Magellan up; on 15-month trip to Venus—Keith Bass
The Space Shuttle Atlantis roared away from its launch pad and into space Thursday, May 4. Its near-perfect launch, after an aborted launch attempt six days earlier, marked a rebirth of interplanetary exploration for the United States.
This flight of the shuttle had one main purpose: to send off Magellan, a spacecraft carrying a Hughes-built radar that is now on a mission to map the Earth’s sister planet, Venus.
Magellan is the first U. S. interplanetary probe in 11 years and the first to be launched by the shuttle. Its successful launch fulfilled NASA’s hopes for a return of stability to interplanetary projects since the Challenger disaster of 1986.
Soon after launch, space agency officials reported that Magellan’s course is to accurate that only a few normal correction maneuvers will be required during the craft’s 15-month trip to Venus.
Magellan is now cruising at about 6000 mph and will travel 795 million miles to reach Venus in August of 1990.
Once there, Magellan will use its Hughes-built radar during 1852 orbits to send back the most detailed images to date of the Venusian surface—a desolate, alien landscape that is perpetually shrouded by layers of dense, poisonous clouds.
Magellan also will relay to Earth information on the chemical composition of Venus, and observation of the craft from Earth will be used to help elicit information about the planet’s gravity.
After giving Magellan a successful send-off, and before their safe return to Earth, the Atlantis crew conducted a few experiments to determine if large, perfect crystals of certain semiconductors could be grown is space. The crew also photographed lightning in support of a project that could lead to better weather prediction, and the shuttles’s thrusters were fired over Hawaii to help the Air Force test an optical tracking system for rockets.
Scientists believe that observations of Venus—and of other planets—enable them to make better conclusions about the origins of Earth and firmer predictions about where planet Earth is headed.
Radar technique key to mission—Bill Andrews
Mapping a planet millions of miles away is no easy task.
It’s hard enough designing and building a complex spacecraft such as Magellan that can leave the Earth, survive a trip across the solar system, and find its way to another celestial rock. In this case, the craft must then settle into a proper orbit and operate automatically by commands from Earth.
Getting to the planet and acquiring orbit are actually well within the technical abilities of NASA, the Jet Propulsion Laboratory, and Hughes Space and Communications Group.
Sometimes the subtle challenges are the toughest to meet. Take radar mapping for example. In order to map the whole planet, the radar signals sent to and received from the planet’s surface by the orbiting spacecraft must be finely tailored.
Radar Systems Group’s Howard Nussbaum, chief scientist in Advance Programs Division and senior member of the RSG team that assisted on the Magellan program, explained the problem of controlling the Synthetic Aperture Radar (SAR) in such a dynamic environment.
“Depending upon the angle of the orbit, which is highly elliptical, the power of the returning signal can vary. When the spacecraft is close to the planet, the radar reception is clearer, so to speak.”
“But if the spacecraft is some distance from Venus, the returning signal is not as strong,” Dr. Nussbaum said. “Therefore, the ‘look’ angle must be adjusted to compensate for the weaker return.”
These altitude variations present a situation somewhat analogous to attempting to take a series of photographs of an object while the camera shifts between close-up and long-distance positions, requiring continuous refocusing.
To compensate for this phenomenon, RSG wrote radar mapping sequence software for the SAR.
The software modifies the mapping sequence instructions that control the signal transmitted by the radar and reception process. The instructions vary according to the predicted orbit of the spacecraft and are used for a three-day period. This ensures a radar return signal that will provide a quality image.
After each three-hour mapping orbit, radar data is transmitted to Earth through the Deep Space Network with stations at Goldstone, Calif., Madrid, Spain, and Canberra, Australia. Upon receipt at JPL, the data is processed to form images. Quality images verify that the mapping sequences devised many days earlier have provided proper planet coverage.