The long-awaited Gaia mission will map the size, shape, and motions of our galaxy to extraordinary precision.
|Update: Gaia launched December 19th without a hitch, blasting off on a Soyuz rocket from Europe’s Spaceport in Kourou, French Guiana at 9:12 UTC (4:12 a.m. Eastern Standard Time). The spacecraft reached its operational orbit in early January. ESA's website has the details on the launch.|
Accurate star positions are the foundation of nearly everything we know about stars and much else in astronomy. A star’s parallax — its tiny apparent looping motion once a year caused by our moving viewpoint on Earth — tells its distance more directly than any other method. Accurate parallaxes of nearby stars form, in turn, the most important base for the entire cosmic distance scale out to the farthest galaxies. And for any object in the universe, you need to know its distance accurately in order to tell its true size, brightness, energy output, and much else about it.The measurement of stars’ positions, motions, and parallaxes is called astrometry. On Thursday morning, the science of astrometry began its first great leap forward since the Hipparcos mission of the 1990s. The European Space Agency (ESA) successfully launched its long-awaited Gaia observatory from Europe's spaceport at Kourou, French Guiana. (Watch here, or check out Gaia’s Twitter feed.)
Gaia is a specialized instrument with a seemingly simple goal: to pinpoint the locations of an enormous number stars better than ever. The plan is to map 1 billion stars down to 20th magnitude, nearly 1% of all the stars in our Milky Way Galaxy. Gaia will measure stars from 5th to 15th magnitude to an accuracy of 25 microarcseconds, or even better on the brighter end of this range. That's like measuring the apparent width of a human hair in Chicago seen from the distance of New York. It's 40 times more precise than Hipparcos could do for stars only down to magnitude 8 or 9. Gaia will measure its faintest stars, from magnitude 15 to 20, to 300 microarcseconds or better.
To do this Gaia will observe each star about 70 times. Additionally, it will measure the stars’ radial velocities, creating a full 3-D map of their motions in space as well as their 3-D locations. It will also catalog their brightnesses from the near ultraviolet to the near infrared (see detailed specs).
The most immediate gain from the mission will be much better parallax distances for many more stars much farther away than those Hipparcos measured two decades ago. The individual accuracy will range from 20% for stars near the center of the galaxy some 30,000 light-years distant, to a remarkable one part in 100,000 for dim stars very nearby. All this will enable Gaia to thoroughly map the size, shape, and kinematics (motions) of our galaxy.
A Long Time Coming
Gaia will have a crisp view from the vacuum of space as it orbits around the Earth’s L2 Lagrangian point, located 1.5 million kilometers from Earth in the direction away from the Sun. This gravitational balance point will keep the craft stable with respect to Earth throughout the planned five-year mission, and its loopings around the L2 point will keep it from ever falling into Earth’s shadow.The spacecraft’s super-precision instrument consists of two identical telescopes pointing 106.5° apart. As the craft rotates once every six hours, each will survey a narrow band of the celestial sphere. A gradual tilt of the spacecraft’s axis will slowly result in full-sky coverage. The telescopes have folded focal lengths of 35 meters (115 feet) and focus onto imaging chips totaling more than 900 megapixels. Gaia is expected to produce more imaging data in its five-year lifetime than the Hubble telescope did in its first 21 years.
Ultimately, Gaia should produce top-quality 3-D coordinates for 10 million of the billion stars observed, some 80 times more stars than the 118,000 for which Hipparcos measured parallaxes. As scientists crunch the incoming data, the Gaia project should issue its first catalogs in mid-2016 and the final one around 2023.
Gaia’s mission planners also hope to gather indirect data on hundreds of planets circling other stars and possibly identify up to 50,000 new brown dwarfs and 20,000 novae.
And, this is actually the downgraded version of the mission! Originally ESA scientists wanted also to search the skies for near-Earth asteroids and measure their orbits. Due to budget reasons, that and other elements of the mission were cut.
Gaia’s long journey to the launch pad took two directors and more than 13 years. Its saga began even before Hipparcos’s ended in 1997, but Gaia was supposed to be the second generation of astrometry after Hipparcos, not the first. NASA designed the Full-Sky Astrometric Mapping Explorer (FAME) to launch in 2004. It would have measured about 40 million stars to as good as 50 microarcseconds. But NASA killed FAME in 2002 for breaking its budget.
Around the same time, the German space agency was planning a mission called DIVA with similar capabilities. Because of the duplication, the Germans canceled DIVA — before FAME was taken off the docket. As a result, the expected next generation of space astrometry never happened.
So a lot is riding on this one.