Reuven Ramaty High Energy Solar Spectroscopic Imager
An artists depiction of the RHESSI spacecraft observing the sun
|Organization||NASA/Goddard Space Flight Center|
|Major contractors||Space Sciences Laboratory
|Launch date||2002-02-05 20:58:00 UTC|
|Launched from||Cape Canaveral Air Force Station|
|Launch vehicle||Pegasus XL|
|Mission length||2 years  (planned)
11 years as February 2013
|Mass||293 kg (650 lb)|
|Length||2.06 m (6.8 ft)|
|Type of orbit||Circular, 38° inclination|
|Orbit height||579 km (360 mi) - 607 km (377 mi)|
|Orbit period||96.5 minutes|
|Telescope style||Coded aperture mask|
|Wavelength||γ-ray / X-ray|
|Diameter||1.18 m (3.9 ft)|
|Collecting area||150 cm2 (0.16 sq ft)|
|Focal length||1.55 m (5.1 ft) (SAS)|
|Spectrometer||9 segmented, hyperpure germanium crystals, cooled to −198 °C (−324.4 °F)|
|Solar Aspect System (SAS)||determines direction to Sun-center within 1.5 arcseconds|
|Roll Angle System (RAS)||determines roll angle within 3 arcminutes|
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI, or more rarely Explorer 81 or originally High Energy Solar Spectroscopic Imager or HESSI) is the sixth mission in the line of NASA Small Explorer missions (also known as SMEX). Launched on 5 February 2002, its primary mission is to explore the basic physics of particle acceleration and explosive energy release in solar flares.
HESSI was renamed to RHESSI on March 29, 2002 in honor of Reuven Ramaty, a pioneer in the area of high energy solar physics - RHESSI is the first space mission named after a NASA scientist. RHESSI was designed and is operated at the Space Sciences Laboratory in Berkeley California.
Mission concept 
RHESSI is designed to image solar flares in energetic photons from soft X rays (~3 keV) to gamma rays (up to ~20 MeV) and to provide high resolution spectroscopy up to gamma-ray energies of ~20 MeV. Furthermore, it has the capability to perform spatially resolved spectroscopy with high spectral resolution.
Scientific objectives 
Researchers believe that much of the energy released during a flare is used to accelerate, to very high energies, electrons (emitting primarily X-rays) and protons and other ions (emitting primarily gamma rays). The new approach of the RHESSI mission is to combine, for the first time, high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, so that a detailed energy spectrum can be obtained at each point of the image.
This new approach will enable researchers to find out where these particles are accelerated and to what energies. Such information will advance understanding of the fundamental high-energy processes at the core of the solar flare problem.
The primary scientific objective of RHESSI is to understand the following processes that take place in the magnetized plasmas of the solar atmosphere during a flare:
- Impulsive energy release,
- Particle acceleration,
- Particle and energy transport.
These high-energy processes play a major role at sites throughout the universe ranging from magnetospheres to active galaxies. Consequently, the importance of understanding these processes transcends the field of solar physics; it is one of the major goals of space physics and astrophysics.
The high energy processes of interest include the following:
- The rapid release of energy stored in unstable magnetic configurations,
- The equally rapid conversion of this energy into the kinetic energy of hot plasma and accelerated particles (primarily electrons, protons and ions),
- The transport of these particles through the solar atmosphere and into interplanetary space,
- The subsequent heating of the ambient solar atmosphere.
These processes involve:
- Particle energies to many GeV,
- Temperatures of tens or even hundreds of millions of degrees,
- Densities as low as 100 million particles per square cm,
- Spatial scales of tens of thousands of kilometers, and
- Magnetic containment times of seconds to hours.
It is impossible to duplicate these conditions in laboratories on the Earth.
The acceleration of electrons is revealed by hard X-ray and gamma-ray bremsstrahlung while the acceleration of protons and ions is revealed by gamma-ray lines and continuum. The proximity of the Sun means, not only that these high-energy emissions are orders of magnitude more intense than from any other cosmic source, but also that they can be better resolved, both spatially and temporally.
