The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.
Find out more about space activities in our 23 Member States, and understand how ESA works together with their national agencies, institutions and organisations.
Exploring our Solar System and unlocking the secrets of the Universe
Go to topicProtecting life and infrastructure on Earth and in orbit
Go to topicUsing space to benefit citizens and meet future challenges on Earth
Go to topicMaking space accessible and developing the technologies for the future
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Jupiter's four largest satellites, including Io, the golden
ornament in front of Jupiter in this image from NASA's Cassini
spacecraft, have fascinated Earthlings ever since Galileo Galilei discovered them in 1610 in one of his first astronomical uses of the telescope.
This true-colour composite frame, made from narrow angle images
taken on 12 December 2000, captures Io and its shadow in transit against the disk of Jupiter. The distance of the spacecraft from Jupiter was 19.5 million kilometres. The
image scale is 117 kilometres per pixel.
The entire body of Io, about the size of Earth's Moon, is
periodically flexed as it speeds around Jupiter. As a
result of its non-circular orbit, it feels the periodically changing gravitational pull of the planet. The heat arising in Io's interior from this continual flexure makes it the most
volcanically active body in the solar system, with more than 100 active volcanoes. The white and reddish colors on its surface are due to the presence of different sulfurous materials. The black areas are silicate rocks.