After nine years of plotting the oldest light in the universe, the Wilkinson Microwave Anisotropy Probe has shut down. The satellite, which single-handedly helped establish the standard model of cosmology, took its last look at the cosmos Aug. 20, and settled into a final parking orbit around the sun Sept. 8.

WMAP launched June 30, 2001, with the goal of sensing subtle temperature differences in the cosmic microwave background, the glow of the first atoms to release their radiation 380,000 years after the Big Bang. Since then, it has provided the most accurate measurement of the age of the universe, proved the existence of dark energy, showed that just 4 percent of the universe is made of ordinary matter and supported the idea that the universe inflated from sub-atomic scale to the size of a soccer ball in its first trillionth of a second.

"It's gone way beyond what I imagined, things I didn't even think about at the time," said cosmologist Charles Bennett of Johns Hopkins University, WMAP's principal investigator.

Before WMAP, much of the universe's history was a blank book. Astronomers had some idea that the universe started with a Big Bang sometime between 8 billion and 20 billion years ago, and rapidly expanded after that. But they had very little notion of exactly when, or exactly how.

"WMAP added an extraordinary amount in terms of nailing things down," Bennett said. Astronomers now know that the universe is 13.75 billion years old, give or take 0.11 billion years, a measurement that was recognized in the Guinness Book of World Records as the "most accurate measure of the age of the universe."

The components of the universe were largely mysterious, too. A study of supernovas suggested in 1998 that the expansion of the universe was accelerating. Some cosmologists blamed a mysterious substance dubbed dark energy for shoving the universe outward, but many were skeptical.

"When the WMAP results first came out in 2003, it really made believers of everybody," Bennett said. By comparing computer models of what hypothetical universes with different compositions should look like to WMAP's view of the actual universe, the team of cosmologists proved that 73 percent of the universe is made of dark energy, 22.4 percent is dark matter and just 4.6 percent is the regular, visible matter that makes up stars, planets and people.

"It was almost miraculous," Bennett said. "All of a sudden, in one fell swoop, we suddenly had all these numbers: the density of atoms, the density of dark energy, the age of the universe, when the first stars formed, the distance light has traveled to get to us.... It was just really stunning to suddenly have all this fall into place."

WMAP takes a direct image of the remnant glow of the early universe. This light has been losing energy and stretching out over the last 13.75 billion years, so by the time it reached the WMAP probe in its orbit between the Earth and the sun, the light was detectable as meters-long microwaves.

The tiny hot spots in this cosmic microwave background are primordial lumps of matter that ultimately grew into the stars and galaxies visible today.

The fact that this glow was visible to WMAP at all is remarkable, Bennett said. It could have easily been too dim, or the universe could have been too dusty. But everything worked out just right, he said.

"I just think it's amazing that it's humanly possible to say things like how old is the universe," he said. "We know it because of the light, and the patterns in the light. Nature has provided us this fossil, and we're really lucky that we can detect it and measure it."

When the satellite first launched as MAP (the W was added later in honor of Princeton University cosmologist David Wilkinson, who died in September 2002), it was designed to last only 2 years.

"We always planned to turn it off, and the question was what was the right time," said cosmologist and WMAP team member David Spergel of Princeton University. "We weren't sure it would last this long."

By the time the team decided to call it quits, the probe's batteries were failing, Spergel said. "I'm not sure it would have made it much longer anyway."

The probe spent its working life in spot of neutral gravity called a Lagrange point, where the pull of the Earth and the sun more or less cancel out. It was the first spacecraft ever to orbit at the point called L2, which is becoming a popular destination for future telescopes like the James Webb Space Telescope.

But the gravity balance is only quasi-stable. Any telescope perched there needs to adjust its position with booster rockets every so often just to sit still. To avoid the possibility that WMAP will hit Earth, scientists sent it into a parking orbit around the sun.

"It's basically as if it was another planet just going around the sun," Bennett said.

Spergel sees the end with "a combination of sadness and satisfaction.... It's definitely for me personally the end of a stage in my career. For all of us, it was something that was a major part of our lives for 15 years."

But Bennett sees it differently. "I don't feel like that at all. I think it's a huge victory," he said. "We didn't want it to fly forever, we wanted to get the answers. And we did that. I'm very happy about it."

He also points out that while the observation stage is over, the work is not. There's still the last two years of data to sift through.

"This is not the end of the mission, only the end of the satellite data-taking," he said. "We're still going to take a couple of years to finish analyzing all the data we took, apply new analysis techniques and get better data than ever."

*Images: 1) WMAP's view of the universe from the first seven years of data. NASA/WMAP Science Team. 2) *NASA/WMAP Science Team.

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