With an electronic command whispered through a deep space antenna in Madrid, Douglas G. Griffith at the Jet Propulsion Laboratory in Pasadena sent the Magellan spacecraft circling to its doom Tuesday in the superheated, acid clouds above the highest mountain on Venus.
Transmitting data until the last, Magellan is expected to leave its distant creators in Pasadena with new insights into the aerodynamics of an alien atmosphere that may help them design more streamlined and fuel-efficient spacecraft.
Since arriving at Venus in 1990, Magellan has overcome technical problems and funding cuts to survey the topography and gravity of the planet in unprecedented detail. Beaming to Earth more than 1 trillion bytes of digital mapping data, it revolutionized the scientific view of the brightest planet in the sky.
By mapping Venus in such detail, the probe has done for the planet what the space agency now is attempting for Earth. Astronauts aboard the space shuttle Endeavour, which landed at Edwards Air Force Base on Tuesday, spent the past 11 days scanning the home planet with a more advanced version of the synthetic aperture radar aboard Magellan. It penetrated the vegetation and crust of civilization obscuring Earth's surface just as Magellan's imaging radar peered through the clouds masking Venus.
After more than 15,000 orbits around the most hellishly hot planet in the solar system, the $900-million spacecraft Tuesday started to spiral deeper into Venus' hothouse atmosphere, where late today it is expected to disintegrate as the air pressure increases to crushing levels found on Earth only in ocean depths. The probe also is gradually losing power because its solar panels are deteriorating.
In five measured rocket bursts Tuesday, the craft slowed and gradually descended to within 90 miles of the planet's surface near Mt. Maxwell, a Venusian peak that dwarfs Mt. Everest. Program manager Griffith and his JPL team expect to lose contact with Magellan well before it reaches the planet's surface.
Sometime late today, "it will slow down to almost nothing and flutter down to its death," Griffith said.
By studying that process, scientists hope to learn more about aerodynamics that might help them save fuel and extend the life of orbiting spacecraft. If NASA engineers, for example, can make the design of the agency's space station even 10% more efficient, they might be able to keep it in orbit longer with fewer refuelings from the space shuttle.
The men and women at JPL have explored the mysterious canyons and lava fields of Venus from small beige cubicles in the laboratory's space operations facility, using Magellan's sophisticated sensors to obtain by remote control detailed views of Venus' volcanic domes, rills and vast craters. On Tuesday, however, they were forced to rely on much more primitive instrument readings to monitor the condition of the craft as it descended into the planet's thick atmosphere.
After each firing of the craft's rocket motors Tuesday, the scientists and technicians left their cubicles and anxiously checked the instrument readings on three small black and white television monitors in the Magellan operations area. They monitored fuel pressure, signal strength and power for any symptoms of distress, like parents trying to diagnose an infant's state of mind from its cryptic cries and gestures.
"Every time we fire a thruster, we are putting more drag on the spacecraft," said mission planner Rob Lock. "Pretty soon, the atmosphere will grab us. Temperatures will rise. . . . This is the exciting part, right when we are on the edge of the cliff."
At the last mission planning meeting on Tuesday, almost the entire project team assembled in a small conference room to debate the spacecraft's final maneuver. As they reviewed the technical merits of sending the spacecraft even deeper into the atmosphere of Venus, mission director James M. Stewart passed a roll of black crepe and a pair of scissors down the table.
One by one, the navigation specialists, operations engineers and mission planners cut themselves strips and fashioned makeshift black armbands of mourning.
"Sentimental? Yes," said Lock. "This has been a fabulous mission."