Stardust Can See Clearly Now – Just Before Earth Flyby
After a few months of foggy vision, NASA’s Stardust mission
team has improved the spacecraft’s navigation-camera resolution to
nearly normal, just as Stardust is preparing to make a close flyby
of the Earth on Monday.
By heating the camera’s optical path, the Stardust team was able
to help its nearsighted spacecraft boil away contaminants that had
been deposited on optical surfaces.
One year ago, the imaging team took pictures of a small lamp
inside the optical path of the camera. The camera will be used to
navigate Stardust to its 2004 encounter with Comet Wild 2
(pronounced “vilt-2”). Apparent contamination of the navigation-
camera prevented a clear test-image of the squiggly line of the
lamp’s filament, and the lens seemed to be covered with a veil of
light-scattering material that produced a blurry image.
The team concluded that the contamination might have been released
with gases escaping from the spacecraft after its launch, and that
heating the optical path of the camera might evaporate the
contaminant covering the camera lens. After a series of heating
cycles, they retested the camera by taking more pictures of the
lamp.
Pictures taken after the heating revealed that the zigzag line of
the lamp’s filament was visible again. Images of stars taken by
the camera are also clearer. The team estimates the camera can now
photograph stars two magnitudes (celestial degrees of brightness)
better. The navigation camera has detected stars as faint as 9th
magnitude, which should allow the spacecraft to perform its final
navigation maneuvers during approach to the comet nearly at the
time originally planned.
Now Stardust, on its journey to collect comet dust, is getting
ready to springboard from Earth — in a maneuver called a
“gravity-assist” — when the spacecraft passes closest to Earth on
January 15, 2001.
Stardust was launched on February 7, 1999, into its first loop
around the Sun. When Stardust passes by Earth at about 22,400
miles per hour (or 10 kilometers per second), it will go into a
slightly wider orbit that will allow it to reach the comet on
January 2, 2004.
On Monday, January 15, Stardust will fly by a point just southeast
of the southern tip of Africa, slightly more than 3,700 miles
(6,000 kilometers) from the surface at about 5:15 a.m. EST (3:15
a.m. PST).
Stardust may be visible to observers using sophisticated
telescopes with charge-coupled device (CCD) detectors from the
Pacific Ocean and the Western United States just before the
spacecraft flies by Earth. Stardust will not be visible using
binoculars.
A gravity-assist works like this: when a spacecraft closely
approaches a planet, the planet’s gravitational pull accelerates
the spacecraft and bends the flight path. Mission designers
account for this extra pull and use it to their advantage to boost
spacecraft speed and direct interplanetary spacecraft to their
targets. Like a windup before the pitch, the Earth gravity-assist
will sling Stardust into the right path to meet Comet Wild 2.
About 15 hours after its closest approach to Earth, the spacecraft
will pass about 61,000 miles (98,000 kilometers) from the Moon.
Because of the greater distance, the Moon’s gravity will have
essentially no influence on the spacecraft’s flight path.
Stardust, a part of NASA’s Discovery Program of low-cost, highly
focused science missions, is managed by the Jet Propulsion
Laboratory (JPL), Pasadena, CA, for NASA’s Office of Space Science,
Washington, D.C. JPL is a division of the California Institute of
Technology, Pasadena. More information on the Stardust mission is
available at:
