Historic images are the dramatic climax of a 25-year dream

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The Independent Online

The Cassini-Huygens mission was launched on 15 October 1997, from Cape Canaveral Florida, but the origins of the £2bn project go back a quarter of century. It is a joint collaboration between the Nasa and the European Space Agency.

The Cassini-Huygens mission was launched on 15 October 1997, from Cape Canaveral Florida, but the origins of the £2bn project go back a quarter of century. It is a joint collaboration between the Nasa and the European Space Agency.

During its seven-year journey to Saturn and its moon Titan, the spacecraft travelled some two billion miles in a series of elliptical moves. To help it on its way, the spacecraft made four "gravity assist swing-bys" ­ two around Venus and one each around Earth and Jupiter ­ when it used the planets' gravitational pull to give it successive, cumulative boosts on its way.

Cassini, the mother spacecraft for the Huygens probe, is about as tall as a two-storey house and is one of the largest and heaviest interplanetary spacecraft ever built ­ only two Soviet built spacecraft were heavier. It arrived at Saturn last July when it successfully went into orbit. Cassini is due to continue orbiting and taking measurements of the planet and its complicated ring system for a further four years.

On Christmas Day, Cassini released the Huygens probe, which is about the size of a domestic washing machine. Powerless and without direct control from Earth, Huygens took a further 22 days to reach Titan, the largest of Saturn's moons.

The probe is named after Christiaan Huygens, a 17th century Dutch astronomer who discovered Titan in 1655. It has an outer carapace protecting an interior packed with six delicate instruments for making measurements of the moon's atmosphere and surface.

On its arrival at the outer fringes of Titan's atmosphere, a series of alarm clocks woke up the Huygens probe from its seven-year slumber. Travelling at 11,200mph, it made its initial entry into the dense atmosphere of the moon ­ which is ten times as thick as that on Earth ­ at an altitude of 789 miles above the surface. Its protective heat shield and set of three parachutes slowed the spacecraft down during its descent to the surface of Titan.

Pilot parachutes and friction with the atmosphere slowed the probe enough for the larger main parachute to be deployed. The probe would have reached the surface travelling at a relatively sedate 12mph.

After the heat shield fell away, the main scientific instruments were exposed, allowing them to gather crucial data ranging from the composition of the surrounding gasses to the velocity of sound. As it descended the probe rotated to allow its cameras to scan panoramic views. Its cameras ­ which took images in digital slices that have to be reconfigured by computers ­ took more than 1,000 pictures.

Before yesterday's events, Professor Ian Halliday, chief executive of the Particle Physics and Astronomy Research Council, said the ambitious targets of the Huygens mission should not be underestimated. "Superlatives can come easy when talking about space missions but this particular voyage of scientific discovery is truly awesome," Professor Halliday said.

"Titan is a mysterious place and raises many scientific questions. Its thick atmosphere is mostly nitrogen, but there are also methane and many other organic compounds. Some of them would be signs of life if they were on our planet.

"Organic compounds form when sunlight destroys methane. If sunlight is continuously destroying methane on Titan, how is methane getting into the atmosphere," he said.

Scientists believe the composition of Titan's atmosphere resembles that of Earth's four billion years ago, when life had yet to evolve here. Monica Grady, a planetary scientist at the Natural History Museum in London, said: "Because of the similarities in the atmospheres, it will give us a fascinating analogue of the type of reactions that would have been happening on Earth."

Scientists calculated that the Huygens probe had a 70 per cent chance of surviving the soft landing. Yesterday's signals show that it has survived the most dramatic fall in the history of interplanetary exploration.


4 October, 1957: Sputnik 1, the first man-made object to orbit the Earth, is launched by the USSR.

12 April, 1961: Yuri Gargarin becomes the first man in space aboard Vostok 1. He orbits the Earth once.

20 July, 1969: Neil Armstrong and "Buzz" Aldrin make the first landing on the Moon using Apollo 11.

15 December, 1970: Soviet Venera 7 is the first probe to land on Venus, transmitting for 23 minutes.

2 March, 1972: Pioneer 10 is launched. It returns the first close-up images of Jupiter in 1973.

5 April, 1973: Pioneer 11 is launched, flying past Saturn in September 1979, where it discovers new rings.

3 September, 1976: Viking 2 lands on Mars where it discovers water frost.

August-September, 1977: Voyager 1 and 2 leave Earth to meet with Jupiter in 1979 and Saturn in 1980.

12 April, 1981: The first manned mission of the original space shuttle, Columbia, is launched.

2 July, 1985: The European Space Agency launches the Giotto spacecraft. It encounters Halley's Comet in 1986.

January, 1986: Voyager 2 flies past Uranus.

24 April, 1990: The space shuttle Discovery deploys the Hubble space telescope.

7 December, 1995: The Galileo spacecraft drops a probe into Jupiter's atmosphere.

31 March, 1997: Contact with Pioneer 10 is terminated. The probe has travelled 6.7 billion miles from Earth.

14 February, 2001: The NEAR probe lands on the asteroid 433 Eros.

25 December, 2005: Scientists lose contact with British-built Beagle 2 after it lands on Mars.

18 May, 2004: Robotic exploration rover Opportunity finds signs of water on Mars.