If one compares two images from SOHO's C3 coronagraph on September 12, 2006, you can detect something has changed. The rather faint and obscure metal armature that holds the centered, blue occulting disk over the telescope lens (where the Sun would be) has moved 180 degrees from one image to the next. Why?
The SOHO spacecraft itself performed a 180 degree rotation and for many hours was taking no imagery that day during the maneuver. For the past few years SOHO has had a jammed antenna. The antenna can no longer be automatically commanded to point toward earth. To increase its ability to send at least some communications back to NASA's Deep Space Network here on Earth, SOHO has to rotate itself so that more of its signals can be received. Some of the time every few months, due to its orbit, it cannot send as much data and images back. We call this its keyhole period.
Such are the aches and pains of a spacecraft that is over 10 years old, a remarkable age of a spacecraft. However, we make sure that the images themselves are rolled right side up here on the ground so the solar north pole is always "up".
In 2003 ESA reported the failure of the antenna Y-axis stepper motor, necessary for pointing the high gain antenna and allowing the downlink of high rate data. At the time, it was thought that the antenna anomaly might cause two to three week data-blackouts every three months.[1] However, ESA and NASA engineers managed to use SOHO's low gain antennas together with the larger 34 and 70 meter DSN ground stations and judicious use of SOHO's Solid State Recorder (SSR) to prevent total data loss, with only a slightly reduced data flow every three months.[2]
The SOHO spacecraft itself performed a 180 degree rotation and for many hours was taking no imagery that day during the maneuver. For the past few years SOHO has had a jammed antenna. The antenna can no longer be automatically commanded to point toward earth. To increase its ability to send at least some communications back to NASA's Deep Space Network here on Earth, SOHO has to rotate itself so that more of its signals can be received. Some of the time every few months, due to its orbit, it cannot send as much data and images back. We call this its keyhole period.
Such are the aches and pains of a spacecraft that is over 10 years old, a remarkable age of a spacecraft. However, we make sure that the images themselves are rolled right side up here on the ground so the solar north pole is always "up".
In 2003 ESA reported the failure of the antenna Y-axis stepper motor, necessary for pointing the high gain antenna and allowing the downlink of high rate data. At the time, it was thought that the antenna anomaly might cause two to three week data-blackouts every three months.[1] However, ESA and NASA engineers managed to use SOHO's low gain antennas together with the larger 34 and 70 meter DSN ground stations and judicious use of SOHO's Solid State Recorder (SSR) to prevent total data loss, with only a slightly reduced data flow every three months.[2]