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(meteorobs) [ASTRO] NEAR Shoemaker Science Update - April 18, 2000
Marginally related to meteorics, but darn interesting results this week!
Lew
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From: Ron Baalke <BAALKE@KELVIN.JPL.NASAdot gov>
Date: Thu, 20 Apr 2000 15:29:35 GMT
Subject: [ASTRO] NEAR Shoemaker Science Update - April 18, 2000
NEAR Shoemaker Science Update
April 18, 2000
http://near.jhuapldot edu/news/sci_updates/000418.html
As NEAR Shoemaker descends ever closer to Eros, the
spacecraft's orbit becomes ever more sensitive to the
details of the gravity field produced by the asteroid.
Just as NEAR Shoemaker must orbit close enough to Eros
to detect any magnetic field from the asteroid (April 7
update), it must also get close to Eros to feel
disturbances from the irregular shape of the asteroid
and to search for any mass concentrations or voids
within it. Both the gravity investigation and the
magnetic field investigation are studying the interior
of Eros, whereas the other investigations - imaging,
laser ranging, infrared, x-ray and gamma ray
spectroscopy - study the surface. The surface for gamma
rays is much deeper than that for visible light (more on
that another time), but even gamma rays see only some
ten centimeters deep.
Although both gravity and magnetism have the property of
decreasing in field strength away from the source of the
field, they are fundamentally different from each other.
As was mentioned on April 7, the simplest possible
configuration of magnetic poles is the dipole consisting
of one north-seeking pole paired with one south-seeking
pole. Isolated magnetic poles (e.g., north-seeking only)
do not exist. However, the exact opposite is true for
gravity. The simplest configuration of gravity is that
of an isolated "pole", which we actually call a "point
mass", and gravitational dipoles do not exist. This is a
fancy way of saying something that everyone knows,
namely, that all masses attract one another by gravity.
This is different from the situation with magnetic poles
- there are two types (north-seeking and south-seeking),
of which opposite types of pole attract each other, but
like poles repel.
The simplest possible gravity field is that of a point
mass which has no structure whatsoever. It turns out
that any spherical mass distribution produces the same
gravity field above its surface as it would if all its
mass were concentrated at the center (making a point
mass there). This simplest possible gravity field obeys
the familiar inverse square law, where the field
strength decreases as the inverse square of the distance
from the center. Since planets like Earth have almost
spherical mass distributions, planetary gravity fields
are very close to those of point masses.
We now know that Eros is not at all close to spherical,
so neither is its gravity field. Since there is no such
thing as a gravitational dipole, the next simplest
gravity field configuration is what we call a
"quadrupole". The degree of distortion of the shape from
spherical is measured by the "quadrupole moment" which
is analogous to the dipole moment mentioned on April 7,
but quadrupoles are more complicated than dipoles, and
indeed they are too complicated to be described as
ordinary vectors. There is more to a quadrupole than one
magnitude and one direction, because there are many ways
to distort a sphere by squashing it flatter or
stretching it into a cigar shape (both of which are
examples of quadrupoles).
We have now encountered the three most basic
configurations of fields - the familiar point mass field
(also called a "monopole field"), the less familiar but
still friendly dipole field, and now the quadrupole
field. The monopole field decreases as the inverse
square of the distance from the center; the dipole field
decreases as the inverse cube as we saw on April 7; and
the quadrupole field decreases as the inverse fourth
power of the distance. Again, quadrupole fields have a
characteristic angular dependence that is distinct from
those of the dipole and the monopole fields (the latter
is spherical).
So the nonspherical shape of Eros distorts its gravity
field, creating in the simplest case a quadrupole field
because there is no gravitational dipole. This distorted
field has a strength that decreases as the inverse
fourth power of the distance, so it is most important
close to the body. In the 100 km orbit around Eros, the
quadrupole field is 16 times stronger than it is in a
200 km orbit. Indeed, it is only in the 100 km orbit,
where NEAR Shoemaker has spent the past week, that the
quadrupole gravitational field of Eros is expected to
become a major factor in disturbing the orbit.
Previously, the effects of solar perturbations were more
important (again, a story for another time).
Our gravity investigators must separate out the effects
of the nonspherical gravity field of Eros. To search for
the possible presence of mass concentrations or voids,
they need to examine not only the mass quadrupole but
even more complicated configurations (or moments of
"higher order" than the quadrupole). Likewise, the
magnetic field investigation must search first for a
dipole but then consider more complicated fields, such
as a magnetic quadrupole field. However, we don't know
if Eros has any magnetic field at all, and that is the
primary issue for the magnetometer team. On the other
hand, the real issue for our gravity investigators is
not whether a nonspherical gravity field exists, but it
is whether that field requires the presence of mass
concentrations or voids. This will be investigated by
comparing the mass quadrupole and higher order moments
with the observed shape of Eros. Which team has the
harder job? I don't know.
Andrew Cheng
NEAR Project Scientist
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