ESAs space science programme in a nutshell
- Sun and Earth
- Comet chasers
- Titan, Mars, Mercury and the Moon
- The cool invisible Universe
- The hot invisible Universe
- Special views of the Universe
- Was Einstein right?
Sun and Earth
Three unprecedented missions, two conducted jointly with NASA but all using spacecraft built in Europe, give ESA a leading role in watching the stormy Sun and understanding how its outbursts affect our home planet. Ulysses (launched in 1990) went over the Sun's poles, where no spacecraft had been before, in order to explore the heliosphere - the huge bubble in space created by the wind of atomic particles that blows non-stop from the Sun. Ulysses made a return visit to the Sun's poles in 2000-2001 when solar activity was particularly frenzied. Every day SOHO (launched in 1995) sends vivid pictures of the Sun's atmosphere, by visible and ultraviolet light, and registers atomic particles en route from the Sun to the Earth. Waves seen by SOHO in the Sun's visible surface reveal what is going on beneath the surface and in the deep interior. Cluster, launched in July and August 2000, is a set of four identical satellites flying in company. They are designed to give the first 3-D picture of events in the Earth's space environment, where gusts in the solar wind shake our planet's magnetic field and drive atomic particles into the air.
Comet chasers
For long a source of wonder, as strange sights in the night sky, comets are now pursued by space scientists as remnants of the raw materials from which the planets were built. ESA's Giotto (1985-92) flew through the dusty head of Comet Halley, gathering remarkable pictures and data. Although badly damaged by the dust, Giotto went on to intercept Comet Grigg-Skjellerup too. Rosetta (due for launch in 2003) will maintain ESA's lead in comet research by going into orbit around the nucleus of Comet Wirtanen. It will observe the surface and emissions in fine detail, and drop onto the surface a lander equipped with instruments. When comets expire, some may continue to orbit as small, dark objects travelling close to the Earth and bringing a small risk of a catastrophic collision.
Titan, Mars, Mercury and the Moon
Targets for ESA missions to other objects in the Solar System offer a deeper understanding of the Earth itself. Huygens (launched in 1997 aboard NASA's Cassini spacecraft bound for Saturn) will parachute through the atmosphere of Saturn's large and peculiar moon, Titan, and make the most distant landing ever attempted on another world. Revelations about primordial chemistry on Titan promise a new perspective on the origin of life on Earth. Mars Express (to be launched in 2003) will make a special contribution to international efforts to explore the planet least different from the Earth, by searching for water. It will also drop a lander on the Martian surface. ESA's mission to Mercury, called BepiColombo, will examine the planet closest to the Sun. Mercury has been relatively neglected in space exploration till now, but it has enigmatic features - dense material, and an unexpected magnetic field - that may give important clues to the formation and behaviour of the Earth itself. The Moon will be the target for SMART-1 (launch expected in early 2003). The main purpose of SMART-1 is to demonstrate solar-electric propulsion and new technologies for future space missions, and its science data will give clues on the formation and evolution of the Earth-Moon system.
The cool invisible Universe
Although light from the stars, and some radio waves, penetrate the air to reach telescopes on the ground, telescopes in space detect many invisible rays that are blocked or confused by the Earth's air. ESA's ISO (1995-98) revolutionised the study of galaxies, stars and planets by infrared light. This is especially good for seeing cool and dusty objects, including newborn stars, and for identifying chemical molecules such as water, which ISO discovered in many places. Herschel (due for launch around 2007) will observe infrared rays with wavelengths longer than ISO's, and promises new insights into the origin of galaxies and the origin of stars. Planck (due for launch at the same time as Herschel) will survey the cool background of radio microwaves which fill the sky, apparently a relic of the Big Bang at the origin of the Universe. Concentrations of matter revealed by Planck will tell of galaxies waiting to be born, and will help physicists to choose between different theories of the Big Bang. Living planets, detectable by infrared light, would be a major objective for Darwin, a possible future project under study by ESA. Several infrared telescopes operating together should be able to see faint planets near other stars and to detect ozone as a signature of an atmosphere like the living Earth's, rich in oxygen.
The hot invisible Universe
Scenes of great heat and violence in the sky release energetic ultraviolet rays, X-rays and gamma rays, which are stopped by the Earth's atmosphere but detectable by satellites. Astronomers still quarry discoveries from the NASA-ESA-UK satellite IUE (1978-96) which for nearly 19 years was the world's chief source of information on ultraviolet emissions. XMM-Newton (launched in December 1999) carries remarkable telescopes conceived in Europe, which make it the most sensitive X-ray observatory ever available. It will give astronomers detailed knowledge of the violent events producing X-rays, including the action of black holes and the amazing explosions called gamma-ray bursts that have puzzled astronomers for many years. The gamma-ray satellite INTEGRAL (to be launched in October 2002) will also see black holes and gamma-ray bursts, and will trace the processes by which Nature cooks the chemical elements in the nuclear kitchens of the stars. Like XMM-Newton, INTEGRAL will be the most sensitive satellite in its chosen energy range.
Special views of the Universe
In a distinctive European contribution to space astronomy, Hipparcos (1989-1993) charted the positions and motions of stars far more accurately than ever before, and ESA released the Hipparcos and Tycho catalogues to the world's astronomers in 1997. Early results have included a revision of the size and age of the Universe, and firmer knowledge about how stars burn and evolve. ESA's Gaia, an ambitious mission to survey more than one billion stars, will achieve even greater precision in star mapping, and give an unprecedented picture of stars in motion throughout the Milky Way Galaxy. Gaia would also check hundreds of thousands of stars for the possible presence of planets. Meanwhile, ESA continues as a partner with NASA in the Hubble Space Telescope (launched in 1990) after providing one of the main instruments and the solar panels. In return, European astronomers have enjoyed generous use of Hubble, with which they have made many discoveries. ESA hopes also to join with NASA in the Next Generation Space Telescope. This is expected to favour short-wavelength infrared rays, and to look for very remote stars and galaxies whose visible light is shifted into the infrared band by the expansion of the Universe.
Was Einstein right?
The force of gravity as described by general relativity is a central feature of 20th Century science, but experts suspect that Einstein's theory will have to be corrected a little to make gravity compatible with other forces of Nature, electric and nuclear. ESA plans to take part in the NASA-led MiniSTEP mission (expected to be launched in 2004) which will look for violations of the principle of general relativity, that all objects and materials fall at the same rate under gravity. By detecting small shifts in the positions of test masses carried in a satellite, MiniSTEP will verify or contradict this principle, far more accurately than it has been checked on the Earth. General relativity also predicts gravitational waves, which squeeze and stretch space itself. ESA is studying a mission called LISA that would detect gravitational waves directly, by measuring the distances between widely separated spacecraft using laser beams. Successful detection would confirm an important aspect of Einstein's theory and open a new window on the Universe, enabling astronomers to examine objects by the gravitational waves they emit.