.jpg) Illustration of a STEREO spacecraft during solar array deployment | |||||||||||
| Mission type | Solar observation | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Operator | NASA | ||||||||||
| COSPAR ID | STEREO-A:2006-047A STEREO-B:2006-047Bn | ||||||||||
| SATCATno. | STEREO-A: 29510 STEREO-B: 29511 | ||||||||||
| Website | stereo stereo | ||||||||||
| Mission duration |
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| Spacecraft properties | |||||||||||
| Manufacturer | Johns Hopkins UniversityApplied Physics Laboratory | ||||||||||
| Launch mass | STEREO-A: 620 kg STEREO-B: 620 kg[1] | ||||||||||
| Dry mass | 547 kg (1,206 lb) | ||||||||||
| Dimensions | 1.14 × 2.03 × 6.47 m 3.75 × 6.67 × 21.24 ft | ||||||||||
| Power | 475 W | ||||||||||
| Start of mission | |||||||||||
| Launch date | October 26, 2006, 00:52 (2006-10-26UTC00:52) UTC | ||||||||||
| Rocket | Delta II 7925-10L | ||||||||||
| Launch site | Cape CanaveralSLC-17B | ||||||||||
| Contractor | United Launch Alliance | ||||||||||
| End of mission | |||||||||||
| Last contact | STEREO-B: September 23, 2016 | ||||||||||
| Orbital parameters | |||||||||||
| Reference system | Heliocentric | ||||||||||
| Period | STEREO-A: 346 days STEREO-B: 388 days | ||||||||||
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 Mission insignia | |||||||||||
STEREO (Solar TErrestrial RElations Observatory) is asolar observation mission.[2] Two nearly identical spacecraft (STEREO-A,STEREO-B) were launched in 2006 into orbits around the Sun that cause them to respectively pull farther ahead of and fall gradually behind the Earth. This enabledstereoscopic imaging of theSun and solar phenomena, such ascoronal mass ejections.
Contact with STEREO-B was lost in 2014 after it entered an uncontrolled spin preventing its solar panels from generating enough power. It was briefly resumed in 2016 before being interrupted and eventually declared lost.
STEREO-B Earth
SunThe two STEREO spacecraft were launched at 00:52 UTC on October 26, 2006, from Launch Pad 17B at theCape Canaveral Air Force Station in Florida on aDelta II 7925-10L launcher into highlyellipticalgeocentric orbits. Theapogee reached the Moon's orbit. On December 15, 2006, on the fifth orbit, the pair swung by the Moon for agravity assist. Because the two spacecraft were in slightly different orbits, the "ahead" (A) spacecraft was ejected to aheliocentric orbit inside Earth's orbit, while the "behind" (B) spacecraft remained temporarily in a high Earth orbit. The B spacecraft encountered the Moon again on the same orbital revolution on January 21, 2007, being ejected from Earth orbit in the opposite direction from spacecraft A. Spacecraft B entered a heliocentric orbit outside the Earth's orbit. Spacecraft A took 347 days to complete one revolution of the Sun and Spacecraft B took 387 days. The A spacecraft/Sun/Earth angle will increase at 21.650° per year. The B spacecraft/Sun/Earth angle will change −21.999° per year. Given that the length of Earth's orbit is around 940 million kilometres, both craft have an average speed, in a rotating geocentric frame of reference in which the Sun is always in the same direction, of about 1.8 km/s, but the speed varies considerably depending on how close they are to their respective aphelion or perihelion (as well as on the position of Earth). Their current locations are shownhere.
Over time, the STEREO spacecraft continued to separate from each other at a combined rate of approximately 44° per year. There were nofinal positions for the spacecraft. They achieved 90° separation on January 24, 2009, a condition known asquadrature. This was of interest because the mass ejections seen from the side on the limb by one spacecraft can potentially be observed by thein situ particle experiments of the other spacecraft. As they passed through Earth'sLagrangian pointsL4 andL5, in late 2009, they searched forLagrangian (trojan) asteroids. On February 6, 2011, the two spacecraft were exactly 180° apart from each other, allowing the entire Sun to be seen at once for the first time.[3]
Even as the angle increases, the addition of an Earth-based view, e.g., from theSolar Dynamics Observatory, still provided full-Sun observations for several years. In 2015, contact was lost for several months when the STEREO spacecraft passed behind the Sun. They then started to approach Earth again, with closest approach in August 2023. They will not be recaptured into Earth orbit.[4]
On October 1, 2014, contact was lost with STEREO-B during a planned reset to test the craft's automation, in anticipation of the aforementioned solar "conjunction" period. The team originally thought that the spacecraft had begun to spin, decreasing the amount of power that could be generated by the solar panels. Later analysis of the received telemetry concluded that the spacecraft was in an uncontrolled spin of about 3° per second; this was too rapid to be immediately corrected using itsreaction wheels, which would become oversaturated.[5][4]
NASA used itsDeep Space Network, first weekly and later monthly, to try to re-establish communications.[4]
After a silence of 22 months, contact was regained at 22:27 UTC on August 21, 2016, when the Deep Space Network established a lock on STEREO-B for 2.4 hours.[6][5][7]
Engineers planned to work and develop software to fix the spacecraft, but once its computer was powered up, there would only have been about 2 minutes to upload the fix before STEREO-B entered failure mode again.[8] Further, while the spacecraft was power-positive at the time of contact, its orientation would drift, and power levels fall. Two-way communication was achieved, and commands to begin recovering the spacecraft were sent through the rest of August and September.[5]
Six attempts at communication between September 27 and October 9, 2016, failed, and a carrier wave was not detected after September 23. Engineers determined that during an attempt to despin the spacecraft, a frozen thruster fuel valve probably led to the spin increasing rather than decreasing.[5] As STEREO-B moved along its orbit, it was hoped that its solar panels may again generate enough power to charge the battery.
