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Planetary News: Venus (2006)

Venus Express Reaches Final Orbit


By A.J.S. Rayl
May 9, 2006
Venus Express in final orbit
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Venus Express in final orbit
Venus Express will make unprecedented studies of the largely unkown phenomena taking place in the Venusian atmosphere. Its suite of instruments will also investigate the interaction between the solar wind and the planetary environment. Finally, the mission will gather glimpses about the planet's surface as it is coupled with the dense atmosphere. Credit: ESA / AOES Medialab

Less than one month after slowing into orbit, and after 16 loops around the planet Venus, the European Space Agency's Venus Express reached its final operational orbit on Sunday, May 7, officials announced early today.

Actually, the spacecraft's success was known late Saturday night, when it communicated to Earth through ESA's ground station at New Norcia in Australia and the Venus Express ground control team at the agency's European Spacecraft Operations Center (ESOC) in Darmstadt, Germany received the advanced confirmation that the final orbit would be successfully achieved about 18 hours later.

Venus Express is the first orbital mission to Earth's "sister planet" since NASA's Magellan in the 1990s, which radar-mapped 98 percent of the surface and collected high-resolution gravity data. Europe's ship is one of a series of "express" spacecraft developed by ESA, so-called because they have been "built more quickly than any other comparable planetary missions," according to agency press kits. The series launched first with Mars Express, which arrived at Mars in December 2003. The agency then capitalized on the technology developed for the Mars mission by utilizing it on Venus Express.

The objectives for Venus Express are to study the thick atmosphere of this hot, hot planet, from the tops of its sulfuric acid clouds, to the roiling heat and crushing pressure of the air at the surface. By studying Venus' atmosphere, scientists hope to determine why it doesn't look more like Earth.

Launched on November 9, 2005, Venus Express arrived at its target on April 11, after a 5-month interplanetary journey to the inner solar system. The initial orbit -- or 'capture orbit' -- was an ellipse ranging from 330,000 kilometers (about 205,000 miles) at its furthest point from Venus surface or apocenter to less than 400 kilometers (about 250 miles) at its closest or pericenter.

As of the 9-day capture orbit, Venus Express had to perform a series of further maneuvers to gradually reduce the apocenter and the pericenter altitudes over the planet. This was achieved by means of the spacecraft main engine -- which had to be fired twice during this period (on April 20 and 23) -- and through the banks of Venus Express' thrusters -- ignited five times (on April 15, 26, 30, and May 3 and 6)

"Firing at apocenter allows the spacecraft to control the altitude of the next pericenter, while firing at the pericenter controls the altitude of the following apocenter," said Andrea Accomazzo, Spacecraft Operations Manager at ESOC in an official statement. "It is through this series of operations that we reached the final orbit last Sunday, about one orbital revolution after the last 'pericenter change maneuver' on Saturday, May 6."

Venus Express' trajectory to Venus
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Venus Express' trajectory to Venus
This artist's impression shows the trajectory of Venus Express to its final destination. In the first week of March 2006, the spacecraft crossed the path of the planet around the Sun. The trajectory took it inside the orbit of Venus to ‘anticipate' the celestial motion of the planet and finally to catch up with it on April 11, 2006. Once at Venus, the spacecraft will have traveled 400 million kilometers. The injection into orbit will put the spacecraft into a first, elongated orbit lasting about 9 days. On May 7, 2006, after a series of maneuvers and 16 ever smaller loops around the planet, Venus Express reached its final operational orbit, lasting 24 hours. Credit: ESA / C. Carreau

Venus Express entered its target orbit at apocenter on May 7, when the spacecraft was at 151 million kilometers (about 94 million miles) from Earth. Now the spacecraft is running on an ellipse substantially closer to the planet than during the initial orbit. The orbit now ranges between 66,000 kilometers  (41,000 miles) and 250 kilometers (155 miles), with the pericenter located almost above the north pole (80 degrees north latitude). The spacecraft currently takes 24 hours to complete one orbit around Venus.

"This is the orbit designed to perform the best possible observations of Venus, given the scientific objectives of the mission. These include global observations of the Venusian atmosphere, of the surface characteristics and of the interaction of the planetary environment with the solar wind," according to Hakan Svedhem, Venus Express Project Scientist. "It allows detailed high resolution observations near pericenter and the north pole, and it lets us study the very little explored region around the south pole for long durations at a medium scale," he concluded.

Until beginning of June, Venus Express will continue its "orbit commissioning Phase", started on April 22. "The spacecraft instruments are now being switched on one by one for detailed checking, which we will continue until mid May. Then we will operate them all together or in groups," said Don McCoy, Venus Express Project Manager. "This allows simultaneous observations of phenomena to be tested, to be ready when Venus Express' nominal science phase begins on June 4, 2006."