Spacecraft Tandem Provide New Views of Venus
August 13, 2007
NASA’s MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft and the European Space Agency’s Venus Express recently provided the most detailed multipoint images of the venusian atmosphere ever seen. The images result from a June 5 flyby of Venus by MESSENGER during its long journey to Mercury. Venus Express already was in orbit at the planet. The two spacecraft carry sets of instruments employing different observation techniques that complement each other.
The data collected at Venus are now being analyzed by teams on both sides of the Atlantic and, as can be appreciated in the first images that have been released, already hints at the potential of the results to come.
The particular orbital geometry of Venus Express when MESSENGER skimmed past Venus on June 5 meant that the two spacecraft were not at the same location (with respect to the surface of the planet) at the exact same time. MESSENGER made its closest approach at a distance of about 338 km from the planet over the planetary coordinates 12.25°S and 165°E, on the nightside of the planet. Meanwhile, Venus Express was behind the horizon, almost right above the South Pole, at about 35,000 km from Venus. So how could they make true joint observations of the same regions and phenomena? Scientists came up with a highly creative solution.
The scientists used a computer simulation based on real atmospheric data about Venus obtained from previous ground and space observations. Knowing the speed of the local winds, which depend both on the altitude and the latitude, they were able to predict where a particular set of clouds would be at a given point in time. For their observation, the Venus Express scientists selected a cloud that — moving west by about 90° longitude every day — was visible to Venus Express and would be in view of MESSENGER 12 hours later, at the time of its closest approach. The same cloud became visible again for Venus Express 12 hours after MESSENGER’s closest approach, this time on the nightside.
The VIRTIS imaging spectrometer onboard Venus Express probed the cloud at several wavelengths. The observations provided a view of the cloud at about 45–50 km altitude from the planet. The Mercury Laser Altimeter (MLA) instrument on board MESSENGER probed the same cloud structure at 50–75 km from the surface. Such an observation — a typical example of atmospheric structure at Venus — with cross-sections obtained at different altitudes and with different instruments, is a unique opportunity for researchers hoping to solve the puzzle of the venusian atmosphere’s dynamics and composition.
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