The Solar and Heliospheric Observatory (SOHO) is a space-based solar observatory developed as a joint mission by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). Launched on December 2, 1995, SOHO was designed to provide continuous observations of the Sun to advance our understanding of solar dynamics, the Sun’s interior, the corona, solar wind, and space weather phenomena. It is positioned at the Lagrange Point 1 (L1), approximately 1.5 million km (932,000 miles) from Earth, ensuring an uninterrupted view of the Sun. Initially planned as a two-year mission, SOHO has far exceeded expectations and remains operational as of 2024.

Historical Background and the Need for the Mission

Early Solar Observations and the Growing Need for Space-Based Missions

Humanity’s interest in the Sun dates back to ancient civilizations, where early astronomers studied solar movements and sunspots. However, it wasn’t until the 20th century that modern scientific techniques allowed for deeper solar exploration. Before SOHO, most solar observations were conducted from ground-based telescopes. However, Earth’s atmosphere significantly limits observations, especially in ultraviolet (UV) and extreme ultraviolet (EUV) wavelengths, which are absorbed before reaching the ground. This limitation hindered detailed studies of the Sun’s outer layers, such as the chromosphere and corona. Recognizing the need for continuous, high-resolution solar observations, space agencies launched several early solar missions:

• Skylab (1973-1974) – The first US space station, which carried a solar observatory and provided valuable images of the corona.
• Helios 1 and 2 (1974, 1976) – A NASA-West German mission that studied the solar wind and interplanetary magnetic field.
• Solar Maximum Mission (SMM) (1980-1989) – Focused on solar flares and coronal mass ejections (CMEs).
• Ulysses (1990-2009) – Investigated the Sun’s polar regions and heliosphere.

While these missions made significant contributions, they were short-lived or had limited observational capabilities. None could provide the long-term, comprehensive study of the Sun that scientists required.

The Birth of SOHO – A Collaborative Effort

During the 1980s, scientists from ESA and NASA recognized the need for a dedicated solar observatory that could provide continuous, high-resolution data on the Sun’s structure, solar wind, and heliosphere. This led to the development of SOHO as part of ESA’s “Horizon 2000” science program and NASA’s Sun-Earth connection strategy.

SOHO was designed to address three critical questions:

1. What are the physical processes inside the Sun? – Understanding the Sun’s internal dynamics using helioseismology.
2. How is the solar corona heated? – Investigating why the corona is millions of degrees hotter than the Sun’s surface.
3. How is the solar wind generated and accelerated? – Studying how solar wind originates and affects space weather.

The mission was unprecedented in its scope, combining multiple advanced instruments to observe different layers of the Sun simultaneously. Unlike previous missions, SOHO would be placed at L1, allowing for continuous monitoring without being affected by Earth’s rotation or atmosphere.

Need for the SOHO Mission

1. Understanding Space Weather and Solar Storms – Solar activity, including CMEs and solar flares, can disrupt satellites, power grids, and communication systems on Earth. SOHO was designed to improve early warnings for such events.
2. Solving Long-Standing Mysteries of Solar Physics – The solar corona’s extreme temperatures and the origin of solar wind remained unexplained before SOHO.
3. Continuous Helioseismology Studies – Mapping the Sun’s internal structure was crucial for understanding solar convection, sunspots, and magnetic activity.
4. Monitoring the Solar Cycle – The Sun follows an 11-year activity cycle, influencing Earth’s climate and space weather. Long-term observations were necessary to track these variations.

By providing real-time data, SOHO became an indispensable tool for space weather forecasting and solar physics research. Its success laid the foundation for later missions, such as the Solar Dynamics Observatory (SDO), Parker Solar Probe, and Solar Orbiter.

Orbit and Position

The Solar and Heliospheric Observatory (SOHO) is positioned at the first Lagrange point (L1), a gravitationally stable location approximately 1.5 million kilometers (932,000 miles) from Earth in the direction of the Sun. This unique orbital placement allows SOHO to have an uninterrupted view of the Sun, free from Earth’s atmospheric distortions and periodic eclipses that affect ground-based telescopes. The L1 point is one of the five Lagrange points in the Sun-Earth system, where the gravitational forces of the two bodies balance with the centrifugal force of an orbiting spacecraft, enabling it to maintain a stable position with minimal fuel consumption. This makes L1 an ideal location for solar observatories like SOHO, as it ensures continuous monitoring of solar activity, coronal mass ejections (CMEs), and space weather phenomena that affect Earth’s magnetosphere and technological infrastructure.

