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Protecting Earth from the Sun:

Sun Watching Satellite SOHO Regains Ability

Solar and Heliospheric Observatory (SOHO), the Sun-watching satellite, has regained most of its capability after having been shut down partially in June-July 2003 when its high-gain antenna froze up reducing its ability to transmit data to Earth. Its cameras and science instruments were shut down as the spacecraft switched into a safe mode.

What happened? SOHO is a watchdog against space weather that can cause problems for communications and power stations on Earth and orbiting satellites.

Movement of the satellite's antenna ground to a halt in June 2003, leaving NASA and the European Space Agency (ESA) without images and data from SOHO for six weeks. If the problem were not fixed by August, SOHO's effectiveness would have been reduced sharply for the balance of its mission.

High-resolution images of the Sun, which SOHO records continuously, can't be sent with the satellite's low-gain antenna, which continued to work.

Extended mission Parked in a distant Earth orbit between Earth and the Sun, SOHO is an international cooperation project of NASA and ESA. When the spacecraft was launched December 2, 1995, it was supposed to work for two years into early 1998. However, in April 1998, its mission was extended five years into 2003 so scientists could observe dark sunspots on the Sun's surface during a period of peak solar activity in 2000-2002.

Previous problem. One of SOHO's attitude control units reset in December 1999, causing the satellite to enter Emergency Sun Reacquisition mode. ESR is a safe mode that protects the satellite by keeping its electricity-generating solar panels pointed at the Sun so its batteries stay charged. That navigation problem left SOHO out of service about two months.

What went wrong in 2003? To transmit high-resolution image telemetry to Earth, SOHO needed to change the position of its high-gain antenna from time to time. However, a mechanical problem with the antenna drive motor prevented that movement. Engineers hoped the motor would warm up enough to become unstuck.

If the antenna problem couldn't be fixed, NASA and ESA still could conduct basic science research during two-thirds of each year and transmit data back to Earth via the low-gain antenna.
  • A low-gain antenna is omnidirectional and does not require pointing toward Earth. Radio signals leave it in all directions.

  • A high-gain antenna is directional so it must be pointed toward a receiver on Earth. Most of its radio energy leaves the antenna in the direction it is pointed.
SOHO travels a long elliptical orbit 900,000 miles away from Earth as it monitors energy ejections from the Sun and provides early warning when magnetic storms are about to hit Earth.

Sometimes the spacecraft is in a place along its orbital path that prevents communication with Earth via the high-gain antenna. Unless the antenna can be moved, it would be pointed away from Earth. It wouldn't move in Summer 2003.

The solution. Elsewhere along its orbital path, ground controllers were able to order the satellite to turn itself upside down so the high-gain antenna would be pointed toward Earth.

Such a change in position allows pointing of the high-gain antenna toward Earth properly about two months out of three. On the other hand, if engineers would have been able to get the antenna to move slightly, they might have been able to get it into a better position that could have reduced its ineffective periods to just 18 days every three months. Unfortunately, the high-gain antenna would not turn.

Without the high-gain antenna to transmit high-resolution images of the Sun, space weather forecasts would be of less quality. There are other satellites that can photograph solar flares, but major events watched by SOHO – coronal mass ejections (CME) – can generate dangerous space weather and sometimes are not accompanied by solar flares.

In mid-July 2003, as the satellite flew along its orbit into the required position, the ground controllers flipped it 180 degrees so its antenna could point toward Earth. Normal operations resumed July 14.

Now, there still will be gaps in the data from SOHO, but up to 98 percent of the information needed by space weather forecasters will be available during the rest of the satellite's working life. No-service periods lasting 9-16 days will occur every three months.

How long will the satellite continue to work in orbit? Ground controllers are hoping SOHO will survive five more years until 2008, when they hope a replacement satellite, the Solar Dynamics Observatory (SDO), might be launched.

Sunspot cycle. Activity on the Sun is cyclical. An 11-year solar cycle peaked in 2001-2002 and is in a downward trend toward a low point in 2006-2007.

LASCO. NOAA's GOES-12 weather satellite also collects real-time solar data, but not with the depth of SOHO. One of the eleven European and American science instruments aboard SOHO is the Large Angle and Spectrometric COronagraph (LASCO), which blocks bright light from the Sun's disk while photographing the powerful but faint bursts of particles that spew out from the Sun.

Charged particles from CMEs generate those beautiful Northern Lights in Earth's sky, but they also can knock out power and communications on Earth, and satellites in space, without warning.

SOHO has seen on the Sun vast tornadoes nearly as wide as the diameter of our planet Earth and gusting at up to 300,000 mph. The satellite has detected dozens of such tornadoes whipping across the surface of the Sun. Most were near the Sun's north and south poles at the time. Knowing about the tornadoes helps scientists understand the impact of the solar wind that buffets Earth, causing auroras and magnetic storms and endangering satellites in space and power supplies on Earth.

Sunspots and solar flares currently are in decline on the eleven-year solar cycle. During SOHO service interruptions, users on Earth turn to data from another satellite, TRACE.

Understanding the Sun. SOHO data helps researchers understand how the Sun works. Also, pictures sent down by SOHO have been used to spot more than 600 comets, including many plunging into the Sun.

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