MIT Physicist Sets Sights on Venus

FOR radar astronomer Gordon Pettengill, the arrival of the Magellan spacecraft at Venus Aug. 10 will symbolize both a high point in his own career and a new era of opportunity for the science he helped establish. The Massachusetts Institute of Technology physicist has been probing the inner solar system with radar for over three decades. He was among the first to bounce a radar signal off Venus.

Now, as principal investigator for Magellan's radar experiment, Professor Pettengill looks forward to mapping that cloud-shrouded planet with a thoroughness that surpasses Earth cartography, given that the latter's sea bottom is not fully charted.

``It's going to be a very complex but very, very interesting undertaking,'' he says in reflecting on his science's progress. ``I can see myself involved in Magellan at least through my retirement.''

Pettengill also looks forward to the Mars Observer craft that the National Aeronautics and Space Administration (NASA) plans to launch in 1992. His team has a laser altimeter on board that he says ``will make exquisitely accurate measurements of the surface heights on Mars'' as it orbits that planet.

This ability to put radar - both microwave and laser radar - in orbit around planets has transformed Pettengill's work. No longer must he struggle to make the most of faint echoes of pulses sent from Earth. He can scan a planet with the ease of a science fiction starship.

But this has not made ground-based radar astronomy obsolete. In fact, Pettengill says, it now is a more important tool than ever for solar system exploration.

He explains: ``Right now, the major interest is in asteroids. They're shooting them off like fish in a barrel. They're up to 20 or 30 already that have been measured by radar ... [out of] probably several hundred that can be seen.''

Such observations are an important complement to telescope sightings. Radar gives a far better size estimate for asteroids than can optical measurements.

Pettengill notes that a powerful unit like the recently upgraded Arecibo instrument in Puerto Rico, with its 1,000-foot diameter, can measure distances to an accuracy of 15 to 20 meters.

To make his point, Pettengill cites the work of a team led by one of his former students, Steven Ostro of the California Institute of Technology's Jet Propulsion Laboratory.

Team members used the Arecibo radar last year to image a newly discovered asteroid named 1989 PB. As published in Science last June, the radar images resolved details clearly enough to show that a pair of asteroids had collided to form what astronomers call a contact binary.

The asteroid came as close as only 11 times the distance to the moon. It is so small that, even at that short distance, it was just a point of light in optical telescopes. Yet the radar measured its overall size - about 1.5 kilometers long by about 0.75 kilometers wide - and showed it consists of two almost spherical bodies, each a little under a kilometer across.

Pettengill expects radar to be valuable for studying comets as well. He notes that, until spacecraft scanned Comet Halley, radar had given the only firm confirmation that a comet has a solid nucleus.

Future spacecraft will probe a few other comets and a few asteroids. NASA, for example, hopes to launch its Comet Rendezvous Asteroid Flyby (CRAF) mission later in this decade.

Radar can help CRAF identify suitable targets. And Pettengill points out that radar will be one of the principal means for studying the hundreds of asteroids and comets that come into the inner solar system and that no spacecraft will visit.

But that is work for a new generation of radar astronomers. Looking forward to Magellan's mission, Pettengill says that his particular interest lies in trying to understand ``some very peculiar'' surface properties on Venus. These show up as areas of high radar reflectivity.

He explains: ``We don't know what the surface is made of in these regions except it's probably some electrically conducting mineral. ... We know it's found predominantly at high altitudes on Venus. But there's now some evidence that, when a meteorite plunges into the surface and kicks up ejecta from below the surface, some of that ejecta is of this same mystery composition.''

Magellan's radar, with its ability to produce detailed images, should help locate these mystery regions precisely. That will be a major step toward finding out what they really are.

Asked what he would like to send to Venus after Magellan, Pettengill replies ``landers, long-lived landers.'' Several Soviet craft have landed on Venus. But they all gave up in about an hour as they cooked in the fierce Venusian heat.

He points out: ``What we need to do is to pursue high-temperature techniques - electronics and other types of machinery that can last. Now whether this is done by going into new solid-state developments that can work at high temperature or whether it's done by super air conditioning that can maintain a cool environment inside, we need to study it.''

Pettengill notes that one of the most important types of information not yet gathered on Venus is seismic data. Geologists can use these to probe a planet's interior. ``We need a lander that can put a sensor into the soil and listen,'' he says.

Another information class is surface material analysis. Soviet landers have made some crude analyses. But, Pettengill notes, while these showed the elements in the material sampled, they did not reveal how these are put together. Again, long-lived automated laboratories are needed.

Pettengill speculates that these need not actually be on the surface. He points out that, a few tens of kilometers up in the atmosphere, there's an environment more or less at room temperature and one (Earth) atmosphere pressure. There is sulfuric acid present. But a balloon-borne laboratory could be designed to shield against the acid. Then, Pettengill adds, devices could be sent down to grab surface samples and return them to the floating laboratory.

He explains: ``There are many ways of doing this. I'm not trying to prejudge it. ... These are areas where I think Venus needs to be tickled in the future. The one problem is there are no present plans to do this.''

Although that may sound like a lament that the United States isn't doing enough in planetary science, it is not.

Pettengill says he actually is optimistic about the program because so many exciting missions now are under way or planned. Magellan is approaching Venus. Galileo is on its way to survey Jupiter and its moons. The Mars Observer is in preparation, to name just three.

``I've seen a lot worse situations than that, believe me, in the last 15 years,'' Pettengill says.

Asked if this is a good time for young scientists to go into the planetary field, he notes that``at the moment, the [job] opportunities are not limitless.'' Yet, he adds: ``My advice ... always would be, if you're seriously interested in it, pursue it. That's going to be your bread and butter and you'll make it work your own way if you're good and really interested in it.''

Second of two articles. The first ran yesterday.