the universe to within one billion years.

"What would be more exciting . . . is if the universe does not fit our current theories," he added. If MAP showed temperature deviations of as small as a thousandth of a degree, the current model of the universe — which predicts that the radiation is equal in all directions — could be challenged.

MAP's mission formally ends in two years, after it observes the entire sky four times.

MAP is a follow-up to 1992's COBE satellite — the Cosmic Background Explorer — which Wilkinson also helped to develop.

In addition to measuring microwave anisotropy, COBE measured the intensity of radiation at different wavelengths and the infrared background. With an angular resolution of seven degrees — so imprecise that the moon hardly appears as a single dot — COBE was unable to make very precise measurements of the anisotropy. Though MAP focuses specifically on the anisotropy at five different wavelengths, its angular resolution is 50 times greater, Wilkinson said.

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University physicist Norman Jarosik, who is also on the MAP team, worked on developing the special radiometers for the satellite. From the start, Princeton researchers knew how to build the detectors and created prototypes to prove it, he said.

Jarosik explained that every day MAP covers about a third of the sky, in a "big donut pattern," but from day to day it covers nearly the same region of the sky, shifting by about a degree each day. After six months, the center of the donut is filled, completing the measurements of the temperature of the entire sky, he said.

There are a lot of repeated measurements, but this "scan strategy" was developed to eliminate some of the systematic error involved in making such measurements. MAP does not directly measure the temperature at a give point in the sky, but instead the temperature is found indirectly through the difference in temperatures between two points in the sky. After many measurements of the same point, its temperature can be compared with points "sideways and diagonally and every which way," Jarosik said.

To make these fine measurements, MAP must continuously face away from the Sun, Earth and Moon — which are much hotter than empty space. MAP has been placed into a special orbit around "L2," an imaginary point in space where the Sun is always behind the Earth. MAP cannot stay precisely at L2, however, as its power source, the Sun, would then be eclipsed by the Earth, explained physics professor Lyman Page, another member of the MAP team at Princeton.

Wilkinson, who helped to craft the design concepts of the satellite's microwave radiometers, described them as looking like a radar receiver. "There's a horn antenna that collects radiation from the reflectors . . . wave guides that look like water pipes, and that goes into very exotic amplifiers." The amplifiers, which were designed and built at the National Radio Astronomy Observatory, were the key to putting it all together.

"We had to have that advanced technology for the thing to work as well as it does," Wilkinson said.

Putting all of the special equipment together was the job of Princeton researchers. "Princeton had the expertise in the instruments itself, but we did not have expertise in the space craft systems," Jarosik said. Researchers at Goddard and the other universities worked out the details of the space flight, the integration of devices and the thorough testing.

It all began, Wilkinson said, as far back as 1991. "We wrote a dummy proposal to NASA at one point," he said.

The new equipment simply called out to be used in such a project. "It was just clear that the satellite was the right thing to do. I think everyone that used these detectors recognized their potential," Page said.

"I really felt like I was being asked to join the A team," Spergel said, recalling when he was first asked by Page if he was interested in the project. Spergel said he knew immediately that he wanted to join the team, but waited a day to give his answer so that his excitement could subside.

And it all came together on the afternoon of June 30, 2001, when the MAP satellite was launched into Earth orbit from Cape Canaveral in a Boeing Delta Rocket. Page and Jarosik watched the launch from Goddard, from where the satellite was being controlled. "The launch team was excellent," Page said.

Page said he was nervous during the launch. The scientists held their breath when the satellite was initially exposed to the intense heat of the Sun. "We're staring at the sun for a few seconds and that was a really trepidatious time." Later, as the solar panels unfolded, the team once again anxiously watched. "It's usually things that break that don't work great," he said.

But, the satellite endured the launch without any incidents.

Wilkinson, too, was pleasantly surprised by the uneventful launch. "We put a lot of work into this thing, five years of intensive effort. We had quite a bit invested in the thing working, and it was a very complicated launch . . . and everything worked like clockwork. I was quite surprised," he said.

Several loops around the Earth, a boost from the Moon and $140 million later, MAP arrived at L2, where it will stay for at least the next two years — though the satellite has enough fuel to last for 50 years.

Though MAP's equipment is now five years old, Jarosik said that "MAP is really still a state-of-the art mission," and it is certain to send back innovative data.

Thanks to the work of the entire MAP team, the answers to science's toughest questions are heading toward us at the speed of light.