Hipparcos

A special tool went into space.

It was a satellite named Hipparcos. It looked at the stars. It found where they are. It also found how far away they sit.

Watching from space is better. The air on Earth can get in the way. Space is very clear.

This tool was very good at its job. It made a big list of stars. This list had many thousands of stars in it.

It helped us learn about the stars. Now we know more about space.

Hipparcos was a special satellite in space.

It belonged to the European Space Agency. It launched in 1989 and worked until 1993. Its job was to do astrometry. This is the way scientists measure the exact positions of stars.

Measuring stars from Earth is hard. The air in our atmosphere can blur the view. Hipparcos lived in space where the view is clear. It used two telescopes to look at the sky. These telescopes were set at a wide angle. This helped the satellite connect different parts of the sky together. It could find the absolute parallax of stars. Parallax is the way we measure how far away a star is.

Hipparcos made very big lists of stars. The Hipparcos Catalogue had more than 118,200 stars.

Another list called the Tycho Catalogue had over one million stars. A later list had 2.5 million stars. These lists help us see how stars move.

The name Hipparcos comes from an ancient Greek astronomer. He was a very famous star expert.

Hipparcos was a very important scientific satellite.

It belonged to the European Space Agency. This mission was the first space experiment for precision astrometry. Astrometry is the careful way we measure where stars are. It also helps us find how far away they are. Scientists needed this to understand how stars move through space. By knowing these details, they could learn about the structure of our galaxy.

Measuring stars from the ground is a hard job. The Earth's atmosphere can blur the view of the sky. Other problems include heat and how the telescope bends. Hipparcos solved this by working in space. It used two telescopes that looked at a wide angle. This angle was 58 degrees apart. These two views were joined onto one single plane. This let the satellite connect different parts of the sky together.

This way of working is called absolute parallax measurement. It is different from relative astrometry used by telescopes like Hubble. Relative astrometry only measures stars by looking at nearby neighbors. This can lead to errors in different parts of the sky. Hipparcos could link stars all over the sky into one rigid frame. It is a bit like using a global network of triangles on Earth. This helped scientists find the true distance to stars.

The mission had a long history of planning. A proposal to use space was first made in 1967. The French space agency, CNES, first looked at the idea. They thought it was too expensive for one country. Later, the European Space Agency accepted the project in 1980. The satellite finally launched in 1989. It worked for four years until 1993. The name Hipparcos comes from an ancient Greek astronomer named Hipparchus. He was a founder of trigonometry.

Hipparcos created several huge lists of star data. The Hipparcos Catalogue had more than 118,200 stars. It reached an accuracy of about 0.001 arcseconds. The Tycho Catalogue included over one million stars. A later version called Tycho-2 had 2.5 million stars. These lists help us see how stars move and change. A follow-up mission called Gaia launched in 2013. These missions help us understand the physical properties of the universe.

Hipparcos was a groundbreaking scientific satellite operated by the European Space Agency (ESA).

Launched in 1989, it functioned until 1993. It was the first space experiment dedicated to precision astrometry. Astrometry is the science of measuring the exact positions and distances of celestial objects. This mission was vital because it provided the first practical attempt at all-sky absolute parallax measurement. Ground-based observatories could not achieve this level of accuracy due to many physical barriers. By measuring stars from space, astronomers could finally understand the physical properties of stars and the structure of our galaxy.

Measuring stars from Earth presents several insurmountable challenges. The Earth's atmosphere causes astronomical seeing, which blurs stellar images. Additionally, telescopes on the ground suffer from thermal and gravitational flexures, where heat or weight causes the instrument to bend slightly. Most existing space telescopes, such as Hubble, perform relative astrometry. This means they measure a star's position by comparing it to nearby background sources. While useful, compiling many local relative measurements creates regional errors. This makes it very difficult to compare distances between different regions of the sky.

Hipparcos solved these problems through a unique mechanical design. The spacecraft used a single all-reflective, eccentric Schmidt telescope. A special beam-combining mirror superimposed two separate fields of view onto one common focal plane. These two viewing directions were separated by a wide "basic" angle of 58 degrees. Because these two views were directed onto the same plane, every measurement made by one telescope was connected to simultaneous measurements by the other. As the satellite scanned the sky, these observations created a dense network of connections. This allowed scientists to deduce a single, global rigid reference frame, much like how Earth's coordinate systems are anchored by a global network of triangles.

The mechanism for capturing data was highly sophisticated. At the focal surface, the telescope used a system of grids. These grids consisted of 2,688 alternating opaque and transparent bands. The period of this grid was 8.2 micrometres. Behind these grids, an image dissector tube converted modulated light into a sequence of photon counts. The satellite sampled these counts at a frequency of 1200 Hz. By analyzing the phase of these photon pulses, scientists could determine the apparent angle between two stars.

The development of Hipparcos was a long and complex process. A formal proposal for a space-based observation mission was first introduced in 1967. Initially, the French space agency, CNES, considered the project too expensive for a single national program. They recommended that it be handled in a multinational context instead. The European Space Agency eventually accepted the project in 1980 after years of study and lobbying. The name "Hipparcos" is an acronym for High Precision Parallax Collecting Satellite. It also honors Hipparchus of Nicaea, an ancient Greek astronomer who discovered the precession of the equinoxes and applied trigonometry to astronomy.

The mission produced several highly significant data sets. The primary Hipparcos Catalogue contained more than 118,200 stars. It achieved a median accuracy of slightly better than 0.001 arcseconds, surpassing its original goal of 0.002 arcseconds.

Another project, the Tycho Catalogue, provided data for over one million stars. This was later expanded into the Tycho-2 Catalogue, which included approximately 2.5 million stars. These catalogues allowed astrophysicists to calculate the six quantities needed to determine stellar motion. By combining position, proper motion, and parallax with radial velocity from spectroscopy, they could map how stars move through space.

Hipparcos fundamentally changed how we study the universe. Its data provided an empirical basis for studying stellar evolution and galactic kinematics. By knowing the exact distances and motions of stars, scientists could better understand how galaxies are built. This mission paved the way for even more advanced technology. In 2013, the Gaia mission was launched as a follow-up to the work started by Hipparcos. Together, these missions continue to refine our map of the cosmos.