Introduction

Introduction

STEREO (Solar TErrestrial RElations Observatory) is the third mission in NASA's Solar Terrestrial Probes program (STP). The mission, launched in October 2006, has provided a unique and revolutionary view of the Sun-Earth System. The two nearly identical observatories - one ahead of Earth in its orbit, the other trailing behind - have traced the flow of energy and matter from the Sun to Earth. STEREO has revealed the 3D structure of coronal mass ejections; violent eruptions of matter from the sun that can disrupt satellites and power grids, and help us understand why they happen. STEREO is a key addition to the fleet of space weather detection satellites by providing more accurate alerts for the arrival time of Earth-directed solar ejections with its unique side-viewing perspective.

History
Instruments

The following four instrument packages are mounted on each of the two STEREO spacecraft:

Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)

SECCHI is a suite of 4 scientific telescopes that will observe the solar corona and inner heliosphere from the surface of the Sun to the orbit of Earth. These unique observations will be made in stereo for NASA's Solar Terrestial Relations Observatory STEREO. The suite has three main parts. The SCIP (Sun Centered Imaging Package - three telescopes), the HI (Heliospheric Imager - two telescopes) and the SEB (Secchi Electronics box).

The STEREO mission is the third in the line of Solar-Terrestrial Probes (STP) and is a strategic element of the Sun-Earth Connections Roadmap. STEREO is designed to view the three-dimensional (3D) and temporally varying heliosphere by means of an unprecedented combination of imaging and in situ experiments mounted on virtually identical spacecraft flanking the Earth in its orbit.

The primary goal of the STEREO mission is to advance the understanding of the three-dimensional structure of the Sun's corona, especially regarding the origin of coronal mass ejections (CMEs), their evolution in the interplanetary medium, and the dynamic coupling between CMEs and the Earth environment. CMEs are the most energetic eruptions on the Sun, are the primary cause of major geomagnetic storms, and are believed to be responsible for the largest solar energetic particle events.

The two spacecraft will be launched together and will use a gravity assist from the moon to slingshot the spacecraft into a heliocentric orbit. The first to enter heliocentric orbit will be the Ahead (STEREO-A) spacecraft and then two weeks later the Behind (STEREO-B) spacecraft. The two spacecraft drift away from Earth at an average rate of about 22.5 degrees per year. After the two year nominal operations phase the spacecraft will be about 90 degrees apart, each about 45 degrees from Earth. STEREO-A will drift ahead of Earth and STEREO-B behind. In order to accomplish this drift, STEREO-A will be traveling faster than Earth around the Sun and so must have an orbit slightly closer to the Sun than Earth's. Similarly, the STEREO-B must be traveling slower than Earth and must have an orbit slightly further than Earth.

Overview:

Coronagraphs:

COR1 and COR2 observe the inner (1.4-4. Rsun) and outer (2-15 Rsun) corona with greater frequency and polarization precision than ever before. COR1 will be the first space borne instrument to explore the inner corona in white light and pB down to 1.4 Rsun. COR2 will image the corona with five times the spatial resolution and three times the temporal resolution of LASCO/C3.


Extreme Ultraviolet Imager (EUVI):

EUVI provides full Sun coverage with twice the spatial resolution and dramatically improved cadence over EIT.

EUVI observes the photospheric magnetic field, chromosphere, and innermost corona underlying the same portions of the corona and the heliosphere observed by COR1, COR2, and HI.


Guide Telescope:

The Guide Telescope acts as a fine sun sensor for the EUVI and provides the error signal for the EUVI fine pointing system.


Heliospheric Imager (HI):

The most novel instrument, HI extends the concept of traditional externally occulted coronagraphs to a new regime, the heliosphere from the Sun to the Earth (12-318 Rsun). HI will obtain the first direct imaging observations of coronal mass ejections in interplanetary space.

