Hipparcos Scans the Skies

By the 1980s, astronomers had convinced their funding agencies that a space observatory to measure star positions in the vacuum of space would be a good investment. The High Precision Parallax Collection Satellite (Hipparcos) went through a series of design studies with the European Space Agency and was launched in 1989 on an Ariane 4 rocket from French Guiana.32 Hipparcos is the only facility in this book not supported or operated by NASA, but its importance transcends its country of origin, and U. S. astronomers have used it extensively for their research. National boundaries melt away in the night sky and international collaboration is the lingua franca of astronomy. Indeed, the stars belong to no one and yet to everyone.

The telescope that transformed the precision with which as­tronomers can map the sky was only 29 centimeters in diameter, not much larger than a dinner plate. Many amateur astronomers use bigger glass for the mirrors of their handmade telescopes. Its mission lasted for just three and a half years, from August 1989 to March 1993, yet the data are still generating scientific results and publications twenty years later.33 Hipparcos was one of the last space missions before the advent of CCD detectors. The satellite swept its gaze across two widely separated patches of sky and the starlight fell on a set of alternating transparent and opaque bands and then onto an old-fashioned photomultiplier tube. The primary goal of the mission was to measure the positions of 100,000 stars with an accuracy of 0.002 arc seconds. How small is this angle?

Five hundred times smaller than the typical angle by which a star image is blurred out by the Earth’s atmosphere, or equal to the angle made by lines to the two opposite sides of a penny in New York as seen from the apex of the triangle in Paris.

Imagine a great city ringed by a fence. It’s nighttime and you’re outside the tall fence looking in. As you walk around the fence, the lights of the city will appear to flicker on and off as they pass behind the slats of the fence and then reappear in the gaps. Now imagine a somewhat different situation: you’re inside the city and wearing a hood. The hole for each eye is covered with extremely thin vertical slats, like a miniature fence. As you turn, the lights from the streets and buildings brighten and fade as they pass in between and behind the slats. Hipparcos worked in this way, scan­ning a great circle on the sky every two hours, with its two imaging fields, or eyes, seeing a particular star 20 minutes apart. The preci­sion of the measurement came about because the angle between adjacent slats was only one arc second, and then combining a hun­dred or more observations of the same star gave a much smaller angular error. In addition to using the data to measure positions, astronomers used the repeated observations to search for variabil­ity in the light of hundreds of thousands of stars as Hipparcos pivoted to scan the entire sky.