BOSTON — WHILE the Hubble telescope team struggles to make the most of a flawed mirror, planetary astronomers are preparing to activate a precision instrument of their own - the Magellan Venus radar mapper. A few seconds after noon Eastern daylight time today, the spacecraft will be 138,465,041 miles from Earth and 2,459,520 miles from Venus. It will go into Venus orbit Aug. 10 ready to map that planet's surface and measure its topography with unprecedented detail.
``In a sense, we know Venus better than we know Earth,'' says Gordon Pettengill, principal investigator for the Magellan radar experiment. Magellan, he adds, will enable us to know Venus even better.
The Massachusetts Institute of Technology radar astronomer explains that ``while we can make quite accurate topographic charts of Earth, the ocean bottoms are not so accurately known.'' The Pioneer Venus craft launched 12 years ago gave a rough radar overview of the topography of the entire planet, and now awaits the new arrival. Magellan's mapping radar and radar altimeter are expected to image surface details as small as a few hundred meters across and to chart heights to a precision of 30 meters (98 feet).
Launched May 4, 1989, the 7,604-pound spacecraft and its 4,721-pound Star 48 braking rocket took the long road to Venus, having orbited the Sun 1.6 times. At Venus, the rocket will insert Magellan into an elliptical orbit inclined 86 degrees to the planet's equator. Magellan's distance from the planet will range from 155 miles to 4,989 miles as it travels around this orbit every 189 minutes.
After a 21-day checkout, Magellan is to begin its mapping task. The 12-foot diameter radar antenna is too small to have the resolution required. It will observe the planet in a series of overlapping strips.
The mapping will be done for a 37-minute period each orbit when Magellan is closest to the planet. These data will be transmitted to Earth to be processed in a way that simulates the capability of an antenna hundreds of meters long. This will resolve details ranging from about 120 meters (390 feet) near the equator to 300 meters (980 feet) near the north pole. By comparison, the best resolution of past American and Soviet Venus radar scans is 2,000 meters (6,560 feet).
Measurements of tiny variations in Magellan's orbit made by monitoring its radio signal will complement the mapping. These changes reflect variations of the planet's gravity due to geological structures beneath its surface.
Venus is dry and rocky with an outer crust of varying thickness and unknown volume. Unlike Earth, it has no moon or magnetic field. Also, for reasons unknown, it rotates slowly and opposite to Earth's spin - the sun rises in the west and the planet takes 243 days to turn once on its axis.
The most striking difference from Earth is Venus's thick, mainly carbon dioxide atmosphere. The surface pressure is 90 times higher than Earth's sea-level air pressure. A thick blanket of cloud, laced with sulfuric acid, hides the surface. This atmospheric blanket has a super greenhouse effect that keeps Venus surface temperature a sizzling 850 degrees F.
Comparing Earth and Venus last March in the magazine Science, planetologist William Kaula of the University of California at Los Angeles pointed out that this insulating atmosphere ``greatly affects the long-term evolution of the bulk of Venus ... and raises the temperature throughout the interior.''
In spite of such differences, scientists often refer to Venus as Earth's ``sister'' planet. Professor Pettengill notes that with 95 percent of Earth's radius, 98 percent of its density, and 90 percent that of its surface gravity, Venus has important similarities to Earth. Processes such as volcano formation or crustal upthrusting should be moderated by gravity in similar ways. Also, he adds, Venus presumably has the same radioactive constitution as Earth so it may be producing internal heat from this source to roughly the same extent as our own planet.
Pettengill says he thinks it is similarities like these ``that are driving the geologists' interest.'' Yet he cautions that Venus may not have some of the processes that Earth has or have them to the same extent. Dr. Kaula made a similar point when he warned that ``hypotheses developed to account for characteristics of Earth in many cases should lead to quite different interpretations when adapted to Venus.''
Plate tectonics may be one of the major Earth processes absent on Venus. Earth's crust is broken into plates whose relative movements generate most of the earthquakes and account for much volcanic action. New crust forms along ocean ridges as material wells up from below and spreads out. Old crust recycles into the interior in the trenches found along many ocean margins. Pettengill says ``we're virtually certain'' Venus doesn't have such plate activity.
He explains: ``If there is any, it's what is called a one-plate planet. It might have had some small amounts initially. But it certainly doesn't have anything like the extent [of plate action] that Earth has, for reasons we don't fully understand.''
Another area of interest is the thick atmosphere. Venus has about the same inventory of carbon dioxide (CO2) as does Earth. Yet on Earth, most of that inventory became absorbed into the sea and locked up in sedimentary rocks. On Venus, the CO2 has made a massive insulating blanket. Finding out how this profound difference between the two planets came about and how it has influenced their development is a major scientific objective.
As Kaula noted, Magellan's detailed mapping will help settle some of the questions ``but will doubtless give rise to many new surprises.''
As of this writing, Magellan is working well and is ready for its assignment. It successfully ran through 100 hours of maneuvers last May. These included a simulation of 28 orbits of Venus with radar mapping and subsequent data relay to Earth. The craft made its last course correction a week ago July24 when controllers used a puff of rocket thrust to change Magellan's speed by 4/10,000ths of a mile per hour.
However, Pettengill notes that the all-important data-processing ground facilities that will produce the pictorial images are not on line yet. The National Aeronautics and Space Administration decided to delay building the facilities for a year to cut costs in the $550 million Magellan program. ``That's a conscious decision they made,'' he says, ``and it has everybody scrambling now.''
Pettengill adds that this is not crippling. In fact, he says, ``We could even delay analyzing the data for a while if we have to.''
He sees little risk of a replay of the Hubble telescope disappointment if the data facilities do not produce the promised high-quality images in timely fashion. He explains: ``It's different from Hubble in the sense that, if there's anything found to be wrong on the ground, we can fix it. Unlike Hubble, we do all of our focusing on the ground. The equivalent of what they do - the formation of images - it's all done on the ground. ... So it gives us a lot more flexibility.''