The Hipparcos Space Mission

Launched in 1989 by the European Space Agency (ESA), the Hipparcos space experiment set out to precisely catalog the placement of stars within our solar system and measure their distances between the earth and each other. During the four years following its launch, it cataloged nearly 120,000 stars. This high-quality scientific data contributed greatly to academic research and scientific literature, helping astronomers and astrophysicists make new findings and improve on existing experiments.

For decades, scientists around the world had longed to accurately catalog the distances of stars within the galaxy. In 1966, a French scientist by the name of Pierre Lacroute proposed the idea of a precision satellite that could accurately map stars while in orbit around the earth.

Facing difficulty due to the high costs and complexities of such an expedition, the project sat dormant until it was finally accepted by the European Space Agency in 1980. The original mission was scientifically motivated. The ESA was interested in using the program to primarily measure the physical properties and structure of stars and celestial bodies.

Production of the satellite began in 1984. The eventual physical build of Hipparcos was a true collaborative effort among European nations. Hardware components, systems, and software came from laboratories and manufacturers across the continent. More than 200 scientists and 2,000 industry personnel contributed to the program and its ultimate success.

Although the planned mission duration was only to last 2.5 years, it continued on through 1993. All in, the mission would cost the ESA approximately 600 million euros.

After years of development, the satellite was finally ready for launch. It almost didn’t make it to space, however. In 1988, as Hipparcos sat in storage in Turin, Italy, a meteorite fell from space and struck the parking lot of the warehouse where the satellite was located. It missed it by a mere 1,000 feet.

The satellite was eventually sent to space on August 8, 1989 from a facility in French Guiana. Although the initial launch was successful, there was some concern when the rocket engine failed to properly set the satellite into its proper orbit. It was meant to orbit the earth over the equator above Africa. Although it ended up in a less-than-ideal, highly elliptical orbit around earth instead, the launch team was able to adjust its position using backup thrusters to account for much of the error. Hipparcos was still able to sweep 80% of the sky to make its observations.

The name “Hipparcos” has a dual meaning. The word itself is a phonetic acronym for High Precision Parallax Collecting Satellite. It’s also a nod to Hipparchus, the ancient Greek astronomer, mathematician, and geographer, who many consider to be the founder of trigonometry. He is also known for drawing up his own star catalog in the 2^nd century BC.

As Hipparcos slowly rotated around the earth, its telescope continually scanned the space around it. Using precisely built hardware, it observed shifts of stars and objects in its observational plane using the emitted light. These observations were then modulated by a grid to finely measure the positions of specific stars and their relative distances.

The Hipparcos mission began with a target list of 118,000 stars. Scientists were hoping to measure the stellar distances of these stars down to an accuracy of 2 milliarcseconds. The project ended up exceeding expectations. Not only were all 118,000 stars on the target list cataloged by Hipparcos, but their accuracy could be measured down to a single milliarcsecond. This highly precise catalog was officially published to the world by the ESA in 1997.

While Hipparcos was in orbit, it was also able to survey millions of additional stars outside of the original target list. The measurements from these stars were published at the same time in a separate catalog named after the 16^th-century Danish astronomer Tycho Brahe. While the measurements of the stars included in the Tycho Catalog weren’t as precise, they still proved to be valuable additions to scientific research.

The published star catalog of Hipparcos has affected numerous areas of astronomical science. Since its release, thousands of scientific papers have been published based on the measured stellar distances of Hipparcos. It has contributed to such research topics as earth’s rotation, the age of the universe, and the structure and masses of dwarf stars. The catalog was also used to develop the Millennium Star Atlas, a precise atlas of one million stars that exist within the galaxy.

While the Hipparcos Catalog was foundational to our understanding of stars and their celestial movements, work is ongoing to improve on its findings. Gaia, a follow-up mission to Hipparcos, launched in 2013 with the mission of cataloging a billion stars at an accuracy 200x better than that of Hipparcos.

Check out other articles from our blog for more astronomy and star facts.