Today marks three weeks until the launch of Gaia, a new space telescope with an ambitious goal: to map the locations of one billion stars in our galaxy. Using this information, Gaia will be able to make an extraordinarily precise three-dimensional map of the Milky Way. Gaia will make repeated observations of these stars over time, letting us study the motions of stars in the galaxy. This will help us learn about the early history of the Milky Way. Stars often continue heading in the direction they started off going after their formation, so information about the young lives of the stars in our galaxy is embedded in their motions today.
Gaia is the successor to Hipparcos, a mission launched in 1989 that mapped the locations and motions of 2 million stars in the Milky Way. Gaia will vastly improve upon the information gleaned from Hipparcos data. Gaia will be observing 500 times as many stars as Hipparcos, and over its five year mission, it will observe each of those one billion stars 70 times!
How can it measure the distances to a billion stars in just five years? Gaia, like Hipparcos before it, uses the principle of parallax. Parallax is actually really easy to demonstrate for yourself. Hold your thumb out at arm's length. Look at it through one eye and then the other. Notice how it moves back and forth? That's the effect of parallax, and the distance side-to-side that your thumb appears to move can be used to compute the length of your arm. By observing stars at two different points in its orbit, Gaia does basically the same thing. But since stars are very far away, they appear to only shift by a tiny amount. Thus, very precise measurements are needed. Gaia will make these observations over 1000 times as precisely as Hipparcos.
The Gaia spacecraft will not orbit the Earth like many space telescopes do - it will be placed at a special position between the Earth and the Sun called a Lagrange Point. At this location, Gaia will be able to orbit around the Sun with the Earth. This will enable Gaia to view the entire sky over the course of a year. Gaia will be roughly 1.5 million kilometers from Earth, or about four times farther from the Earth than the Moon.
Stay tuned over the next few weeks for more information about Gaia, and what it will be able to teach us!