28 Sep 1999
"It was the equivalent of an emergency-room procedure to stabilize a patient," said Michel Verdant, ESA's SOHO Programme Manager. "But now we have new software. This is the medicine we need to cure the patient once and for all."
Indeed, the new software will make SOHO much more robust and reliable in case orientation problems ever happen again, Verdant said. On 27 September, ground controllers beamed the new software up to SOHO. Both software and spacecraft were developed mainly by prime contractor Matra Marconi Space. Now even if SOHO loses sight of the guide star it needs to orient itself to remain stable on all three axes, it won't have to resort to the Emergency Sun eacquisition mode (ESR), which up to now automatically meant halting all scientific observations.
"It's a major accomplishment to be out of this situation," said Bernhard Fleck, ESA's SOHO Project Scientist. "Before this new software, losing the guide star would have meant that we had no way of measuring the spacecraft's roll rate or of controlling it."
In case of trouble, the new software will now allow engineers to determine SOHO's drift by measuring the slight changes in the speed of the so-called momentum wheels. In a way, these wheels, which are normally used to impart the necessary force to point the spacecraft, will be used as if they were gyroscopes and to return SOHO to its correct orientation.
"It is very similar to using the gyroscopes, but here we are using the whole spacecraft as a gyroscope," said Verdant. And this sets a real space record. For the first time, a spacecraft is operating in space without the gyroscopes it was designed to rely upon.
To avoid possible problems with other programs already installed, the software was uploaded to a part of the spacecraft memory never before used. "The software is something we must always respect," said Francis Vandenbussche, SOHO System Engineering Manager. "I am very confident everything will work fine."
"This new software will provide an extra safety net to the satellite." In fact, if SOHO now lost its guide star, the spacecraft would automatically switch to others loaded in its memory. Should that method fail, control of the spacecraft would be regained by using the momentum wheels as reference. And that would still allow the satellite to continue its observations.
Finally, there will always be ESR mode, as a last resort. Besides halting all observations, it takes several days to bring the spacecraft back to top gear. "With this new software, we'll sleep much better," said Verdant.
About a week of so-called flight commissioning will be required before scientific observations -- which had been partly suspended -- can resume in full.
"What really amazes me is the competence and skills of these engineers who are capable of coming up with solutions to solve incredible problems," said Fleck.
Barring unforseen problems, SOHO should continue its mission until at least 2003, seven years after its launch, Fleck said. "But what I would really like to see is the spacecraft carry out observations for a full solar cycle of 11 years."
SOHO, the most sophisticated satellite ever to study the Sun, is part of a joint programme between ESA and NASA.
IMAGE CAPTION:
This EIT full sun image, taken on 14 September 1999 in the He II emission line at 304 A shows the upper chromosphere/lower transition region at a temperature of about 60,000 K. The bright features are called active regions. A huge erupting prominence escaping the Sun can be seen in the upper right part of the image. Prominences are "cool" 60,000 K plasma embedded in the much hotter surrounding corona, which is typically at temperatures above 1 million K. If an eruption like this is directed toward the Earth it can cause a significant amount of geomagnetic activity in the Earth's environment with a following spectacular aurora.
Instrument: EIT; Taken: Sept 14 1999, 07:19UT
"Sunquake" telescope appears to be OK - The Solar and Heliospheric Observatory (SOHO) instruments are getting checked out after SOHO's deep-freeze. First look is encouraging. (Space Science News, October 14, 1998)
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
European Space Agency Headquarters, Paris, France
October 14, 1998
Images from the Michelson Doppler Imager and the Extreme Ultraviolet Imaging Telescope on SOHO are posted on the Internet at: http://sohowww.nascom.nasa.gov.
"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 the Extreme Ultraviolet Imaging Telescope (EIT).
"As of today, nine of the 12 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 July 23, SOHO was located using radar techniques with the 305-meter Arecibo, Puerto Rico, radio telescope of the U.S. National Astronomy and Ionosphere Center as a transmitter and a 70-meter dish of the NASA Deep Space Network as a receiver. SOHO first responded to radio transmissions on August 3, and telemetry from SOHO was received August 8, telling controllers the condition of the spacecraft and its instruments. The spacecraft's frozen hydrazine fuel was gradually thawed, and on September 16, 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, an activity representing over a terabyte (a trillion bytes) of data. After its launch on Dec. 2, 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 enables SOHO to observe intense solar activity, expected when the count of sunspots rises to a maximum around the year 2000.
