Consolidated but Undifferentiated: NEAR Shoemaker Science Update 4 October 2000

Will there ever be an Olympics held on an asteroid? Back in the 1950’s there
were confident predictions that the year 2000 would see permanently
inhabited space stations, lunar bases, and manned voyages to the planets.
After all, we had just witnessed the developments of television and the
atomic bomb, and if we could progress from the first heavier-than-air flying
machines to artificial satellites in 54 years, then who knew where we would
be in another 50 years? Now that year 2000 is mostly behind us, even manned
voyages to other planets are not in prospect for at least the next decade.
Holding the Olympics on an asteroid sounds little more realistic now than it
would have sounded fifty years ago. But science and technology have
continued to move forward at an exponentially increasing rate, and what we
are doing in space now, on NEAR, would have been unimaginable fifty years
ago. Using a laser to measure topography? – lasers hadn’t been invented yet
fifty years ago. Measuring the x-ray spectra of an asteroid to confirm that
it is linked to primitive meteorites? – this notion would have sounded as
crazy to a scientist of fifty years ago as holding the Olympics on an
asteroid sounds to us now. At that time, scientists had no reason to expect
that asteroids would even be significant sources of x-rays.

Science and technology therefore advance in directions that we cannot
predict and often do not expect. We are not sending people to other planets,
but we are sending instrumented probes to explore new worlds, without
sending people. Not that we wouldn’t send people if we could – but it costs
too much. NEAR, for example, had a total cost, for the entire mission
including launch vehicle, spacecraft, and operations, that amounts to $0.80
for each person in the USA, spread over the past seven years. For that
money, enough to buy a candy bar if saved over the whole period, NEAR has
recorded two principal findings about Eros – that Eros is a primitive,
undifferentiated object, and that Eros is a consolidated body as opposed to
a rubble pile, meaning a loose agglomeration of much smaller pieces held
together mostly by gravity. There are more findings, of course, but they
will be our topic for another time.

Eros, as an undifferentiated body, is made of material that has been
scarcely altered since the beginning of the solar system, and probably since
before the formation of the Earth. The relative proportions of rock-forming
elements in Eros is similar to those found in the Sun itself. Eros was never
subjected to the melting and the separation into compositionally distinct
layers that Earth, Mercury, Venus and Mars have all experienced. Eros is
made of the same material as certain primitive meteorites, which are pieces
of ancient asteroids that have fallen to the surface of the Earth, and which
have also never been subjected to melting, although some degree of
metamorphism is typical (meaning that these meteorites have been subjected
to considerable heating). No rocks form naturally on Earth that are of the
same composition as Eros or these meteorites. Even though Eros has about the
same overall density as the Earth’s crust, NEAR could never mistake a
32.7-km chunk of Earth’s crust for Eros. The x-ray spectra would appear
totally different.

The reason for this difference is that Earth’s crust is the outermost layer
of a differentiated body. Iron, nickel and other metals (including gold and
platinum) were once finely dispersed throughout the primitive material from
which Earth formed, but melting occurred early in Earth’s history, and these
metals settled for the most part into the core of the Earth. Only a small
proportion of these metals can still be found in Earth’s crust. On Eros,
however, these metals would still be present, dispersed throughout the body
of the asteroid. There have been serious suggestions that commercial mining
of asteroids may someday become viable. While undifferentiated Eros may be
10% metallic nickel-iron by weight (with most of the iron in chemically
bound forms), there are also differentiated asteroids, like 1986DA, that are
almost pure nickel-iron.

NEAR’s other principal finding is that Eros appears to be consolidated,
meaning that it is held together by material strength. Huge tectonic
features are found, which cannot occur in the absence of material strength.
Steep slopes are found, where unconsolidated material would be expected to
slide off. Many craters have shapes that indicate the influence of material
strength in their formation, such as the square craters shown in the image
of the day for
September 19.
Eros has certainly been battered by innumerable
impacts, but it has not been battered into a rubble pile. This finding is of
interest because it constrains the collisional history of Eros, but someday
it may also be interest if we ever discover an asteroid that is on a
collision course with Earth. If we decide to deflect such an asteroid, we
surely need to know if it is a loose pile of rubble, ready to come apart
with little provocation. Breaking such an asteroid into many pieces, most of
which are still heading for Earth, would be disastrous.

But surely we would send an instrumented probe first, before committing to a
particular means of dealing with a rogue asteroid – provided that we had
enough time. NASA is funding small programs to search the skies for
near-Earth asteroids, and other countries are considering such programs.
There is no known asteroid at this time that is on a collision course with
Earth, but the next asteroid impact with enough explosive force to threaten
the future of civilization is likely to occur within 100,000 years. Eros
itself may strike the Earth millions of years from now, or it may not – we
cannot say for certain. Perhaps we need to do more to learn about near-Earth
asteroids and to make sure we have enough warning time before the next large
asteroid impact. It may even allow us someday to hold the Olympics on an
asteroid.