Solar and Heliospheric Observatory

The Solar and Heliospheric Observatory (SOHO) is a special spacecraft created by the European Space Agency to study the Sun. It was launched on December 2, 1995, and has been giving us amazing information about our star for over 30 years! SOHO works together with scientists from both Europe and the United States.

Besides learning about the Sun, SOHO has also discovered more than 5,000 comets — little icy objects that sometimes have beautiful tails when they get close to the Sun. This makes SOHO one of the best comet-hunters in space!

SOHO sits at a special spot between the Earth and the Sun called the L1 point. This spot lets SOHO watch the Sun very closely and give us important warnings about space weather — like big bursts of energy from the Sun that can affect satellites and even our power grids here on Earth. It works with other spacecraft such as Aditya-L1, Wind, Advanced Composition Explorer, and Deep Space Climate Observatory to keep an eye on our star.

Scientific objectives

The Solar and Heliospheric Observatory (SOHO) has three main scientific goals. First, it studies the outer layers of the Sun, including the chromosphere, transition region, and corona, using special instruments. Second, it observes the solar wind and related events near a special point in space called L₁, using tools to measure conditions right there. Third, it explores the Sun's inner structure using techniques that study vibrations on the Sun's surface.

Orbit

The SOHO spacecraft travels in a special path called a halo orbit around a point between the Sun and the Earth, known as the L1 point. This point is where the Sun's strong gravity balances with Earth's weaker gravity. While SOHO is near L1, it doesn’t stay exactly there. Instead, it moves in a loop that passes through L1 and stays in a plane perpendicular to the line connecting the Sun and Earth. This path allows SOHO to orbit L1 once every six months while staying in contact with Earth.

Communication with Earth

The Solar and Heliospheric Observatory (SOHO) sends a steady stream of images and measurements about the Sun to Earth. This information helps scientists predict when big bursts of solar material, called coronal mass ejection, might hit our planet. These events can affect electrical grids and satellites, so having early warnings helps keep our technology safe.

In 2003, a part of SOHO’s antenna stopped working correctly. This could have caused periods without data. But experts found a way to keep getting information by using smaller antennas and special recording systems on the spacecraft. This solution allowed SOHO to continue sharing important solar data with only small delays.

Instruments

The Solar and Heliospheric Observatory (SOHO) has twelve instruments that study the Sun. These instruments observe different parts of the Sun and its surroundings, helping scientists learn more about our star.

Some of the important instruments include the Coronal Diagnostic Spectrometer (CDS), which looks at the Sun’s outer atmosphere, and the Large Angle and Spectrometric Coronagraph (LASCO), which creates an artificial solar eclipse to study the corona. The Michelson Doppler Imager (MDI) examined the Sun’s surface and magnetic fields until 2011. Other tools measure the solar wind and other aspects of the Sun’s activity.

Public availability of images

Observations from the Solar and Heliospheric Observatory (SOHO) can create images that are available on the internet for everyone to see. These images help scientists study the Sun and are useful for research. Some types of data, like spectra and measurements of particles in the solar wind, are not easy to turn into pictures. The images show different colors because they capture light at various wavelength or frequency levels, from regular light (optical, including Hα) to special Extreme ultraviolet (EUV) light. These special images are often shown in false color to make them easier to understand.

People interested in using SOHO's instruments can contact the teams through email or the SOHO website. There is no set time for personal observations, but a formal process called the "JOP" program allows using multiple instruments together. These requests are reviewed by science teams during their meetings.

Comet discoveries

See also: List of Kreutz sungrazers

The Solar and Heliospheric Observatory (SOHO) has helped scientists discover many comets. By studying the Sun, SOHO’s camera blocks the Sun’s bright light, making it easier to spot comets nearby. Over the years, more than 70 people from 18 countries have found comets using SOHO’s images.

SOHO has reached many exciting milestones in comet discoveries. It found its 1,000th comet in 2005, its 2,000th in 2010, its 3,000th in 2015, its 4,000th in 2020, and its 5,000th in 2024. As of December 2025, SOHO has discovered 5,204 comets!

Near loss of SOHO in 1998

In June 1998, the Solar and Heliospheric Observatory (SOHO) stopped working and lost contact with controllers. The spacecraft was spinning and not pointing at the Sun, which meant it was losing power.

Experts from ESA rushed to Europe and the United States to try to fix it. After many days, they found SOHO using special radar from the Arecibo Observatory and the Goldstone Solar System Radar. They finally heard a signal from SOHO in early August and began working to restart it.

By September, SOHO was pointed at the Sun again and started working normally by the end of October. However, one of its important parts called a gyroscope stopped working later that year. After that, controllers had to find new ways to control SOHO without it.