STEREO
Solar Terrestrial Relations Observatory 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 lagging, the other leading the Earth in heliocentric orbit around the Sun – to provide the first-ever stereoscopic measurements to study the Sun and the nature of its coronal mass ejections, or CMEs. These events are responsible for large solar energetic particle events in interplanetary space and are the primary cause of major geomagnetic storms at Earth.
The two spacecraft were launched to drift slowly away from the Earth in opposite directions at about 10 degrees per year for the lagging spacecraft and 20 degrees per year for the leading one. Optimal longitudinal separation of about sixty degrees was achieved after two years. Afterwards the separation gradually increased beyond the design lifetime of two years with the possibility of extended mission observations at larger angles.
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.
The two STEREO observatories are nearly identical with selective redundancy. Each spacecraft bus was built by the Johns Hopkins University Applied Physics Laboratory (APL), in Laurel, Md., and NASA Goddard Space Flight Center procured the instruments. Each observatory and its instruments were integrated at APL. The STEREO mission consists of two solar-powered, 3-axis-stabilized spacecraft, with a launch masses – including propellant – of 623 and 658 kilograms. In their stowed configuration, each had a length, width and height of 2.0 × 1.2 × 1.1 m. Upon solar-array deployment, its width increased to 6.5 m, and with all of its instrument booms and antennae deployed, its dimensions are 7.5 × 8.7 × 5.9 m.
Each spacecraft has a primary and backup miniature inertial measurement unit, that contain three ring laser gyroscopes to detect angular changes. Additional attitude information is provided by the star tracker and the SECCHI instrument Guide Telescope. The solar panels can produce an average of 596 watts of power, and the spacecraft consumes an average of 475 watts. 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. The spacecraft is communicating with the APL-based Mission Operations Center via NASA's Deep Space Network.
Scientific Instruments on each spacecraft:
1. Sun Earth connection coronal and heliospheric investigation (SECCHI):
a. extreme ultraviolet imager (EUVI)
b. inner coronagraph (COR1)
c. outer coronagraph (COR2)
d. heliospheric imager (HI)
2. interplanetary radio burst tracker (SWAVES)
3. in-situ measurements of particles and CME transients (IMPACT)
4. plasma and suprathermal ion composition (PLASTIC)
The two STEREO observatories stacked at Astrotech in Florida on August 11, 2006. Credit: NASA/George Shelton
Orbital view of STEREO's “B" observatory with instruments labeled.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory
Both spacecraft were launched together aboard a single Boeing Delta II rocket (Delta 7925-10L no. D319) from Cape Canaveral Air Force Station, site SLC-17B, at 00:52:00 UT on October 26, 2006. They were inserted into an initial orbit around Earth at about 165 × 171 kilometers. After a second and third burn, the two spacecraft were sent into a translunar orbit, planned at 182 × 403,810 kilometers at a 28.5-degree inclination. Just after the final burn, at 01:19 UT, STEREO A separated from STEREO B. On the fifth orbit for both, on December 15, 2006, both spacecraft swung by the Moon (STEREO A at 7340 km and STEREO B at 11,776 km) and used a gravitational assist maneuver to head into different orbits.
STEREO A was in a solar orbit inside Earth’s orbit (and was “ahead”) while STEREO B remained in a high Earth orbit. STEREO B encountered the Moon again on January 21, 2007, at 8818 km, and was accelerated into the opposite direction from STEREO A. It entered heliocentric orbit outside of Earth’s orbit (and was “behind”). STEREO A orbit was 0,95 au × 0,97 au x 0,12°, and STEREO B was 0,99 au × 1,09 au x 0,03°. The orbital periods of STEREO A and STEREO B were 347 days and 387 days, respectively. The two spacecraft separate from each other at a (combined) annual rate of 44 degrees.
Night launch of the Delta 2 rocket with pair of STEREO spacecraft. Credit: NASA
Artist's vision of STEREO satellites deploing solar panels on Earth orbit.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory
A star tracker failed briefly on STEREO B but this had no impact on the mission. Later, in May 2009, the spacecraft was successfully rebooted with new guidance and control software. A similar reset was implemented with STEREO A in August of the same year. A transponder malfunction in July 2013 briefly interrupted science activities on STEREO B. More seriously, the spacecraft suffered a failure of its inertial measurement unit in January 2014 but controllers managed to quickly revive the spacecraft.
During the first years after launch, the dual-spacecraft mission achieved its landmark goal: providing the first stereoscopic, or multiple-perspective, view of our closest star. On February 6, 2011, the mission achieved another landmark: STEREO-A and -B reached a 180-degree separation in their orbits, allowing the entire Sun to be seen at once for the first time by any set of spacecraft. On July 23, 2012, during an extreme solar storm more powerful than anything seen in the past 150 years, STEREO A was able to collect significant data on the phenomenon.
Unanticipated high temperatures in the high gain antenna feed horns of both spacecraft were detected in June 2014, effectively reducing the data return rate, thus curtailing the science program. Because of this problem, mission scientists formulated a reduced program of science operations for STEREO A in August 2014, one that was further thwarted by a massive proton storm (caused by a large solar flare on the far side of the Sun) on September 3, 2014. The energy particle fluxes were so high that star trackers on both STEREO spacecraft were reset.
Later, on October 1, 2014 communications were lost with STEREO B immediately after a planned reset of the spacecraft. All attempts to recover contact were in vain and it is thought that anomalies in the guidance and control system of the spacecraft might have rendered it powerless as a result of drift away from direct exposure of the Sun to its solar panels. Controllers hoped at the time that eventually STEREO B would drift into proper orientation (much like SOHO spacecraft in 1998) and would power up and resume its mission.
Remarkably, 22 months after the loss of contact, on August 21, 2016, NASA’s Deep Space Network reestablished communications with STEREO B (having tried once a month through this period). Controllers concluded that STEREO B was probably spinning out of control around its principal axis of inertia. This uncontrolled orientation allowed some power generation but not enough time to upload a software fix. The attempt to recover the spacecraft was not successful. STEREO B has been out of contact since September 23, 2016. Four years after the initial loss of communications with STEREO B, NASA directed that periodic recovery operations cease with the last support October 17, 2018.
STEREO A meanwhile was put into safe mode in March 2015 for several months during a superior solar conjunction, a period when the spacecraft is on the opposite side of the Sun from Earth. Communication was re-established with the spacecraft July 11, 2015, when images were received again. The science program remained at a low status until November 17, 2015, when STEREO A began to operate at full capacity again. The key element here was the transmission of real-time data, known as beacon data from coronagraph imagery.
On August 12, 2023, STEREO-A spacecraft passed between the Sun and Earth, marking the first Earth flyby of the nearly 17-year-old mission. The visit home brought a special chance for the spacecraft to collaborate with NASA missions near Earth. As of late 2025, STEREO A continues to operate without problems.
References:
Siddiqi, Asif A. Beyond Earth: A
Chronicle of Deep Space Exploration, 1958-2016.
Washington, DC: NASA History Program Office,
2018. ISBN 978-1-62683-042-4
Hatfield, Miles. After
Seventeen Years, A Spacecraft Makes Its
First Visit Home. Aug 10, 2023.
NASA’s Goddard Space Flight Center, Greenbelt,
Md.
NASA Solar
System Exploration: STEREO A
& B
NASA. NSSDCA: STEREO
A, STEREO
B
NASA Goddard Space Flight Center: STEREO Website
Johns Hopkins Applied Physics Laboratory: STEREO
Website
© 2025, Andrew Mirecki
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