Highly Eccentric Orbiting Satellite; examined magnetic fields outside of Earth's magnetosphere.

Typical orbit: 10,213 km x 221,472 km at 60.5 to 89.9 deg inclin. Mass: 116 kg (255 lb). Associated Launch Vehicle: Delta E, Delta L.

• 1968 December 5 - HEOS 1 - Launch Site: Cape Canaveral. Launch Vehicle: Delta E. Mass: 108 kg (238 lb). Perigee: 20,020 km (12,430 mi). Apogee: 202,780 km (126,000 mi). Inclination: 60.50 deg. Period: 6,704.30 min.

  Highly Eccentric Orbiting Satellite; examined magnetic fields outside of Earth's magnetosphere. Spacecraft engaged in research and exploration of the upper atmosphere or outer space (US Cat B).

• 1972 January 31 - HEOS 2 - Launch Site: Vandenberg. Launch Vehicle: Delta L. Mass: 123 kg (271 lb). Perigee: 405 km (251 mi). Apogee: 240,164 km (149,230 mi). Inclination: 89.90 deg. Period: 7,477.10 min.

  Highly Eccentric Orbiting Satellite; particles and fields data. Fifth satellite of ESRO. Also registered as US object 1972-05A in A/AC.105/INF.248 with orbit 7835.4 min, 439 x 248160 km x 90.2 deg, category B.

• USAF Satellite Database, USAF Institute of Technology, 1995. Accessed at: ftp://archive.afit.af.mil/pub/space/.

• McDowell, Jonathan, Jonathan's Space Home Page, Harvard University, 1997-present. Jonathan McDowell's complete on-line listing of all objects orbited and over 20,000 rocket launches Accessed at: http://www.planet4589.org/jsr.html.

• Krebs, Gunter, Gunter's Space Page, University of Frankfurt, 1996. Accessed at: http://www.skyrocket.de/space.

• JPL Mission and Spacecraft Library, Jet Propulsion Laboratory, 1997. Accessed at: http://msl.jpl.nasa.gov/home.html.

• McDowell, Jonathan, Launch Log, October 1998. Jonathan McDowell's complete on-line listing of all objects orbited since Sputnik. Accessed at: http://hea-www.harvard.edu/~jcm/space/log/launch.html.