Europe's 'discovery machine' soars to map the Milky Way
BY STEPHEN CLARK
Posted: December 19, 2013
The sharp-sighted Gaia observatory, carrying the largest camera ever flown in space, rocketed into a predawn sky from French Guiana on Thursday to survey a billion stars and reveal the structure of the Milky Way in finer detail than any mission before.
Dubbed a "discovery machine" by Gerry Gilmore, one of Gaia's founding fathers from Cambridge University, the mission blasted off at 0912:19 GMT (4:12:19 a.m. EST; 6:12:19 a.m. local time) on a Soyuz rocket from the Guiana Space Center in South America.
The 15-story Russian launcher, modified with digital controls and upgraded range safety systems, tilted east from the European-run space base in French Guiana, quickly becoming a brilliant point of light in the sky over Sinnamary, the nearest town to the launch pad.
As the powerful launcher rose into the first rays of sunlight, its teardrop-shape exhaust plume was illuminated, putting on a spectacular sky show for observers around the tropical space base.
Officials reported no problems in the launch sequence, and the three-stage Soyuz booster finished its work less than 10 minutes after liftoff. A Russian Fregat upper stage took over, firing two times to inject the 4,484-pound Gaia spacecraft on an energetic trajectory toward an operating post at the L2 Lagrange point, a gravitationally stable location more than 900,000 miles from Earth.
The satellite was deployed at 0954 GMT (4:54 a.m. EST), and ground antennas in Australia acquired the first signals from Gaia moments later.
Following an automatic preprogrammed sequence, Gaia unfolded an insulating thermal sunshield half the size of a tennis court. The deployed shade gives Gaia a distinctive hat-shaped appearance and keeps the probe's sensitive camera at a chilly minus 148 degrees Fahrenheit at all times.
"Gaia will take a census of about one billion stars with an extraordinary precision that exceeds not only the dreams of ancient astronomers but also goes beyond the performance of its predecessor, Hipparcos, launched by Ariane 4 almost 25 years ago," said Stephane Israel, chairman and CEO of Arianespace, the French launch services provider overseeing the Soyuz flight.
On Friday, Gaia will get a boost from its own rocket engine to put the probe on course to arrive at its operating post around the L2 Lagrange point around Jan. 7.
By spring 2014, the mission will begin its five-year survey of the cosmos, detecting every star, supernova, quasar, planet and asteroid brighter than 20th magnitude -- 400,000 times dimmer than visible with the naked eye.
Gaia will plot the locations and movements of a billion stars through its mission.
"The estimate of the number of stars in the Milky Way is 100 to 200 billion stars, so we observe between one-half and 1 percent of these stars," Prusti said. "Because of the completeness of Gaia to a limiting magnitude, this proving of 1 percent of these objects will help us reconstruct the remaining part. We're not going to take a full census of the Milky Way, but we are going to look at a billion stars and we'll have enough statistical power to deduce the structure of the Milky Way."
For 150 million of the brightest objects, Gaia will see how fast they are moving toward or away the us, giving astronomers a snapshot of the galaxy in three dimensions.
"We can measure two positions in space [at different times], the third position via the parallax, and then the true speed on the plane on the sky. But we are missing the fifth dimension, which is the radial speed, the speed of the star either approaching or receding from the spacecraft. We do this with a radial velocity spectrometer," said Giuseppe Sarri, ESA's Gaia project manager.
First proposed in the 1990s, Gaia was selected for development by ESA as a cornerstone mission in 2000. It follows the European Hipparcos mission, which was the first probe to plot the positions and motions of stars - a measurement known as astrometry.
Gaia will see 10,000 times more stars than Hipparcos, which launched in 1989.
"Gaia promises to build on the legacy of ESA's first star-mapping mission, Hipparcos, launched in 1989, to reveal the history of the galaxy in which we live," said Jean-Jacques Dordain, ESA's director general. "It is down to the expertise of Europe's space industry and scientific community that this next-generation mission is now well and truly on its way to making ground-breaking discoveries about our Milky Way."
Officials say Gaia's total cost is approximately 940 million euros, or nearly $1.3 billion, including contributions from ESA and academic institutions in the United Kingdom, Italy, Germany, France and Spain responsible for processing the mission's immense data archive.
That comes to a cost of approximately one euro for each star seen by Gaia.
The chemical make-up of millions of stars will also be revealed by Gaia, adding more information to the mission's data haul, which will exceed one petabyte. That's equivalent to a million CDs, or 1,000 million million bytes.
