STEREO

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.

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 gravitational slingshot. 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, ejecting itself 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 and, 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 will be lost for several months when the STEREO spacecraft pass behind the Sun. On October 1, 2014, contact was lost with STEREO-B during a planned reset to test the craft's automation, in anticipation of this Solar 'conjunction' period. , contact had not been re-established, but efforts were still underway to regain communication and control.

They will then start to approach Earth again, with closest approach sometime in 2023. They will not be recaptured into Earth orbit.

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 from the Earth itself, 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:
 * Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) has five cameras: an extreme ultraviolet imager (EUVI) and two white-light coronagraphs (COR1 and COR2). These three telescopes are collectively known as the Sun Centered Instrument Package or SCIP, and image the solar disk and the inner and outer corona.  Two additional telescopes, heliospheric imagers (called the HI1 and HI2) image the space between Sun and Earth. The purpose of SECCHI is to study the 3-D evolution of Coronal Mass Ejections through their full journey from the Sun's surface through the corona and interplanetary medium to their impact at Earth.
 * In-situ Measurements of Particles and CME Transients (IMPACT) will study energetic particles, the three-dimensional distribution of solar wind electrons and interplanetary magnetic field.
 * PLAsma and SupraThermal Ion Composition (PLASTIC) will study the plasma characteristics of protons, alpha particles and heavy ions.
 * STEREO/WAVES (SWAVES) is a radio burst tracker that will study radio disturbances traveling from the Sun to the orbit of Earth.

Spacecraft subsystems

 * Structure
 * Launch weight including propellants was 1364 pounds (620 kg).


 * Propulsion and attitude control
 * 3-axis control


 * Attitude determination
 * Each STEREO spacecraft has a primary and a backup Miniature Inertial Measurement Unit (MIMU), provided by Honeywell, which measure changes to the spacecraft attitude. Each MIMU is equipped with three ring laser gyroscopes to detect angular changes. Additional attitude information is provided by the Star Tracker and the SECCHI Guide Telescope.


 * Power
 * 475 Watts from solar panels.


 * Telecommunications
 * Data downlink: 720 kilobits per second.


 * Flight computers
 * 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 25 megahertz RAD6000 CPUs: one for Command/Data-handling, and one for Guidance-and-Control. Both are radiation hardened 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.
 * Data handling
 * 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.