NASA’s Pioneering Helios Mission: A Milestone in Solar Exploration
On December 10, 1974, NASA embarked on a groundbreaking journey to study the Sun closely by launching Helios 1, the first of two spacecraft designed for this purpose. This mission was one of the most significant international collaborations of its time. The Federal Republic of Germany, then known as West Germany, contributed the spacecraft while NASA’s Goddard Space Flight Center in Greenbelt, Maryland, coordinated the U.S. participation. Furthermore, NASA’s Lewis, now known as the Glenn Research Center in Cleveland, offered the launch vehicle. The Helios 1 mission was equipped with ten scientific instruments and made its initial close approach to the Sun on March 15, 1975, surpassing all previous spacecraft in terms of proximity and speed. Following in its footsteps, Helios 2 was launched in 1976 and ventured even closer to the Sun. Both missions exceeded their expected 18-month lifespan, delivering unprecedented data from their unique solar orbits.
The two Helios probes were constructed by the West German company, Messerschmitt-Bƶlkow-Blohm, for the West German space agency DFVLR, now known as the German Aerospace Center (DLR). These probes were notable for being the first non-Soviet and non-American spacecraft to be placed in a heliocentric orbit. Each Helios probe, weighing 815 pounds, carried a suite of instruments from both the U.S. and West Germany, with a combined weight of 158 pounds. These instruments were designed to study various aspects of the Sun and its surrounding environment. They included high-energy particle detectors to measure the solar wind, magnetometers to study the Sun’s magnetic field and its fluctuations, and micrometeoroid detectors. Once the spacecraft were launched and their systems checked, operators at the German control center near Munich took over the control and data collection. The spacecraft were designed to spin on their axis once every second to evenly distribute solar radiation, while optical mirrors on their surfaces reflected most of the heat away.
Helios 1 was launched at 2:11 a.m. EST on December 10, 1974, from Launch Complex 41 at Cape Canaveral Air Force Station, which is now known as Cape Canaveral Space Force Station. This launch marked the first successful flight of the Titan IIIE-Centaur rocket, which was, at that time, the world’s most powerful rocket. The success of this launch was crucial as it followed the failure of the Centaur upper stage during the rocket’s inaugural launch on February 11, 1974. The successful deployment of Helios 1 bolstered confidence in the Titan IIIE-Centaur, which was later used to launch the Viking orbiters and landers to Mars in 1976, and the Mariner Jupiter-Saturn spacecraft, later renamed Voyager, in 1977 to explore the outer solar system.
The Centaur upper stage successfully placed Helios 1 into a solar orbit with a period of 190 days, with its closest approach, or perihelion, orbiting well inside Mercury’s orbit. The spacecraft’s ten instruments were activated within days of the launch, and it was declared fully operational by January 16, 1975. Helios 1 made its closest approach to the Sun on March 15, 1975, reaching a distance of 28.9 million miles, closer than any previous spacecraft. The Mariner 10 previously held the record during its Mercury encounters. Helios 1 also set a speed record for spacecraft, traveling at 148,000 miles per hour at perihelion. Despite parts of the spacecraft reaching temperatures of 261 degrees Fahrenheit, the instruments continued to function effectively. During its second perihelion on September 21, the temperatures rose to 270 degrees, impacting some instruments. Helios 1 continued to transmit valuable data until its primary and backup receivers failed, and its high-gain antenna could no longer point towards Earth. Ground controllers deactivated the spacecraft on February 18, 1985, with the final contact occurring on February 10, 1986.
Helios 2 was launched on January 15, 1976, following a trajectory similar to Helios 1 but venturing even closer to the Sun. On April 17, it came within 27 million miles of the Sun, setting a new speed record of 150,000 miles per hour. At this proximity, Helios 2 experienced 10% more solar heat than its predecessor. Unfortunately, Helios 2’s downlink transmitter failed on March 3, 1980, resulting in the loss of usable data. Controllers subsequently shut it down on January 7, 1981. Scientists correlated data from the Helios instruments with similar data from other spacecraft, such as the Interplanetary Monitoring Platform Explorers 47 and 50 in Earth’s orbit, and the Pioneer solar orbiters, as well as Pioneer 10 and 11 in the outer solar system. Besides solar observations, Helios 1 and 2 studied the dust and ion tails of comets such as C/1975V1 West, C/1978H1 Meier, and C/1979Y1 Bradfield. The insights obtained from the Helios probes significantly expanded our understanding of the Sun and its environment, while also raising new questions to be explored by subsequent missions.
The joint ESA/NASA Ulysses mission offered a new perspective by studying the Sun from above its poles. Launched from the space shuttle Discovery during STS-41 on October 6, 1990, Ulysses used Jupiter’s gravity to alter its trajectory, enabling it to fly over the Sunās south polar region from June to November 1994 and then over the north polar region from June to September 1995. Ulysses continued its unique observations during several more polar passes until June 30, 2009, nearly 19 years after launch, far exceeding its anticipated lifespan.
NASA’s Parker Solar Probe, launched on August 12, 2018, is another mission that has made increasingly close passes to the Sun, even flying through its corona and breaking the distance record set by Helios 2. The Parker Solar Probe made its first perihelion at just 15 million miles on November 5, 2018. It is expected to make its closest approach of only 3.86 million miles from the Sun’s surface on December 24, 2024, which is just 4.5% of the distance between the Sun and Earth.
The ESA Solar Orbiter, launched on February 10, 2020, began its scientific operations in November 2021. It carries ten instruments, including cameras that have captured the highest resolution images of the Sun, including its polar regions from as close as 26 million miles away.
These ongoing missions build on the legacy of the Helios program, continuously expanding our knowledge of the Sun and enhancing our understanding of solar phenomena that have profound effects on our solar system and Earth itself.
For more details on the Helios mission, visit NASA’s Science Mission Directorate.
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