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Probe to study Venus with radar

By WILLIAM HARWOOD UPI Science Writer

CAPE CANAVERAL, Fla. -- NASA's Magellan probe, cobbled together from spare parts left over from other programs, will use radar beams to create a detailed, photo-quality map of cloud-shrouded Venus's hidden surface.

Launched May 4, 1989, from the shuttle Atlantis, Magellan is to slip into orbit around Venus early Friday after a 15-month space voyage.

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Its goal: to spend at least 243 days mapping the cloudy planet's hidden surface using a high-resolution radar capable of distinguishing objects as small as a football field.

Built by Martin Marietta Astronautics Co. of Denver, Magellan will be placed in a highly elliptical three-hour, nine-minute orbit around Venus, one tilted 86 degrees to the equator with a low point of about 170 miles and a high point of about 5,000 miles.

As the probe travels through the low-altitude part of each orbit, its 12-foot dish antenna will be used to fire radar pulses at the ground, 'illuminating' a 15.5-mile-wide, 9,900-mile-long swath, or 'noodle.'

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After each 37.2-minute mapping run, Magellan will re-orient itself, pointing the antenna toward distant Earth so the recorded radar data can be beamed back to the Jet Propulsion Laboratory in Pasadena, Calif., for analysis.

The probe then will swing back around, aim its antenna at Venus and make another mapping pass.

'Radar' is an acronym for 'radio detecting and ranging.' As the name implies, it uses radio waves, a form of electromagnetic radiation like X-rays and visible light, to determine an object's range, or distance. And as every motorist knows, radar also can be used to determine an object's velocity.

As it turns out, radar is an ideal tool for 'seeing' through the clouds that shroud Venus. The trick is turning that data into a useable image.

As Magellan sails through the low-point of its orbit, 26.5-millisecond radar pulses will be fired at the surface at the speed of light, 186,000 miles per second. After a very short but measurable delay, the pulse's reflection from the surface will be detected by the spacecraft.

By measuring the time between the pulse and the arrival of its reflection, computers can determine the distance to the point on the ground where the pulse was reflected. But that alone is not enough to assemble a map, because more than one point will lie at any given distance from the spacecraft.

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To further pin down a point's location, Magellan also will measure the 'Doppler shift' of the reflected radar pulse, that is, the amount the radar beam's wavelength has changed because of the spacecraft's orbital motion.

To anyone present when a fire engine goes past, the pitch of the siren, which depends on the sound's wavelength, will change.

Likewise, a returning radar pulse will have a slightly different wavelength based on whether Magellan is approaching a point on Venus or receding from it.

The Doppler shift, then, provides data about the location of a point on the surface with respect to Magellan's line of flight.

Each point in the Magellan radar image will thus have a unique distance and Doppler shift. By carefully combining that data, along with data giving the precise orientation of the antenna, computers can assemble a photograph-like map of the surface.

The level of detail that can be distinguished by an imaging radar system depends on the size of the antenna, just as the performance of normal telescopes depends on the size of the lens or mirror.

Magellan's antenna is 12 feet wide, but because the spacecraft moves a considerable distance between the time a radar pulse is emitted and its reflection is received, the antenna has an 'effective' diameter of some 3,000 feet.

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For this reason, Magellan is said to be equipped with a 'synthetic aperture radar' system and, under the best possible conditions, surface features as small as 350 feet or so can be distinguished. The best previous Venus radar maps have resolutions of about 1 mile.

It would greatly simplify Magellan's mission if the spacecraft could be fired into a circular orbit around Venus, because the probe's velocity and altitude would remain constant. But a tight budget forced planners to settle for a cheaper, less-powerful braking rocket, one that can only achieve an elliptical orbit.

As it is, Magellan's speed will increase as it approaches the low-point of its orbit, called periapsis, and decrease as it climbs to a higher altitude.

With Magellan, the timing of the radar pulses must be constantly adjusted as the craft speeds along to make sure a transmission does not interfere with a reflection. It is a tremendously complex task and one that requires some 950 computer commands per pass to accomplish.

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