SOHO does not remain completely stationary at L1 but follows a halo orbit around this point. A halo orbit is a three-dimensional, elliptical trajectory around a Lagrange point, allowing the spacecraft to maintain a stable position without being directly in line with the Sun and Earth. This positioning prevents radio interference between SOHO and Earth-based communication systems, ensuring a constant and clear data transmission link with ground stations. The size of SOHO’s halo orbit spans approximately 600,000 km by 200,000 km (370,000 miles by 124,000 miles), taking about six months to complete one revolution. The spacecraft’s trajectory is carefully maintained through small station-keeping maneuvers, which adjust its position and compensate for minor gravitational disturbances. This stability enables long-term observations without the need for frequent course corrections, making the mission highly efficient and cost-effective.

The selection of the L1 point for SOHO’s orbit was based on several scientific and operational advantages. First, its proximity to Earth allows for high-speed data transmission, enabling near-real-time monitoring of solar events that could impact space weather and terrestrial systems. Second, being located outside Earth’s magnetosphere ensures that SOHO’s instruments can observe the undisturbed solar wind and interplanetary magnetic field, providing crucial insights into the Sun’s influence on the heliosphere. Third, the stability of the L1 orbit minimizes thermal fluctuations and mechanical stress on the spacecraft, enhancing the longevity of its scientific instruments. Compared to a low Earth orbit (LEO) or a geostationary orbit, which would be subject to Earth’s atmospheric drag, eclipses, and limited observation windows, the L1 orbit offers an unparalleled vantage point for continuous solar studies.

The success of SOHO’s mission at L1 has set a precedent for subsequent space observatories. Several later missions, including the Advanced Composition Explorer (ACE), the Deep Space Climate Observatory (DSCOVR), and the Solar and Heliospheric Observatory (Solar Orbiter), have also been positioned near L1 to benefit from its stable viewing conditions. SOHO’s precise orbital placement has allowed it to operate far beyond its initial two-year mission lifespan, providing nearly three decades of invaluable solar data. This longevity has significantly enhanced our understanding of solar physics, space weather forecasting, and the Sun-Earth connection, proving that L1 remains the most strategic location for future solar observation missions.

Scientific Instruments

SOHO is equipped with 12 scientific instruments, each designed to study different aspects of the Sun, from its internal structure to its outer atmosphere and solar wind. These instruments are categorized into three main groups based on their primary function; Helioseismology Instruments: Study the Sun’s interior using solar oscillations, Solar Atmosphere Instruments: Observe the Sun’s corona, chromosphere, and transition region and Solar Wind and Energetic Particles Instruments: Measure the solar wind and its interaction with the interplanetary medium. Below is a detailed description of each instrument and its contribution to the SOHO mission.

Helioseismology Instruments (Studying the Sun’s Interior)

Michelson Doppler Imager (MDI)

• Objective: Measures oscillations on the Sun’s surface to map its internal structure and dynamics.
• How it Works: MDI detects Doppler shifts in sunlight caused by pressure waves (p-modes) traveling inside the Sun. By analyzing these oscillations, scientists can infer the temperature, density, and motion of materials within the solar convection zone and even detect subsurface sunspots before they emerge.
• Key Discoveries:
  • Provided the first high-resolution maps of solar internal flows.
  • Helped identify the tachocline, a transition layer between the Sun’s radiative and convective zones.

Solar Atmosphere Instruments (Studying the Corona and Chromosphere)

Extreme Ultraviolet Imaging Telescope (EIT)

• Objective: Captures images of the Sun’s outer layers in extreme ultraviolet (EUV) wavelengths to study the transition region and low corona.
• How it Works: EIT takes images at four different EUV wavelengths, each corresponding to different layers of the solar atmosphere.
• Key Discoveries:
  • Observed coronal loops and active regions responsible for solar flares.
  • Played a crucial role in tracking the Sun’s 11-year solar cycle by imaging sunspot activity.

Large Angle and Spectrometric Coronagraph (LASCO)

• Objective: Observes the solar corona and detects coronal mass ejections (CMEs).
• How it Works: LASCO has three coronagraphs (C1, C2, C3) that block the bright solar disk, allowing faint coronal structures to be observed.
• Key Discoveries:
  • Provided the first continuous, detailed observations of CMEs.
  • Helped predict space weather by detecting solar storms before they reach Earth.
  • Discovered over 4,500 comets, many of which were “sungrazers” that passed close to the Sun.

