BOSTON — MAGELLAN - the Venus radar mapping spacecraft - has begun its third exploration cycle. This is a major triumph for the Magellan project team.
Failure of the craft's primary data transmitter Jan. 4 and noise in its backup transmitter had threatened to end the mission. Now ground controllers have found a way to work around the noise problem.
"The data coming in look clean. We're back in business but at half the data-transmission rate," Magellan project manager James Scott says. This means that Magellan cannot do as much mapping in a given amount of time as in the past. But it still can cover a great deal of Venusian ground. In particular, it can take a second look at areas already covered and send back data for constructing three-dimensional stereo views of important features.
Planetary scientists would have called Magellan's mission successful even if it had come to an abrupt end in January. It fulfilled almost all of its primary objectives by mapping more than 95 percent of the surface with the 3,880 orbits it made of Venus during the previous 15 months.
"For the first time in human history, we've produced a map of a planet the size of the Earth, and we've done it in a very short time," says Magellan project scientist R. Stephen Saunders at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena. The laboratory manages the project for the National Aeronautics and Space Administration (NASA).
Magellan radar scientist Gordon H. Pettengill at the Massachusetts Institute of Technology agrees. He explains: "We've certainly met the [mapping] objectives we set out, which represent the medium of our expectations. We've exceeded them and then some."
This was the original plan: The project was to fulfill a nominal set of primary objectives - such as a nearly complete planetary map - during its first one or two survey programs. If the spacecraft continued to function, it would carry out additional survey cycles. These aimed for such important but secondary objectives as stereo mapping. Now only one primary objective - making a detailed map of Venus's gravity field - remains. Dr. Scott says, "We look forward to doing that" on schedule in the autumn.
Obtaining that objective, however, is in doubt. In a move that caught the Magellan team by surprise, the Bush administration's new fiscal 1993 budget (which begins Oct. 1, 1992) has no money for the mission. By saving about $80 million in operating funds, this would force the project to halt radar mapping in June and cease operations by October.
"The whole thing seems so absurd," Stephen Saunders laments. He says he is not sure this is a final decision, adding that JPL will fight it. He explains that he means "we want to make sure everybody understands the high scientific value this mission now is producing at relatively low cost."
Meanwhile, planetary scientists are delighted with the information they now have. Dr. Pettengill explains: "It's the old story. We ask certain questions in the beginning. Now we have a whole host of new questions we didn't know how to ask before.... Mysteries abound!" he exclaims with the enthusiasm of an investigator who faces a new challenge.
In some ways, Venus is Earth's twin. It has the same density and roughly the same surface gravity. It has the same amount of carbon dioxide, although, unlike Earth, this is all in the Venusian atmosphere. Yet there are intriguing differences between the two planets, besides the fact that Venus doesn't have surface water.
Earth's crust is broken into giant plates. These move about relative to each other. Much heat escapes from the interior where lava wells up between plates. Magellan's maps show this doesn't seem to happen on Venus. "Why is that?" Pettengill asks.
He adds that a key question now is how Venus gets rid of its internal heat. Magellan has shown that there is extensive vulcanism. But this doesn't seem an adequate heat transport mechanism.
JPL's Saunders says the Venusian surface "may be showing us what Earth was like before plate tectonics began."
Then there is the 4,200-mile channel. It is longer than the Nile River, making it the longest channel known in the solar system. Saunders calls its cause "a major mystery."
Even with the new questions, scientists no longer consider Venus an unknown body. In terms of a comprehensive planetary chart, Magellan's mission has made it the best-mapped planet in the solar system.