SPACECRAFT MISSIONS TO COMETS AND ASTEROIDS - OVERVIEW

Spacecraft missions are required to understand the detailed chemical compositions and structures of comets and asteroids. There are wide differences between comets and asteroids and even wide differences between different types of asteroids. Some asteroids are likely to be fragile and rich in carbon-based molecules while others are thought to be solid iron. Whether looking for the richest source of raw materials or trying to nudge an Earth threatening object out of harms way, it makes a big difference whether we're dealing with a 50-meter sized fluff ball or a one-mile slab of solid iron. Because comets and asteroids differ so widely in their characteristics, missions have been planned to visit a widely diverse group of objects. After a brief summary of various spacecraft instruments, the seven current missions to comets and asteroids are briefly described.

Typical Spacecraft Instruments:

• Imager: An imager is a television camera system using a charged-coupled device (CCD) detector (the same type of detector in a camcorder).

• IR Spectrometer and UV Spectrometer: This infrared (IR) detector measures the light received in several different infrared portions of the light spectrum. In so doing, this instrument can often infer the minerals resident in a neighboring comet or asteroid. The ultraviolet (UV) spectrometer measures a comet's light in several narrow regions of the ultraviolet spectral region in an effort to identify gases that emit radiation in this region of the spectrum.

• Lidar: The Lidar instrument sends a pulse of light to the target body and measures the round trip time required for the light signal to travel to and from the bounce point on the target body. By measuring this so-called "light travel time," the distance between the spacecraft and target body can be accurately determined. This instrument is often used to define the shape of the target body and to help navigate the spacecraft when it is in close proximity to the target body.

• X-Ray Spectrometer, Gamma Ray Spectrometer, Alpha/X-ray Spectrometer: These spectrometer instruments are all designed to determine the chemical (elemental) composition of the target body's surface materials (what is the chemical makeup of the surface rocks and soils?). When highly energetic radiation or particles fall upon the asteroid's surface materials, each type of atom in these materials responds by emitting radiation in a characteristic frequency. These characteristic frequencies then identify the atoms that are present in the surface soils. X-rays from the sun provide the incident radiation for the x-ray spectrometer. High-speed atomic nuclei (cosmic rays) from space are responsible for inducing the high frequency radiation measured by the gamma ray spectrometer. The alpha particle spectrometer measures the radiation resulting from the bombardment of helium nuclei (alpha particles) upon the asteroid's surface soils; a radioactive source material (e.g., Curium) in the instrument itself provides the source for these alpha particles.

• Impact dust mass spectrometer: Once a dust particle hits this detector at high velocity, the resulting charged particles (ions) interact with a magnetic field within the instrument. Measuring the amount their subsequent trajectories are modified by this magnetic field then identifies the specific ions.

  [Illustration of Brownlee particle]

• Magnetometer: This instrument measures the intrinsic or induced magnetic field of the target body. The charged particle environment in a comet's atmosphere can trap solar magnetic fields and some asteroids, in analogy with the Earth, may have magnetic fields due to their metallic cores.

• Plasma Package: A plasma is a cloud of electrically charged atoms or molecules. Plasma instruments are designed to measure the densities and characteristics of plasma clouds that are created when the neutral molecules in cometary atmospheres become charged as a result to the sun's radiation. Plasma instruments will also be used to monitor the plasma (ion) engines used to drive the DS1 spacecraft.