The NEAR spacecraft is on its way to an asteroid named for the god of love. Once it arrives in 1999, scientists expect to learn about the role asteroids played in forming our solar system as well as the physics and chemistry of the solar system bodies that hit and left their marks on the surfaces and atmospheres of all the planets.
by Lucy A. McFadden, Department of Astronomy, University of Maryland, College Park, Md.; Christopher T. Russell, Department of Earth and Space Sciences, University of California, Los Angeles, Calif.; and Andrew F. Cheng, Applied Physics Laboratory, Johns Hopkins University, Laurel, Md.
Fig. 1. Near-Earth asteroid mission travels to 433 Eros
Just after Valentine's Day the Near-Earth Asteroid Rendezvous (NEAR) spacecraft embarked on a 3-year-long journey to the asteroid Eros (see figure 1) to study its geophysical state and its geochemistry. The spacecraft, which is cooled by exposure to the low temperatures of space and powered with fixed solar panels (see figure 2), was built by the Johns Hopkins Applied Physics Laboratory. To further our understanding of asteroids and their role in the formation of the solar system, the spacecraft will orbit Eros for a year after its trajectory through the inner solar system (see figure 3). Once it arrives in February 1999, NEAR will use five scientific instruments to make measurements and quickly transmit data to Earth through an antenna.
Fig. 2. Schematic of the NEAR spacecraft showing its basic components, the solar panels providing power to run the spacecraft and its scientific instruments, the 1.5-m antenna which transmits data to Earth, one of four rocket thrusters which are used to maneuver the spacecraft, and the instrument platform where all the instruments are mounted. Note, the magnetometer is mounted above the radio antenna. The computers controling the spacecraft and instruments cannot be seen.
G. Witt discovered 433 Eros, the first asteroid found to cross Mars' orbit and to approach Earth's, in 1898. With diameters of approximately 40 x 14 x 14 kilometers, Eros is the second largest planet-crosser and larger than the Martian moons Phobos and Deimos. The asteroid was named for the son of Aphrodite, Eros, the god of love. The mortal beauty Psyche, Eros' wife, was forbidden to gaze upon him by the light of day. Psyche's curiosity eventually got the best of her, and she defied the order. Like Psyche, scientists, driven by curiosity and daring to break barriers, in this case technological, have sent a spacecraft to Eros. They will gaze at the asteroid with their instruments.
Fig. 3. The path NEAR will take during its journey to the asteroid 433 Eros.
Eros was selected as a target for space exploration because of its proximity to Earth, which makes the mission less expensive in terms of fuel expenditure and spacecraft mass, both large factors in the monetary cost of a mission. The data NEAR collects will expand our knowledge of the small bodies near Earth. We will learn more about the role asteroids have played in forming our solar system as well as the physics and chemistry of the impactors that have affected the surfaces and atmospheres of all the planets. The instruments on board NEAR may allow us to establish the relationship between ordinary chondrite meteorites, those most abundant on Earth, and S-type asteroids found in the inner portion of the main asteroid belt. These asteroids seem to be composed of the same minerals as the meteorites, minerals called olivine, pyroxene, and metallic iron. Data from NEAR may settle the debate over whether S-type asteroids are chemically and mineralogically similar to the ordinary chondrites or whether they are instead the source of more chemically evolved bodies. If they are like the ordinary chondrites, then they are examples of the starting material from which the solar system formed. If they are more evolved, then we infer an additional heat source in the asteroid belt after the early stages of formation.
The mission will provide scientists with the most comprehensive analysis to date of the surface and interior of any solar system body beyond the Moon, and it will undoubtedly force them to modify their models of solar system formation. It is also an experiment in paring down costs; its development and first 30 days of operation will cost less than the $150 million budgeted. For comparison, the Galileo mission to Jupiter cost more than a billion dollars.
The NEAR mission will achieve a number of both scientific and technological firsts. Perhaps its most daring technological accomplishment will be its navigation in orbit around a highly irregularly shaped planetesimal. Eros will be the smallest body in the solar system to have its mass measured and its gravity field mapped from orbit. It will also be the first small body to have its elemental composition probed with X ray and gamma ray spectrometers, to have its shape measured, and to be magnetically surveyed.
