INFO 35-1998: SOHO is nearly back in business
15 October 1998Brilliant new pictures of the Sun from the solar spacecraft SOHO show that its ordeal is coming to a happy ending, nearly four months after the ESA/NASA mission seemed lost in space on 24 June. Images from the Michelson Doppler Imager and the Extreme Ultraviolet Imaging Telescope on SOHO (the Solar and Heliospheric Observatory) can be already seen on the Internet.
"Scientists on both sides of the Atlantic have waited anxiously for the recovery of SOHO," commented Roger Bonnet, ESA's director of science. "Thanks to the extraordinary determination and skill of ESA and NASA personnel, with industrial contractors and scientific teams also playing their part, the world has recovered its chief watchdog on the Sun. SOHO is needed more than ever, because the Sun is rapidly becoming stormier with a mounting count of sunspots."
"It's very exciting to see these images again after so many weeks of concern. We hope that all the SOHO scientific instruments can be returned to the same level of health, so we can resume normal scientific operations in the near future," said Dr. Joseph Gurman, the U.S. project scientist for SOHO, and co-investigator on EIT.
"As of today, nine of the twelve instruments on board SOHO have been turned on. Four of them are already fully functional, the other five are still undergoing careful recommissioning activities. But so far no signs of damage due to thermal stress during the deep freeze have been detected. I tip my hat to the engineers who built this spacecraft and these sensitive but robust instruments," said Dr. Bernhard Fleck, the ESA project scientist for SOHO. The remaining three instruments will be switched on over the next few weeks.
The images are the latest success for the team during a complex, challenging recovery sequence. On 23 July, SOHO was located using radar techniques with the 305-metre Arecibo, Puerto Rico, radio telescope of the US National Astronomy and Ionosphere Center (NAIC) as a transmitter and a 70-meter dish of the NASA Deep Space Network (DSN) as a receiver. SOHO first responded to radio transmissions on 3 August, and telemetry from SOHO was received 8 August, telling controllers the condition of the spacecraft and its instruments. The spacecraft's frozen hydrazine fuel was gradually thawed, and on 16 September, SOHO's thrusters were fired to stop its spin and to place it in the correct orientation towards the Sun.
Prior to the interruption, instruments on SOHO had taken about two million images of the Sun, which represents over a terabyte (a trillion bytes) of data. After its launch on 2 December 1995, SOHO revolutionized solar science by its special ability to observe simultaneously the interior and atmosphere of the Sun, and particles in the solar wind and the Sun's outer atmosphere.
SOHO observations have been the subject of more than 200 papers submitted to refereed, scientific journals. Apart from discoveries about flows of gas inside the Sun, giant "tornadoes" of hot, electrically charged gas, and clashing magnetic field-lines, SOHO also proved its worth as the chief watchdog for the Sun, giving early warning of eruptions that could affect the Earth.
SOHO operates at a special vantage point 1.5 million kilometers (about one million miles) out in space, on the sunward side of the Earth. The spacecraft was built in Europe and it carries both European and American instruments, with international science teams. SOHO was launched on an Atlas IIAS rocket and is operated from NASA's Goddard Space Flight Center in Greenbelt, Maryland.
In April 1998, SOHO's scientists celebrated two years of successful operations and the decision of ESA and NASA to extend the mission to 2003. The extension would enable SOHO to observe intense solar activity, expected when the count of sunspots rises to a maximum around the year 2000. It would remain the flagship of a multinational fleet of solar spacecraft, including the ESA/NASA Ulysses and Cluster II missions.