Science: Houston, we have a result
When scientists lost contact with a solar observatory spaceship, it looked like disaster. But then they received a call. By Alexander Hellemans
Friday 11 September 1998
Last week, the investigation into what went wrong concluded that the prospects look good. The joint investigation board of the American National Aeronautics and Space Administration and the European Space Agency found that there were at least no ``on-board anomalies" but rather ``a number of ground errors" which led to the satellite spinning out of its carefully allotted position between the Earth and Sun.
Radar signals sent from Earth pinpointed Soho's position and found that it was spinning at a rate of about one revolution per minute. This rotation prevented the solar panels recharging the spacecraft's dead batteries but now engineers are hoping to regain control by slowly bringing life back to the frozen satellite. "It is now in reach to get it back," says Bernard Fleck, ESA's Deputy Project Scientist for Soho at the Goddard Space Flight Center.
Before the mishap, solar scientists had already heralded Soho a resounding success. "We collected a lot of data, and very good data," says Russell Howard at the US Naval Research Laboratory in Washington, and the principal investigator of an American instrument designed to take images of the solar corona - the superheated halo surrounding the Sun. "All the instruments were working well."
Built by the French-British company Matra Marconi Space, the craft was launched in December 1995 and was placed in a solar orbit at a point about a million miles away, where the gravity of the Sun and Earth cancel each other out.
During the past two-and-half years, its 11 instruments have harvested an impressive amount of information. Cambridge University's Douglas Gough says that in the two years Soho was operational, he and his colleagues managed to get a much more precise idea of both the structure and the rotation of the Sun - it rotates at different speeds depending how far you are from the solar equator.
Among the most important observations concern the coronal mass ejections - huge clouds of ionised gas ejected from the solar atmosphere and hurled over vast distances into space. In the past they have been observed only from Earth, but the instruments aboard Soho have made it possible to study the clouds as they head towards Earth - which takes them about 80 hours to reach. Because these masses of electrically charged gases carry magnetic fields, they cause "magnetic storms" in the Earth's magnetosphere, disrupting radio communications and interfering with electrical power supplies.
Several of Soho's instruments are able to detect vibrations of the solar surface. In analogy with the study of vibrations of the Earth's crust, the study of solar vibrations is called solar seismology. Just like a bell, the Sun "rings" at several pitches simultaneously. Unlike the smaller planets in our solar system, the Sun lacks a properly defined surface. "It is practically a perfect gas that simply becomes more dense as you go down," explains Richard Bogart, an astronomer at Stanford University in California.
The hot masses of gas in the outer layers of the Sun constantly move up and down like boiling water in a tea kettle- a process called convection. This convective movement causes the Sun to vibrate constantly. Several of the instruments on board Soho are able to detect the oscillations of the hot gasses on the Sun by looking at how the light emitted by these gases changes in frequency while they are moving, just like the pitch of the horn of a passing train changes - a phenomenon known as the Doppler effect.
As the Earth's seismic waves tell us about its interior, so do the seismic waves on the Sun tell astronomers about what is going on inside our own star. These waves are called pressure waves or p-waves and they travel through the Sun just like sound waves, but have much lower periods than the sound waves we are used to: the periods of the different detected vibration are in the range of several minutes. The study of the p-waves has revealed a complex array of motion in the outer solar layers: the Sun rotates at different speeds, and huge flows of gas stream under the visible solar surface.
With Soho's instruments, astronomers also discovered how huge flares, eruptions in the solar atmosphere mainly visible at X-ray wavelengths, cause quakes on the Sun. Such a flare is caused by disturbances in the solar magnetic field, whereby vast numbers of electrons are accelerated downwards from the solar atmosphere. As these electrons hit the solar surface they cause seismic waves that propagate over the solar surface "like ripples from a pebble thrown in a pond", says Valentina Zharkova of Glasgow University, one of the astronomers who observed such a large x-ray flare in 1996. Soho has also allowed astronomers to observe comets striking the Sun, and hopes were high that Soho would observe ripples caused by their impact.
The loss of the spacecraft would be an even bigger disappointment for the solar astronomers because the observations period of two years that was originally planned was extended until 2003. This would have made it possible for the spacecraft to study the Sun when it goes through the maximum activity of the 11-year solar cycle. At this point, the number of sunspots reaches a maximum and the Sun's magnetic field goes into reverse. "We wanted to find out how the structure was going to change through the solar cycle," says Gough, who is coinvestigator on three instruments that detect solar vibrations. "We are really interested in being able to observe the huge expulsions of plasma as the Sun picks up in activity," said Howard. If Soho cannot be revived, scientists will lose the opportunity to redesign their research in response to what has been found out up to now. "We planned on doing different things that we never even planned on doing. That is where the real discovery starts," said Gough.
A post-mortem into the disaster found that contact was lost because of errors in pre-programmed command sequences during maintenance operations, whereby the spacecraft, instead of remaining pointed at the Sun, entered a slow spin. Flywheels control the spacecraft's orientation and they gradually spin faster during attitude corrections and so need to be slowed down at regular intervals. This is done in a controlled fashion during maintenance sessions whereby the slowing down of the flywheels is compensated by the firing of hydrazine thrusters that also control the orientation of the craft. The investigation board confirmed that during the last maintenance session, a wrongly programmed sequence caused the loss of control of the spacecraft. However, its report also points out that several other factors - such as the display of housekeeping data that is "not userfriendly", and the overburdening of staff during the maintenance operation - contributed to the mishap.
The hope of recovering the spacecraft has increased dramatically since first contact was established on 3 August. "So far the recovery went fairly smoothly," Fleck said. The major problem was that because of its slow spin, about one revolution per minute, the solar panels were not operating at full capacity, and the batteries became discharged. The solar panels now deliver about 40 per cent of their capacity because the craft is in a more advantageous position in its orbit around the Sun than two months ago.
Now the housekeeping operations are working continuously, but before the spacecraft can be taken out of its spin, the hydrazine in the main tank and the pipes leading towards the thrusters have to be unfrozen with electric heating elements. "We completed the thawing of the hydrazine tank," said Fleck. "We are now heating the pipes that connect the hydrazine tank to the thrusters outside."
Fleck expects that the thawing of the pipes can take up to two weeks because a quick thawing may cause the pipes to burst due to the expansion of the hydrazine. The next steps are the testing of the gyros that control the craft's position. Then the spin of the spacecraft will be slowed down in an attempt to point it back to the Sun, where its solar panels will receive their full complement of sunshine. However, none of the 11 solar instruments have yet been tested and Fleck acknowledges that this month will be crucial to future of Soho. "We aren't home yet, but a couple of miracles happened in the last few weeks."
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