Magellan Unveils Venus
NASA spacecraft uses radar to map planet's cloud-covered surface
PASADENA, CALIF. — BRILLIANT Venus punctuates the evening sky this spring. But the planet shines even brighter at Magellan project headquarters. Narrow strips of high-resolution radar images of its surface adorn the walls of an analysis room. They reveal vivid details of an alien landscape that had been largely a cloud-shrouded mystery a year ago. When the Magellan spacecraft, which began mapping the planet last Sept. 15, finishes its primary radar mapping mission May 15, its managers expect that some 80 percent of that surface will be laid bare.
Scientists here at the California Institute of Technology's Jet Propulsion Laboratory, which manages Magellan for the National Aeronautics and Space Administration (NASA), are ecstatic at that prospect.
``We've been feeling around in the dark prior to this. Now we're getting data,'' says Kathi Beratan, a science support-team analyst. She adds, ``We're actually going to end up with a more complete data set [of the Venus surface] than we have of the Earth [where ocean bottoms are poorly known] in terms of the resolution of the data.''
Magellan science and mission design manager Thomas Thompson calls Magellan's ability to see geologic features ``which weren't even hinted at in previous data'' a ``major step forward'' in planetary science.
Those features include such surprises as sinuous, river-like channels that run for hundreds of kilometers and may be carved by the flow of highly liquid lava. They also include dome-like hills that may represent thick lava outflows.
And for Dr. Thompson, who is interested in craters, the big surprise was the fact that the craters seen so far ``all, basically, looked fresh.'' There seems to be no surface weathering going on.
Magellan's radar resolution - the ability to see detail - is some 10 times better than any surface view of Venus available before. Thick clouds, laced with sulfuric acid, cover the planet. But radar has a clear view.
Magellan images show surface features ranging in size from about 120 meters (390 feet) near the equator to 300 meters (980 feet) near the north pole. The finest detail American and Soviet radar scans have produced before this is 1,000 to 2,400 meters (0.7 to 1.5 miles) across.
The difference between Magellan's sharper view and that of early probes is comparable to the difference between scanning Earth and seeing that Mt. St. Helens is clearly a volcanic mountain and not being able to distinguish it from the blurred image of a meteorite crater.
Members of the mission team are also very pleased with the spacecraft. They see its glitches in a different perspective from the impression of a troubled mission that may be conveyed by recurring reports of mishaps. These include loss of one of the two tape recorders used to store data for later transmission and intermittent dropout of radio contact.
Thompson says that to him, ``Magellan looks absolutely super.'' Dr. Beratan observes: ``People hear about our problems. But they don't realize it's been a fabulously successful mission.''
Mission analyst Rob Lock explains that this success is due partly to the elaborate safeguards built into the spacecraft. He notes that ``anything like the kinds of problems we've had would have caused just about any other nation's spacecraft to go belly up.'' He says Magellan's fault-protection system and the contingency planning it represents has enabled the mission team to take these challenges in stride.
He describes this on-board fault-protection system as being ``almost as complex as the whole spacecraft'' itself. It includes whole sets of computer programs - and even whole computers - that monitor the spacecraft, take corrective action where possible, and call for help from Earth when needed. He explains that it takes ``eight to 10 years to design a mission like this,'' because no single failure or any two simultaneous failures are to be allowed to ruin it. ``So we spend a hundred million dollars on software to protect a half-billion-dollar mission,'' he says.
This ability to stumble and keep going has enabled Magellan nearly to complete its primary mission with loss of only a few percent of the planned data. Mission team members expect to recoup that loss later.
FOR unknown reasons, Venus rotates slowly in a sense opposite to Earth's spin. The sun rises in the west and the day is 243 Earth days long. Thus Magellan's overall mission is broken into cycles of 243 Earth days each.
This pattern allows the planet to make one complete turn under the orbiting spacecraft during each observing cycle. The first cycle, which ends May 15, is devoted to the so-called primary mission that aims to produce a global map of about 80 percent of the planet's surface.
It will take several more months to process the data and build up that global map.
But the images already in hand have given planetary scientists plenty to think about as they try to understand what is happening on a planetary surface with a temperature of 450 degrees C. (850 degrees F.) lying under a carbon-dioxide atmosphere at 90 times the surface air pressure on Earth.
Crater specialist Thompson explains that, for incoming meteorites, passage through that atmosphere ``is equivalent to the passage through a kilometer or so of solid rock in terms of mechanical forces.'' Many smaller projectiles never make it through. Only the shock wave they produce hits the ground. Some meteorites break up into impacting fragments like cluster bombs. Only big projectiles, measuring kilometers across, come through intact. Magellan images show evidence of all three cases.
A recent Jet Propulsion Laboratory announcement reports that Venus may be venting interior heat through volcanic features called coronae. These are circular or oval features up to more than a kilometer high surrounded by a ring of ridges and troughs.
Magellan project geologist Ellen Stofen says the high-resolution data ``have revealed many exiting new aspects of coronae'' that point to a volcanic nature.
While scientists scan each new image for more clues, mission planners look ahead to the next mission cycles. They plan to fill in the blanks of their map. Then, mission analyst Lock says, ``we're going to have freedom.'' Instead of being tied to a pre-arranged mapping schedule, they will be free to revisit interesting features, change radar aiming, and otherwise follow intriguing research leads.
They would like, eventually, to circularize Magellan's orbit. Right now, it is in an eliptical orbit that, by design, enables it to map at varying distances from the planet. There is no authority yet for such a radical orbit change. And there is no main rocket power to do it with. But there is enough maneuvering-thruster fuel available to allow Magellan to skim through the outer atmosphere and use aerobraking to go into a tight circular orbit.
``A circular orbit would allow us to get a very uniform data set and let us be much more clear and accurate,'' explains Beratan. This, then, is the ultimate dream of project scientists. As Thompson puts it, after the currently planned jobs are done, ``I think that's the greatest thing Magellan could do.''