Contour (Comet Nucleus Tour), USA

Contour is part of NASA's Discovery programme and is a joint project between the Johns Hopkins University Applied Physics Laboratory (APL), Maryland, and Cornell University, New York. Its aim is to send a small spacecraft to examine comets Encke and Schwassmann-Wachmann 3 and bring samples back to Earth. Other participants in the $154 million project are Von Hoerner & Sulger and NASA's Goddard Space Flight Center.

THE CONTOUR SPACECRAFT

The design of the spacecraft is simple and has few articulated mechanisms. The solar array is body mounted and does not require drive motors. The mission geometry allows it to use fixed, passive, existing antenna designs. Other than the Contour Remote Imager Spectrograph (CRISP), all instrument and antenna pointing is controlled by moving the spacecraft. A dust shield made of Nextel and Kevlar protects against impacts for the dust sizes and densities expected at the three encounters.

THE CONTOUR MISSION

On 12 November 2003, the Contour spacecraft will visit Encke, which has the shortest orbital period of any known comet - 3.2 years to circle the sun. Schwassmann-Wachmann 3 has probably come close to the Sun less often than Encke has so it is likely that a smaller fraction of its ice has evaporated. Several large pieces split off the nucleus in 1996, and it is expected that when Contour flies by on 18 June 2006, it will probably see areas of exposed, relatively unaltered subsurface material.

Three Earth-gravity assist manoeuvres will be used to achieve the two comet encounters over the three year period. The mission design is flexible enough to allow the spacecraft to visit and examine any as yet unknown comets that come close enough to the Earth. To identify suitable candidates for interception as early as possible, the Contour project will support a worldwide early warning search programme for comets approaching from the fringes of the solar system.

CONTOUR REMOTE IMAGER/SPECTROGRAPH (CRISP)

The Contour Remote Imager/Spectrograph (CRISP) weighs 8.6kg and uses 29.1W of electricity. CRISP will actively track the comet's nucleus near the spacecrafts closest approach, taking high resolution images, colour images through ten filters and an infrared spectral map. The imager can pick up wavelengths from 450 to 770nm and takes pictures with a resolution of 1,024 by 1,024 pixels. The spectrograph picks up wavelengths of 800 to 2,550nm and its picture pixel resolution is 256 spatial by 256 spectral.

CRISP's scan mirror is the critical component that allows the instrument to take high-resolution images (as good as 4m per pixel) while passing as close as 100km from its target at high velocity. The imager and spectrometer both 'look' at a scan mirror. The mirror steers the fields of view to track the comet nucleus and take out its relative motion so that images can be obtained without smears.

OTHER CONTOUR COMPONENTS

The Contour Aft Imager (CAI) weighs 1.8kg and uses 2.5W. Both the CRISP and the CAI will be supplied by the John Hopkins University APL.

The Dust Analyser (CIDA) weighs 11kg and requires 2.5W and is being supplied by Von Hoerner & Sulger.

The Neutral Gas Ion Mass Spectrometer (NGIMS) weighs 9.3kg and uses 23.3W. It will measure the abundance and isotope ratios for many neutral and ion species in the coma of each comet during the flyby. These measurements together with data from the dust experiment will contribute to the understanding of the chemical composition of the nucleus itself and allow differences between the comets to be studied. This instrument will be supplied by NASA's Goddard Space Flight Center (GSFC).