The Magellan spacecraft will go into orbit around Venus Friday, August 10, and three weeks later will begin mapping with imaging radar up to 90 percent of the cloud-covered planet.
Magellan was lifted into near-Earth orbit by the shuttle Atlantis May 4, 1989, for its 15-month-long cruise to Venus, traveling one and one-half times around the sun.
Friday, its solid rocket motor with about 15,000 pounds of thrust will fire 83 seconds, to slow down the spacecraft sufficiently for Venus to capture it in its orbit.
Magellan approaches Venus over its north pole at a speed of 24,600 miles per hour. The retrorocket will slow its velocity to 18,675 mph at the spacecraft's closest approach to the planet.
The Earth-Venus-spacecraft geometry is such that the rocket firing will occur behind Venus from Earth's point of view. Earth operators will lost the radio signal from Magellan at 9:41 a.m. PDT and regain it at 10:06 a.m. as the spacecraft begins its initial orbit of the planet.
The orbit is designed to bring the spacecraft to within 170 miles of the planet's surface with its closest point, or periapsis at 10 degrees north latitude. At its farthest point in the orbit, or apoapsis, it will be 5,054 miles away. Each orbit takes 3 hours and 9 minutes.
Although more than 30 missions have been flown to Venus since the early 1960s by the United States and the Soviet Union, no mission has to date acquired a detailed map of Venus. Optical cameras cannot be used because Venus is shrouded in thick clouds. The United States' Pioneer and the Soviet Union's Venera missions used imaging radar, but not at the resolution of Magellan's synthetic aperture radar (SAR).
Two Venera missions provided good images of Venus' surface but at best their resolution was one kilometer, sixth-tenths of a mile. Pioneer acquired a global map of Venus but at a coarse resolution of 60 miles. The Magellan resolution will be in the range of 500 feet per pixel, or picture element.
One cycle of mapping will take 243 days, or one rotation of Venus. On each orbit, Magellan will point its high-gain antenna, used both for mapping and for Earth communications, to the surface and for 37 minutes acquire swath of data about 16 miles wide and extending about 10,000 miles from the north pole to about 70 degrees south latitude.
The swaths of data will be processed into images and then pieced together into mosaics which will be used to make a global map.
From 70 to 90 percent of Venus will be mapped in the first cycle, and in succeeding 243-day cycles, the rest of the planet will be mapped.
Magellan has only one instrument, the SAR but it also provides altimetry data and will be used to measure the temperature of the surface to help scientist determine its composition.
This is the fourth imaging radar to be placed into space under the management of the Jet Propulsion Laboratory, but the first to be placed in orbit around another planet. The first SAR was flown on SEASAT in 1978, followed by the Shuttle Imaging Radar, or SIR-A and SIR-B which were improvements on the SEASAT design.
All of the radar experiments were successful, and Magellan has benefitted from their experience. An imaging radar cannot be tested on the ground because it must move over its target area to acquire an image. That is because both Doppler data, generated by the motion of the spacecraft, as well as range-delay data are used to create an image.
The radar is tested through end-to-end system analysis and computer modeling based on mission requirements. Additionally, a target simulator was used for sensor testing and spacecraft integration before launch from Kennedy Space Center. Data flow tests also were done with the spacecraft at KSC, communicating with the Deep Space Network Tracking Station there which relayed the data to the JPL operations and ground data processing center.
The spacecraft also simulated orbital operations and performed successful radar tests for four days last May while enroute to Venus.
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