Because NEAR launched on time, it will fly by the 60-km diameter, main belt asteroid named 253 Mathilde in June 1997. This target of opportunity is anticipated with excitement. It could give the planetary community its first glimpse of the surface of an asteroid that is similar in brightness—it is very dark—and color to the most typical of the main belt asteroids. These asteroids are designated C-types after their similarity to carbonaceous chondrites. Because Mathilde's colors appear gray in the visible and near-infrared, it is difficult for scientists to deduce which minerals its surface is composed of as there are many candidates. Who knows what surface features might appear on the surfaces of dark, main belt asteroids? Recent ground-based brightness measurements indicate that Mathilde has the third-longest rotation period of any known asteroid: 417 hours. It is difficult to imagine how such a long rotation period is possible. Some process is reducing this asteroid's rotation rate. Mission scientists eagerly await images of Mathilde that might provide some explanation.
Just after launch, the engineers performed their spacecraft check-out and proceeded to turn on each instrument. The Multispectral Imager (MSI) pointed toward the Moon and took pictures to compare the MSI response with the known properties of the lunar surface. This is known as a calibration procedure. We also had a chance to image the bright comet Hyakutake at the end of March, again primarily for calibration purposes. Back on Earth, while the spacecraft is en route to Eros, scientists and mission planners are finalizing the sequence of observations that will be made in 1999. The MSI will be used for optical navigation as the spacecraft approaches Eros and also to determine the asteroid's shape. It will return images from which one can distinguish the difference between features that are 6 meters in size. The MSI will also be used to search for satellites during the approach phase. Our interest in this search is piqued by the Galileo spacecraft's unexpected discovery of a satellite named Dactyl at the asteroid Ida in August 1993.
After the spacecraft goes into orbit about Eros, its position will be monitored, and Eros' total mass and distribution of mass will be derived from the radio science experiment using the spacecraft's main antenna. A model of the asteroid's interior structure will be made as the spacecraft accelerates around the asteroid. The MSI and NEAR Laser Rangefinder (NLR), which sends out a laser beam and times how long it takes to get to the asteroid and back to the spacecraft will then be used to build and shape models of the asteroid. Once we know the shape, we can compute its volume and then, with the mass determination, the bulk density, the measure of mass per unit of volume, will be derived. Data on Eros' mass distribution will shed light on one of the fundamental debates in the planetology community: Are asteroids solid fragments, or are they loosely bound conglomerate of fractured debris? Knowledge of the bulk density will constrain these hypotheses.
While in orbit, NEAR's magnetometer, a Near-Infrared Spectrometer (NIS), and an X ray and a gamma ray spectrometer, will begin mapping. The magnetometer measures the orientation of the magnetic field, revealing whether the asteroid has an intrinsic or remanent field. This knowledge will help determine the possible temperature range to which the asteroid was initially heated when it formed. The NIS will measure reflected sunlight in a region sensitive to electronic transitions in major rock-forming minerals. The distribution of minerals holds clues to what goes on at the asteroid's surface.
As the spacecraft is lowered into closer and closer orbits, the X ray spectrometer will map the resonance fluorescence spectra from elemental Mg, Al, and Si, S, Ca, Ti, and Fe. Two solar monitors will continuously measure the X ray output from the Sun so that the abundance of elements can be measured. The gamma ray spectrometer will independently measure the abundance of elements by counting emissions from elements that are stimulated by cosmic rays and energetic solar particles. Naturally occurring gamma radiation from K, U, and Th will also be measured.
After NEAR gazes at Eros for a year, enlightening mortal scientists with a deeper understanding of the early solar system, what will be the spacecraft's fate? If NEAR is like Psyche and mythology holds true, the spacecraft will crash to the surface, and the two will remain tightly bound forever. Celestial mechanics, however, provides an alternate ending in which the spacecraft is ejected from its chaotic orbit in Eros' gravitational sphere. This scenario might present an opportunity for the spacecraft to travel to other asteroids and extend the mission.
For more information about the NEAR mission, access the NEAR home page on the World Wide Web at http://hurlbut.jhuapl.edu:80/NEAR/
Source: Eos, Feb. 20, 1996, p. 73.
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