Re: NEAR flyby of Asteroid 253 Mathilde on June 27

[Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>]

>     On June 27, 1997 the NEAR spacecraft will pass within about 1200
>     km of main belt asteroid 253 Mathilde. Complementing the Galileo
>     flyby's of S-asteroids Gaspra and Ida, this will be the first ever
>     close observation of a C-asteroid. 

S-asteroids are the most common type of asteroids in the main asteroid
belt.  They are thought to be parent bodies of iron and stony-iron
meteorites.  In fact, data from Galileo's flyby of Gaspra in 1991 indicates
the asteroid may have a magnetic field.

The C-asteroids are the second most common type of asteroid, and
their spectra match closely with carbonaceous meteorites (CI & CM).  
Phobos and Deimos, moons of Mars, are suspected the be C-asteroids from
the main asteroid belt that have somehow been captured by Mars.

>     Mathilde has attracted recent
>     attention due to its extremely slow rotation period of 17.5 days.

Only two other asteroids are known to have longer periods: 288 Glauke (48
days) and 1220 Clocus (31 days).  Because of the slow spin, NEAR will only
be able to image one side of the asteroid.

>     Primary science objectives during this 10 km/sec flyby include
>     high- resolution imaging, as well as albedo and spectral mapping
>     of the illuminated surface of the large (50x50x70 km) asteroid.
>     The best monochrome images will achieve resolutions of 200
>     meters/pixel. Global imaging in seven colors between 0.4 and 1.1
>     micron will be carried out at resolutions of 400-500 m/pixel. On
>     departure a satellite search will be made in which bodies as small
>     as 100 meters across could be detected. A determination of the
>     mass of Mathilde to about ±10% will be carried out by the Radio
>     Science experiment.

This will be the first mass determination of an asteroid by a spacecraft
flyby.  Galileo did not pass close enough to Ida or Gaspra to determine
their masses.  Also, Mathilde is a rather large asteroid at 38 miles (61 km),
which is twice the size of Ida and four times the size of Gaspra.  Mathilde
appears to be a spherical body, while Ida and Gaspra were irregularly shaped.

>     Due to the encounter geometry (approach phase angle 139°,
>     departure at 39°) the best imaging of Mathilde will occur around
>     and just after closest approach. Locating Mathilde with sufficient
>     accuracy to insure the the highest resolution observations are
>     obtained near closest approach requires optical navigation updates
>     of Mathilde's position as late as 12 hours before encounter. 

And to further complicate matters, Mathilde is a very dark object - it
only reflects 4 percent of the light falling on it.  
The asteroid will appear as a thin  crescent as the spacecraft approaches
and won't get favorable lighting until near closest approach.  Since the
entire spacecraft has to be turned to point its camera at Mathilde, the
solar arrays won't be pointed directly at the Sun and will be generating
power at 25% of its maximum.  Because of this power constraint, the camera
will be the only science instrument operating during the encounter.

Ron Baalke

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