 

  • SECCHI EUVI: Extreme UltraViolet Imager (LMSAL)
  • SECCHI COR1: Inner Coronagraph (GSFC)
  • SECCHI COR2: Outer Coronagraph (NRL)
  • SECCHI HI: Heliospheric Imager
  • Sungrazing Comets

Solar imaging on STEREO will be accomplished with the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument package. 2 A series of telescopes are used to image CMEs from the solar surface to the terrestrial environment. The first is an extreme ultraviolet imager (EUVI) that will image the chromosphere and low corona in four emission lines characteristic of coronal temperatures. Two white light coronagraphs extend observations from the EUVI field-of-view out to 15 solar radii. Because of the large gradient in coronal brightness, the inner corona is observed by the COR1 coronagraph, while the outer corona is observed with the COR2 coronagraph. Finally, Earth-directed CMEs are observed from the near-Sun to near-Earth environments by the side-viewing Heliospheric Imager (HI).

SECCHI EUVI: Extreme UltraViolet Imager (LMSAL)

1. INTRODUCTION

The Extreme Ultraviolet Imager (EUVI) is part of the SECCHI instrument suite currently being developed for the NASA STEREO mission. As an integral element of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), the EUVI plays a critical role in addressing the SECCHI science objectives, in particular: • Investigate the initiation of Coronal Mass Ejections (CMEs): How flux systems interact during the CME initiation, the role of reconnection, and coronal dimming • Investigate the physical evolution of CMEs: Their 3-dimensional structure, how they are accelerated, and the response of the low corona • Investigate the 3-dimensional structure of Active Regions

The STEREO mission consists of two identically instrumented spacecraft in heliocentric orbits, drifting away from Earth in opposite directions at 22 degrees per year. The two observatories will provide stereoscopic imaging of the sun as their separation angle increases. The STEREO launch is scheduled for November 2005.

The SECCHI investigation is led by the Naval Research Lab (NRL). The EUVI telescope is being developed at the Lockheed Martin Solar and Astrophysics Lab (LMSAL). The EUVI mirrors are being figured and coated at the Institutd’Optique (IOTA) and calibrated at the Institut d’Astrophysique Spatiale (IAS), the focal plane assembly is being developed at NRL and the University of Birmingham, the camera electronics are being developed at the Rutherford Appleton Lab, and the aperture door is being supplied by the Max-Planck Institut für Aeronomie (MPAe).

The SECCHI instrument suite includes five telescopes covering a broad range of fields of view, starting at the solar surface and extending all the way to the interplanetary space between the sun and Earth. The EUVI covers the innermost portion of this range, from the solar chromosphere to the inner corona at 1.7 solar radii. The EUVI builds on its predecessor, EIT on SOHO 1 , with several performance improvements including better spatial resolution and a higher image cadence. The following is an overview of the design and the capabilities of this new telescope.

2. EUVI TELESCOPE OVERVIEW

The EUVI observes the chromosphere and low corona in four different EUV emission lines between 17.1 and 30.4 nm. It is a small, normal-incidence telescope with thin metal filters, multilayer coated mirrors, and a back-thinned CCD detector. Figure 1 shows the EUVI on the SCIP platform and Figure 2 is a cross section through the telescope.

EUV radiation enters the telescope through a thin metal film filter of 150 nm of aluminum. This filter suppresses most of the UV, visible, and IR radiation and keeps the solar heat out of the telescope. During launch, the filter is protected by a door. The radiation then passes through an aperture selector to one of the four quadrants of the optics. Each quadrant of the primary and secondary mirror is coated with a narrow-band, multilayer reflective coating, optimized for one of four EUV lines. The radiation continues through a filter wheel, with redundant thin-film aluminum filters to remove the remainder of the visible and IR radiation. A rotating blade shutter controls the exposure time. The image is formed on a CCD detector. The main parameters for the EUVI telescope are summarized in Table 1.

Instrument type Normal incidence EUV telescope (Ritchey-Chrétien)
Wavelength selection Narrow-band EUV reflective multiplayer coatings
Four passbands selected by different coatings in each mirror quadrant
Bandpass He II 30.4 nm
Fe IX 17.1 nm
Fe XII 19. 5nm
Fe XV 28.4 nm
IR/visible/UV rejection Thin metal film filters
Aperture 98 mm at primary mirror
Effective focal length 1750 mm
Field of View Circular full sun field of view to ±1.7 solar radii
Spatial Scale 1.6” pixels
Detector Backside illuminated CCD (e2v CCD42-40), 2048 x 2048 pixels
Mechanisms

