Ramaty High-Energy Solar Spectroscopic Imager

The Primary Mission Objective is to explore the basic physics of particle acceleration and energy release in solar flares.

Science Objectives


  • Determine the frequency, location, and evolution of impulsive energy release in the corona
  • Study the acceleration of electrons, protons, and heavier ions in flares
  • Study the heating of plasma to tens of millions of degrees and determine its relationship to particle acceleration
  • Study the propagation and evolution of energetic particles in flares
  • Determine the relative abundances of accelerated and ambient ions in flares


  • Obtain images and spectra of the Crab Nebula with 2 arcsecond spatial resolution and ~1 keV spectral resolution
  • Detect and obtain high resolution spectra of gamma-ray bursts and cosmic and terrestrial transient sources over a large fraction of the sky
  • Search for cyclotron line features in gamma-ray bursts and cosmic transient sources
  • Obtain high resolution spectra and search for line features in steady X-ray and gamma-ray sources

Primary Observations

Simultaneous, high resolution imaging and spectroscopy of solar flares from 3 keV X-rays to 17 MeV gamma rays with high time resolution

Expected Numbers of Flares:

  • Tens of thousands of microflares
  • Over a thousand X-ray flares with crude imaging and spectra to >100 keV
  • Hundreds of flares with >1000 counts per second above 20 keV, allowing spatial changes to be followed on timescales of 0.1 seconds.
  • Tens of flares sufficiently intense to allow the finest possible imaging spectroscopy
  • Up to 100 flares with the detection of gamma-ray lines
  • Tens of flares with detailed gamma-ray line spectroscopy and the location and extent of the source determined to ~40 arcseconds

Mission Details

RHESSI (Ramaty High-Energy Solar Spectroscopic Imager)Mission Class

Small Explorer (SMEX)


Date: February 5, 2002
Time: 3:58 PM EDT
Vehicle: Orbital Sciences Corporation (OSC) Pegasus XL
Site: L1011 aircraft takes off from Kennedy Space Center


Altitude: 600 km (373 miles)
Inclination to the equator: 38 degrees

Spacecraft Pointing

Spin stabilized
Spin axis within 0.2 degrees of Sun center
Spin rate:15 revolutions per minute

Ground Stations

University of California, Berkeley

Wallops Flight Facility, Virginia
Santiago, Chile
Weilheim, Germany

Operations Lifetime

2 years (3 years desirable)



Nine segmented, hyperpure germanium crystals
Cooled to ~75 K (-198 degrees Celsius)

Energy Range

~3 keV – ~17 MeV

Spectral Resolution

~1 keV (FWHM) in the front segment up to ~100 keV
~3 keV in the rear segment up to ~1 MeV increasing to ~5 keV at 20 MeV



Fourier-transform imaging
9 rotating modulation collimators (grid pairs)

Field of View

Full Sun (~1 degree)