Stardust Can See Clearly Now - Just Before Earth Flyby
11 Jan 2001
(Source: Jet Propulsion Laboratory)
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After a few months of foggy camera 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 re-tested 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 in brightness, 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. The Earth will not be in the navigation camera's field-of-view during the flyby, so no images of Earth will be taken.
Stardust was launched on February 7, 1999, into its first loop around the Sun. When Stardust passes by Earth at about 10 kilometers per second (22,400 miles per hour), 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 6,000 kilometers (3,700 miles) from the surface at about 3:15 a.m. PST (6:15 a.m. EST).
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 after 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 98,000 kilometers (61,000 miles) 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, Calif. 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 http://stardust.jpl.nasa.gov/index.html.