NEAR's Magnetometer Investigation (MAG) will conduct the first closeup search for coherent bodywide magnetism at an asteroid. Tantalizing hints of magnetic signatures were detected during Galileo's flybys of Gaspra and Ida, so 433 Eros may have a measurable magnetic field.



Fluxgate Sensor & Electronics

Antenna Dish Assembly

Magnetometer Objectives

Eros is an S-asteroid, representative of the important class which dominates the inner asteroid belt. Scientists remain uncertain whether S- asteroids are undifferentiated bodies related to ordinary chondrite meteorites, or whether they are geochemically processed bodies akin to stony-irons. The issue is an important test of our understanding of the connection between asteroids and meteorites. If S-asteroids are unrelated to ordinary chondrites, then where do these most common of all meteorites come from? If the two are related, then why don't they look spectrally identical?

The NEAR magnetometer may also help determine whether small asteroids are solid fragments of rock with densities like those of meteorite samples, or whether they tend to be porous piles of collisionally fragmented rubble with considerable void space. Many meteorites show measurable magnetization, but if Eros were a randomly re-accreted pile of fragments, one would not expect a significant coherent bodywide magnetic field.

Magnetometer Description

To minimize costs and spacecraft impacts, no booms are used. The sensor is mounted approximately 1 meter from the main spacecraft bus, on the high gain antenna feed support.

The heritage of this magnetometer includes APL and NASA/GSFC instruments aboard Voyager, UARS, Geotail, WIND, Freja, Viking, Pioneer, POGS, Giotto, AMPTE/CCE, Mars Observer, and DMSP. The NEAR magnetometer sensor mounting configuration is similar to that used on the successful Giotto mission to Comet Halley.

Incorporated in the design is a complete calibration of the magnetometer as well as the effect on the instrument of spacecraft magnetic interference. Using proven methods and the statistics of nearly a year in orbit, even magnetic signals due to Eros that are smaller than spacecraft noise can be reliably isolated.

If Eros exhibits small natural remnant magnetization (NRM), the magnetic sphere of influence will be smaller than the 35-km NEAR rendezvous orbit, and the strength of the moment must be assessed indirectly by studying the magnetic perturbations associated with the Eros bow shock.

For intermediate NRM, the spacecraft may intermittently cross the Eros magnetopause, and magnetic fluctuation measurements will be used to assess these events.

For large NRM, the spacecraft will remain entirely within the Eros magnetosphere for extended periods of time.

Magnetometer Management

Magnetometer Science Team:
Mario H. Acuña (NASA/GSFC), Leader
Christopher Russell, UCLA
Magnetometer Instrument Scientist:
Lawrence J. Zanetti (JHU/APL)
Lead Engineer:
David A. Lohr (JHU/APL)
NEAR Payload Manager:
Robert E. Gold (JHU/APL)

Links to the other NEAR instruments:

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