The HELIOS Mission

Image 1: One of the Helios space probes


HELIOS was a joint German-American scientific mission. The launches took place on December 10, 1974, and January 15, 1976 from the Kennedy Space Center with a TitanIIIE/Centaur rocket combination. The probes were brought into orbit around the sun at a distance of 0.31 AU (HELIOS-1) and 0.29 AU (HELIOS-2) from the sun, or 46.5 and 43.5 million km

It was planned that the probes reach a distance of 50 million km from the sun, which is about a third the distance that the earth has to the sun. The probes had the form a gigantic spool of thread with special mirrors and solar cells covering the surface. HELIOS spun on its axis at a rate of one revolution per second in order to disperse the heat evenly. None the less, the surface reached temperatures of several hundred degrees. Both probes operated simultaneously to give an abundance of data from different positions. Because the experiments were practically identical, comparison of the data helped gain additional insight. The probes have survived the “hellish” conditions against expectations and, even after ten years, were still sending data to earth. Even today they are still circling the sun at a rate of 190 days for one orbit. The mission ended in 1978 (Helios-2) and 1986 (Helios-1).

The scientific goals and equipment of HELIOS

  • Exploration of the sun and its influence on space
  • Exploration of solar wind, magnetic fields, high energy particles, dust and micrometeorites
  • Origin and dispersion of objects in the heliosphere close to the sun

The HELIOS probes were equipped with ten experiments each: analysers to observe the speed, direction and quantity of the particles of the solar wind (protons, alpha particles, electrons) (experiment 1); two magnetometers to measure the permanent magnetic field and its slow changes (experiments 2 and 3); one magnetometer (coil magnetometer) to observe faster magnetic field variations (experiment 4); a measuring antennae with a radio wave receiver attached to observe slow and fast variations in electric field strength (experiment 5); several particle telescopes to determine the quantity, direction and energy of penetrating atomic particles of moderately high to high energy (cosmic radiation) (experiments 6, 7 and 8); HELIOS-2 also has an additional electronic set-up to detect the intensity variations of extremely short wave gamma radiation: a monitor to observe the sun with respect to of X-rays (experiment 7); three photometers to detect the sky’s brightness in three directions (zodiac-light photometer) (experiment 9); two instruments to detect and analyse impacting dust particles.

The experiments provided a vast quantity of data that took scientists years to evaluate. The new findings about the sun and the effects of the solar wind were numerous enough to easily fill up a whole textbook. HELIOS was one of the most successful German-American space projects in two ways. Aside from the extreme scientific benefit, the project gave the new German space industry the chance to exercise itself in the management of complex programs..

HELIOS turned the solar system into a physics laboratory of gigantic proportions. Here phenomena could be researched that for many reasons couldn’t be examined in laboratories on earth, and for the most part still can’t. Because the processes of acceleration and dispersion of charged particles take place in distant regions of space where they can’t be observed, the data form HELIOS has far-reaching astrophysical impact for the formation and history of the universe.

Kiel’s Contribution

Also on board HELIOS-1 and its sister probe HELIOS-2 was an experiment from the University of Kiel, developed by a work group under the leadership of Horst Kunow, who was later a member of the Department of Extraterrestrial Physics in the Institute for Experimental and Applied Physics. Kiel’s experiment measured the cosmic radiation (electrons, protons and heavy atomic nuclei) that is accelerated on the sun, in interplanetary space or in the galaxy outside of the solar system and found their way, along magnetic field lines, and at unimaginable speeds of 13,000 to 300,000 km/s, to the measuring system of HELIOS.