Sky & Telescope February 1984 [antikvár]

Radar Views of VenusTHE SOVIET UNION has two scientific spacecraft in orbit around Venus, and American planetary scientists are puz-zled. When Veneras 15 and 16 reached the planet last year (on October lOth and 14th, respect àvely), t here was widespread spéculation that each craft carried a radar system to map the cloud-covered surface below. The rumors proved true in the months that followed, a handful of radar images were included among the infre-quent releases from the Tass news agency. Unfortunately, Soviet scientists gave their American colleagues little else to work with. One Venus specialist, looking over the images, conceded, "We're not even sure where they are on the planet."The situation is unlike that surrounding the previous Venera missions, whose instrumenteel probes dropped onto the plan-et's hellish surface in March, 1982, to pho-tograph and analýze the surroundings (ST: May, 1982, page 452). Americanswere even invited to assist in choosing the Venera 13 and 14 landing sites, bringing to bear radar data gained by an instrument aboard NASA's Pioneer Venus orbi-ter during the late 1970's.This time, however, Western observers are still trying to piece together what is go-ing on. The twin spacecraft are apparent-ly more advanced than those used on earli-er flights. For example, their communication antennas are larger and their teleme-try rates higher (up to 100,000 bits per second) than those on Veneras 13 and 14. In addition to radar mappers, the orbiters carry infrared spectrometers built in East Germany for atmospheric studies and radar altimeters for determining the height of topographie features to within 50 meters. The spacecraft also orbit Venus in identical, 24-hour polar orbits; these highly eccentric tracks come to within about 1,000 km of the surface at a point 60° north of the equator.Each Venera carries a "synthetic-aper-ture" radar system with a rectangular antenna 6.0 by 1.4 meters in size. It bounces microwave pulses off the groundj area roughly 7° to 17° to the side of the track directly below the spacecraft, jthen listens for the echoes. According to Ste-phen Saunders of the Jet Propulsion Labo-ratory, the use of such small incidence an-gles will reveal subtle topography well, but generally they are not the best ones for ge-ological study. (See the discussion of radar imaging on page 138 of the February, 1982, issue of this magazine.)Saunders is the JPL project scientist for NASA's Venus Radar Mapper (VRM) mis-sion, to be launched in 1988. VRM will also carry synthetic-apertuie radar, though the mission plan calls for it to pass much closer to the cloud tops and employ incidence angles as high as 45°.Since the on-board radar systems can operate effectively only when close to the planet, the Veneras' highly elliptical orbits necessarily limit coverage to the northern hemisphere. Each radar scan is 150 km wide and up to 9,000 km long, which means that Venus' previously unobserved north pole will be scanned but anything within about 18° of its equator will not. American scientists find this particularly puzzling, since the equatorial region con-tains some of the most interesting geologi-cal terrain. It is also where the Russians plan to drop off two instrumented landing probes in June, 1985, en route to a flyby of Halley's cornet.The Venera spacecraft seem to be oper-ating as planned. Each has been relaying long, narrow swaths of data to Earth daily. As Venus rotates, the radar tracks appear to "walk" eastward around the planet about 1 Vj 0 per day; at this rate, coverage through all longitudes will not be complete until mid-June. An American scientist visiting Moscow during December was shown two complete scans and told that many more were being processed.Geologists are keenly interested in how well the Venera radar systems will resolve features on Venus' surface many feel a planet's geologie history cannot be unrav-eled unless surface details are observed with 1-km resolution or better. The Venera images released late last year and re-produced here have picture elements (pixels) about 1 Vj km across, though the actu-al surface resolution is somewhat coarser. This puzzles Saunders even more: "Our radar people have been at a real loss to understand why these images are appar-ently as poor as they are, because their antenna is something we'd love to have on VRM," which will use a leftover Voyager dish antenna 3.7 meters across.In fact, the Venera images may be noIn this pair of Venera images, the multiringed features are roughly 100-200 km in diameter and are probably large impact eraters. Note the central peak in the one at left.110 Sky Telescope, February, 1984This radar image of Venus* surface was recorded by one of two Soviet spacecraft now in orbit around the planet. It was released by the Tass news agency on November 16th, about one month after Veneras 15 and 16 began their investigations. The official caption calls the prominent circuler feature, a "volcanic cupola [caldera] in the region of Metida." American scientists believe "Metida" refers to a highland area named Metis Regio near Venus' north pole. Streaming radially outward from the caldera are ridgelike features that may be the results of volcanic flows.