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Asteroid Mathilde Reveals Her Dark Past

Don Savage
Headquarters, Washington, DC               June 30, 1997 
(Phone:  202/358-1547)

Helen Worth
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD
(Phone:  301/953-5113)

RELEASE:  97-147

ASTEROID MATHILDE REVEALS HER DARK PAST

         More than 100 years after her discovery, asteroid 253 
Mathilde has been sharing her secrets with scientists in the 
Science Data Center at the Johns Hopkins University Applied 
Physics Laboratory in Laurel, MD.  A 25-minute flyby of the 
asteroid by NASA's Near Earth Asteroid Rendezvous (NEAR) 
spacecraft on June 27 has resulted in spectacular images of a 
dark, crater-battered little world assumed to date from the 
beginning of the solar system.    

       The Mathilde flyby is the closest encounter with an 
asteroid to date and the first with a C-type asteroid.  The 
asteroid's mean diameter was found to be 33 miles (52 kilometers), 
which is somewhat smaller than researchers originally estimated.  
A study of the asteroid's albedo (brightness or reflective power) 
shows that it reflects three percent of the Sun's light, making it 
twice as dark as a chunk of charcoal.  Such a dark surface is 
believed to consist of carbon-rich material that has not been 
altered by planet-building processes, which melt and mix up the 
solar system's original building block materials.  

     The Mathilde flyby met all its initial goals: getting a clear 
image of the sunlit side of the asteroid, getting color images 
that will give clues to the types of rock that make up the 
asteroid, and getting images that will help researchers determine 
if Mathilde has any moons.  In the next month, scientists expect 
to complete initial analysis of their data and have improved 
measurements of Mathilde's volume, mass, and density.  

       "The Mathilde encounter was one of the most successful flybys of
all time," said Dr. Robert W. Farquhar, of the Applied Physics Laboratory,
NEAR Mission Director.  "We got images that were far better than we 
thought possible, especially since the spacecraft was not designed 
for a fast flyby."

     Only the multispectral imager, one of six instruments on the 
spacecraft, was used during the flyby in order to conserve power 
provided by solar-powered panels.  The spacecraft was 
approximately 186 million miles from the Sun, too far to provide 
power for NEAR's other instruments.

       "Even though this was a very difficult undertaking," said 
Dr. Stamatios M. Krimigis, head of the APL Space Department that 
managed the program for NASA, "the NEAR Operations Team was so 
well prepared there was little doubt that it would succeed; not 
only that, but this was the smallest operations team of any 
planetary encounter, proving that the Discovery Program paradigm 
of 'smaller, faster, cheaper' is alive and well."  

       Although Mathilde proved to be rounder than asteroids such as Gaspra
and Ida, Dr. Joseph Veverka of Cornell University, Ithaca, NY, who leads 
the mission's imaging science team, said, "Mathilde turned out to 
be more irregularly shaped than most of us expected.  The degree 
to which the asteroid has been battered by collisions is 
astounding.  At first glance there are more huge craters than 
there is asteroid."   

       The imager found at least five craters larger than 12 miles 
(20 kilometers) in diameter just on the lighted side of the 
asteroid.  Scientists wonder how the asteroid can remain intact 
after having been hit by this many projectiles, each probably at 
least a mile wide.  

       The craters reveal evidence of the asteroid's makeup.  "We 
knew that C-asteroids are black, but we did not expect their 
surfaces to be as uniformly black and colorless as Mathilde's 
surface turned out to be," Veverka said.  "This global blandness 
is an important clue telling us that asteroids such as Mathilde 
are made of the same dark, black rock throughout because none of 
the craters, which are punched deep into the asteroid, show 
evidence of any other kind of rock."  Such uniformity seems to 
confirm that C-type asteroids are in fact pristine samples of the 
primitive building blocks of the larger planets.   

       Dr. Donald K. Yeomans of the Jet Propulsion Laboratory, 
Pasadena, CA, who heads the radio science team formed to determine 
Mathilde's mass said, "Mathilde is an asteroid with a very 
tortured past."  By determining the bulk density of the asteroid, 
researchers will have a clue to how it was formed.  A composite of 
objects would have a lower density than a solid chunk from a 
larger asteroid.  Data analysis to determine density will not be 
complete until later this year, but Dr. Yeomans said, "Preliminary 
results suggest that Mathilde is much less dense than we had thought."  

       One mystery that remains is Mathilde's extraordinarily slow 
(17.4 days) rotation rate.  Its collision history could be a 
factor, but more research needs to be done to determine what role 
such collisions have played.  The search for Mathilde moons 
continues; none has yet been discovered.   

       The next major event of the NEAR mission will occur on July 
3, when the spacecraft's bi-propellant engine is fired to head 
NEAR back toward Earth.  This deep-space maneuver will be the 
first time the engine has been fired and will keep both engineers 
and scientists in suspense for 11 minutes before they know if the 
maneuver was successful.  An Earth gravity-assist maneuver on Jan. 
23, 1998, will send the spacecraft toward its primary target, 
asteroid 433 Eros.  NEAR will reach Eros nearly a year later and 
will remain locked in orbit around the asteroid until Feb. 6, 
2000, when the mission ends.  

       Commenting on the success of the Mathilde flyby soon after 
the first images were received, Dr. Wesley T. Huntress Jr., NASA 
Associate Administrator, Office of Space Science, said,  "It's 
today that the Discovery Program really begins. NEAR was the first 
of our Discovery missions to be launched and it's the first to 
return scientific results."  He said the APL-led team that managed 
the NEAR program proved the concept behind the Discovery Program:  
that exciting planetary missions can be done at low cost, in a 
short time.  

       The NEAR spacecraft was launched Feb. 17, 1996, from Cape 
Canaveral Air Station in Florida.  NEAR Science Team Group Leaders 
are: Joseph Veverka, Cornell University; Jacob I. Trombka, 
NASA/Goddard Space Flight Center, Greenbelt, MD; Mario H. Acuna, 
NASA/Goddard; Maria T. Zuber, MIT and NASA/Goddard; and Donald K. 
Yeomans, NASA/Jet Propulsion Laboratory, Pasadena, CA.  Andrew 
Cheng, JHU/APL, is the Project Scientist.  The Johns Hopkins 
University Applied Physics Laboratory operates the mission for 
NASAÕs Office of Space Science, Headquarters, Washington, DC.

                            -end-

EDITOR'S NOTE:   Images of Mathilde and the NEAR spacecraft are 
available for media representatives by calling the Headquarters 
Imaging Branch on 202/358-1900, or the JHUAPL Public Affairs 
Office at 301/953-5113.

NASA Photo numbers (black and white only) are:
97-H-446; 97-H-447; 97-H-448; 97-H-449; and 97-H-450

Mathilde flyby images and updates can be obtained on the Mathilde 
homepage at:  

                http://sd-www.jhuapl.edu/NEAR/Mathilde