1 aperture selectorm, 1 filter wheel, 1 focal plane shutter

Image stabilization Active secondary mirror (tip/tilt)
3.1 Optical Design

The EUVI optics is a Ritchey-Chrétien system with a secondary mirror magnification of 2.42. This system provides pixel limited resolution across the entire field of view in all four optical quadrants. The low secondary mirror magnification reduces the telescope’s sensitivity to shifts in the mirror separation and eliminates the need for a focus mechanism. The telescope is fully baffled to prevent charged particles entering the front aperture from reaching the CCD. The telescope pupil is located just in front of the primary mirror and is defined by an aperture mask. The baffles and aperture mask have been designed for an unvignetted field of view to ±1.7 solar radii. The optical prescription of the system is summarized in Table 2. Figure 3 shows ray trace results for a single quadrant, both on-axis and at the edge of the field, and up to 0.15 mm inside and outside of nominal focus. The system has a minor amount of field curvature; the nominal focus location is chosen to minimize the aberrations across the field. 

SECCHI COR1: Inner Coronagraph (GSFC)

We report here on the inner coronagraph, labeled COR1. COR1 is a classic Lyot 3 internally occulting refractive coronagraph, adapted for the first time to be used in space. The field of view is from 1.3 to 4 solar radii. A linear polarizer is used to suppress scattered light, and to extract the polarized brightness signal from the solar corona.

Optical Layout

Sunlight enters through the front aperture, where the objective lens focuses the solar image onto the occulter. To keep scattering to a minimum, a singlet lens is used for the objective, made of radiation hardened BK7-G18 glass. The primary photosphere light suppression mechanisms in COR-1 are the objective lens to occulter imaging system and the aperture stop to field lens to Lyot stop imaging system. Rather than rely on the on the out of band rejection of the filter, these primary suppression mechanisms are designed to work over the full sensitivity wavelength band of the instrument.

The solar image from the objective will be chromatically aberrated, so the occulter must be sized to block all the solar photospheric light from the near UV to infrared (350–1100 nm). The cut-on at 350 nm is set by the transmission of the BK7-G18 glass in the objective lens, and the cut-off at 1100 nm is set by the band gap of the silicon detector. Subsequent lenses in the optical train balance the chromatic aberration from the objective. The narrow bandpass of the instrument also minimizes the effect of chromatic aberration in the final image.

The occulter is mounted on a stem mounted at the center of the field lens (Figure 2). The tip of the occulter is cone shaped, to direct the sunlight into a light trap which surrounds the occulter. The radius was chosen to block all wavelengths (350–1100 nm) out to a radius of 1 : 1 R Ø . At the design wavelength of 656 nm, the solar image is completely occulted out to 1.30 R Ø , and partially vignetted out to 1.64 R Ø . The wedge-shaped design of the light trap ensures that any ray entering it must reflect many times, so that the light will be absorbed before it can find its way out again.

Diffracted light from the edge of the front aperture is focused onto a Lyot stop and removed. This eliminates the largest source of stray light in the system. Additional stray light rejection is accomplished by placing baffles at various points between the front aperture and Lyot stop. A Lyot spot is also glued to the front surface of the doublet lens immediately behind the Lyot stop, to remove ghosting from the objective lens.

Two doublet lenses are used to focus the coronal image onto the CCD detector. The first, a positive power achromat, is placed immediately behind the Lyot stop, while the second, a negative power acromat, is placed further down the optical path. Together, these act as a telephoto-lens system, focusing the coronal image onto the detector plane, while maintaining diffraction-limited resolution. A bandpass filter 10 nm wide, centered on the H Æ line at 656 nm, is placed just behind the first doublet. Thus, the Lyot stop, spot, first doublet, and bandpass filter form a single optical assembly. A linear polarizer on a hollow core motor rotational stage is located between the two doublets. The current design calls for Corning’s Polarcor to be used as the polarizing material, and the placement of the polarizer was chosen to be as close to the first doublet as possible within the dimension constraints of the largest diameter piece of Polarcor that can be obtained (33 mm clear aperture). Normal operations call for three sequential images to be taken with polarizations of 0 ± and ß 60 ± , to extract the polarized brightness. A focal plane mask is located between the shutter and the focal plane detector, and is used to remove diffracted light from the edge of the occulter, as discussed below. The detector is an EEV model 42-40 CCD, 2 with 2048 £ 2048 pixels, 13.5 π m on a side. Typically, the COR1 images will be 2 £ 2 binned onboard before telemetering to the ground. The CCD is backside illuminated, with an anti-reflective coating for high quantum efficiency. A radiator at the back of the instrument ensures low-noise operation by keeping the CCD temperature below -75 ± C. The single-pixel full well capacity will be greater than 100,000 electrons, digitized to 14 bits.

