JPL and NASA News

Bill Wheaton, IPAC

1998 November

NEAR Approaches 433 Eros:

The Near Earth Asteroid Rendezvous will perform its first rendezvous burn at 20:00 UTC on 20 December, leading (after 3 more adjustments) to Eros Orbit Insertion on 10 January, nearly 3 years after its launch on 17 February, 1996. NEAR is the first mission to actually orbit an asteroid. The history of Solar System exploration suggests that the transition from brief fly-by missions to orbiters is typically accompanied by a qualitative leap in our understanding. Lunar Orbiter for the Moon, Mariner 9 for Mars, Magellan for Venus, and Galileo (crippled though it is by the grievous loss of its high-gain antenna) for Jupiter -- each has produced a disproportionate gain in our knowledge, by replacing observations sampled in time and space by a much more global view. Eros is one of the most interesting asteroids, because it is both fairly large and yet also approaches the Earth as closely as about 20 million km at minimum. (Its 1.76 year orbit carries it out 1.78 AU, into the innermost part of the Main Belt, and brings it to perihelion at only 1.13 AU.) Readers may remember also NEAR's Earth gravity-assist flyby on 23 January, 1998, which brought it down the east coast of South Africa (unfortunately during daylight hours, so it could not be seen) a few minutes after its closest approach to Earth over Iran.

NEAR trajectory is in red; orbits of 433 Eros and 253 Mathilde are green.

Because so little is known about the environment, the approach must be performed with some care, lest NEAR collide with a satellite of Eros, or with debris orbiting in its gravitational sphere of influence. Also, Eros is the first body that is not even axially symmetric ever to be orbited by a spacecraft. Like most asteroids, Eros is far too small for gravity to have crushed its materials into a spherical shape; its estimated dimensions are about 14 X 14 X 40 km. The slight polar flattening of the Earth (1 part in 297) has an important effect on the motion of low-earth satellites, causing the orbits to precess. Eros, however, is grossly unsymmetrical. If the size of the orbit is large (say, 1000 km), the effect becomes small and spacecraft will orbit in nearly the familiar Keplerian way, save for perturbations by the Sun. But when the orbit is close to the primary, there are at least two profound effects. First, even if Eros were non-rotating, a close orbit would in general trace a complex non-periodic curve. Conservation laws would limit it to a certain allowed range of distance, which the orbit would probably more or less fill without ever repeating itself exactly.

Second, however, Eros, shaped something like a dumpy cigar, seems to be in a flat spin about its shorter (14 km) axes. Evidently the gravitational potential near such a rotator is not constant, so that the spacecraft's energy and angular momentum will not even be conserved. In principle the spacecraft might either gain or lose energy and angular momentum (at Eros's expense, of course) so that some orbits escape, and others crash. It seems that navigation of NEAR without having to use a great deal of propellant is not quite trivial, and the details cannot even be specified with precision until the gravitational potential (dependent on the detailed shape and distribution of mass) is measured. In any event, the current mission plan calls for initial orbit insertion into a 14,285 km X 400 km orbit on 10 January, 1999, followed by a change to a 401 km X 200 km orbit on 20 January, and then to 205 X 195 km on 27 January. Clearly initial characterization of Eros's gravitational properties will be a high priority during this period. Later plans are to bring the orbiter down to about 100 km in mid-February, to 50 km in mid-March, and then to 35 km in April. Finally, sometime before the nominal end-of-mission on 6 February 2000, the plan is to actually attempt to land the orbiter at a point to be selected based on the information previously gathered.

NEAR will conduct six experiments. The Multispectral Imager (MSI) will return images in one broad band plus seven spectral channels between 0.45µ and 1.05µ, with about 10 m spatial resolution at a range of 100 km. Imaging is of course crucial in the sense that the manifold significance of the size, shape, texture, and color information MSI will return gives a kind of framework for interpreting many of the clues provided by the other instruments. The color bands have been chosen to permit discrimination among the common minerals found in meteorites, and expected on asteroids. The scanning Near-IR Spectrograph (NIS), which yields spatially resolved spectra, operates in two overlapping spectral bands with 32 spectral channels each. The first extends from 0.804µ to 1.506µ, and the second from 1.348µ to 2.732µ. The instrument will scan slowly over a 140° range, giving spectra in successive spots with a size of down to about 300 m, depending on the distance to Eros. By repeated scans, detailed maps of spectra and hence mineralogy can be built up. Since the visible and IR spectra of minerals have been extensively studied, NIS should be the definitive source of information about the mineralogy of the surface, although with lower spatial resolution than MSI.

The X-ray/Gamma-Ray Spectrometer (XGRS), will map the distribution of various elements and isotopes down to a depth of several centimeters. X rays from the Sun, and high-energy particles from both cosmic rays and solar flares, stimulate the emission of fluorescent atomic X rays and nuclear gamma rays. Thus this instrument studies atomic and isotopic composition independent of chemistry. Because gamma rays are penetrating, it also gives a compositional average over the top half meter or so of the surface, complementary to the surface information provided by the optical and IR observations. This instrument is also sensitive to cosmic gamma-ray bursts and a key member of the Interplanetary Network of burst detectors which, by intercomparison of burst arrival times at different spacecraft, gives us our most precise knowledge of their celestial positions.

The Magnetometer (MAG) will characterize the magnetic field of Eros. Even more than the gamma-ray spectrometer, it is sensitive to the deep interior properties. If Eros contains a large fraction of iron-nickel alloy, as expected for a Type "S" asteroid, then much will depend on whether it preserves the magnetization of some earlier parent body, to what extent it has been heated since it was magnetized (since iron loses its magnetization if sufficiently heated), and whether it consists of a few large pieces or an incoherent jumble of small fragments. If the field is strong enough that the magnetosphere extends well out from Eros, then NEAR may orbit through it or even entirely within it, allowing direct investigation of the bowshock, transition region, and magnetotail, assuming these exist.

The Laser Rangefinder (NLR), will give detailed information (range accuracy: 6 m) about Eros's shape and rotational dynamics, by bouncing pulses off the surface and timing their return. Finally, the Radio Science experiment requires no instrumentation beyond the spacecraft X-band (8.84 GHz) communications equipment. It can measure the spacecraft velocity along the line of sight with an accuracy of 0.1 mm/sec. By precise measurement of NEAR's orbit it will permit the mapping of the gravity and hence mass distribution of Eros. As described above, the information it will provide is crucial to management of the orbit and the mission, besides its scientific importance. Next month I hope to have more to say about the asteroid science NEAR will pursue, and why Eros is likely to become a kind of Rosetta stone for interpreting much data we have already collected.

NEAR has been designed, built, and managed by the Applied Physics Laboratory (APL), which has a long and illustrious history of building small but productive spacecraft, at Johns Hopkins University. Much more information about all aspects of the project can be found at:
 
http://sd-www.jhuapl.edu/NEAR/

Follow-ups:

I regret an error in my report on SIRTIF, the Space Infrared Telescope Facility, last month. The 85 cm telescope for SIRTF is actually being constructed by Ball Aerospace, not by Lockheed-Martin as I stated. As I write Deep Space 1 (July Canopus) is mounted on its Delta II launch vehicle at the pad at Cape Canaveral; by the time you read this it should be on its way.

Bill Wheaton
Waw@ipac.caltech.edu