RHESSI (Ramaty High Energy Solar Spectroscopic Imager) is an American (NASA) solar flare observatory that was launched at 20:58 UT on 2002 February 5 by a Pegasus XL rocket which was released from a L-1011 aircraft flying out of the Cape Canaveral AFS. The initial orbital parameters were period 96.5 min, apogee 607 km perigee 579 km, and inclination 38.0 deg.
The 293-kg (with fuel), 414-W satellite is equipped to image at high resolution solar flares in X-rays and gamma rays. The X-rays and gamma rays cover an energy range of 3 keV-20 MeV with an energy resolution of about one keV and a spatial resolution of just a few seconds of arc.
The imaging is accomplished by a 45 cm x 1.7 m tube containing nine pairs (one behind the other, spaced 1.5 m apart) of tungsten or molybdenum wire grids of width 9 cm mounted parallel to the rotation axis of the tube pointing at the sun. The tube rotates about its axis as the spacecraft spins at a rate of 15 rpm. During a rotation, a photon from any point on the sun can either pass through a grid-pair or be blocked by one or other of the grids. This causes a modulation of the intensity of photons emanating from that point. The depth of modulation is zero for the photons arriving exactly along the spin axis and gradually increases to the off-axis photons.
Behind each grid-pair is a cryogenic (75 K) germanium detector of 7.1 cm diameter and 8.5 cm thickness. The output from each of the nine detectors, at any given energy, can be Fourier-analyzed to provide a full two-dimensional spatial spectrum of an extended source region on the sun. The full spatial spectrum is possible because each wire grid pair has a different slit width, spacing, and wire thickness.
Data accumulation is about 16 Gb during a 10-min rotation. The telemetry data is collected at Berkeley (CA), Wallops (VA), Santiago (Chile), and Weilheim (Germany). Science analysis of the data will involve close collaboration with many dedicated ground-based and satellite-based solar observatories.
Alternate Names
Explorer 81
HESSI
Reuven Ramaty High Energy Solar Spectroscopic Imager
SMEX/RHESSI
Small Explorer/RHESSI
Facts in Brief
Launch Date: 2002-02-05
Launch Vehicle: Pegasus XL
Launch Site: Cape Canaveral, United States
Mass: 230.0 kg
Nominal Power: 414.0 W