Eros or Bust
| Tweet |  |
February 8, 2000 -- If all goes
well on Valentines Day, 2000, NASA's Near Earth Asteroid Rendezvous
(NEAR) spacecraft will go into orbit around asteroid 433 Eros.
Thruster firings on February 3 slowed NEAR to a leisurely 18
mph and it is now less than 3000 miles from the asteroid.
When NEAR enters orbit around Eros on February 14, it will become
the first spacecraft to circle an asteroid. NEAR will use its
instruments to scrutinize the potato-shaped space rock, which
is about twice the size of Manhattan Island, for an entire year.
Above: In 1998 NASA's NEAR spacecraft
captured this mosaic image of the Earth and Moon after it flew
by Earth in a gravity
assist maneuver that sent it hurtling toward asteroid Eros.
It will arrive at the asteroid on February 14, 2000.
Not all asteroids are far away in the asteroid belt. Some, called Near Earth Asteroids (or NEAs), have orbits that bring them very close to Earth. 433 Eros is one of these. NEAs are thought to be fragments ejected from the main asteroid belt by asteroid-asteroid collisions or by gravitational perturbations from Jupiter. Some NEAs might also be the nuclei of dead, short period comets.
As any dinosaur can tell you, it's important
to keep an eye on Near Earth Asteroids. Many NEAs have struck
Earth and its moon in the past. One widely accepted theory blames
the impact 65 million years ago of an asteroid or comet at least
6 miles (10 kilometers) in diameter for mass extinctions among
many life forms, including the dinosaurs. Other theories suggest
that the chemical building blocks of life and much of EarthÂ’s
water arrived on asteroids or comets that bombarded the planet
in its youth.
Left: This diagram shows the orbits of Earth, Mars,
the asteroid belt, and Jupiter. The asteroid belt lies between
Mars and Jupiter.
On June 30, 1908, a small asteroid 330 feet (100 meters) in diameter
exploded over the remote region of Tunguska in Siberia, devastating
more than half a million acres of forest. One of the most recent
close calls occurred on March 23, 1989, when an asteroid 0.25
mile (0.4 kilometer) wide came within 400,000 miles (640,000
kilometers) of Earth. Surprised scientists estimated that Earth
and the asteroid - weighing 50 million tons and traveling at
46,000 mph (74,000 kilometers per hour) -had passed the same
point in space just six hours apart!
Approximately 800 NEAs have been found to date, probably only
a small percentage of their total population. The largest presently
known is 1036 Ganymed, with an approximate diameter of 25.5 miles
(41 kilometers). Estimates suggest that at least 700 NEAs may
be large enough - 0.6 mile (1 kilometer) or more in diameter
- to threaten civilization if they were to strike the Earth.
The target of the NEAR mission, 433 Eros, was the first near-Earth asteroid to be discovered and is the second largest known. It is one of only three known NEAs with diameters of more than 6 miles (10 kilometers). With dimensions 21 by 8 by 8 miles (33 by 13 by 13 kilometers), Eros is one of the most elongated asteroids.
Right: On February 4, NEAR's multispectral
imaging camera captured this sequence from a distance of 4620
miles (7700 km). The images of Eros were acquired every 15 degrees
of rotation for one Eros "day", which is 5.27 hours
long. Eros's overall shape has been compared to a boat, a shoe,
a peanut, and a banana with a bite taken out of it. This sequence
of images is the first to show the major geographic features
of the northern and equatorial latitudes of Eros. (click
for more info from Johns Hopkins Applied Physics Laboratory)Into Orbit
On Feb. 14, at 10:33 a.m. EST, when NEAR is 207 miles (333
kilometers) from the center of Eros, it will fire its hydrazine
engines to slow it enough to be captured by the asteroid's weak
gravitational pull. Confirmation of a successful orbit is expected
to come at about 11:30 a.m. EST to waiting team members in the
Mission Operations Center on the Applied Physics Laboratory campus
of Johns Hopkins University.
That these critical events will happen on Valentine's day is
appropriate because Eros is named for the Greek god of love.
