海角大神

For long-time residents, we Earthlings still know little about our galactic neighborhood. But a new satellite from the European Space Agency (ESA) could change that, as it surveys the skies for answers to some of the most compelling and still unanswered questions about our local cosmos.

In a culmination of some two decades of planning, the ESA聽launched its Gaia satellite聽from the European spaceport in聽French Guiana on Thursday morning (9:12 GMT).聽The 鈧�740 million ($1 billion) mission, now en route some 930 million miles toward a solar orbit, is tasked with prospecting our star-brimmed galaxy for the data points that will furnish the largest ever three-dimensional map of the Milky Way. It鈥檚 also a map that scientists will be able to run as a time-lapse film, whirling it backwards to the galaxy鈥檚 origins and then spinning it forward again to trace how, and why, the Milky Way came to be.

鈥淭here has never been anything on this scale before Gaia,鈥� says Gerry Gilmore, an astronomer at the University of Cambridge and the UK鈥檚 principle investigator on the Gaia mission. 鈥淚t鈥檚 the only thing like it.鈥�

From myth to reality

In 1989, the ESA launched its Hipparcos satellite 鈥� so named for the ancient Greek astronomer, the compilier of perhaps the world鈥檚 earliest star catalog鈥� into the sun鈥檚 orbit. For four years, the satellite swiveled through skies, watching the some 118,000 stars that passed through its rotating field of vision. It measured their distance to the Earth. It measured how fast they were going, and in what direction.

For the ESA, the mission was also a proof-of-concept project: Hipparcos showed that it was possible to measure the distance from our star to other stars, and it was possible to precisely measure their movements. That meant, in turn, that it was possible to collect the numbers that could be visualized as a galactic landscape, a map of the neighborhood in which Earth has an address.

So, in 1993, a team of European scientists proposed a successor mission to Hipparcos: 鈥淕aia.鈥� The name, at the time, was an acronym for Global Astrometric Interferometer for Astrophysics. It was later jettisoned for just 鈥淕aia,鈥� in reference to the Greek goddess, the daughter of Chaos and the mother of everything.

鈥淚t鈥檚 an appropriate name for the first mission deign to study the whole of what we can see in the skies,鈥� says Dr. Gilmore.

Gaia is designed to watch one billion of the Milky Way鈥檚 estimated 100 billion stars 鈥� or, about 10 thousand times the number of stars that Hipparcos surveyed. Some of these stars will be about 100 times further afield as those that Hipparcos saw, since technological limitations had kept Hipparcos鈥檚 sight within 1,000 light years from Earth. Gaia, though, was created to see out to the Milky Way鈥檚 fringes, some 100,000 light years afield.

Gaia is expected to make its measurements 鈥� the stars鈥� distances and movements 鈥� with unprecedented precision. It has a dual-telescope system and the coffee-table-sized camera boasts about a billion-pixels. Gaia absorbs about 30 times the light of its predecessor, and its stellar measurements are expected to be about 200 times more accurate.

鈥淗ipparcos was measuring our home鈥檚 backyard,鈥� says Jos de Bruijne, deputy project scientist on the Gaia mission. 鈥淕aia is measuring the whole city.鈥�

Galactic Archeology

Scientists know that our galaxy is a big spiral with a big bulge at its middle, like a sprawling kingdom whose capital is bloated with centralized power. They know, too, that our galaxy is in fact a conglomeration of thousands of smaller galaxies, each of which was, with classic imperial malevolence, ripped up and arranged into a cosmic empire: the Milky Way.

But the nuances of what our galaxy looks like, and how it came to look this way, are still unknown.

The 3D map that Gaia鈥檚 data is expected to produce will be the most complete map yet of our galactic neighborhood. When scientists rewind it back to see the galaxy in its humbler years, and then wind it forward again, they鈥檒l be scrolling though a billions-of-years-long narrative of territorial gains, geographic jockeying, and cataclysms both major and minor. It鈥檚 a tale that鈥檚 epic on the cosmic scale, and it鈥檚 one that scientists hope will answer questions like: What forces produced the galaxy鈥檚 layout? What events shaped its architecture? And how 鈥� in a universe of bangs and change 鈥� has the Milky Way maintained its spiral shape for billions and billions of years?

鈥淭his is galactic archeology,鈥� says Dr. de Bruijne.

鈥淲e have a paradigm for the how the galaxy formed, and now we鈥檒l be able to test that paradigm,鈥� he says. 鈥淕aia will tell us what really happened in the past to our galaxy and why it has the structure it has.鈥�

Scientists are unsure, for example, if the Milky Way does, in fact, have wisps of thousands and thousands of shredded-up galaxies emanating from its main heft, as theories predict it should. These small galaxies, which went into the Milky Way鈥檚 making, should still faintly waft from the congealed whole, like stubborn and un-swallowed legs protruding from a monster鈥檚 jaws.

So far, though, just a few ghost traces of the galaxies鈥� once-independent existences have been found. But, if the models are right, there should be thousands of them, and it鈥檚 now up to Gaia to prove the models either right or wrong, says Gilmore.

鈥淓ither Gaia will find them, or there is something seriously wrong with our theories,鈥� says Gilmore.

Bonus Missions

Gaia is designed to orbit the sun for five years. That means it will pass each of the billion stars it watches some 70 times. And that means it will see a lot.

鈥淭here is a large variety of bonus science,鈥� says de Bruijne, of the countless discoveries that he expects Gaia to make.

Gaia is expected to count stars鈥� planets 鈥� it should find more than 10,000 exoplanets, the team says. It should note which stars have just been born, and which are dying, and which have died. It will likely find objects that are behaving as expected, and it will find some 鈥� it is expected 鈥� that are not behaving as expected at all. It will happen upon, in all likelihood, bodies and phenomena that demure from all the models, that dispute all the theories.

All of these activities, over five years, should be enough data to fill 1.5 million CD ROMS.

Each of these finds, too, is expected to be put online. In what is called Project Supernova, the Gaia team plans to ask citizen astronomers to look through their own telescopes, observe the oddball phenomena that Gaia has just seen, upload their own pictures, and weigh in on what it is: Supernova? Brown dwarf? Quasar?

In another 鈥渇ree bonus,鈥� Gaia will likely help furnish a catalogue of the dangerous class of asteroids 鈥� known as Apollo asteroids 鈥� that orbit between the Earth and the sun, says Gilmore. Little is known about this group of asteroids, since they鈥檙e obscured in the sun鈥檚 light, except that they are there and that they are worth being concerned about. The asteroid that banged into remote Russia over the winter, in the middle of the afternoon, was such an asteroid. No one had seen it coming.

鈥淭hese are the really dangerous, dinosaur-killing ones,鈥� says Gilmore.

鈥淕aia might even save your life,鈥� he says.