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Orion the Hunter, bright and distinctive even through city skyglow, is the best-known of the 88 constellations that cover the sky. You’ll find Orion climbing the southeastern heavens at this coldest time of the year. Sirius, his Dog Star, follows brightly down below. The planet Jupiter, even brighter, glares off to Orion’s left this season.

Even if you’ve never recognized Orion before, you may have noticed Orion’s Belt, the eye-catching row of three stars in his middle. They look like such a neat row up there in space. In fact, they’re not. The stars of Orion’s Belt are at different distances from us and, if you could see them from the side, would form some kind of very unrelated-looking arrangement.


But here’s the rub. Even in this age of precision spaceflight, a thousand known exoplanets, and a date for the Big Bang that’s accurate to 1 percent, astronomers still have such poor information on the distances to most stars that we don’t know how Orion’s Belt is really arranged.

It’s a problem all over the sky.

Two weeks ago, the European Space Agency launched a specialized space observatory to fix the star-distance problem with spectacular thoroughness. The mission is named Gaia. It promises to revolutionize not just this one field but, because star distances are fundamental to nearly everything else we know beyond the solar system, much of the rest of astronomy, too.

Its Dec. 19 launch received little attention. But once Gaia settles into its assigned post far from Earth, it will start measuring star positions, an ancient endeavor called astrometry, with extraordinary new precision and abundance. It will singlehandedly increase the data we possess about where stars are located in space by thousands of times compared to all previous such measurements in history.

Gaia will measure the directions to 1 billion stars with a precision equivalent to measuring the different directions to the two edges of a grain of salt in Chicago as seen from Boston. For about 10 million stars, the precision will be better: matching the width of a salt grain in Hawaii as seen from here.


Gaia will measure each of these billion stars about 70 times during its planned five-year mission. This will allow data analysts to map the microscopic loops of “parallax motion” that stars appear to trace on the sky once a year due to our moving viewpoint on Earth as we circle the Sun. When you do the geometry for these loops, you get the most direct measurements of star distances possible.

Star parallaxes have been measured from the ground since 1838, but only for stars nearby and not nearly as well as astronomers would like. Distances to farther stars used to be pathetically uncertain. This really mattered, for two reasons. The entire cosmic distance scale beyond the solar system — not just for stars but nebulae, galaxies, and everything else — was built on a foundation of nearby star distances. If you don’t know an object’s distance well, you usually can’t tell its true size, brightness, energy output, mass, or much else about it very well either.

To address this problem, in 1989, the European Space Agency launched an earlier star-measuring craft named Hipparcos. In the clarity above Earth’s atmosphere, it worked a revolution, raising the number of stars with reliable distances from a few thousand to more than 100,000 and reaching about 10 times farther out.


NASA planned to launch the next-generation star measurer, a satellite named FAME, in 2004. German astronomers planned a similar mission of their own named DIVA but dropped it because FAME would duplicate it. Then in 2002, NASA killed FAME because it broke its budget. So the planned second generation of space astrometry never happened.

Gaia was to be the third generation, which is why it will be such an enormous leap forward now. Its data-crunching teams expect to start issuing its first results in 2016 and the complete catalog for a billion stars in 2021. It can’t come soon enough.

Whole-sky maps

Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.