The Helios mission consisted of two spacecraft that were used to make in-situ observations of the inner heliosphere. Helios 1 was launched on December 10, 1974, and Helios 2 on January 15, 1976. The orbits of the Helios 1 and 2 spacecraft had periods of 190 and 187 days, aphelia of 0.99 and 0.98 AU, and perihelia of 0.31 and 0.29 AU, respectively. In terms of design and instrumentation, the two spacecraft were nearly identical; each was equipped with several instruments,two booms and a 32-m electric dipole antenna. Each spacecraft spun once per second about an axis normal to the ecliptic plane. Operations ceased for Helios 2 on March 21, 1980, and for Helios 1 in March of 1986.
Helios 1 & 2 rank among the most important missions in Heliophysics, and, although more than 40 years old, the more than 11 years of data returned by its spacecraft remain of paramount interests to researchers. Their unique trajectories, which brought them closer to the Sun than any spacecraft before or since, enabled their diverse suite of in-situ instruments to return measurements of unprecedented scientific richness. Analyses of these measurements produced groundbreaking insights into the large-scale spacial and temporal variations in the inner heliosphere [Marsch1991a, Marsch1991b, Marsch2006], solar wind turbulence across both MHD and kinetic scales [Marsch1991b, Bruno2005, Bruno2013], the effects of kinetic microinstabilities [Marsch1982b, Marsch1987], the process of collisional thermalization [Marsch1982a], and ongoing heating processes [Schwartz1983].
This archive will be a tremendous asset to the entire solar and heliophysics community but especially to the Solar Probe Plus (SPP) and Solar Orbiter (SolO) instrument and science teams. Indeed, the importance of Helios to the upcoming SolO and SPP missions is evidenced even in the planned trajectories of these new spacecraft: in rough terms, the former mirrors that of Helios and the latter extends it even closer to the Sun. In addition, in anticipation of the new, groundbreaking, measurements that will be performed by SPP and SolO, it will be worthwhile revisiting some of the most important outcomes from Helios related to solar and solar wind physics with more modern state-of-the-art analysis techniques.