When NASA’s Near Earth Asteroid Rendezvous (NEAR)
spacecraft left for asteroid 433 Eros five years ago, scientists weren’t
certain what they would find when the probe arrived. Was Eros a 30-km
fragment from a planet that broke apart billions of years ago? Or perhaps a
jumble of space boulders barely held together by gravity? Was Eros young or
old, tough or fragile … no one knew for sure.

But now, after a year in orbit and a daring landing on the asteroid itself,
NEAR Shoemaker is beaming back data that could confirm what many scientists
have lately come to believe: Asteroid Eros is not a piece of some long-dead
planet or a loose collection of space debris. Instead, it’s a relic from the
dawn of our solar system, one of the original building blocks of planets
that astronomers call “planetesimals.”

As NEAR Shoemaker was heading for its historic landing on Feb. 12, 2001,
team members hoped the spacecraft –which was designed to orbit, not land–
would simply survive. When it did survive, they set their sights a little
higher. From its perch on the surface of the asteroid, NEAR’s gamma-ray
spectrometer (GRS) can detect key chemical signatures of a planetesimal —
data that scientists are anxious to retrieve.

“The gamma-ray instrument is more sensitive on the ground than it was in
orbit,” says Goddard’s Jack Trombka, team leader for the GRS. “And the
longer we can accumulate data the better.” NASA recently gave the go-ahead
for NEAR’s mission to continue through Feb. 28th, tacking four days onto an
extension granted just after the spacecraft landed.

To do its work the GRS relies partly on cosmic rays, high-energy particles
accelerated by distant supernova explosions. When cosmic rays hit Eros, they
make the asteroid glow, although it’s not a glow you can see with your eyes;
the asteroid shines with gamma-rays.

“Cosmic rays shatter atomic nuclei in the asteroid’s soil,” explains
Trombka. Neutrons that fly away from the cosmic ray impact sites hit other
atoms in turn. “These secondary neutrons can excite atomic nuclei (by
inelastic scattering) without breaking them apart.” Such excited atoms emit
gamma-rays that the GRS can decipher to reveal which elements are present.

“We can detect cosmic-ray excited oxygen, iron and silicon, along with the
naturally radioactive elements potassium, thorium and uranium,” says
Trombka. Measuring the abundances of these substances is an important test
of the planetesimal hypothesis.

Planetesimals came to be when the solar system was just a swirling
interstellar cloud, slowly collapsing to form the Sun and planets. Dust
grains condensed within that primeval gas. The grains were small, but by
hitting and sticking together they formed pebble-sized objects that fell
into the plane of the rotating nebula. The pebbles accumulated into
boulders, which in turn became larger bodies, 1 to 100 km wide. These were
planetesimals — the fundamental building blocks of the planets.

For reasons unknown Eros was never captured by a growing protoplanet. It
remained a planetesimal even as other worlds in the solar system grew and

Fully-developed planets like Earth are chemically segregated — that is,
they have heavier elements near their cores and lighter ones at the surface.
Planetary scientists call this “differentiation.” If Eros were a chip from a
planet that broke apart, perhaps in the asteroid belt, it would exhibit
chemical signatures corresponding to some layer from a differentiated world.

For example, Eros might be iron-rich if it came from the core of such a
planet or silicon-rich if it came from the crust.

Instead, “orbital data from the x-ray spectrometer (a low-energy cousin of
the GRS) showed Eros is very much like a type of undifferentiated meteorite
we find on Earth called ordinary chondrites,” says Andrew Cheng, the NEAR
project scientist at Johns Hopkins University Applied Physics Laboratory
(APL), which manages the mission for NASA.

Eros seems to harbor a mixture of elements that you would only find in a
solar system body unaltered by melting (an unavoidable step in the process
of forming rocky planets). But, says Cheng, there is a possible discrepancy.

“The abundance of the element sulfur on Eros is less than we would expect
from an ordinary chondrite. However, the x-ray spectra tell us only about
the uppermost hundred microns of the surface, and we do not know if the
sulfur depletion occurs only in a thin surface layer or throughout the bulk
of the asteroid.”

The GRS can go deeper, as much as 10 cm below the surface. Although the
instrument can’t detect sulfur, it is sensitive to gamma-ray emissions from
other elements such as radioactive potassium that are indicators of melting.
Like sulfur, potassium is a volatile element — it easily evaporates when a
rock is heated. Finding plenty of potassium would strengthen the conclusion
that Eros is an unmelted and primitive body.

On the other hand, a widespread dearth of “volatiles” would hint that Eros
isn’t so primitive after all.

It might sound like an ivory-tower question, but knowing the makeup of this
asteroid — both its internal structure and its chemical composition– has a
practical application. The solar system is littered with space rocks more or
less like Eros, and many come uncomfortably close to Earth. One day we may
need to blow one apart (or deflect one without blowing it apart) to avoid an
unpleasant collision. Near-Earth asteroids are also potential mining
resources as humans expand into space. In either case, knowing more about
them is a good idea!

“Our first four data sets are here and they look great,” says Jack Trombka.
“John Goldsten, the lead engineer for the gamma-ray spectrometer at the
Johns Hopkins Applied Physics Laboratory, has done a fabulous job making the
instrument work on the surface, which is a different environment than orbit.

“We’re just hoping to get as much data as we can before the mission ends.”

NEAR Shoemaker launched on Feb. 17, 1996 – the first in NASA’s Discovery
Program of low-cost, scientifically focused planetary missions — and became
the first spacecraft to orbit an asteroid on Feb. 14, 2000. The car-sized
spacecraft gathered 10 times more data during its orbit than originally
planned, and completed all the mission’s science goals before its controlled
descent on February 12, 2001. Funding for the mission extension comes from
the NEAR project.