3. MECHANICAL AND THERMAL DESIGN The COR1 mechanical structure is designed as a series of tube sections (Figure 1), which are bolted and pinned together for stability. The individual optics are aligned, mounted, and pinned within these tube sections. COR1 assembly starts from the front tube section, with the objective, occulter, light trap, and field lens, and then each subsequent section is added, together with its associated optical or mechanical components. Because the individual tube sections are pinned together, sections can be taken off and back on again without changing the optical alignment.

Three mechanisms are included in the COR1 instrument package. At the front of the instrument will be a door, to protect the instrument before and during launch, and during spacecraft maneuvers. This door is being supplied by the Max-Planck-Institut f ̈ur Aeronomie in Lindau, Germany, and is made to the same design as the doors used for the LASCO 4 and EIT 5 instruments on SOHO. On the front of this door will be a diffuser so that the operation of the instrument can be tested when the door is closed, and to provide a flat-field calibration signal.

The other two mechanisms are the hollow core motor to rotate the linear polarizer, and a rotating blade shutter mounted just in front of the focal plane detector assembly, both supplied by Lockheed Martin. All the mechanisms, together with the focal plane CCD detector, will be operated from a centralized control system for all the SECCHI instruments.

Because the two STEREO spacecraft are in elliptical orbits about the Sun, the COR1 instruments will experience considerable variation in solar load, from 1264–1769 and 1068–1482 W/m 2 for the ahead and behind spacecraft respectively. When these loads are combined with the modeled changes in the material thermal properties from beginning to end of life, and with the most extreme differences in the thermal loads from the surrounding structure, the worst-case temperature variation in the COR1 instrument is from 2.5 to 30 ± C. There’s also an axial gradient in temperature from the front to the back of the instrument, varying from 3 ± C in the cold case, to 7 ± C in the hot case. Strategically placed software controlled proportional heaters with programmable set points, are used to keep the instrument within the 0–40 ± C operational temperature range. There are also survival heaters on mechanical thermostat control to keep the instrument within the -20–55 ± C non-operational range.

Specialized composite coatings of oxides over silver are used to help manage the intense solar fluxes which COR1 will be experiencing. The oxide coatings are deposited onto many of the exposed surfaces around the aperture area, such as the objective lens holder assembly and door assemblies, as well as on the front layer of the multilayer insulation. This coating exhibits very low solar absorbtivities, is very stable, and has relatively high IR emissivity values depending on the thickness of the deposited oxide layers. The majority of the solar load collected by the front objective is concentrated on the occulter tip (Figure 2). In the worst-case analysis, the tip can reach a temperature of 125 ± C. This tip is made of titanium, and is diamond turned to direct the sunlight into the light trap. It is coated with a Goddard composite silver coating for high reflectivity. The occulter shaft is coated with black nickel to radiate away the heat. A thin cross-section titanium shaft is used to thermally isolate the occulter from the field lens. A passive radiator at the back of the spacecraft will be used to maintain the CCD detector at a temperature of about -80 ± C. Because this is the side of the spacecraft facing away from the Sun, the radiator will be looking out into empty space. The CCD is conductively isolated by using a thin titanium labyrinth structure to cantilever it off of the main instrument structure. It is radiatively isolated by using multilayer insulation and low emissivity gold coatings on the internal surfaces of the focal plane assembly to minimize parasitic heat into the CCD.

SECCHI COR2: Outer Coronagraph (NRL)

 

SECCHI HI: Heliospheric Imager

The HI is a wide-angle visible-light imaging system for the detection of coronal mass ejection (CME) events in interplanetary space and, in particular, of events directed towards the Earth.

