HEOS (Highly Eccentric Orbiting Satellite) 1 and 2 were ESRO (European Space Research Organisation) satellites that traveled in extremely elongated orbits in order to study the interplanetary magnetic field and solar and cosmic ray particles outside Earth's magnetosphere.

 

Nine scientific groups in five countries provided experiments for the HEOS 1 satellite. Under a 30 December 1966 memorandum of understanding and a 8 March 1967 contract with ESRO, the mission was the first cost-reimbursed NASA launch of a foreign scientific satellite.

 

HEOS 2, the second satellite in the series, continued the study of the interplanetary medium.

 

spacecraft	launch vehicle	launch date	launch site	solar orbit	mass (kg)
HEOS 1	Delta E	Dec 5, 1968	Cape Canaveral	20,020 × 202,780 km × 60.5°	108
HEOS 2	Delta L	Jan 31, 1972	Vandenberg	405 × 240,164 km × 89.9°	123