Summary

It will launch a mission to look inside stars named after the famous scientist. It will determine the precise chemical composition of the stars and, in some cases, the size of the energy-generating heart. This will enable astronomers to age the stars accurately for the first time.

Later, in 1933, Arthur Eddington also wrote: "It would indeed be rash to assume that nowhere in the Universe has Nature repeated the strange experiment which she performed on the Earth."

In other words, he expected there to be inhabited planets around other stars. Eddington, the spacecraft, cannot prove or disprove this expectation but it will make a start on finding the answer. It will search for Earth-sized worlds around other stars that may be the sites of extraterrestrial life.

Eddington's orbit

Instead of an orbit around the Earth, Eddington will be placed far away, beyond the Moon. Its location, at a distance of 1.5 million kilometres from Earth, pointing directly away from the Sun, is usually known as L2.

L2 is the second Lagrangian point, named after its discoverer, Joseph Louis Lagrange (1736-1813). He was a great Italian mathematician who discovered that there were five points of equilibrium in an orbital system containing two massive bodies. In this case, the Sun and the Earth make up the two orbiting bodies and the five points are labelled, L1 to L5.

One of the principal advantages of an L2 orbit is that it offers uninterrupted observations, since the Earth, Moon and Sun remain 'behind' at all time. Because the L2 point moves around the Sun, keeping pace with the Earth, the entire celestial sphere can be observed in the course of one year. The spacecraft must perform small manoeuvres every month, to ensure it stays at L2.

The L2 orbit is also far away from the detrimental effects of Earth's immediate environment. Around our world, the radiation given out by the planet and the particles trapped in its magnetic field can make sensitive observations difficult or even impossible. At L2, these problems are greatly reduced.

Eddington will not be the only ESA mission going to L2; Herschel, Planck, JWST and Gaia will all make their way there during the coming years.

Before Eddington

Eddington will be the culmination of an international attempt to perform asteroseismology from space. Two small precursor space missions are planned, mostly funded by national space agencies.

MOST is a Canadian mission using a 15 cm telescope that will launch in 2003. It will monitor a handful of the very brightest stars such as Procyon and Eta Boötes for about a month each.

COROT is a French mission, with some funding from ESA, that uses a 27 cm telescope. It will be launched in 2005. COROT will make long-term observations of between 20-30 stars, leading to high precision data. It will also attempt to discover transiting planets, larger than a few times the size of the Earth.

These smaller missions will pave the way for ESA's Eddington which will perform an extensive and far-reaching survey, beginning in 2008. From a better vantage point, further from the Earth, ESA's mission will study the interior composition of 50 000 stars. It will also survey 500 000 stars, looking for planets down to the size of Mars (that is about one-third the size of the Earth).

Who was Arthur Eddington?

Born in England, Arthur Stanley Eddington (1882-1944) studied physics and mathematics in Manchester before winning a scholarship to Cambridge University. In 1913, he was appointed the Plumian Professor of Astronomy there. During the following year, he became director of the Cambridge Observatory and was elected a Fellow of the Royal Society.

Arthur Stanley Eddington (1882-1944)

Eddington was one of the first physicists to grasp the theory of relativity and made many contributions to the field, including an expedition to view a total eclipse of the Sun in 1919 which resulted in the first confirmation of Einstein's theory that gravity bends light when it passes near a massive star. He published a book entitled 'Mathematical Theory of Relativity' in 1923.

Apart from this work, Eddington made important contributions to the study of stellar evolution, motion and interior structure. He discovered the mass-luminosity relationship for stars, calculated the abundance of hydrogen throughout the Universe and produced a theory to explain how the Cepheid class of variable stars pulsated. Perhaps his most famous book is 'The Internal Constitution of the Stars', published in 1926.

ESA's Eddington mission has been named in recognition of the pioneering role, played by Arthur Stanley Eddington in the field of stellar structure and evolution.