The first EIT image taken in the Fe IX/X line at 171 A is available at:
http://sohowww.nascom.nasa.gov/operations/Recovery/eit_171_981013.gif
The MDI image can be found at:
The latest SOHO EIT images can be found on the Web at:
http://umbra.nascom.nasa.gov/eit/eit_full_res.html
Details about the operations and about SOHO in general, can be found at:
http://sohowww.nascom.nasa.gov
Information on the recovery of SOHO can be found at:
http://sohowww.nascom.nasa.gov/operations/Recovery/operations/Rec overy/
"It's like putting a bottle of water in the freezer," said Dr. David Hathaway of NASA's Marshall Space Flight Center. "In one of these telescopes we have optical elements with metal mountings that contract differently than the optical elements, so we're worried that they may be cracked."
The instrument is the Michelson Doppler Interferometer - MDI - that is a key instrument for measuring magnetic fields and vibrations on the surface of the sun. Hathaway is an associate investigator on the MDI and used it to discover giant convective cells (right) that may play a major role in the sun's 22-year-long cycle.
SOHO has been high drama - literally - for the solar science community since June 24 when a communications error pointed it away from the sun. It lost electrical power and the thermal control that keeps the electronics, telescopes, and propellant at just the right temperatures.
Engineers scrambled first to locate the craft , and then to send a signal loud enough to be heard by the antenna which now was pointed away from Earth. Between Aug. 3 and Sept. 16 they received tentative signs that SOHO was alive, and then slowly brought it under ground control, pointed it in the right direction, and then thawed it out.
Over the last two weeks, scientists have been gradually reactivating the telescopes to see which ones work and whether SOHO can continue its phenomenal series of discoveries about the sun.
"It has been nice to see these other instruments come back on line," Hathaway said. "Some aspects of our helioseismology can be done by GOLF and VIRGO [see instruments listed at left], but they have more limited capabilities, and they have very narrow fields of view. The MDI shows the whole sun at high resolution, and gives us velocities and magnetic fields. It will be sorely missed if its lost."
One of the keys, Hathaway explained, is a calcite crystal within the MDI. The crystal is softer than glass and absolutely crucial to extracting from the sun's intense light a narrow set of measurements.
"If it's been cracked by the mounting contracting too tightly around the crystal, then it will just deliver scattered light and the MDI won't work," he said.
The MDI is scheduled to be powered up on Monday. The electronics and some moving parts will be exercised first, he said, followed by the camera itself. Scientists will know right away if the crystal survived the deep freeze. They will either get a clear image, or one that looks like a broken jumble.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
European Space Agency Headquarters, Paris, France
September 17, 1998
The SOHO flight operations team reported success in the maneuver, which is called attitude recovery, at 2:29 p.m. EDT Wednesday, the first time the joint European Space Agency (ESA) and NASA spacecraft has been controlled from the ground since SOHO spun out of control and communication was lost on June 24.
"It's a big step forward in our recovery plan for SOHO," says Dr. John Credland, ESA's head of scientific projects. "We were never quite sure that we would manage to make the spacecraft point back towards the Sun, which is essential for its proper operation. I congratulate our joint ESA-NASA team, helped by our industrial contractors, who have accomplished it."
"This is the best news we've had from SOHO in a long time," said Dr. George Withbroe, Director of the Sun-Earth Connection science theme at NASA Headquarters, Washington, DC. "Despite the gloomy early days after the loss, we always stayed hopeful that the resourceful people on the team could save the day. We're not there yet -- we still have to see if the scientific instruments survived. But this gives us reason to hope."
"Now we start a comprehensive check of all the spacecraft's systems and scientific instruments," said Dr. Bernhard Fleck, ESA's project scientist for SOHO. "We shall take our time and go step by step, in consultation with the 12 scientific teams in Europe and the United States, who provided the instruments. In some cases the instruments have been through an ordeal of heat or cold, with temperatures approaching plus or minus 100 degrees Celsius (212 degrees Fahrenheit). But I'm cautiously optimistic that SOHO can win back much of its scientific capacity for observing the Sun."
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 American and European instruments, with international science teams. NASA launched SOHO and has responsibility for operations at NASA's Goddard Space Flight Center, Greenbelt, MD.
After its launch on Dec. 2, 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 heliosphere. Apart from amazing 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.