The protective covers, or doors, of the HI instruments were opened at the end of 2006 and early in 2007 on the two spacecraft revealing an unprecedented view of the space between the Sun and the Earth. Within the first months of operation, HI had observed the dark-side of the Moon, several CMEs, asteroids and, in the very first images received from HI-B, a spectacular view of comet McNaught.

Since then, in addition to observing and cataloging many more CME, the HI instruments have made a number of spectacular observations and discoveries, including:

- The impact of a CME on Venus and the interpretation of its effects by comparing HI images with data received from spacecraft in orbit around the planet

- The collision of a CME with a comet, resulting in the stripping off (or "disconnection") of the comet's tail

- The discovery of the element iron, in atomic form, in the tail of a comet

- The imaging for the first time of the very faint optical emission associated with so-called Corotating Interaction Regions (CIRs) in interplanetary space, where fast-flowing Solar wind catches up with slower wind regions

The HI instrument has been developed by a UK-led consortium which includes the Centre Spatial de Liege, Belgium, and the Naval Research Laboratory, USA. The UK Team

STEREO/WAVES (SWAVES)

SWAVES is an interplanetary radio burst tracker that traces the generation and evolution of traveling radio disturbances from the Sun to the orbit of Earth. Principal Investigator Dr. Jean Louis H. Bougeret, Centre National de la Recherche Scientifique, Observatory of Paris, and Co-Investigator Dr. Robert J. Macdowall of Goddard, lead the investigation.

STEREO-WAVES Instrumentation

The instrument complement is similar to experiments that our group has flown on Ulysses, Wind and Cassini. This instrument package gives the essential coverage in frequency and in dynamic range for the tracking of emissions from the sun, as well as for understanding the plasma processes that give rise to them. The instrument as proposed requires 10.8 kg, 5.6 W, and 455 b/s.

SWAVES Antenna System

As its primary sensors, SWAVES will use three mutually orthogonal monopole stacer antenna elements, each 6 meters in length. The three monopoles will be deployed away from the sun so that they remain out of the fields of view of sunward looking instruments as shown in Figure 6. The preferred configuration has all three antennas mounted at the same location on the spacecraft body.

Stacer antennas were chosen for their excellent thermal properties, stiffness, reliability, and heritage. Several hundred stacer elements have been used successfully on NASA Wallops sponsored sounding rocket flights. They also have been used on many tens of scientific and classified satellite missions. They have been employed as electric field sensors/antennas, as well as to support deployed instruments or spacecraft ballast masses. Stacer elements used on the recent Polar and FAST missions as spin axis booms provide an impressive example of what can be achieved. Ground testing indicated that the deviations from a straight line were less than 2 cm. This was confirmed in the on orbit deployment, where the spacecraft sensors were unable to detect any spin axis change, or any level of jitter.

The antenna design optimizes the radio burst tracking in the 16MHz - 30kHz range and maintains a high signal-to-noise ratio for expected solar type II, type III, and other solar and interplanetary radio emissions. The first antenna null occurs at ~83 MHz placing the minimum noise levels between 200kHz and 16MHz.

Each 6-meter stacer antenna element has 1.0 kg mass and requires 0.3 kg of mounting hardware. The boom element identified as most suitable for the STEREO mission is fabricated from 0.005 inch thick by 4-inch wide Beryllium Copper strip material. Deployed lengths up to six meters have been produced in this geometry. The element's diameter at the base is about one inch and it tapers to 0.6 inch at the tip. The deployed antenna elements are very stiff with a cantilever resonance frequency of 0.22 Hz. A ~3.0 meter tip deflection is required to buckle the boom. When stowed, the spring element fits compactly into a two-inch diameter cylinder. This design was first developed for the University of Surrey microsatellite program, UOSAT. It has since been used as a gravity gradient boom for about a dozen of these missions.

Each of the antenna elements will be restrained for launch and released on orbit by a single, small (50 gram), non-explosive nickel titanium shape memory alloy pin puller. This boom mechanism will also include spring-loaded nozzles that both expand to accommodate the boom taper, and telescope outward to provide bending support for the deployed boom. The spring driven telescoping action also provides a substantial push off force, so as to assure reliable deployments. A lightweight string or wire that is stretched along its centerline closely controls the deployment stroke of the stacer. This line is stowed prior to deployment in an axial feed bobbin, or on a small rotating spool

Preamplifiers

There will be a high input impedance preamplifier connected to each of the three electric monopoles. These are required in order to prevent loading of the antennas which are electrically equivalent to small capacitors in the voltage mode used by the instrument. The preamplifiers will be of a very classical and frequently used design with a low-noise JFET as the first stage followed by a PNP bipolar transistor and an op-amp.

