Illustration of a STEREO spacecraft during solar array deployment
|Mission type||Solar observation|
|COSPAR ID||STEREO-A: 2006-047A |
|SATCAT no.||STEREO-A: 29510 |
|Manufacturer||Johns Hopkins University Applied Physics Laboratory|
|Launch mass||619 kg (1,364 lb)|
|Dry mass||547 kg (1,206 lb)|
|Dimensions||1.14 × 2.03 × 6.47 m|
3.75 × 6.67 × 21.24 ft
|Start of mission|
|Launch date||October 26, 2006, 00:52UTC|
|Rocket||Delta II 7925-10L|
|Launch site||Cape Canaveral SLC-17B|
|Contractor||United Launch Alliance|
|End of mission|
|Last contact||STEREO-B: September 23, 2016|
|Period||STEREO-A: 346 days |
STEREO-B: 388 days
STEREO (Solar Terrestrial Relations Observatory) is a solar observation mission. Two nearly identical spacecraft 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 enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.
Contact with STEREO-B was lost in 2014, but STEREO-A is still operational.
The two STEREO spacecraft were launched at 00:52 UTC on October 26, 2006, from Launch Pad 17B at the Cape Canaveral Air Force Station in Florida on a Delta II 7925-10L launcher into highly elliptical geocentric orbits. The apogee reached the Moon's orbit. On December 15, 2006, on the fifth orbit, the pair swung by the Moon for a gravity assist. Because the two spacecraft were in slightly different orbits, the "ahead" (A) spacecraft was ejected to a heliocentric 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 will take 347 days to complete one revolution of the Sun and Spacecraft B will take 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 shown here.
Over time, the STEREO spacecraft will continue to separate from each other at a combined rate of approximately 44° per year. There are no final positions for the spacecraft. They achieved 90° separation on January 24, 2009, a condition known as quadrature. This is of interest because the mass ejections seen from the side on the limb by one spacecraft can potentially be observed by the in situ particle experiments of the other spacecraft. As they passed through Earth's Lagrangian points L4 and L5, in late 2009, they searched for Lagrangian (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.
Even as the angle increases, the addition of an Earth-based view, e.g., from the Solar Dynamics Observatory, will still provide full-Sun observations for several years. In 2015, contact was lost for several months when the STEREO spacecraft passed behind the Sun. They will then start to approach Earth again, with closest approach sometime in 2023. They will not be recaptured into Earth orbit.
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 its reaction wheels, which would become oversaturated.
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. 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.
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 decreasing. 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.
The principal benefit of the mission is stereoscopic images of the Sun. In other words, 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 for coronal mass ejections — massive bursts of solar wind, solar plasma, and magnetic fields that are sometimes ejected into space.
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. Before STEREO, the detection of the sunspots that are associated with CMEs on the far side of the Sun was only possible using helioseismology, 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.
STEREO program scientist Madhulika Guhathakurta expects "great advances" in theoretical solar physics and space weather forecasting with the advent of constant 360° views of the Sun. STEREO's observations are already being incorporated into forecasts of solar activity for airlines, power companies, satellite operators, and others.
On July 23, 2012, STEREO-A was in the path of the solar storm of 2012, which was similar in strength to the Carrington Event. Its instrumentation was able to collect and relay a significant amount of data about the event. STEREO-A was not harmed by the solar storm.
Each of the spacecraft carries cameras, particle experiments and radio detectors in four instrument packages:
Each STEREO spacecraft had a dry 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). 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). The solar panels can produce an average of 596 watts of power, and the spacecraft consumes an average of 475 watts.
The STEREO spacecraft are 3-axis-stabilized, and each has a primary and backup miniature inertial measurement unit (MIMU) provided by Honeywell. These measure changes to a spacecraft's attitude, and each MIMU contains three ring laser gyroscopes to detect angular changes. Additional attitude information is provided by the star tracker and the SECCHI Guide Telescope.
STEREO's onboard computer systems are based on the Integrated Electronics Module (IEM), a device that combines core avionics in a single box. Each single-string spacecraft carries two CPUs, one for command and data handling and one for guidance and control. Both are radiation-hardened 25-megahertz IBM RAD6000 processors, based on POWER1 CPUs (predecessor of the PowerPC chip found in older Macintoshes). The computers, slow by current personal computer standards, are typical for the radiation requirements needed on the STEREO mission.
For data storage, each spacecraft carries a solid-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 an X-band downlink capacity of between 427 and 750 kbit/s.
STEREO probes stacked at Astrotech in Florida
August 11, 2006
Launch of the STEREO probes on a Delta II rocket
October 26, 2006
One of the first images of the Sun taken by STEREO
December 4, 2006
The Sun's South Pole. Material can be seen erupting from the Sun in the lower right side of the image.
A three-dimensional time-for-space wiggle image taken by STEREO
Jupiter as seen by STEREO-A HI1
November 23, 2008
Nearly the entire far side of the Sun
February 2, 2011
Nearly the entire surface of the Sun, taken in extreme ultraviolet at 19.5 nm, with white lines showing solar coordinates (0° is directly towards Earth)
February 10, 2011
A full day of Sun data from the STEREO satellites
February 13–14, 2011
For STEREO's 10th anniversary, Deputy Project Scientist Terry Kucera gives an overview of the mission's top 5 success stories.
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