The two NASA STEREO spacecraft have, over the last few months, been in superior conjunction ─ a time during which they were completely out of Earth-contact due to their being behind the Sun ─ from which they are now emerging.
Launched in 2006, the STEREO mission comprises two near-identical space-based observatories in heliocentric orbits ─ one leading (STEREO-A) and the other lagging (STEREO-B) the Earth ─ with the aim of providing the first ever stereoscopic measurements to study the Sun and the nature of its coronal mass ejections (CMEs). After some 8 years of highly successful and scientifically productive operation, both spacecraft passed behind the Sun from the vantage point of Earth.
On approach to superior conjunction, it was, however, found that the operational temperature of the STEREO high-gain antenna (HGA) on each spacecraft was increasing markedly, due to their near-Sun pointing. To mitigate the potentially damaging temperature increase, strategies were devised for continued operation ─ either side of superior conjunction ─ based on the use of the HGA first and second side-lobes, rather than the main beam. Off-pointing the HGA away from the Sun has enabled the continued downlink of an, albeit reduced, data stream either side of the inevitable, but relatively brief, complete loss of downlink associated with superior conjunction.
On July 11, the first images in over three months were received from STEREO-A, the spacecraft having emerged from superior conjunction. The spacecraft, and all of its instruments, have been deemed fully operational. It should be noted that, unfortunately, STEREO-B encountered a severe problem prior to entering superior conjunction ─ associated with a planned reset performed as a test of solar conjunction operations ─ resulting in a complete loss of contact. NASA will shortly be resuming efforts to re-establish contact with STEREO-B, and there is continued hope that the spacecraft will be recovered. However, with or without STEREO-B, we still have a heliophysics observatory that rated extremely highly in the recent NASA Senior Review.
All of the imaging instruments on STEREO incorporate UK camera systems (RAL-e2v) and the STEREO/Heliospheric Imager (HI) instruments are led from the UK. The STEREO/HI instruments are not only the principal tools used for imaging CMEs, and other solar wind transients, in the heliosphere, but are also regularly used for the study of comets, stellar variability, dust and other phenomena. We look forward to the continuation of this highly productive mission and, as always, are very happy to discuss the exploitation of HI and other STEREO data with potential users.
For additional information on the emergence of STEREO-A from solar conjunction, please see:
http://stereo.gsfc.nasa.gov/news/news.shtml
and links therefrom.
For additional information regarding the superior conjunction, and the reduced STEREO science operations associated with HGA side-lobe operations, please see
http://stereo-ssc.nascom.nasa.gov/solar_conjunction.shtml
and links therefrom.
For additional information regarding the loss of contact with STEREO-B, please see
http://stereo-ssc.nascom.nasa.gov/behind_status.shtml
and links therefrom.
Richard Harrison, STEREO/HI Principal Investigator
Jackie Davies, STEREO/HI Project Scientist
RAL Space