During the first few weeks after achieving orbit the spacecraft
will slowly descend toward the asteroid. Because it is irregularly
shaped and rotating, this early stage of the mission can be very
tricky.
"No one has ever orbited a small body in space,"says
Dr. Robert Farquhar, NEAR mission director. "The orbital
stability is rather tenuous, and as we travel around Eros our
navigation maneuvers must be perfect to keep us from crashing
into it." 
Using
a multispectral
imager, laser
rangefinder, and the onboard radio
science experiment, mission scientists and engineers will
collect enough information about Eros's shape, mass and gravity
field to allow the spacecraft to come closer.
Left: A diagram of the NEAR spacecraft, showing locations
of the magnetometer, multi-spectral imager, infrared spectrometer,
the laser rangefinder, the X-ray spectrometer, and the Gamma-ray
spectrometer.
"Soon after we go into orbit we should know the asteroid's
mass and therefore its density to within 5 percent," says
Dr. Andrew Cheng, mission scientist.
The onboard
magnetometer will determine the strength of the asteroid's
magnetic field -- if there is one.
"This will give the scientific community the first definitive
measurement of an asteroid's magnetism, which contains clues
to its thermal and geologic history," Dr. Cheng says. "The
results of these measurements and others that we will take over
the next year will help us to determine the origin of the asteroid
and give us an unprecedented understanding of asteroids in general."
For the first two months NEAR will slowly descend to within 31
miles (50 kilometers) from Eros. During this low-orbit phase
scientists will use the x-ray/gamma-ray
spectrometer to measure the abundance of various elements
that make up Eros. X-rays from the sun striking the asteroid
can produce significant count rates of fluorescence X-rays from
surface elements such as magnesium, aluminum, and silicon. The
elements sulfur, calcium, titanium, and iron are also present
in asteroids, but count rates will be lower and data will take
longer to accumulate. Similarly, cosmic ray protons (and energetic
particles associated with solar flares) can interact with the
asteroid surface to produce gamma rays characteristic of the
nuclear energy levels of a given element. Gamma rays also can
be spontaneously emitted by naturally occurring radioactive elements
such as potassium, uranium, and thorium. 
Right: A distant image of Eros is
shown for comparison with a montage of main belt asteroids. The
picture of Mathilde was obtained by NEAR during a flyby on June
27, 1997. The pictures of Gaspra and Ida were captured by the
Galileo spacecraft during flybys in 1991 and 1993, respectively.
Brief flybys are not sufficient to answer many outstanding questions
about asteroids. The NEAR mission will remedy that with a year-long
study of Eros.
In late August the spacecraft will begin to climb from 31 to
311 miles (50 to 500 kilometers) above the center of Eros. During
this ascent the spacecraft's camera will continue to snap pictures
of the asteroid's surface that will be compiled into a complete
map. In December NEAR will descend, possibly to less than a mile,
from the surface of the asteroid. From that vantage point the
near- infrared spectrometer can collect extremely high resolution
data of the asteroid's surface, making it possible to distinguish
the composition of rocks as small as a grapefruit. Final events
of the mission, which will end in February 2001, will be determined
sometime this summer.
NEAR was launched Feb. 17, 1996, from Cape Canaveral Air Station,
FL. Its original rendezvous date of Jan. 10, 1999, was postponed
when a firing of the spacecraft's bipropellant engine, designed
to put the spacecraft on target for the rendezvous, exceeded
preset acceleration limits and caused the spacecraft to retreat
into safe mode. But valuable information about the asteroid was
collected by a hastily programmed flyby of Eros on Dec. 23, 1998.
Early images can be found on the Internet at: http://near.jhuapl.edu
The NEAR (Near Earth Asteroid Rendezvous) mission, a NASA
Discovery Program being conducted by the Johns Hopkins University
Applied Physics Laboratory, Laurel, MD, is the first mission
to orbit an asteroid.
Near Earth Asteroid Rendezvous mission - NEAR home page from Johns Hopkins University
NASA Press Release - 8 Feb. 2000