With co-located antennas, these preamplifiers will be included in the main SWAVES electronics package itself. If the antenna units cannot be co-located, external preamplifier enclosures will be required. Each of three enclosures would be in a separate small box weighing about 80g each. The box size is estimated to be 8x5x3H cm. The total extra harness weight is estimated to be 200g (TBC). The difference in weight for the main box between the two options is only 60g.

Radio Receivers

There are five radio receivers in the SWAVES instrument. They cover the following frequency ranges:

LFR Lo 10-40 kHz
LFR Hi 40-160 kHz
HFR 0.125-16.075 MHz
FFR1 50 MHz fixed frequency
TDS 250,000 samples/second time series snapshots

All of the receivers except the TDS are controlled by the ADSP processor which itself is controlled by the common DPU. The TDS is controlled directly by the DPU. If SWAVES does not share a common DPU with IMPACT, the TDS will be controlled by the ADSP.

In-situ Measurements of Particles and CME Transients (IMPACT)

IMPACT sample the 3-D distribution and provide plasma characteristics of solar energetic particles and the local vector magnetic field. Principal Investigator: Dr. Janet G. Luhmann, University of California, Berkeley.

Solar Wind Plasma Electron Analyzer (SWEA)

SWEA is designed to measure the distribution function of the solar wind core and halo electrons from below an eV to several keV, with high spectral and angular resolution over practically the full spherical range. This capability allows the distinction between these components in detail during both undisturbed periods and the passage of CME generated disturbances, when the interplanetary field rotates far out of the ecliptic plane. SWEA consists of a hemispherical top-hat electrostatic analyzer (ESA) that provides a 360 deg. field of view in a plane, combined with electrostatic deflectors to provide nearly 4 pi coverage when SWEA is mounted at the end of the STEREO boom. The inner plate radius is 3.75 cm and the plate separation is 0.28 cm. The resulting energy resolution dE/E is 18%, and the geometric factor is 0.01 cm2 ster E (eV). SWEA compensates for the effects of spacecraft potential on the lowest energy particles by having an outer hemisphere that can be biased according to the plasma density measured by the PLASTIC solar wind ion instrument.

Suprathermal Electron Telescope (STE) UC Berkeley

STE is a new instrument that covers electrons in the energy range ~2-20 keV which are present as a superhalo on the solar wind electrons, and as CME shock-accelerated, or flare-accelerated populations extending beyond the SWEA range. STE utilizes passively cooled silicon semiconductor devices (SSDs) which measure all energies simultaneously. The STE consists of two arrays of four SSDs in a row, each ~0.1 cm2 area and ~500 microns thick. Each array looks through a rectangular opening that provides a ~20X80 degree field of view for each SSD with the 80 degree direction perpendicular to the ecliptic. Adjacent FOVs are offset for a total FOV of ~80X80 degrees. The two arrays are mounted back-to-back, looking in opposite directions, centered about 25 deg. from the average Parker Spiral field direction. STE is located just inboard of SWEA on the STEREO boom to clear its field of view and remain in shadow.

Magnetometer (MAG) GSFC

The magnetometer system is a simplified version of the magnetometers flown on Mars Global Surveyor and Lunar Prospector. It is a tri-axial flux gate design that will be mounted on the STEREO ~4m boom just inboard of the SWEA and STE instruments. The flux gate sensors use a ring core geometry, with magnetic cores consisting of molybdenum alloy. The units are compact, low power, and ultra-stable. To optimize sensitivity at the low field values to be found in interplanetary space, the magnetometer dynamic range is divided into 8 ranges that are automatically switched whenever the field being measured exceeds a predetermined level. The maximum range is sufficiently large to allow IMPACT MAG magnetic field measurements during the commissioning phase Earth orbits.