Four years after the initial loss of contact, NASA terminated periodic recovery operations effective October 17, 2018.[9]
The principal benefit of the mission wasstereoscopic images of the Sun. Because the satellites are at different points along the Earth's orbit, but distant from the Earth, they can photograph parts of the Sun that are not visible from the Earth. This permits NASA scientists to directly monitor the far side of the Sun, instead of inferring the activity on the far side from data that can be gleaned from Earth's view of the Sun. The STEREO satellites principally monitor the far side forcoronal mass ejections — massive bursts ofsolar wind, solarplasma, and magnetic fields that are sometimes ejected into space.[10]
Since the radiation from coronal mass ejections, or CMEs, can disrupt Earth's communications, airlines, power grids, and satellites, more accurate forecasting of CMEs has the potential to provide greater warning to operators of these services.[10] Before STEREO, the detection of thesunspots that are associated with CMEs on the far side of the Sun was only possible usinghelioseismology, which only provides low-resolution maps of the activity on the far side of the Sun. Since the Sun rotates every 25 days, detail on the far side was invisible to Earth for days at a time before STEREO. The period that the Sun's far side was previously invisible was a principal reason for the STEREO mission.[11]
STEREO program scientistMadhulika Guhathakurta expected "great advances" in theoreticalsolar physics and space weather forecasting with the advent of constant 360° views of the Sun.[12] STEREO's observations are incorporated into forecasts of solar activity for airlines, power companies, satellite operators, and others.[13]
STEREO has also been used to discover 122eclipsing binaries and study hundreds morevariable stars.[14] STEREO can look at the same star for up to 20 days.[14]
On July 23, 2012, STEREO-A was in the path of the Carrington class CME of thesolar storm of 2012.[15] This CME, it is estimated, if it had collided with Earth's magnetosphere, would have caused a geomagnetic storm of similar strength to theCarrington Event, the most intense geomagnetic storm in recorded history.[16] STEREO-A's instrumentation was able to collect and relay a significant amount of data about the event without being harmed.
Each of the spacecraft carries cameras, particle experiments and radio detectors in four instrument packages:
Each STEREO spacecraft had adry mass of 547 kg (1,206 lb) and a launch mass of 619 kg (1,364 lb). In their stowed configuration, each had a length, width and height of 2.0 × 1.2 × 1.1 m (6.67 × 4.00 × 3.75 ft). Upon solar-array deployment, its width increased to 6.5 m (21.24 ft).[20][21] With all of its instrument booms and antennae deployed, its dimensions are 7.5 × 8.7 × 5.9 m (24.5 × 28.6 × 19.2 ft).[22] The solar panels can produce an average of 596 watts of power, and the spacecraft consumes an average of 475 watts.[20][21]
The STEREO spacecraft are 3-axis-stabilized, and each has a primary and backupminiature inertial measurement unit (MIMU) provided byHoneywell.[23] These measure changes to a spacecraft's attitude, and each MIMU contains threering laser gyroscopes to detect angular changes. Additional attitude information is provided by thestar tracker and the SECCHI Guide Telescope.[24]
STEREO's onboard computer systems are based on the Integrated Electronics Module (IEM), a device that combines coreavionics in a single box. Each single-string spacecraft carries two CPUs, one for command and data handling and one for guidance and control. Both areradiation-hardened 25-megahertzIBM RAD6000 processors, based onPOWER1 CPUs (predecessor of the PowerPC chip found in olderMacintoshes). The computers, slow by currentpersonal computer standards, are typical for the radiation requirements needed on the STEREO mission.
STEREO also carries ActelFPGAs that usetriple modular redundancy for radiation hardening. The FPGAs hold theP24 MISC and CPU24soft microprocessors.[25]
For data storage, each spacecraft carries asolid-state recorder able to store up to 1 gigabyte each. Its main processor collects and stores on the recorder images and other data from STEREO's instruments, which can then be sent back to Earth. The spacecraft have anX-band downlink capacity of between 427 and 750 kbit/s.[20][21]
3D red cyan glasses are recommended to view this image correctly.
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