A NASA animation showing the approximate orientation of the space probe with Venus in the background.
Credit: NASA
SYDNEY: NASA's Messenger spaceprobe came within 350 km of the surface of Venus and is now on target to reach Mercury in January 2008 - the first probe to visit the innermost planet for 30 years.
Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft) has now used the gravitational pull of Venus twice to propel it towards its final destination.
The first flyby of Venus, which took place in October 2006, occurred during a 'blackout' period when no data could be gathered, as radio signals were blocked by the Sun.
This time the U.S. space agency scientists hope to have gathered valuable new data about Venus – but they won't know until Thursday, when radio signals containing images and other information are returned to Earth.
Tandem effort
This will be the first time two spacecraft have studied Venus in tandem. In 2005, the European Space Agency launched the Venus Express spacecraft. This has been collecting detailed data on the planet since April 2006 when it took up residence in orbit.
Having both spacecraft in orbit together offers "an unprecedented opportunity to study Venus's atmospheric circulation, cloud structure, atmospheric chemistry and solar wind interaction," said Sean Solomon, Messenger's principal investigator at NASA and a planetary scientist at Washington DC's Carnegie Institution.
Venus Express will gather information about the same regions of Venus both before and after Messenger's flyby. This will provide scientists with two sets of comparative data within a small time period.
In addition to generating detailed atmospheric data, Messenger has the capacity to map the composition and topography of the planet's surface, and to measure variations in the thickness of the crust.
Venus is the closest planet to us, and as such has become the most often visited by spacecraft from Earth. Although a great deal is known about the planet, one perplexing question remains: why does a planet so similar to Earth in terms of size, composition and solar distance have such a radically different climate and geological history?
"Each new mission to Venus makes small steps toward an eventual answer to that overriding question," said Solomon.
A runaway greenhouse effect is thought to have given the planet an atmosphere that is mostly carbon dioxide and an atmospheric pressure 90 times that of Earth. Surface temperatures are a scorching 400°C.
Mercury, unveiled
Launched in August 2004, Messenger is on track to becoming the first spacecraft to orbit the even hotter planet of Mercury. The spaceprobe is protected by a 'sunshade' of heat-resistant ceramic cloth. On the sunlit side of Mercury, it will endure temperatures of over 350°C and the Sun will appear 11 times brighter than on Earth.
Only one spacecraft has previously visited Mercury; NASA's Mariner 10 flew past Mercury three times between 1974 and 1975.
Mariner 10 carried no instruments capable of determining the composition of Mercury's surface and was only able to image 45 per cent of the planet. In contrast, Messenger will carry out remote chemical sensing of the surface, study the atmosphere and magnetic field, and capture colour images of the surface, said Solomon.
While the Venus flyby is useful in itself, it is also a critical step in moving Messenger towards Mercury. Being pulled into Venus' gravitational field will not only change its trajectory, putting it on course for its encounter with Mercury, it also slows the spacecraft down from 36.5 km/s to 27.8 km/s. This puts it on target to reach Mercury in January 2008 after an epic 7.9 billion km voyage.
"Typically, spacecraft have used planetary flybys to speed toward the outer solar system," said Andy Calloway, Messenger's mission operations manager at NASA. "Messenger – headed in the opposite direction – needs to slow down enough to slip into orbit around Mercury."