Solar Electron Proton Telescope (SEPT) ESTEC, University of Kiel

SEPT consists of two dual, double-ended magnet/foil solid state detector particle telescopes that cleanly separate and measure electrons in the energy range 20-400 keV and protons from 20-7000 keV, while providing anisotropy information through use of several fields of view. Each SSD detector in SEPT is 300 microns thick and 0.53 cm2 in area. A rare-earth permanent magnet is used to sweep away electrons for ion detection, while a parylene foil transmits electrons but stops protons. SEPT is divided into two pieces for field-of-view reasons. The SEPT-E telescope is housed with the rest of the SEP Package on the body of the spacecraft. It looks in the ecliptic plane along the Parker Spiral magnetic field direction, both forward and backward. SEPT-N/S is housed separately at a different spacecraft location, and looks out of the ecliptic plane perpendicular to the nominal magnetic field, both north and south. The viewing cones for the SEPT telescopes are each ~60 degrees.

Suprathermal Ion Telescope (SIT) GSFC, MPI for Solar System Research, University of Maryland

SIT is a time-of-flight ion mass spectrometer that measures elemental composition of He-Fe ions over the energy range ~30 keV/nucleon to 2 MeV/nucleon. The Field of View angles are 17X44 degrees, with the 44 deg angle in the ecliptic plane, centered ~60 deg from the spacecraft-Sun line to avoid sunlight while still intercepting insignificant numbers of Parker Spiral field controlled energetic ion fluxes The telescope analyzes ions that enter through thin entrance foils and stop in a solid state detector. A time-of-flight approach for determining the composition utilizes start and stop times obtained from secondary electrons entering a microchannel plate system. The MCP and SSD areas are each 6.0 cm2. The SIT geometric factor allows study of even small SEP events.

Low Energy Telescope (LET) Caltech, GSFC, JPL

LET is a special double-fan arrangement of 14 solid state detectors designed to measure protons and helium ions from ~1.5 to 13 MeV/nucleon, and heavier ions from ~2 to 30 MeV/nucleon. LET uses a standard dE/dx vs. E technique, identifying particles that stop at depths of ~20-70 microns and ~70-2000 microns corresponding to two general energy ranges. The large field of view spans from 20 deg above to 20 deg below the ecliptic plane, and extends 65 deg to either side of the forward and backward Parker Spiral field directions in the ecliptic plane. LET's large geometric factor also ensures the detection of even small SEP events.

High Energy Telescope (HET) Caltech, GSFC, JPL

HET also uses the solid state detector, dE/dx vs. E approach, but in a six-detector, more traditional linear arrangement designed to measure protons and helium ions to 100 MeV/nucleon, and energetic electrons to 5 MeV. HET identifies particles that stop at depths of 1 to 8 mm in the detectors. Some information will also be obtained on heavier nuclei up through Fe using the dE/dx vs E signatures and ranges together, and penetrating particles will be analyzed. HET's field of view covers a 47.5 degree cone around the Parker Spiral field direction.

Mission

STEREO (Solar TErrestrial RElations Observatory) is the third mission in NASA's Solar Terrestrial Probes program (STP). It employs two nearly identical space-based observatories - one ahead of Earth in its orbit, the other trailing behind - to provide the first-ever stereoscopic measurements to study the Sun and the nature of its coronal mass ejections, or CMEs.

STEREO's scientific objectives are to:

  • Understand the causes and mechanisms of coronal mass ejection (CME) initiation.
  • Characterize the propagation of CMEs through the heliosphere.
  • Discover the mechanisms and sites of energetic particle acceleration in the low corona and the interplanetary medium.
  • Improve the determination of the structure of the ambient solar wind.

Why the need for STEREO?

Coronal mass ejections (CMEs), are powerful eruptions that can blow up to 10 billion tons of the Sun's atmosphere into interplanetary space. Traveling away from the Sun at speeds of approximately one million mph (1.6 million kph), CMEs can create major disturbances in the interplanetary medium and trigger severe magnetic storms when they collide with Earth's magnetosphere.

Large geomagnetic storms directed towards Earth can damage and even destroy satellites, are extremely hazardous to Astronauts when outside of the protection of the Space Shuttle performing Extra Vehicular Activities (EVAs), and they have been known to cause electrical power outages.