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.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
European Space Agency Headquarters, Paris, France
August 4, 1998
Signals sent yesterday through the NASA Deep Space Network (DSN) station at Canberra, Australia, were answered by SOHO at 6:51 p.m. EDT in the form of bursts of signal lasting from two to ten seconds. These signals were recorded both by the NASA DSN station and the ESA station at Perth, Australia. Contact is being maintained through the NASA DSN stations at Goldstone, CA; Canberra; and Madrid, Spain.
Although the signals are intermittent and do not contain any data information, they show that the spacecraft is still capable of receiving and responding to ground commands.
"This is an excellent sign," said Dr. Joe Gurman, NASA SOHO Project Scientist at the Goddard Space Flight Center, Greenbelt, MD. "It means the spacecraft still has a heartbeat and gives us added optimism that we may be able to restore SOHO to scientific operation. Our next step, already underway, is to continue the careful process of attempting to re-establish control of the spacecraft. We will be attempting, in the near future, to begin data transmissions in order to get an assessment of SOHO's condition."
hr>
Space Science News: Radio astronomers find a lost satellite
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
European Space Agency Headquarters, Paris, France
July 27, 1998
Radio contact with SOHO, a joint mission of the European Space Agency (ESA) and NASA, was interrupted on June 24, an event under review by a joint ESA/NASA investigation board.
With the encouragement of Dr. Alan Kiplinger of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, CO, researchers at the U.S. National Astronomy and Ionosphere Center (NAIC) in Arecibo, Puerto Rico, used the facility's 305-meter (990-foot) diameter radio telescope to transmit a signal toward SOHO on July 23. The 70-meter dish of NASA's Deep Space Network in Goldstone, CA, acted as a receiver, locating the spacecraft's echo and tracking it using radar techniques for more than an hour.
Preliminary analysis of the radar data, which is ongoing, indicates that SOHO is still in its nominal halo orbit near the so-called "L-1" Lagrangian point in space, (a gravitationally stable vantage point 1.5 million kilometers ahead of the Earth) and is turning slowly at a rate of roughly one revolution per minute. Staff members of NAIC and the Deep Space Network, in close cooperation with ESA and NASA, are continuing to analyze the radar data to extract more precise information on SOHO's location and motion, which in turn could help in future recovery efforts, as SOHO's solar panels turn toward the Sun.
ESA and NASA engineers also are continuing their efforts to re-establish radio data communication with the spacecraft, encouraged by the radar measurement of a slow spin rate, which suggests minimal structural damage has occurred.
The NAIC team was led by Dr. Donald Campbell. The NAIC is operated by Cornell University, Ithaca, NY, under a cooperative agreement with the U.S. National Science Foundation, Washington, DC. The Deep Space Network is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
European Space Agency Headquarters, Paris, France
July 16, 1998
Meanwhile, the NASA/ESA investigation board concentrates its inquiry on three errors that appear to have led to the interruption of communications with SOHO on June 24. Officials remain hopeful that, based on ESA's successful recovery of the Olympus spacecraft after four weeks under similar conditions in 1991, recovery of SOHO may be possible.
The SOHO Mission Interruption Joint ESA/NASA Investigation Board has determined that the first two errors were contained in preprogrammed command sequences executed on ground system computers, while the last error was a decision to send a command to the spacecraft in response to unexpected telemetry readings. The spacecraft is controlled by a joint ESA/NASA Flight Operations Team, based at NASA's Goddard Space Flight Center, Greenbelt, MD.
The first error was in a preprogrammed command sequence that lacked a command to enable an onboard software function designed to activate a gyro needed for control in Emergency Sun Reacquisition (ESR) mode. ESR mode is entered by the spacecraft in the event of anomalies. The second error, which was in a different preprogrammed command sequence, resulted in incorrect readings from one of the spacecraft's three gyroscopes, which in turn triggered an Emergency Sun Reacquisition.
At the current stage of the investigation, the board believes that the two anomalous command sequences, in combination with a decision to send a command to SOHO to turn off a gyro in response to unexpected telemetry values, caused the spacecraft to enter a series of Emergency Sun Reacquisitions, and ultimately led to the loss of control.
The efforts of the investigation board are now directed at identifying the circumstances that led to the errors, and at developing a recovery plan should efforts to regain contact with the spacecraft succeed.
