Hipparcos (HIgh Precision PARallax COllecting Satellite): Measuring the Milky Way

Hipparchus, the ancient Greek astronomer, compiled the first known star catalog in 129 BCE, noticing that star positions change over time. This discovery paved the way for mapping the night sky. After refinements by Ptolemy and others, photographic plates in the twentieth century allowed precise star measurement, enabling audacious plans to map the Milky Way. However, Earth’s atmosphere obscured celestial objects. Launching telescopes into orbit overcame this obstacle. The satellite Hipparcos, named after Hipparchus, was designed to accurately measure the positions and distances of stars. Despite nearly failing after launch in 1989 when its final engine misfired, Hipparcos persevered in an elliptical orbit. Using trigonometric parallax, it achieved accuracy 500 times greater than earthly telescopes. Hipparcos compiled a catalog of over 100,000 stars and measured 7,000 distances accurately. This revealed our galaxy’s barred spiral shape. While seemingly basic, this data enabled discoveries about extraterrestrial life, galactic history, general relativity, and more. Hipparcos laid the foundation for the billion-star map of its successor Gaia Space Observatory. Though most people now live divorced from the night sky due to light pollution, satellites like Hipparcos help reconnect humanity to the stars’ mysteries.