Learning about the composition of stars "helps us with the scientific questions we have about the Milky Way because we do not only know where the stars are, but we can say what kind of stars they are. That really helps to find where they come from, where are they going, when were they born, and so forth," Prusti said in an interview before launch.
The extraordinary precision is made possible by an array of CCD detectors the size of a table top, giving Gaia the imaging power to resolve a strand of hair 1,000 miles away.
Gaia's twin telescopes will scan the sky as the spacecraft spins four times each day, registering an average of 40 million stars every day and beaming their light onto 106 CCDs comprising nearly a billion pixels with an area of 0.38 square metres, the largest imaging focal plane ever deployed in space.
The sensors for Gaia's imaging array were made by e2v in the United Kingdom.
"The key technological reason we can do this job so much better is that since Hipparcos, CCDs can now be used in space, allowing us to observe many targets at the same time by imaging the sky, which gives us an advantage, together with a bigger satellite," Prusti says. "But it's really the detector technology that allows us to benefit from Gaia."
Gaia's instrument, which actually serves three functions, will plot the position and movement of a star's point light source across the plane of CCDs. Two prisms will split light into red and blue spectra, allowing researchers to use a process known as chromatic correction to ensure they know exactly a star's color, temperature, mass and other things.
"With Gaia, Astrium built the largest space instrument ever in ceramics, thus allowing Gaia to offer an exceedingly high stability for science observations," said Eric Beranger, CEO of Astrium Satellites, Gaia's prime contractor. "With Gaia, Astrium built the biggest focal plane ever, 100 times better than today's common digital cameras. And for this masterpiece, this jewel of space hardware, Astrium gathered and led an industrial consortium made of 50 companies -- 47 European and three American."
To keep stable, Gaia will use an innovative micro-propulsion system with cold gas thrusters. Spinning reaction wheels or conventional rocket engines would disturb the science mission, and the probe will use electronic actuators to point its antenna toward Earth, minimizing vibrations, according to Sarri.
Speaking to reporters in French Guiana after the launch, Prusti said Gaia will go through several months of commissioning before starting its science mission. Operational observations are due to begin in early May, he said, with the first release of data expected in late 2015.
"Gaia represents a dream of astronomers throughout history, right back to the pioneering observations of the ancient Greek astronomer Hipparchus, who catalogued the relative positions of around a thousand stars with only naked-eye observations and simple geometry," said Alvaro Gimenez, ESA's director of science and robotic exploration.
Much more is in the offing from Gaia, including discoveries closer to home.
"Over 2,000 years later, Gaia will not only produce an unrivaled stellar census, but along the way has the potential to uncover new asteroids, planets and dying stars," Gimenez said.
"Because Gaia is doing this totally unbiased survey, anything that looks point-like and is brighter than 20th magnitude will be observed," Prusti said. "That means we will get a huge number of asteroids in our solar system. They come like a free bonus for Gaia. This will allow us to determine the orbits of these asteroids by orders of magnitude than can be done with ground-based observations."
Gaia will look inside Earth's orbit and could uncover asteroids hiding from other telescopes, according to Prusti.
"We are not looking directly into the sun but close enough to do something that is not possible from the ground," Prusti said. "This will allow us to look inside Earth's orbit, so we'll look for asteroids that are locked in Earth-trailing or leading Lagrange point and we can make a census of what type of objects these are and how many there are."
Data collected by Gaia will also contain hints of large gas giant planets around other stars.
"The sensitivity of Gaia will be high enough that we will have a census of Jupiter-sized planets around nearby stars in orbits between two to seven or eight years," Prusti said. "That's a parameter space that's very poorly studied from ground based observations. We will see a star that has an extra bubble with respect to the anticipated parallax and proper motion, and from measuring that extra bubble we can deduce whether it has a companion which is another star or perhaps a Jupiter-sized planet."
Scientists expect Gaia will discover tens of thousands of brown dwarfs, or failed stars, and 20,000 supernovas. And models of the motion of stars throughout the galaxy could yield insights into the distribution of dark matter, the poorly-understand glue that binds the universe together.
"It's the first ever genuine census machine where we don't need to know anything about what's in the sky to start with," Gilmore said. "That is literally the requirement. The requirement is we don't bias ourselves with thinking we know what's up there. We're actually going to see what nature's telling us. We'll see everything."
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