ESA and NASA engineers believe the spacecraft is currently spinning with its solar panels nearly edge-on towards the Sun, and thus not generating any power. Since the spacecraft is spinning around a fixed axis, as the spacecraft progresses in its orbit around the Sun, the orientation of the panels with respect to the Sun should gradually change. The orbit of the spacecraft and the seasonal change in the spacecraft-Sun alignment should result in the increased solar illumination of the spacecraft solar arrays over the next few months. The engineers predict that in late September 1998 illumination of the solar arrays and, consequently, power supplied to the spacecraft, should approach a maximum. The probability of successfully establishing contact reaches a maximum at this point. After this time, illumination of the solar arrays gradually diminishes as the spacecraft-Sun alignment continues to change.
In an attempt to recover SOHO as soon as possible, the Flight Operations Team is uplinking commands to the spacecraft via NASA's Deep Space Network, managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, approximately 12 hours per day with no success to date. A recovery plan is under development by ESA and NASA to provide for orderly restart of the spacecraft and to mitigate risks involved.
The recovery of the Olympus spacecraft by ESA in 1991 under similar conditions leads to optimism that the SOHO spacecraft may be recoverable once contact is re-established. In May 1991, ESA's Olympus telecommunications satellite experienced a similar major anomaly which resulted in the loss of attitude, leading to intermittent power availability. As a consequence, there was inadequate communication, and the batteries and fuel froze. From analysis of the data available prior to the loss, there was confidence that the power situation would improve over the coming months.
A recovery plan was prepared, supported by laboratory tests, to assess the characteristics of thawing batteries and propellants. Telecommand access of Olympus was regained four weeks later, and batteries and propellant tanks were thawed out progressively over the next four weeks. The attitude was then fully recovered and the payload switched back on three months after the incident. Equipment damage was sustained as a result of the low temperatures, but nothing significant enough to prevent the successful resumption of the mission. The experience of Olympus is being applied, where possible, to SOHO and increases the hope of also recovering this mission.
Estimating the probability of recovery is made difficult by a number of unknown spacecraft conditions. Like Olympus, the hydrazine fuel and batteries may be frozen. Thermal stress may have damaged some of the scientific instruments as well. If the rate of spin is excessive, there may have been structural damage. SOHO engineers can reliably predict the spacecraft's orbit through November 1998. After that time, the long-term orbital behavior becomes dependent on the initial velocity conditions of the spacecraft at the time of the telemetry loss. These are not known precisely, due to spacecraft thruster activity that continued after loss of telemetry, so orbital prediction becomes very difficult.
Summing up the scientific returns from SOHO, which completed its two-year primary mission in April, Dr. George Withbroe, NASA's Director of the Sun-Earth Connections science program at NASA Headquarters said, "In the last two years, SOHO revolutionized our understanding of the Sun in many ways. It was a unique set of instruments devoted to the study of the most important star to us on Earth -- our Sun -- and we are very hopeful that the project engineers will be able to return this world-class observatory to science operations again."
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
European Space Agency Headquarters, Paris, France
June 26, 1998
SOHO went into emergency sun reacquisition mode, and ground controllers lost contact with the spacecraft at 7:16 p.m. EDT on June 24. This mode is activated when an anomaly occurs and the spacecraft loses its orientation toward the Sun. When this happens, the spacecraft automatically tries to point itself toward the Sun again by firing its attitude control thrusters under the guidance of an onboard Sun sensor.
Efforts to re-establish contact with SOHO did not succeed and telemetry was lost. Subsequent attempts using the full NASA Deep Space Network capabilities have so far also not been successful.
Engineers from NASA and ESA are attempting to reestablish contact with the spacecraft.
SOHO is a joint mission of the European Space Agency and NASA. It was launched aboard an Atlas IIAS rocket from Cape Canaveral Air Station, FL, on Dec. 2, 1995, and mission operations are directed from NASA's Goddard Space Flight Center, Greenbelt, MD.
In April 1998, SOHO successfully completed its nominal two-year mission to study the Sun's atmosphere, surface and interior. Major science highlights include the detection of rivers of plasma beneath the surface of the Sun; the discovery of a magnetic "carpet" on the solar surface that seems to account for a substantial part of the energy that is needed to cause the very high temperature of the corona, the Sun's outermost layer; the first detection of flare-induced solar quakes; the discovery of more than 50 sungrazing comets; the most detailed view to date of the solar atmosphere; and spectacular images and movies of coronal mass ejections, which are being used to improve the ability to forecast space weather.
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