NASA Launches the Solar and Heliospheric Observatory

Date December 2, 1995

The Solar and Heliospheric Observatory, the product of a joint effort by the U.S. National Aeronautics and Space Administration and the European Space Agency, was the first spacecraft to provide uninterrupted observations of the Sun, with scientific instruments monitored by more than fifteen hundred scientists around the world.

Also known as SOHO

Locale Cape Canaveral, Florida

Key Figures

• Roger Bonnet (b. 1938), director of science at the European Space Agency, 1983-2001
• Vicente Domingo (fl. late twentieth century), SOHO project scientist at the European Space Agency
• Bernhard Fleck (fl. late twentieth century), SOHO project scientist for the European Space Agency at NASA’s Goddard Space Flight Center
• Michel Verdant (fl. late twentieth century), SOHO program manager for the European Space Agency

Summary of Event

The Sun is only one of uncountable billions of stars scattered through space. It is an average star in size, temperature, and other factors. It is mostly hydrogen, and it is estimated to be 4.5 billion years old; at 860,000 miles in diameter, it would take 110 Earths lined up side by side to span it. Life on Earth depends on the Sun’s light and heat, which originate in nuclear reactions deep inside its core. As stable as it is, the Sun can affect radio communications on Earth with sunspot activities, and its plasma eruptions can cause geomagnetic storms that disrupt the communications of human-made satellites in orbit around Earth, radio communications, and power systems.

On December 2, 1995, the Solar and Heliospheric Observatory (SOHO) was launched on an Atlas II-AS rocket from Cape Canaveral, Florida. The purpose of the billion-dollar mission was to provide scientists with their first uninterrupted observations of the Sun. The SOHO spacecraft was built in Europe under the management of the European Space Agency. The twelve instruments it carried were developed by scientists in both Europe and the United States; nine were the work of European scientists, and three were created by teams led by U.S. scientists. The National Aeronautics and Space Administration (NASA) handled the launch and then took charge of SOHO’s operations from Goddard Space Flight Center in Greenbelt, Maryland.

SOHO was designed to send data on the Sun back to Earth at the rate of one thousand images a day, beamed to the radio dishes of NASA’s Deep Space Network around the world. In all, more than fifteen hundred scientists have been involved with monitoring the data gathered by SOHO.

The SOHO spacecraft has two parts: a service module that houses the power, communication, and navigation systems; and a payload module that contains the scientific instruments. Previous Sun-observing instrument packages had been placed on spacecraft that orbited Earth and therefore experienced data interruptions when the planet came between them and the Sun. SOHO, in contrast, moves around the Sun at the same pace the Earth does. It orbits Earth at a distance about four times that between Earth and the Moon, at a point where the gravities of the Earth and the Sun keep SOHO locked into a constant position relative to the Sun.

Because life on Earth is in many ways directly dependent on the Sun, it is important for scientists to know as much as possible about our solar system’s star. The instruments aboard SOHO constantly monitor phenomena as wide-ranging as the interior heat of the Sun, its visible surface and stormy atmosphere, and the solar wind in sectors of outer space distant from the Sun, where incoming atoms from more distant stars are encountered. SOHO also sends data back to Earth on the complex flow of gases formerly hidden beneath the Sun’s visible surface and on changes in the patterns of the Sun’s magnetic fields.

Discoveries that scientists have made based on data provided by SOHO include the existence of forces that affect the solar wind in various directions and possible ways to predict the occurrence of the Sun’s plasma eruptions, which react with Earth’s magnetic field to affect radio communications, television and telephone signals, air and sea navigation systems, satellites in space, and power sources on Earth. Gradually, data gathered by SOHO have clarified the complexities of the seemingly constant Sun.

When SOHO was launched, the craft’s expected life span was two to three years, and in June, 1998, transmissions from SOHO ceased. Engineers in Europe and the United States went to work on the problem and, after weeks of effort, were able to restore communication with the satellite. Then, however, the last gyroscope on the craft that had been working went bad. This made it hard to orient SOHO correctly, and precious fuel was wasted in attempts to get it pointed in the right direction again.

