The Magellan spacecraft's orbit at its closest approach to Venus was lowered Monday and today it began a full 243-day cycle of gravity mapping, project officials at NASA's Jet Propulsion Laboratory said.
Magellan has now mapped 99 percent of the surface of Venus. Monday, controllers ordered a one-hour orbit adjustment burn to lower its periapsis -- closest approach to the planet -- altitude from 258 kilometers (160 miles) to 182 kilometers (113 miles).
"That will help us obtain the best possible resolution in the equatorial latitude gravity map," Project Manager Doug Griffith said.
The objective of cycle 4, which extends to May 15, 1993, is to obtain a global map of the Venus gravity field from the elliptical orbit. The orbit apoapsis, or furthest point from the planet, remains the same, 8,543 kilometers (5,296 miles).
During this fourth cycle, variations in the gravitational pull experienced by the spacecraft are being recorded by carefully tracking the Doppler shift of a radio signal that Magellan will constantly beam to the Deep Space Network tracking stations.
When Magellan passes over a dense region of Venus' interior, for example, the spacecraft accelerates in its orbit, and the location of the denser region is mapped.
Over the course of the 243-day cycle, one rotation of Venus, variations in the planet's density will be mapped at a resolution much higher than achieved by previous missions.
Looking at the interior with gravity observations is expected to provide an improved understanding of the forces of tectonics and volcanism that shape the planet.
Magellan is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science and Applications.
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