HESSI, the sixth Small Explorer, was a Spectrum Astro satellite derived from the SA-200S design. It carried a rotating modulation collimator transform telescope.

The satellite rotated at 15 rpm, imaging hard X-ray flares from the Sun by reconstructing the Fourier components from the time modulation of the flux through a set of 9 grids each 9 cm in diameter. It was expected to make images with a resolution of 2 arcseconds at 40 keV energies and 36 arcseconds at 1 MeV energies. Launch delays meant HESSI missed some of the best flares at solar max.

Mass: 304 kg (670 lb).

• 2002 February 5 - HESSI - Launch Site: Mayport DZ. Launch Complex: -. Launch Vehicle: Pegasus. Mass: 449 kg (989 lb). Perigee: 574 km (356 mi). Apogee: 599 km (372 mi). Inclination: 38.00 deg. Period: 96.40 min.
  HESSI, the sixth Small Explorer, carried a rotating modulation collimator transform telescope, imaging solar flares in the hard X-ray spectrum. The launch marked the return to flight of Pegasus after the Hyper-X failure. The launch was originally to have occurred on 28 March 2001. The L-1011 launch aircraft took off at 19:29 GMT from the Cape Canaveral Skid Strip RW30/12, and headed out to the drop area at 28.0 N 78.5 W over the Atlantic. Drop of the Pegasus in the Atlantic Drop Zone at 28.0 N 78.5 W was at 20:58 GMT, with ignition 5 seconds later. The Pegasus reached orbit at 21:07 GMT. On the first pass it was confirmed that the solar panels had opened.

  The satellite rotated at 15 rpm, imaging by reconstructing the Fourier components from the time modulation of the solar x-ray flux through a set of 9 grids each 9 cm in diameter. It was expected to make images with a resolution of 2 arcseconds at 40 keV energies and 36 arcseconds at 1 MeV energies. The launch delays meant that HESSI missed some of the best flares at solar max.