Eventually, the engineers and controllers were able to perform the equivalent of a computer upgrade, even though the computer in question was about a million miles away. The accomplishment left SOHO performing better than it had originally, because it was able to reorient itself in a different way, no longer relying on the guide star it had needed to locate to orient itself correctly. With the new software, SOHO could be oriented through measurements of changes in the speed of its momentum wheels, devices used to provide the force necessary to position the craft. In effect, this procedure turned the entire spacecraft into a gyroscope, and SOHO became the first spacecraft to function without the original gyroscopes with which it was launched.

The procedure also fixed a related problem. One of SOHO’s telescopes, used to observe phenomena such as the Sun’s corona and plumes, had been experiencing problems with blurred images caused by frozen water vapor, hydrocarbon residue, and other contaminants. When SOHO was spinning out of control, the mission’s ground personnel worried that the alternating heat and cold it was experiencing while spinning would harm the sensitive instrumentation on board. That did not happen; rather, the heat burned off the contaminants that had plagued the telescope, improving its sensitivity from before the loss of control by some 60 percent. When SOHO experienced additional problems in 2003, project scientists found that they could continue to receive data from the satellite by using radio dishes that were more powerful than those used previously.

Far exceeding its initial expected life span, SOHO continued operating into the early years of the twenty-first century, allowing scientists to observe some solar phenomena in more long-term fashion than they had originally hoped. One example is the information SOHO provided on sunspots, including how they behave and how their behavior affects the Sun. In 1996, soon after SOHO was launched, the Sun had relatively few sunspots. By 2000, sunspots were at their peak in terms of numbers. SOHO was able to provide more long-term data on sunspot phenomena than originally anticipated.

Significance

SOHO gave physicists their first uninterrupted look at the Sun and then went on to exceed its projected three-year life span by more than a decade. The full significance of the data gathered by SOHO may not be known for many years, until all the analyses are done and scientific papers are published that bring together the findings from scientists’ examinations of the data from all twelve of its instruments. SOHO’s data have provided hints of what forces lie behind the solar wind as well as information on the relationship between magnetic changes in the Sun and solar flares that affect telecommunications and power grids on Earth, so that scientists can predict these changes and preparations can be made for the problems they cause. SOHO made it possible for scientists to see what lies within the Sun through the detection and measurement of sound waves at its surface. An unanticipated extra came when SOHO sent back to Earth outstanding images of Comet Hyakutake, providing data for another field of space study.

As the product of two space agencies operating on different continents, SOHO demonstrated how cooperative space projects can turn out. When SOHO began to fail in 1998, the project staff provided a textbook study on how new programming can be uploaded into a spacecraft as far away from Earth as a million miles. More than ten years after its launch, SOHO continued to provide a window on the Sun that was expected to lead to nothing less than a revolution in solar science.

Bibliography

Birney, D. Scott, Guillermo Gonzalez, and David Oesper. Observational Astronomy. 2d ed. New York: Cambridge University Press, 2006. Textbook provides an introduction to observational astronomy, covering telescopes, types of stars, and solar observations, including SOHO.

Fleck, Bernhard, and Zdenek Svestka, eds. The First Results from SOHO. New York: Springer, 1998. Presents detailed descriptions of SOHO’s twelve scientific instruments as well as reports on the first results from SOHO.

Hanslmeier, Arnold, Astrid Veronig, and Mauro Messerotti, eds. Solar Magnetic Phenomena: Proceedings of the Third Summer School and Workshop Held at the Solar Observatory Kanzelhöhe, Kärnten, Austria. New York: Springer, 2005. Collection of documents about magnetic phenomena in the atmosphere of the Sun, including the physics of solar flares, coronal mass ejections, and high-energy solar radiation. Presents highlights from the SOHO findings.

Wimmer-Schweingruber, Robert F., ed. Solar and Galactic Composition: A Joint SOHO/ACE Workshop. Melville, N.Y.: American Institute of Physics, 2001. Collection of papers from a workshop that involved representatives of SOHO and the Advanced Composition Explorer. Both projects focus on the composition of the Sun and its immediate environment.