Space probe to begin comet quest

First comet encounter is next year

By Dr David Whitehouse BBC News Online science editor

Final pre-launch adjustments are being made to Contour, the American space agency (Nasa) probe that will chase down and study two comets.

We really have more ideas about comets than facts

Joseph Veverka, Contour's principal investigator

Contour (Comet Nucleus Tour spacecraft) will provide the first detailed look at the differences between these primitive building blocks of the Solar System.

If the countdown goes well, Contour will be taken into space by a Boeing Delta 2 rocket no earlier than 3 July. The spacecraft will orbit Earth until 15 August, when it will then fire its main engine and begin its comet quest.

It will rendezvous with comets Encke in 2003 and Schwassmann-Wachmann-3 in 2006.

"Contour will be Nasa's second mission dedicated solely to exploring these largely unknown members of our Solar System," said Colleen Hartman, director of the Solar System Exploration Division at Nasa.

Closer to the comet

"Contour joins our other operating mission, Stardust, which is on its way to bring a sample of a comet back to Earth," she added.

Contour will make frequent Earth flybys

"Next year, the Deep Impact mission will launch to join our fleet of comet-exploring spacecraft. These missions all help us find answers to the fundamental questions of how our planet may have formed and evolved, and how life may have began on Earth and perhaps elsewhere in the Universe."

The target comets were chosen because of their relative closeness to Earth during encounter time - less than 50 million kilometres (31 million miles).

Comet Encke has been seen from Earth more than any other comet. Researchers believe it is a relatively old object that gives off comparatively little gas and dust.

Inside look

In contrast, Comet Schwassmann-Wachmann 3 recently split into several pieces, intriguing scientists with hopes that they might see fresh, unaltered surfaces and materials from inside the comet.

Last minute preparations

"The key to the Contour mission is to visit a diverse range of comets, from an evolved comet such as Encke, to a younger comet like SW3, or even a new comet never seen in this part of the Solar System," said Mary Chiu of the Johns Hopkins University Applied Physics Laboratory.

When it leaves the Earth in August, Contour's orbit will loop around the Sun and back to Earth for a gravitational slingshot manoeuvre to take the probe on to its targets. The procedure alters Contour's trajectory and helps it reach several comets without using much fuel.

Contour will fly as close as 100 kilometres (62 miles) to the heart of each comet. To protect it against debilitating high-velocity impacts, it has a five-layer dust shield of heavy metallic fabric.

Close-up pictures

"Comets are the Solar System's smallest bodies, but among its biggest mysteries," said Joseph Veverka, Contour's principal investigator.

"We believe they hold the most primitive materials in the Solar System and that they played a role in shaping some of the planets - but we really have more ideas about comets than facts."

Contour will travel closer to a comet nucleus than any spacecraft ever has before. Its four scientific instruments will take pictures and measure the chemical composition of the comets' nuclei as well as the surrounding gases and dust.

Its main camera, the Contour Remote Imager/Spectrograph, will provide high-resolution images showing rocks and other features on a nucleus as small as four metres (13 feet) across.