Venus Science Options

[ljk4-1]

Sunspot, NM (Feb. 7, 2006) -- The planet Venus is best known for the thick layers of clouds that veil its surface from view by telescopes on Earth. But the veil has holes, and a New Mexico State University scientist plans on using a solar telescope to peer through them to study the weather on Venus.

"Observations of Venus from a nighttime telescope at a single location are very difficult because Venus is so close to the Sun in the sky," said Dr. Nancy Chanover, a planetary scientist at NMSU in Las Cruces, NM. "You can observe it for about two hours at most." Then the Sun rises and blinds the telescope (or Venus sets, depending on the time of year).

http://www.nso.edu/press/venus06/

[Guest_BruceMoomaw_*]

In the case of Mariner 1 & 2, there was never any question of it -- it was necessary to reduce the spacecraft's mass, and therfore its complexity, to an absolute minimum for the Atlas-Agena B to launch it to Venus at all. But even the earlier 500-kg Mariner A -- which carried a tape recorder and was originally planned for launch to Venus in 1962 by Atlas-Centaur, before it became excruciatingly clear in mid-1961 that the extremely troubled Centaur wouldn't be ready by then -- carried a multichannel microwave radiometer and a UV spectrometer, but no camera. It was simply not felt to be justified for Venus precisely because, while we'd known about the existence of the UV cloud patterns since about 1910 (!), they were not adequate justification scientifically for something as complex and data-hungry as a camera. If a third instrument had been added to Mariner A, it would certainly have been an IR spectrometer for more atmospheric composition and temperature data.

The Mariner 5 story is stranger. It was added to NASA's schedule on very short notice in December 1965 (along with the 1969 Centaur-launched Mars Mariners) after it became painfully clear that otherwise there would be no planetary probes at all between 1964 and 1973 (to which the gargantuan "Voyager" Mars landers had been delayed by then). But that very short notice for a mission which consisted of hastily reworking the backup third 1964 Mars Mariner spacecraft meant that they simply did not have time to develop and test many of the new experiments they would otherwise have put on it (UV and IR spectrometers and/or an improved microwave radiometer). Further budget cuts also meant that they had to quickly remove half of the 1964 Mariners' particle and field instruments, and cancel an Earth occultation photometer that could have measured cloud-top altitude. In fact, from the start, its top science goal was listed as determining Venus' surface air pressure (since Mariner 2 was thought to have nailed down its high surface temperature firmly, despite a few scientific dissenters) -- and so its top-priority science experiment was S-band radio occultation, which had been added to the 1964 Mariners as an afterthought!

But JPL neverthless recommended retaining the TV camera, with the red and green color filters replaced by one UV and one visible filter. The reason was that there was still some hope then that there might be gaps in the clouds that might allow the craft to get a glimpse of the surface -- plus the fact that it was fairly easy to retain the already-existing instrument and there was always a chance that it might make some useful observations on cloud patterns. However, NASA HQ overrode JPL and ordered the camera replaced by Stanford's dual-frequency radio occultation experiment (an improved version of the one on Pioneer 6), which was power-hungry enough that it and the camera could not both be flown, and which could both double-check the S-band experiment and obtain some additional data on ionospheric structure.

This is STILL the subject of dispute among scientists -- Bruce Murray, in his 1977 book on Mariner 10, bemoans the fact that the camera wasn't carried, but an article I saw in "Icarus" some time in the 1980s defends the decision. The one thing that puzzles me is that, had the camera been flown, its photos would apparently have been taken at close range like those on Mariner 4 -- although one would think that more distant whole-disk photos of Venus' cloud patterns would have been better. Some engineering reason, I suppose.

And of course we have the Soviets' Venera 2, which (like Venera 3 with its entry probe) came very close to successfully reaching Venus in early 1966, with a scanned-film camera system. I imagine this was an attempt to recycle one of the spacecraft the Soviets had planned to launch to Mars in 1964 along with Zond 2, and cancelled after that craft suffered a failure of one of its solar panels to deploy. (Zond 3, which successfully obtained the second set of lunar farside photos in 1965, was also obviously a recycled 1964 Mars craft -- its other instruments included IR and UV spectrometers which had little use for the Moon, but which were copies of the ones carried along with a camera on the 1962 Mars 1 probe. And it repeatedly replayed the lunar photos from distances up to several tens of millions of km, as an engineering test. There is, however, apparently still dispute as to whether Zond 2 itself was another flyby craft or an actual attempt to land a small parachute-equipped lander on Mars -- which would, of course, have crashed.)