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50 Years Ago: Launch of Helios 1 to Explore the Sun 


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On Dec. 10, 1974, NASA launched Helios 1, the first of two spacecraft to make close observations of the Sun. In one of the largest international efforts at the time, the Federal Republic of Germany, also known as West Germany, provided the spacecraft, NASA’s Goddard Space Flight Center in Greenbelt, Maryland, had overall responsibility for U.S. participation, and NASA’s Lewis, now Glenn, Research Center in Cleveland provided the launch vehicle. Equipped with 10 instruments, Helios 1 made its first close approach to the Sun on March 15, 1975, passing closer and traveling faster than any previous spacecraft. Helios 2, launched in 1976, passed even closer. Both spacecraft  far exceeded their 18-month expected lifetime, returning unprecedented data from their unique vantage points. 

The fully assembled Helios 1 spacecraft prepared for launch.
The fully assembled Helios 1 spacecraft prepared for launch.
Credit: NASA

The West German company Messerchmitt-Bölkow-Blohm built the two Helios probes, the first non-Soviet and non-American spacecraft placed in heliocentric orbit, for the West German space agency DFVLR, today’s DLR. Each 815-pound Helios probe carried 10 U.S. and West German instruments, weighing a total of 158 pounds, to study the Sun and its environment. The instruments included high-energy particle detectors to measure the solar wind, magnetometers to study the Sun’s magnetic field and variations in electric and magnetic waves, and micrometeoroid detectors. Once activated and checked out, operators in the German control center near Munich controlled the spacecraft and collected the raw data. To evenly distribute the solar radiation the spacecraft spun on its axis once every second, and optical mirrors on its surface reflected the majority of the heat. 

Workers encapsulate a Helios solar probe into its payload fairing.
Workers encapsulate a Helios solar probe into its payload fairing. 
Credit: NASA

Launch of Helios 1 took place at 2:11 a.m. EST Dec. 10, 1974, from Launch Complex 41 at Cape Canaveral Air Force, now Space Force, Station, on a Titan IIIE-Centaur rocket. This marked the first successful flight of this rocket, at the time the most powerful in the world, following the failure of the Centaur upper stage during the rocket’s inaugural launch on Feb. 11, 1974. The successful launch of Helios 1 provided confidence in the Titan IIIE-Centaur, needed to launch the Viking orbiters and landers to Mars in 1976 and the Mariner Jupiter-Saturn, later renamed Voyager, spacecraft in 1977 to begin their journeys through the outer solar system. The Centaur upper stage placed Helios 1 into a solar orbit with a period of 190 days, with its perihelion, or closest point to the Sun, well inside the orbit of Mercury. Engineers activated the spacecraft’s 10 instruments within a few days of launch, with the vehicle declared fully operational on Jan. 16, 1975. On March 15, Helios 1 reached its closest distance to the Sun of 28.9 million miles, closer than any other previous spacecraft – Mariner 10 held the previous record during its three Mercury encounters. Helios 1 also set a spacecraft speed record, traveling at 148,000 miles per hour at perihelion. Parts of the spacecraft reached a temperature of 261 degrees Fahrenheit, but the instruments continued to operate without problems. During its second perihelion on Sept. 21, temperatures reached 270 degrees, affecting the operation of some instruments. Helios 1 continued to operate and return useful data until both its primary and backup receivers failed and its high-gain antenna no longer pointed at Earth. Ground controllers deactivated the spacecraft on Feb. 18, 1985, with the last contact made on Feb. 10, 1986. 

Helios 1 sits atop its Titan IIIE-Centaur rocket at Launch Complex 41 at Cape Canaveral Air Force, now Space Force, Station in Florida.
Helios 1 sits atop its Titan IIIE-Centaur rocket at Launch Complex 41 at Cape Canaveral Air Force, now Space Force, Station in Florida.
Credit: NASA

Helios 2 launched on Jan. 15, 1976, and followed a path similar to its predecessor’s but one that took it even closer to the Sun. On April 17, it approached to within 27 million miles of Sun, traveling at a new record of 150,000 miles per hour. At that distance, the spacecraft experienced 10% more solar heat than its predecessor. Helios 2’s downlink transmitter failed on March 3, 1980, resulting in no further useable data from the spacecraft. Controllers shut it down on Jan. 7, 1981. Scientists correlated data from the Helios instruments with similar data gathered by other spacecraft, such as the Interplanetary Monitoring Platform Explorers 47 and 50 in Earth orbit, the Pioneer solar orbiters, and Pioneer 10 and 11 in the outer solar system. In addition to their solar observations, Helios 1 and 2 studied the dust and ion tails of the comets C/1975V1 West, C/1978H1 Meier, and C/1979Y1 Bradfield. The information from the Helios probes greatly increased our knowledge of the Sun and its environment, and also raised more questions left for later spacecraft from unique vantage points to try to answer. 

