Because comets and asteroids are relatively unchanged leftovers from the solar system formation process, it is important to discern their structures and chemical makeup. These objects formed under the same conditions as did the planets, but unlike the planets, they remained relatively unchanged since their formation. Thus, knowledge of the chemical composition and structure of these objects should offer clues as to the chemical mix and conditions under which the solar system's planets formed 4.6 billion years ago. In addition, information on their compositions and structures will be important should one of them be discovered to be on an Earth threatening trajectory. Knowledge of their compositions and structures will also be important in order to make intelligent choices as to which objects would offer the richest sources of raw materials.
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.
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.
Spacecraft Missions to Comets and Asteroids
Near-Earth Asteroid Rendezvous (NEAR)
Launch: February 17, 1996
Asteroid Mathilde Flyby: June 27, 1997
Asteroid Eros Initial Flyby: December 23, 1998
Asteroid Eros Rendezvous: February 14, 2000
The NEAR mission flew within 1200 km of asteroid Mathilde and spent nearly one year in orbit about asteroid Eros in 2001-2001.
Mission Web site: http://near.jhuapl.edu/
Launch: December 30, 2004
Comet Tempel 1 Impact/Flyby: July 4, 2005
Deep Impact mission will impact the surface of comet Tempel 1 thus creating a fresh crater larger than the size of football field and deeper than a seven-story building. The spacecraft will study the crater formation process and examine the subsurface structure of one of the solar system's most primitive objects, a remnant from the outer solar system formation process.
Mission Web site: http://deepimpact.jpl.nasa.gov
DEEP SPACE 1
Launch: October 25, 1998
Asteroid 9969 Braille Flyby: July 28, 1999
Comet Borrelly Flyby: September 22, 2001
The primary Deep Space 1 mission objectives were to test space technologies. The spacecraft flew within 2000 km of Comet Borrelly on September 22, 2001.
Mission Web site: http://solarsystem.nasa.gov/missions/profile.cfm?MCode=DS1
Launch: February 6, 1999
Comet Wild-2 Flyby: January 2, 2004
Earth Sample Return: January 15, 2006
The STARDUST spacecraft will image the nucleus of Comet Wild-2, collect dust from both the comet's coma and from interplanetary space and bring these dust samples back to Earth for study.
Mission Web site: http://stardust.jpl.nasa.gov
Launch: December 2002
Asteroid 25143 Itokawa Rendezvous: September 2005
Earth Sample Return: June 2007
A cooperative mission between Japan and the U.S., the Hayabusa spacecraft rendezvoused with near-Earth asteroid (25143) Itokawa and return asteroid surface samples to Earth for analysis. Itokawa, a 600 meter sized, potato-shaped asteroid, is named after Hideo Itokawa, a Japanese rocket pioneer.
Hayabusa Project (JAXA main site): http://www.isas.ac.jp/e/enterp/missions/hayabusa/index.shtml
Launch: March 2, 2004
Comet Churyumov-Gerasimenko Rendezvous: May 2014
After three Earth gravity assists and a Mars gravity assist, the Rosetta spacecraft rendezvoused with, landed upon, the surface of a comet in an effort to study its composition and structure.
Mission Web site: http://sci.esa.int/rosetta
Launch: June 20, 2007
Asteroid Vesta Rendezvous: 2011
Asteroid Ceres Rendezvous: 2015
The Dawn mission will orbit two asteroids on a single voyage. Ceres and Vesta evolved under radically different circumstances in different parts of the solar system more than 4.6 billion years ago. By observing both protoplanets with the same set of instruments, Dawn will provide new insight into the formation and evolution of our solar system.
Mission Web site: http://dawn.jpl.nasa.gov/
To be continued...
NASA's Near-Earth Object (NEO) Program coordinates NASA-sponsored efforts to detect, track and characterize potentially hazardous asteroids and comets that could approach the Earth. To learn more, visit the home page of NASA's Near-Earth Object Program: http://neo.jpl.nasa.gov/