Page last updated at 12:00 GMT, Monday, 4 July 2005 13:00 UK

Q&A: Deep Impact comet mission

An image of Tempel 1 on impact (Nasa)
The flyby craft watched from a distance of about 500km
The US space agency's Deep Impact mission has punched a crater in the surface of a comet to reveal the ancient material beneath.

BBC News looks at how the mission worked and what scientists hope to learn from it.

How did the mission blast a hole in Comet Tempel 1?

The Deep Impact spacecraft carried a 370kg (820lbs) "impactor" into space, which it released directly into the path of Comet Tempel 1.

The impactor was a battery-powered craft that operated independently of the Deep Impact mothership for one day. After its release, the impactor took over its own navigation and manoeuvred into the path of the comet.

As Tempel 1 approached, it was travelling at 37,000km/h (23,000mph) relative to the impactor.

The projectile shattered on impact and excavated a crater in the comet's crust, releasing dust and gas from within and revealing "pristine" material beneath the surface.

The event took place some 130 million km from Earth.

What will Deep Impact do during the encounter?

After releasing its projectile into the path of Tempel 1 on Sunday, Deep Impact manoeuvred out of the way on to an "observation trajectory".

The "impactor" in sight of its target     Image credit: Maas Digital

This trajectory passed 500km (300 miles) from the comet at its closest approach and gave Deep Impact a front row seat as the impactor crashed into the solid part of Tempel 1 - its nucleus.

The Deep Impact flyby spacecraft was observing the collision and its aftermath with onboard science instruments.

These recorded information on the material blasted from the crater, and the structure and composition of the comet's interior. The impactor spacecraft was composed mainly of copper, because this metal is not expected to appear in the pristine material revealed by the collision.

Shields protected Deep Impact as it passed through the comet's dust tail. The flyby spacecraft then turned to look at the comet again, gathering additional information from the other side of the nucleus and observing changes in Tempel 1's post-collision activity.

Professional and amateur astronomers on Earth also observed the encounter, broadcasting their results over the internet.

What information will the flyby spacecraft obtain?

Deep Impact has two main science instruments: the High Resolution Instrument (HRI) and the Medium Resolution Instrument (MRI).

The HRI is the main science camera for the mission. It consists of a telescope, an infrared (IR) spectrometer, and a multi-spectral CCD camera.

In addition to taking normal images of the impact in the visible spectrum, the HRI provides an infrared "fingerprint" of the material from inside the comet's nucleus.

Now that this data has been collected, it can be analysed and compared with the known spectral signatures of materials likely to be found in comets. If matches are found, then identification of the material inside the comet would be accomplished.

The MRI takes pictures in the visible light spectrum and provides back-up for the HRI. It has a slightly wider field of view. Both instruments assist the spacecraft with navigation.

The impactor used simpler versions of the flyby spacecraft's hardware and software and sent back images of Tempel 1 up to three seconds from impact.

How are images and data returned to Earth?

The flyby spacecraft uses an X-band radio antenna to communicate with Earth; it listened to the impactor on a different frequency.

For most of the mission, the flyby spacecraft has communicated with the ground-based listening stations of Nasa's Deep Space Network. During the short period of encounter and impact, when there was an increase in the volume of data, overlapping antennas around the world were used.

Deep Impact transmitted the most important data to Earth immediately. The rest was to be sent over in the following week.

What do scientists hope to learn from this mission?

Many comets travel around the Sun in long, oval orbits, releasing dust and gas.

They consist of a solid, icy core - or nucleus - surrounded by a cloudy atmosphere called a coma and one or two tails.

Though scientists suspect that the material inside them is unchanged since the formation of the Solar System 4.6 billion years ago, very little is known about the true nature of this material. Neither do they know much about the structure of cometary nuclei.

Analysis of different forms, or isotopes, of hydrogen in water on Earth suggests that much of our H2O was brought here as ice on comets. Scientists think Earth was bombarded by asteroids and comets early in its history.

These mysterious objects are also thought to have seeded Earth with the complex organic molecules that gave rise to biology.

Therefore, by studying the pristine material excavated by Deep Impact's projectile, scientists will learn much about the building blocks of the Solar System and the origins of life.

video and audio news
See images of the comet's collision

Nasa probe strikes Comet Tempel 1
04 Jul 05 |  Science & Environment
Chase is on for comet spacecraft
28 Apr 05 |  Science & Environment
Comet mission's images are blurry
29 Mar 05 |  Science & Environment
Comet mission set for 2005 launch
15 Dec 04 |  Science & Environment
Nasa to crash probe into comet
27 May 01 |  Science/Nature

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