Since X-rays are not easily reflected or refracted, imaging in X-rays is difficult. One solution to this problem is to selectively block the X-rays. If the X-rays are blocked in a way that depends on the direction of the incoming photons, then it may be possible to reconstruct an image. The imaging capability of RHESSI is based on a Fourier-transform technique using a set of 9 Rotational Modulation Collimators (RMCs) as opposed to mirrors and lenses. Each RMC consist of two sets of widely-spaced, fine-scale linear grids. As the spacecraft rotates, these grids block and unblock any X-rays which may be coming from the Sun modulating the photon signal in time. The modulation can be measured with a detector having no spatial resolution placed behind the RMC since the spatial information is now stored in the time domain. The modulation pattern over half a rotation for a single RMC provides the amplitude and phase of many spatial Fourier components over a full range of angular orientations but for a small range of spatial source dimensions. Multiple RMCs, each with different slit widths, provide coverage over a full range of flare source sizes. Images are then reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry.
RHESSI provides spatial resolution of 2 arcseconds at X-ray energies from ~4 keV to ~100 keV, 7 arcseconds to ~400 keV, and 36 arcseconds for gamma-ray lines and continuum emission above 1 MeV.
RHESSI can also see gamma rays coming from off-solar directions. The more energetic gamma rays pass through the spacecraft structure, and impact the detectors from any angle. This mode is used to observe gamma ray bursts (GRBs). The incoming gamma rays are not modulated by the grids, so positional and imaging information is not recorded. However, a crude position can still be derived by the fact that the detectors have front and rear pickups. Also, the detectors near the burst shield the ones away from the burst. Comparing signal strengths around the nine crystals, and front-to-back, then gives a coarse, two-dimensional position in space.
When combined with high-resolution time stamps of the detector hits, the RHESSI solution can be cross-referenced on the ground with other spacecraft in the IPN (Interplanetary Network) to provide a fine solution. The large area and high sensitivities of the Ge crystal assembly make RHESSI a formidable IPN component. Even when other spacecraft can provide burst locations, few can provide as high-quality spectra of the burst (in both time and energy) as RHESSI.
Rarely, however, a GRB occurs near the Sun, in the collimated field of view. The grids then provide full information, and RHESSI can provide a fine GRB location even without IPN correlation.
Spacecraft and instrument 
The entire spacecraft rotates to provide the necessary signal modulation. The four, fixed solar panels are designed to provide enough gyroscopic moment to stabilize rotation about the solar vector. This largely eliminates the need for attitude control.
The instrument detectors are nine high-purity germanium crystals. Each is cooled to cryogenic temperatures by a mechanical cryocooler. Germanium provides not only detections by the photoelectric effect, but inherent spectroscopy through the charge deposition of the incoming ray. The crystals are housed in a cryostat, and mounted with low-conductivity straps.
A tubular telescope structure forms the bulk of the spacecraft. Its purpose is to hold the collimators above the Ge crystals at known, fixed positions.
RHESSI observations have changed our perspective on solar flares, particularly on high-energy processes in flares. RHESSI observations has led to numerous publications in scientific journals and presentations at conferences. As of September 2008, RHESSI is mentioned in 970 publications, books, and presentations (as listed on NASA ADS). Between February 2006 to 2008, 200 publications have been published about RHESSI observations.
Some significant RHESSI results follow:
- RHESSI was the first satellite to accurately measure terrestrial gamma-ray flashes that come from thunder storms, and RHESSI found that such flashes occur more often than thought and the gamma rays have a higher frequency on average than the average for cosmic sources.
BATSE was the first instrument to detect Terrestrial Gamma-ray Flashes (TGFs), intense gamma-radiation from thunderstorms. See Fishman 1994 paper.
- New solar flare satellite gets renamed Spaceflight Now article
- RHESSI Imaging Explained
-  First Gamma-Ray Images of a Solar Flare (Hurford et al. 2003)
-  Live spacecraft status information
-  2008 RHESSI Senior Review Report
- RHESSI Home Page hosted by the Goddard Space Flight Center
- RHESSI Home page hosted by ETH
- RHESSI Science Nuggets A biweekly series of Web articles explaining recent results
- High Energy Solar Spectroscopic Imager Mission Profile by NASA's Solar System Exploration
- HEDC a scientific data warehouse for RHESSI data
- Browser a simple RHESSI data browser