NEAR Shoemaker has successfully completed the first phase of
its exploration of Eros, focusing on global mapping from an
approximately circular 200 km orbit. The spacecraft is now in a
transfer orbit that will take it to its next stage of
exploration, mapping from 100 km orbit, that will start on April
12. Tucked in amidst the many thousands of images and infrared,
x-ray and gamma ray spectra, not to mention the hundred thousands
of laser returns, there is another data set garnered in the 200
km orbit that deals with an entirely different aspect of the
nature and history of Eros; the magnetic field.

NEAR
Shoemaker’s magnetometer is searching for a magnetic field
generated by Eros. So far, no asteroid has been shown
conclusively to produce a magnetic field, although there were
hints of such fields from the asteroids Gaspra and Ida that were
the targets of Galileo flybys in 1992 and 1993 respectively. Many
meteorites, which are pieces of asteroids that fall to Earth, are
magnetized, having picked up and retained magnetization from
their parent bodies. If Eros turns out to have a magnetic field
as strong as that of many meteorites, NEAR Shoemaker should
detect this field once close enough to the asteroid.

Most
magnetized objects, when studied from far enough outside their
surfaces, have a magnetic field like that of an ordinary bar
magnet, forming what is called a “magnetic dipole”. A compass
needle has such a field, and it has a “north-seeking” pole that
points to (magnetic) north on Earth. An arrow that points from
the south-seeking pole (S) to the north-seeking pole (N) of a
compass needle is aligned with the “magnetic dipole moment” of
the needle. This is a quantity with a direction (S to N) and a
magnitude defining the strength of the magnetic field (i.e., it
is a “vector”). The magnetic fields of the Sun and the Earth
have prominent magnetic dipole moments, as well as most of the
planets (all but Mars and Venus which have unmeasurably small
dipole moments, and Pluto whose dipole moment is unknown).
Earth’s own magnetic dipole moment is “upside down” — it points
roughly in the direction from geographic north to geographic
south.

Close to the surface of a magnetized body, it is
commonly found that the field is more complex than that of a
magnetic dipole, as if there were additional pairs of
north-seeking and south-seeking poles on the surface of the body
besides the main pair. Ordinary refrigerator magnets, for
example, have more complex fields than do simple bar magnets —
often the north-seeking poles and south-seeking poles of a
refrigerator magnet are arranged in alternating stripes. This has
the effect of concentrating the magnetic field close to the
surface where it’s needed to cause the magnet to stick to the
refrigerator. The magnetic field of Mars turns out to have some
similarities — the main dipole moment of Mars is almost or
completely absent, but there are numerous north-seeking and
south-seeking poles located in “magnetic stripes” in the southern
highlands.

The magnetic field strength of a magnetic dipole
has the property of decreasing as the inverse cube of the
distance from the body (aside from an angular dependence). That
is, at twice the distance, the field strength is reduced by 2x2x2
= 8. The field strength at Earth’s surface near the equator is
about 30000 nanotesla (nT). At one Earth radius above the
surface, the field strength has decreased by a factor of 8, and
by two radii above the surface (which is three times the distance
from the center) the field has decreased by a factor of 27. The
magnetic dipole has the property of being the arrangement of
magnetic poles that causes magnetic fields to stretch farthest
from the body — any more complex arrangement of poles would cause
field strength to decrease even more rapidly away from the
planet.

This is why NEAR Shoemaker has to orbit close to Eros —
to be able to detect any magnetic field that may be produced.
NEAR’s magnetometer can detect an external magnetic field of only
1 nT, but Eros would have to be almost as strongly magnetized as the
Earth in order to generate a 1 nT field as far away as the 200 km
orbit. An Eros surface field of hundreds to thousands of nT,
like that of many meteorites, would be detectable only in lower
orbits. There are complications of course – the spacecraft itself
generates a magnetic field, and the solar wind carries a magnetic
field — but so far, no Eros magnetic field has been detected.
Will Eros turn out to be magnetic? We shall see.

Andrew Cheng,
NEAR Project Scientist