CMEs: a Fundamental Science Challenge

Solar ejections are the most powerful drivers of the Sun-Earth connection. Yet despite their importance, scientists don't fully understand the origin and evolution of CMEs, nor their structure or extent in interplanetary space. STEREO's unique stereoscopic images of the structure of CMEs is enabling scientists to determine their fundamental nature and origin.

Mission Concept

STEREO provides a unique and revolutionary views of the Sun-Earth system. The satellites trace the flow of energy and matter from the Sun to Earth as well as reveal the 3-D structure of coronal mass ejections and help us understand why they happen. STEREO also provides alerts for Earth-directed solar ejections, from its unique side-viewing perspective adding it to the fleet of Space Weather detection satellites.

STEREO Capabilities:

  • First stereo viewing of Sun from out-of-Earth-orbit vantage points.
  • First imaging and tracking of space weather disturbances from Sun to Earth.
  • First continuous determination of interplanetary shock positions by radio triangulation.
  • First simultaneous imaging of solar activity with in-situ measurement of energetic particles at 1 AU.

Mission Design | The basics:

  • Two Sun-pointed observatories with identical instrument complements.
  • Each in a heliocentric orbit drifting away from the Earth, one leading and one lagging.

The most efficient and cost-effective method to place the twin observatories, launched aboard a single rocket, into their respective orbits was to use what is known as "lunar swingbys." This was the first time this technique has been used to manipulate orbits of more than one spacecraft at the same time. Mission designers use the Moon's gravity to redirect the observatories to their appropriate orbits - something the launch vehicle alone is not able to do.

For the first three months after launch, the two observatories flew in highly elliptical orbits extending from very close to Earth to just beyond the Moon's orbit. STEREO Mission Operations personnel at the Johns Hopkins University's Applied Physics Laboratory (APL) in Laurel, Maryland, synchronized spacecraft orbits so that about two months after launch they encountered the Moon, at which time one of them was close enough to use the Moon's gravity to redirect it to a position "behind" Earth. Approximately one month later, the second observatory encountered the Moon again and was redirected to its orbit "ahead" of Earth.

Thus, the two STEREO spacecraft provide a stereoscopic view of the Sun and its atmosphere, similar to the way our two eyes allow us to see the three-dimensional world around us. When combined with data from observatories on the ground and in low-Earth orbit, STEREO's data allows scientists to track the buildup and lift-off of magnetic energy from the Sun and the trajectory of Earth-bound CMEs in 3-D.

Orbital Insertion
Orbit

Science
Sapcecraft
The two STEREO observatories are nearly identical with selective redundancy. The building of the spacecraft bus and the integration of the instruments were done by the Johns Hopkins University Applied Physics Laboratory (APL).

The two solar-powered observatories with 3-axis-stabilization, each had a mass at launch of approximately 1,364 pounds (620 kilograms, including propellant). The spacecraft communicate with the APL-based Mission Operations Center via NASA’s Deep Space Network.

The significant challenge in spacecraft design is the large number and extent of the instrument fields-of-view, coupled with the various instruments’ competing design requirements to ensure successful science observations.

The major design drivers to support the science instrument performance are a conductive outer surface for the energetic particle experiments, stringent electromagnetic compatibility and interference requirements for the radio burst tracker, and contamination control of both volatiles and particulates for the imager experiment.

The spacecraft bus consists of six operational subsystems supporting two instruments and two instrument suites. This combination provides a total of 16 instruments per observatory. The subsystems include: command and data handling; radio frequency communications; guidance and control; propulsion; power; and thermal.


Key Characteristics of Twin Observatories

  • Mass: 1,364 pounds (620 kilograms)
  • Dimensions:
    • 3.75 feet (1.14 meters)
    • 4.00 feet (1.22 meters) wide (launch configuration)
    • 21.24 feet (6.47 meters) wide (solar arrays deployed)
    • 6.67 feet (2.03 meters) deep
    • Power consumption: 475 watts
    • Data downlink: 720 kilobits per second
    • Memory: 1 gigabyte
    • Attitude:
    • Control – within 7 arcseconds (0.0019 degrees)
    • Knowledge – within 0.1 arcsecond (0.000028 degrees)

*1 arcsecond = 1/3,600 of a degree

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