Venus has always held a certain fascination for sky-gazers. It’s the brightest object aside from the Sun and the Moon and it’s been named for three goddesses of love: the Roman Venus, the Greek Aphrodite, and the Babylonian Ishtar. As naked eye astronomy and myth gave way to scientific observation, Venus took on a different personality. Early Earth-based observations suggested it was a younger world and a tropical paradise, but better technology revealed it was hot and carbon dioxide rich. But there’s no better way to learn about a planet than to visit it. (Left, an artist’s concept of Mariner 2 – the first interplanetary spacecraft.)
In 1967, NASA developed a mission to send men to Venus. But before getting into the proposed manned mission, it’s worth stepping back to look at the state of NASA’s knowledge of Venus and its understanding of the interplanetary space a mission would have to go through to get there. Before this manned mission proposal, NASA had only sent one mission to Venus – Mariner 2.
Humans have given Venus a very interesting life. Our current understanding if Venus has its roots in the late 18th century. In 1761, Russian astronomer Mikhail Vasilievich Lemonosov discovered Venus’ atmosphere. Tracking its transit in front of the Sun in an attempt to determine its diameter, he observed a fuzziness around its edges. The only explanation was that Venus had an atmosphere with thick clouds. The myth of a habitable Venus was born. (Right, transit of Venus in front of the Sun.)
Venus as a habitable world – and every planet for that matter – became a focus in the late 19th century. In the wake of Darwin’s On the Origin of Species, scientists were quick to apply evolution to every world. If life arose on Earth, why not everywhere else? Orbiting closer to the sun and under thick clouds assumed to be made of water vapour like those on Earth, many imagined Venus as a swamp-like world. Its equatorial region was likely too hot for humans, but its equivalent of subarctic regions were thought to be quite pleasant.
It wasn’t until the 1920s that astronomers and scientists began applying spectroscopy to Venus. Spectroscopic observations, which can reveal a planet’s composition by registering an element’s interference pattern on reflected sunlight, uncovered a world shrouded in carbon dioxide. The image of a tropical swamp paradise was shattered though some held on the hope that it was a carbon-rich swamp, almost like an early Earth. No one could know for sure what was happening on Venus until there was a way to observe the planet up close. (Left, Venus with its thick cloud cover.)
The chance came in the 1960s. The dawn of the space age saw three separate races between the major players – the Soviet Union and the United States. Most popular is the race to put a man on the moon, but both countries were also racing to land a probe on Venus and Mars.
In 1962, NASA began cutting its teeth in interplanetary space with the Mariner program. The first target was Venus. Mariner 1 failed soon after launch, but Mariner 2 was a striking success.
The mission’s primary goal was to return the right data to allow scientists to determine wether Venus was a paradise off of Earth or something else. To answer this central question, the Mariner 2’s principle scientific payload were a microwave radiometer to determine the planet’s temperature and atmospheric details, and an infrared radiometer to determine the cloud structure and temperatures of their layers. (Left, Venus as seen by Mariner 10. This image has been enhanced to simulate Venus’ natural colour as the human eye would see it.)
But the spacecraft’s time near the planet was short. It was a fly-by mission; it wouldn’t go into orbit. The whole mission only gave Mariner 2 only a 45 minute window in which its microwave and infrared radiometers could measure Venus directly. The rest of its life would be spent in interplanetary space. Not wanting to waste any mission time, four other instruments on board gathered data on cosmic radiation and magnetic fields.
Mariner 2 launched August 27, 1962. Two days later, it was happily settled on its Venusian trajectory. Its cruise experiments were functioning and the spacecraft had established strong communications with Earth. Data was flowing back to NASA.
On December 7, Venus’ gravity began exerting a pull on Mariner 2 for the first time. A week later, the spacecraft made its short fly-by. After 109 days in space, Mariner 2 came within 21,645 miles (34, 854 km) of the surface traveling at 4.19 miles per second (6.743 km per second). Its microwave and infrared radiometers sprung to life.