Solar Ultraviolet Measurements of Emitted Radiation (SUMER)

• Objective: Analyzes the Sun’s upper chromosphere and transition region in ultraviolet (UV) light.
• How it Works: SUMER measures the intensity and Doppler shifts of UV spectral lines to determine temperature, velocity, and density of solar plasma.
• Key Discoveries:
  • Provided crucial data on how plasma is heated and flows in the corona.
  • Helped identify regions where the solar wind originates.

Ultraviolet Coronagraph Spectrometer (UVCS)

• Objective: Studies the extended solar corona and the acceleration of the solar wind.
• How it Works: UVCS uses ultraviolet spectroscopy to measure the temperature, density, and flow of coronal gases up to several solar radii from the Sun.
• Key Discoveries:
  • Confirmed that coronal heating occurs at altitudes higher than previously thought.
  • Showed how fast solar wind particles accelerate in the corona.

Coronal Diagnostic Spectrometer (CDS)

• Objective: Measures the temperature and density of different layers of the Sun’s corona.
• How it Works: CDS captures X-ray and EUV spectral lines emitted by hot plasma in the corona, allowing scientists to track the evolution of solar flares and active regions.
• Key Discoveries:
  • Provided the first direct evidence of plasma flows in active regions.
  • Helped study how energy is transferred from the lower atmosphere to the corona.

Solar Wind and Energetic Particle Instruments (Studying the Solar Wind and Interplanetary Medium)

Charge, Element, and Isotope Analysis System (CELIAS)

• Objective: Measures the composition and charge states of solar wind particles.
• How it Works: CELIAS detects ions in the solar wind, providing information about their origin and how they interact with the Earth’s magnetosphere.
• Key Discoveries:
  • Confirmed the existence of high-speed solar wind streams associated with coronal holes.
  • Helped understand how different types of solar wind affect Earth’s space environment.

Comprehensive SupraThermal and Energetic Particle Analyzer (COSTEP)

• Objective: Measures energetic particles emitted by the Sun, especially during solar storms.
• How it Works: COSTEP detects electrons and protons in the supra-thermal energy range and monitors sudden bursts of energetic particles associated with solar flares and CMEs.
• Key Discoveries:
  • Provided data on how solar energetic particles (SEPs) propagate through space.
  • Helped improve space weather forecasting by detecting early warning signals of solar storms.

Energetic and Relativistic Nuclei and Electron Experiment (ERNE)

• Objective: Studies high-energy particles from the Sun and cosmic rays from beyond the solar system.
• How it Works: ERNE detects protons, helium nuclei, and heavier ions to analyze their energy spectra and origins.
• Key Discoveries:
  • Helped differentiate between solar particles and cosmic rays from distant supernovae.
  • Contributed to understanding how the Sun’s magnetic field influences energetic particle acceleration.

Data Transmission and Ground Stations

SOHO continuously transmits its data to ground stations, including the Deep Space Network (DSN). The data is then processed and analyzed at various research centers, including ESA’s European Space Operations Centre (ESOC) and NASA’s Goddard Space Flight Center (GSFC).

Current Status

Despite several technical challenges over the years—including a loss of contact in 1998 that was later restored and issues with its gyroscopes—SOHO has remained a reliable solar observatory. Engineers and scientists have adapted its operations, allowing it to continue functioning even with degraded equipment. ESA has extended SOHO’s mission multiple times, but a new solar observation mission, NOAA’s Space Weather Follow-On Lagrange 1 (SWFO-L1), is set to launch in 2025. This mission will take over some of SOHO’s functions, and with no additional funding expected beyond 2025, SOHO is likely to conclude its operations. While the ESA has planned a two-year post-operations phase starting in 2026, it is unclear if the spacecraft will remain in use in a limited capacity or if it will be decommissioned completely. Regardless, SOHO’s scientific impact and its contributions to space weather research will continue to influence future missions and studies.

In conclusion, SOHO is one of the most successful solar observatories ever built, revolutionizing our understanding of the Sun. Its continuous monitoring of solar activity has helped in both scientific research and practical applications such as space weather forecasting. Despite being in operation for nearly three decades, SOHO continues to be a vital tool in solar and heliospheric research.