STEREO

Mission profile

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 degree/year. The B spacecraft/Sun/Earth angle will change −21.999 degrees 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 degrees per year. There are no final positions for the spacecraft. They achieved 90 degrees 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 degrees 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.

Loss of contact

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 the spacecraft had begun to spin, decreasing the amount of power that could be generated by the solar panels. NASA used its Deep Space Network, first weekly and later monthly, to try to re-establish communications. 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.

Later analysis of the received telemetry concluded that the spacecraft was in an uncontrolled spin of about three degrees per second; this is too rapid to be immediately corrected using its reaction wheels, which would become oversaturated. Engineers will work to develop software to fix the spacecraft, but once its computer is powered on there will only be about two minutes to upload the fix before STEREO-B enters failure mode again. Further, while the spacecraft was power positive at the time of contact, its orientation will drift and power levels will fall. Another opportunity to fix the vehicle will not happen until mid-2017 when power charge levels rise sufficiently.

Mission benefits

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-degree views of the Sun. STEREO's observations are already being incorporated into forecasts of solar activity for airlines, power companies, satellite operators, and others.

STEREO has also been used to discover 122 eclipsing binaries and study hundreds more variable stars. STEREO can look at the same star for up to 20 days.

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.

Science instrumentation

Each of the spacecraft carries cameras, particle experiments and radio detectors in four instrument packages:

Spacecraft subsystems

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.

STEREO also carries Actel FPGAs that use triple modular redundancy for radiation hardening. The FPGAs hold the P24 MISC and CPU24 soft microprocessors.

For data storage, each spacecraft carries a solid state recorder able to store up to one 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 kbps.