Good news, bad news: GRAIL science underway, Venus Express suffers storm damage
Posted by Emily Lakdawalla
08-03-2012 18:54 CST
UPDATE TO THIS UPDATE: Venus Express' star trackers are back online, hooray!
Two brief mission updates. First, the good news: NASA announced yesterday that the twin GRAIL spacecraft have begun the science phase of the mission, transmitting precisely timed signals to each other in order to map the Moon's gravity field. An interesting tidbit from the press release: During the science phase, their orbit will range in altitude from 51 down to 16(!!) kilometers, and the spacecraft separation will range from 65 to 225 kilometers.
And now, the bad news. According to ESA, since the recent solar storm passed Venus, both of Venus Express' star trackers are suddenly unable to detect stars. Star trackers are used by spacecraft to precisely determine their orientation in space. They also have internal gyroscopes, but over time, small errors accumulate and the gyroscopes no longer provide correct orientation data. Star trackers use sightings on stars to periodically recalibrate their gyroscopes. (Even Opportunity does this, using a special neutral density solar filter on her Pancam to photograph the Sun and compare its position to where it was expected to be.)
Orientation is absolutely crucial information for a spacecraft; it's needed to point instruments at the planet, angle solar panels to the Sun, and aim radio signals at Earth. Some spacecraft can navigate using pictures of star fields taken by sensitive high-resolution cameras. But Venus Express does not have a high-resolution camera, and the one it has is designed to image Venus' bright clouds; I am not sure that it is sensitive enough to see stars.
According to a space.com article, there is reason to hope: the star trackers have "experienced similar glitches in the past," 5 to 10 times. But the "blindness" (as ESA described it) has never before lasted longer than 32 hours; at the time the report was issued this morning, the star trackers had been out of commission for 40.