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20th Century

Due to the considerable amount of time between transits, there were no transits of Venus in the 20th century. However, one of the most important scientific goals of observing the transit was to determine the distance to Venus and therefore the Sun. During the 20th Century we have been able to make use of new techniques to determine this distance and have even used these numbers to send probes to Venus itself.

Radio measurements

With the construction of large radio telescopes, such as the Mk1 telescope (now known as the Lovell Telescope) completed at Jodrell Bank in 1957, an entirely new way of measuring the astronomical unit became possible - that of making a direct measurement by radar of the distance of Venus at its closest approach of around 41 million km.

In time for the closest approach of Venus in 1961, suitably powerful radars were installed on the Mk 1 telescope and also at the 70m Goldstone dish in New Mexico. We knew that the Sun would be about 3.6 times further away than Venus at closest approach, and hence (using the precise values) a value of the astronomical unit was calculated. The Jodrell Bank value was 149,600,000 km.

At the next closet approach of Venus we used a different approach in which the relative motion of Venus and the Earth was measured over a 6 week period about the time of closest approach. This used a "carrier wave" or CW radar similar to that uses by Gatso cameras to measure the speed of cars passing them. The result of this experiment was 149,596,600 +/- 900 km.

Going there

The earliest mission to Venus was a fly-by of the Mariner 2 spacecraft in 1962. The probe took measurements of the atmosphere and magnetic field of the planet and also measured the "inter-planetary medium", the tenuous mixture of particles that exists in the space between the planets. There were two further fly-bys of NASA spacecraft with Mariner 5 and Mariner 10, each carrying more sophisticated instruments than it's predecessor. The Soviet Union also launched a series of probes to Venus from 1961 to 1983. Known as the Venera series, some were designed as orbiters while others actually landed on the surface and sampled the surface rock. In 1985, there were fly-bys of two further Soviet probes, known as Vega 1 and 2, on their way to an encounter with comet Halley. NASA's Pioneer project consisted of both a lander and orbiter which functioned sucessfully from 1978 until 1992. In 1990 the Galileo probe also passed by the planet on it's way to Jupiter. Finally, the Magellan spacecraft conducted several experiments, including mapping of the planet by radar, from 1990 until it's signal was lost in 1994. There are several future missions to Venus, both in the construction and early planning stages. One such mission is ESA's Venus Express which is scheduled to launch on a Russian Soyuz rocket in 2005.

Internet Links
The Lovell Telescope (Jodrell Bank)
Jet Propulsion Laboratory (NASA)
Mariner 2 (NASA)
Mariner 5 (NASA)
Mariner 10 (NASA)
Halley's Comet (SolarViews)
Pioneer (NASA)
Galileo (NASA)
Venus Express (ESA)
Venera
How Far to the Sun? (Ohio State University)
Description of radar (Wikipedia)