helios-1-9-spacecraft-illustration-from-
llustration of a Helios probe in flight, with all its booms deployed. 
Credit: NASA

The joint ESA/NASA Ulysses mission studied the Sun from vantage points above its poles. After launch from space shuttle Discovery during STS-41 on Oct. 6, 1990, Ulysses used Jupiter’s gravity to swing it out of the ecliptic plane and fly first over the Sun’s south polar region from June to November 1994, then over the north polar region from June and September 1995. Ulysses continued its unique studies during several more polar passes until June 30, 2009, nearly 19 years after launch and more than four times its expected lifetime. NASA’s Parker Solar Probe, launched on Aug. 12, 2018, has made ever increasingly close passes to the Sun, including flying through its corona, breaking the distance record set by Helios 2. The Parker Solar Probe reached its first perihelion of 15 million miles on Nov. 5, 2018, with its closest approach of just 3.86 million miles of the Sun’s surface, just 4.5 percent of the Sun-Earth distance, planned for Dec. 24, 2024. The ESA Solar Orbiter launched on Feb. 10, 2020, and began science operations in November 2021. Its 10 instruments include cameras that have returned the highest resolution images of the Sun including its polar regions from as close as 26 million miles away. 

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John J. Uri

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Dec 10, 2024
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      Launch of space shuttle Discovery on mission STS-63. STS-63 Commander James Wetherbee on Discovery’s flight deck. STS-63 Pilot Eileen Collins on Discovery’s flight deck. On Feb. 3, Discovery and its six-person crew lifted off from Launch Pad 39B at 12:22 a.m. EST, the time dictated by orbital mechanics – Discovery had to launch into the plane of Mir’s orbit. Within 8.5 minutes, Discovery had reached orbit, for the first time in shuttle history at an inclination of 51.6 degrees, again to match Mir’s trajectory. Early in the mission, one of Discovery’s 44 attitude control thrusters failed and two others developed minor but persistent leaks, threatening the Mir rendezvous.  

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      STS-51C Commander Thomas “T.K.” Mattingly films the Earth from Discovery’s overhead flight deck window. STS-51C crew members Loren Shriver, left, Ellison Onizuka, and James Buchli on Discovery’s flight deck. STS-51C Payload Specialist Gary Payton on Discovery’s flight deck. Sunlight streams through Earth’s upper atmosphere, with Discovery’s tail and Orbital Maneuvering Engine pods outlined by sunlight. The Pacific coast of Guatemala and southern Mexico. New Orleans and the Mississippi River delta. Discovery touches down at NASA’s Kennedy Space Center in Florida. The STS-51C astronauts are greeted by NASA officials as they exit Discovery. To maintain the mission’s secrecy, NASA could reveal the touchdown time only 16 hours prior to the event. On Jan. 27, Mattingly and Shriver brought Discovery to a smooth landing at KSC’s Shuttle Landing Facility after a flight of three days one hour 33 minutes, the shortest space shuttle mission except for the first two orbital test flights. The astronauts orbited the Earth 49 times. About an hour after touchdown, the astronaut crew exited Discovery and boarded the Astrovan for the ride back to crew quarters. Neither NASA management nor the astronauts held a post mission press conference. The U.S. Air Force announced only that the “IUS aboard STS-51C was deployed from the shuttle Discovery and successfully met its mission objectives.” Later in the day, ground crews towed Discovery to the Orbiter Processing Facility to begin preparing it for its next planned mission, STS-51D in March. 
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      Following the recovery of SRBs after each shuttle mission, engineers conducted detailed inspections before clearing them for reuse. After STS-51C, inspections of the critical O-ring seals that prevented hot gases from escaping from the SRB field joints revealed significant erosion and “blow-by” between the primary and secondary O-rings. Both left and right hand SRBs showed this erosion, the most significant of the program up to that time. Importantly, these O-rings experienced weather colder than any previous shuttle mission, with overnight ambient temperatures in the teens and twenties. Even at launch time, the O-rings had reached only 60 degrees. Engineers believed that these cold temperatures made the O-rings brittle and more susceptible to erosion. One year later, space shuttle Challenger launched after similarly cold overnight temperatures, with O-rings at 57 degrees at launch time. The Rogers Commission report laid the blame of the STS-51L accident on the failure of O-rings that allowed super-hot gases to escape from the SRB and impinge on the hydrogen tank in the ET, resulting in the explosion that destroyed the orbiter and claimed the lives of seven astronauts. The commission also faulted NASA’s safety culture for not adequately addressing the issue of O-ring erosion, a phenomenon first observed on STS-2 and to varying degrees on several subsequent missions.  
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