Telescopes and Earth

Not every space mission goes to another world. Many of ESA's most valuable missions stay close to home, looking outward at the stars or back at Earth. This chapter covers both: the space telescopes, from its part in the Hubble Space Telescope to the James Webb Space Telescope and the star-mapping missions Hipparcos and Gaia, and the Earth-observation satellites that watch the weather, ice, oceans and land. It is a less glamorous side of the agency than comets and landers, but it is where much of the science and much of the everyday value lies. Mission details are given from general knowledge and flagged where they should be checked.

Why Put a Telescope in Space?

The Earth's atmosphere blurs and blocks light, and some kinds of light, such as X-rays and much of the infrared, hardly reach the ground at all. A telescope in orbit sees more sharply and in more wavelengths. The price is that it must be launched, cannot easily be repaired and has to work on its own. ESA's science programme (Chapter 3) funds a series of space observatories chosen by competition among scientists.

Hubble and the James Webb Space Telescope

Two of the most famous telescopes were joint projects with NASA, and ESA's share in them is an example of the partnership running through the course.

• Hubble Space Telescope. Launched in 1990 by NASA. ESA took part from the start, supplying equipment including its solar panels and an instrument, and receives a share of the observing time (the share is often quoted as around a sixth, but check).
• James Webb Space Telescope. A joint project of NASA, ESA and the Canadian Space Agency. It was launched in December 2021 on a European Ariane 5 rocket from Kourou (Chapter 4), which was ESA's contribution to the mission, along with instruments. It observes mainly in the infrared and works far from Earth.

Mapping the Stars: Hipparcos and Gaia

One of Europe's real strengths has been astrometry, the precise measurement of the positions and movements of stars.

• Hipparcos. Launched in 1989, it measured the positions of over a hundred thousand stars very accurately. Its catalogue was published in the 1990s.
• Gaia. Launched in 2013, it set out to map positions and motions for around two billion stars in the Milky Way, which makes it a three-dimensional map of part of our galaxy. It is generally reported to have ended its observations in 2025 (check), but the data is still being processed and released.

The practical consequence is that astronomers all over the world use the Gaia catalogue as a reference, and it has been one of the most cited data sets in astronomy.

Other Space Observatories

Looking Back at Earth

ESA has a long record in Earth observation. Its early ERS satellites, launched in the 1990s, used radar to measure the sea, ice and land through cloud and in the dark. The large Envisat satellite, launched in 2002, carried a suite of instruments to study the atmosphere, oceans, ice and land, and it operated for about ten years before contact was lost in 2012 (check). Later missions focused on specific questions, for example CryoSat on the thickness of polar ice, and Aeolus on measuring winds.

Copernicus

The largest present effort is Copernicus, a European programme for observing the Earth. It is an EU programme, and ESA builds and develops the satellites, the Sentinel family, while other organisations run the services that use the data. Different Sentinels carry radar, optical cameras and instruments to monitor the atmosphere and oceans. The data is made freely available, and it is used for:

• tracking floods, fires and other emergencies;
• monitoring farming, forests and land use;
• watching sea ice, sea level and ocean conditions;
• measuring air quality and the climate.

This is a good example of the division of roles described in Chapter 3: the EU funds and owns the programme and ESA provides the technical expertise. Chapter 8 returns to it, and Chapter 9 looks at how the United Kingdom's access changed after it left the EU.

Exercises

Next chapter: People in Space, which turns to the European astronauts.