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Crippled space probe bound for second chance at Venus
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: November 21, 2011


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Japan's Akatsuki probe completed a three-part series of course correction maneuvers Monday, lining up the spacecraft for a second chance to reach Venus in late 2015 after an engine failure curbed plans to orbit the sweltering, cloud-covered planet one year ago.


Artist's concept of the Akatsuki spacecraft. Credit: JAXA
 
Flying on a spiraling trajectory through the inner solar system, Akatsuki fired its reaction control system thrusters three times in November to adjust its trajectory, setting up the spacecraft for another chance to enter orbit around Venus in November 2015.

The first two burns occurred Nov. 1 and Nov. 10, and each maneuver lasted almost 10 minutes. The final thruster firing Monday finished the task Monday as Akatsuki reach perihelion, or the closest point to the sun in its orbit.

All three maneuvers went well, according to Seiichi Sakamoto, a mission spokesperson at the Institute of Space and Astronautical Science.

Engineers turned to Akatsuki's smaller attitude control thrusters after failing to recover the craft's main engine, which shut down too early during a critical firing in December 2010 to enter orbit around Venus.

The engine burned for less than three minutes Dec. 6, much shorter than the length necessary to slow the spacecraft enough to slip into orbit.

A faulty valve or obstruction near the throat of the main engine nozzle may have caused the failure, and Japan's space agency said the engine "may have gradually been damaged" due to the anomaly.

Controllers tried to fire the engine again in September, but it generated a small fraction of its total thrust. The outcome of the engine test compelled Japanese officials to use Akatsuku's healthy, but less powerful thrusters.

Engineers sent commands in October to the probe to dump some of its propellant to reduce its mass ahead of the thruster firings this month.

The three maneuvers were crucial for Akatsuki to have a chance to reach Venus again, but Sakamoto said it's still not certain if the probe would be able to enter orbit in 2015. Other opportunities are available in subsequent years, he said.


Image of Akatsuki's main engine, which failed during the mission's Venus orbit insertion maneuver. Credit: JAXA
 
Akatsuki is Japan's second interplanetary mission to miss its target.

The Nozomi mission missed two chances to orbit Mars. The robotic orbiter was supposed to arrive at Mars in 1999, but a valve malfunction left the probe without enough propellant to reach the Red Planet. Officials replanned the mission to enter Martian orbit in 2003, but a solar flare zapped the spacecraft in 2002 and left it too damaged to attempt any arrival maneuvers at Mars.

Japan's Hayabusa mission missed its first chance to return to Earth with precious samples from the surface of an asteroid, but engineers devised a novel way to control the craft with ion thrusters and the probe limped back to Earth in 2010.

Akatsuki means dawn in Japanese, honoring the common position of Venus as the morning star in twilight skies.

Before the probe's launch in May 2010, Japanese space officials called Akatsuki the first interplanetary weather satellite, and it carries a suite of five cameras each designed to study a specific slice of the Venusian atmosphere, according to the Japan Aerospace Exploration Agency, or JAXA.

With a runaway greenhouse effect driving surface temperatures to nearly 900 degrees Fahrenheit and whipping high-altitude winds reaching 225 mph, Venus is a planet like no other in the solar system.

Previously called Planet-C and Venus Climate Orbiter, the $300 million mission was designed to observe the high-speed jet stream, study the source of sulfuric acid clouds, snap the first pictures of lightning at Venus, and search for active volcanoes.

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