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Engineers struggle to fix deep-space observatory

Posted May 15, 2013

Mission managers with NASA's Kepler space observatory announced Wednesday that a second reaction wheel failed. The mission team wrote: "Reaction wheel #4 remained at full torque while the spin rate dropped to zero. This is a clear indication that there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing." This comes after a failure of another of the spacecraft's four reaction wheels last summer and threatens to end the mission.

Kepler was launched by NASA in March 2009 to hunt for planets outside our solar system, called exoplanets. The primary mission of 3.5 years was extended through 2016 to give scientists more time to continue the hunt. The telescope watches distant stars detecting a tiny decrease in the brightness when planets pass in front. This requires precise aiming of the telescope as it orbits the sun, trailing just behind Earth.

Reaction wheels are mounted inside spacecraft like Kepler and the Hubble Space Telescope to keep them aimed at the target they are observing as well as keep them lined up with ground stations on Earth to transmit images and receive commands. Reaction wheels take advantage of the conservation of angular momentum to rotate a spacecraft in the microgravity of space.

An electric motor, powered by the spacecraft's solar panels, rotates a flywheel causing the spacecraft to rotate in the opposite direction. 

Equipped with four reaction wheels, spacecraft like Kepler can be efficiently pointed in any direction without using precious fuel for thrusters. At least three functioning wheels are required for normal spacecraft operations.

Kepler's sensors have been indicating increased friction in one of the wheels since the beginning of the year, prompting engineers to give the spacecraft a rest for several weeks. When that didn't help, they tried higher speeds and higher operating temperatures for the wheels in an attempt to nurse a bit more life out of the lubricants.

As efforts to bring the failed wheels back online continue, engineers are making preparations for a backup plan dubbed "hybrid mode."   This makes use of the two remaining reaction wheels along with the spacecraft's thrusters. While this is hoped to squeeze a bit more life out of the spacecraft, it may limit its effectiveness to only the largest exoplanets. Discovery of more Jupiter-sized or larger planets might be possible, but Kepler's days of finding Earth-sized planets may be behind it.

At the time of Kepler's launch, 340 exoplanets were known. In 4+ years of operations, Kepler has added 2,740 candidates and 132 confirmed exoplanets to that list. Kepler is credited with discovery of Earth-sized planets in the habitable zone around their stars. The mission also gained fame in 2011 with discovery of Kepler-16b a rocky, icy planet orbiting twin suns. Star Wars fans quickly dubbed the planet "Tatooine."

All that from a tiny patch of sky. This has led astronomers to an estimate of over 17 billion stars in our galaxy alone with Earth-like planets orbiting them.

You can find the area of the night sky Kepler has been studying.

Shortly before midnight, the constellation Cygnus the swan will rise in the northeastern sky, an X shaped constellation located among the Milky Way. The area Kepler has been watching is centered over Cygnus's right wing or the upper left portion of the X (see the diagram on the left). Extend your arm and cover the area with your thumb for a rough idea of how small a patch of the night sky Kepler has been watching.


Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.

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