Mission’s scientists were primarily looking for “limb darkening,” a phenomenon that indicates a planet’s temperature. Seen head-on, like the perspective Mariner 2 had approaching Venus, a planet’s atmosphere is thicker at its edges than on its face. If the heat of a planet is generated by a thick atmosphere, its sides or limbs will emit more energy and will register as being brighter. But if the planet’s heat comes from its surface, its limbs will be darker than the face. The heat from the surface will be brightest where it has less atmosphere to pass through. (An artist’s concept of the relative magnetic field shields of Venus and Earth. Credit: ESA.)
Both Mariner 2’s infrared and microwave radiometers observed limb darkening. Venus’ surface was hot, and the tropical paradise theory took a big hit. Limb darkening was only the beginning. Venus was an oven. The temperature on Venus’ sunlit side was 187 degrees Celsius (367 Fahrenheit), its dark side was 127 degrees Celsius (261 Fahrenheit), and along the terminator (the zone where the light side gives way to the dark) was 296 degrees Celsius (566 Fahrenheit).
Measurements of Venus’ atmosphere found very that carbon dioxide had very little effect on the sunlight reflected from the planet. Since scientists knew from Earth observations that Venus’ clouds were composed largely of the gas, its small effect on light meant that the sunlight Mariner 2 was analyzing was bouncing right off the cloud layer. Had it been coming through the clouds from the surface, the absorption of carbin dioxide would have been much stronger.
Part of the mission’s science goals were to measure radiation, but Mariner 2 failed to find any radiation coming from the planet. But this wasn’t conclusive proof that Venus lacked a magnetic field. There was a possibility that it might have a weak magnetic field that Mariner 2 couldn’t reach. Venus’ proximity to the Sun and consequently strong solar winds could have the effect of constricting the magnetic field to a close sphere around the planet. But if Venus did have a magnetic field, project scientists reasonably assumed it was somewhere in the neighbourhood of 0.1 percent of Earth’s. (Venus’ surface as seen by the Magellan spacecraft. 1994.)
Mariner 2 also found that Venus didn’t have a strong effect on any charged particles in interplanetary space. The mission also helped scientists determine Venus’ mass. It was found to be the closest in size to Earth with a mass a little more than o.8 times that of Earth.
Mariner 2 continued to send data back to NASA after the fly by. But the organization lost contact with the spacecraft in January 1963. It failed to respond to 50 commands sent in one day. Subsequent efforts in May and August were equally unsuccessful. Mariner 2 is silent now, somewhere in a heliocentric orbit.
In the mid-1960s, NASA turned its attention to Mars. The hostile environment on Venus made it impossible to harvest life, and finding traces of life is what NASA was after. Mariners 3, 4, 6, 7, 8, and 9 were all sent to Mars. NASA did return to Venus with Mariner 5 in 1967 and Mariner 10 made a fly-by of Venus and Mercury in 1974. (A schematic diagram of the Mariner 10 spacecraft.)
The Soviet Union, on the other hand, turned its attention to Venus in the mid 1960s. After experiencing the first of many failure on Mars in 1962, it pursued a landing on Venus and the return of surface images as the ultimate goal. Venera 9 accomplished these goals in the early 1970s.
NASA’s final report on Mariner 2 was published in 1965. It answered a lot of questions, but also asked many more. With this as the primary knowledge base, NASA began looking into the possibility of a manned mission to Venus. The idea was to build on Mariner 2’s findings and at the same time prove that humans are capable of long duration spaceflight.
The manned mission to Venus is a daring and fascinating proposal that really deserves to be treated on its own. I’ll give it my undivided attention in an upcoming post.
Richard Corfield. Lives of the Planets. Basic Books: New York. 2007.
Mariner-Venus 1962 Final Report. National Aeronautics and Space Administration. 1965.