The Hipparcos Spacecraft

Dr Alan Cooper

Listening carefully

It's amazing how much you can tell about a place from a distance - with the right equipment. We explain the successes of remote sensing.

Boldly going

Mankind has always yearned to know what lay beyond our planet - but only in the last fifty years has it really been possible to start to explore outside our atmosphere. David Hughes introduces our study of how we've been exploring space.

About our expert

Dr Alan Cooper worked first in Harwell's Nuclear Reactor Division and then at the newly inaugurated CERN in Geneva. He moved to the Zero Gradient Synchrotron, joined the Open University in 1971, where he worked on several of the new physics and astronomy courses.

Hipparchus deserves to be called the first astronomer. Over many years between 146 and 127 BC he carefully logged and measured the positions of stars in a thoroughly scientific way, and was rewarded by discovering and measuring the precession of the Earth's axis. He was the first to have some understanding of spherical trigonometry, and for that alone claims the respect of students up to the present day! The European Space Agency (ESA) launched a spacecraft in 1989 with the sole aim of making the best star catalogue possible - what better name for it than Hipparcos?

The design of Hipparcos depends entirely on the advantages of being in space: it would not work on Earth. In the absence of atmospheric distortions, no telescope is needed. Hipparcos has a set of masks, rotating with a constant speed in the absence of friction. Its photometers record the times of appearance and disappearance of stars through these masks. Over many months the whole sky is covered, and many millions of timings recorded. A huge calculation identifies the stars (with reference to existing catalogues) and finds improved "least squares" estimates of their positions and luminosities. This is also the prelude to applying the method of parallax used by Bessel to estimate distances of the nearer stars - but the Hipparcos results reach out much further, surveying a substantial part of the Galaxy. The final results are available for anyone to use, in the form of a CD. For the layperson, the results for the brightest stars are available in a form which re-uses parallax. With the distance known, it is possible to calculate what the sky would look like if the separation of our eyes was not 65mm but 200,000 AU (or 100,000 times the diameter of the Earth's orbit). This is shown as a stereoscopic pair of images, one red and one green. Viewed through red/green spectacles, the Galaxy appears, truly for the first time on this planet, in full 3 dimensional depth. Stereoscopy, after all, is just parallax by another name.

This huge database has proved so useful that a new spacecraft, GAIA, is planned (for 2009) to reach ten times further than Hipparcos - about 1000 times as many stars. This is still within our Galaxy, and quite different methods (not directly AU based) are needed to get to other galaxies, and hence into the realm of cosmology.