Gaia’s on a mission to create a 3-D map of the Milky Way’s stars

As the Gaia space observatory blasted into space from French Guiana Thursday morning, Mark McCaughrean watched nervously from the European Space Agency (ESA) operations center in Darmstadt, Germany. 

"I mean, any time you stick 740 million euros ($1.02 billion) worth of scientific hardware on top of a controlled bomb, you get nervous," joked McCaughrean, scientific advisor in Science and Robotic Exploration at the ESA.

But not to worry, the launch went well.

The basic aim of the Gaia mission, he says, is to create a giant 3-D star catalog of the Milky Way. The catalog is expected to help scientists understand the formation of the Milky Way, and how it has changed over the years. It should also help researchers see planets around distant stars. "We expect to discover thousands of those," he says.

Gaia's two main telescopes will also look for supernovae, or exploding stars, and asteroids, in hopes of spotting any rogue space rocks that may be headed our way.

But the mission's first task will be to find a parking place in space. Gaia will take about three months to get to its assigned spot behind the Earth, as seen from the Sun. Then, what McCaughrean calls the "astronomical census" will begin — as Gaia's array of scientific instruments repeatedly measure the positions, motions, temperatures, luminosity and composition of a billion stars.

And McCaughrean said that should lead to new discoveries. Some scientists are interested in using Gaia's measurements of distant stars to test the general theory of relativity.

"In order to make this amazing catalog, we actually have to factor in the gravitational influence of all the planets and the Sun in our solar system," he says. Their gravitational pull will affect the light coming from distant stars and thus the measurements of the light taken by Gaia.

"As the light comes past those planets, it slightly disturbs them," notes McCaughrean. "We'll have such accuracy that we'll actually be able to see if Einstein's theory of relativity was correct to very high precision."

McCaughrean says achieving a 3-D view of the night sky just isn't possible from Earth. "When you look out at the night sky, it appears that all stars are at the same distance. You can't tell the distances just by looking at night," he said.

"We can measure the distances with Gaia because as Gaia goes around the Sun with the Earth once a year, we shift perspectives slightly, so we'll actually be able to see the nearby stars. But by watching over five years, we'll be able to watch the stars moving slowly against the sky, and we can also measure their speed coming toward us or going away. So, in a way, we get a six-dimensional map, three positions and three velocities, and in that way we'll be able to rewind the Milky Way and see what it looked like in the past."

He also says Gaia should tell us about how stars are born in our galaxy. "Many stars have been around for billions of years, but stars are being born every day in the Milky Way and by placing them with Gaia's results, we'll understand much better how star formation propagates throughout the Milky Way, like wild fire setting [another] fire — or one star-forming region can set off another one. To be able to really define that, we need to know distances and Gaia will tell us that extremely accurately."

Accuracy is obviously one of Gaia's strengths. McCaughrean says its 3-D map of the Milky Way will generate more than 30,000 CD-ROMs worth of data. That flood of data should keep scientists very busy, well beyond Gaia's five-year mission.