Welcome to the STEREO Learning Center

Frequently Asked Questions

What is STEREO and its purpose?
What are coronal mass ejections and why is it important to study them?
How have the STEREO observatories obtained the first-ever "stereo" or 3-D views of the Sun?
How were the twin STEREO observatories placed into their respective orbits?
Is this the first time lunar swingbys have been used to manipulate spacecraft orbits?
Why do you use "nearly identical" to describe the twin observatories?
How does STEREO fit into NASA's science program?
How many and what types of instruments are onboard the observatories?
What are the characteristics of the STEREO spacecraft (size, mass, etc.)?
When and from where were the observatories launched?
How long will the mission last?
What are the mission costs?
Who is sponsoring and managing the mission? Who is building and operating the spacecraft?
Why are the SECCHI/EUVI images different colors and what do they mean?
What is that odd looking thing I see in this STEREO image?
What's the latest on STEREO-B?

1. What is STEREO and its purpose?

STEREO is an acronym for Solar TErrestrial RElations Observatory. This mission employs two nearly identical observatories to provide the first-ever, 3-D stereoscopic images to study the nature of coronal mass ejections (CMEs). These powerful solar eruptions are a major source of the magnetic disruptions on Earth and a key component of space weather, which can greatly affect satellite operations, communications, power systems, the lives of humans in space, and even global climate over the long term.

The science objectives of the STEREO mission are to understand the causes and mechanisms of the origins of CMEs and characterize their propagation through the heliosphere; discover the mechanisms and sites of energetic particle acceleration in the low corona and the interplanetary medium; and develop a 3-D time-dependent model of the magnetic topology, temperature, density and velocity structure of the ambient solar wind.

2. What are coronal mass ejections and why is it important to study them?

Coronal mass ejections (CMEs) are powerful eruptions that can blow up to 10 billion tons of the sun's atmosphere into interplanetary space. Typically, CMEs eject about one billion tons of solar particles into space and travel at about one million mph. They can create major disturbances in the interplanetary medium (the space between planets containing electromagnetic radiation, dust, gas and plasma) and trigger severe magnetic storms if they reach Earth.

CME-driven shocks may also play a significant role in the acceleration of solar energetic particles, which can damage spacecraft and harm astronauts. CMEs are often associated with solar flares and prominence eruptions, but they can occur when neither is present.

Despite their importance, scientists don't fully understand the origin and evolution of CMEs, nor their structure or extent in interplanetary space. STEREO's unique 3-D images and movies of the structure of CMEs will enable scientists to determine their fundamental nature and origin.

3. How have the STEREO observatories obtained the first-ever "stereo" or 3-D views of the sun?

The STEREO mission offers a totally new perspective on solar eruptions by imaging coronal mass ejections and background events from two nearly identical observatories simultaneously. To obtain unique views of the sun, the twin observatories have been placed into a rather challenging orbits where they will be offset from one another. One observatory is placed ahead of Earth in its orbit and the other behind. Just as the slight offset between your eyes provides you with depth perception, this placement allows the STEREO observatories to obtain 3-D images of the sun.

4. How were the twin STEREO observatories placed into their respective orbits?

The twin STEREO observatories were launched together aboard a Delta II rocket. To place them into orbit in the most efficient and cost-effective manner possible, STEREO mission designers used lunar gravity to redirect the observatories to their appropriate orbits -- something the launch vehicle alone cannot do.

For the first three months after launch, the observatories flew an orbit from a point close to Earth to one that extends just beyond the moon. Mission Operations personnel at The Johns Hopkins University Applied Physics Laboratory, in Laurel, Md., synchronized observatory orbits so they encountered the moon about two months after launch, at which time one of them was close enough to use the moon's gravity to redirect it to an orbit lagging behind Earth. Approximately one month later, the second observatory encountered the moon again and was redirected to its orbit ahead of Earth.

5. Was this the first time lunar swingbys have been used to manipulate spacecraft orbits?

While lunar swingbys have been used before, the STEREO mission is the first to use lunar swingbys to manipulate orbits of more than one spacecraft at a time. This technique enables multiple payloads to be launched aboard a single vehicle, saving a great deal of money.

6. Why do you use "nearly identical" to describe the twin observatories?

The twin STEREO observatories fly as mirror images of each other in order to obtain "stereo" views of the sun's activities. This requires the identical instruments on each craft to be placed in slightly different locations.

The "B" observatory's main structure is a little thicker so that it can support the weight of the "A" observatory during launch. The "B" observatory will retain a portion of the separation fitting or ring used to connect the two during their ride into space.

7. How does STEREO fit into NASA's science program?

Heliophysics is one of the four principal science divisions within NASA's Science Mission Directorate. Heliophysics focuses on explaining the physical processes that link the sun with the Earth and the rest of the solar system.

STEREO is part of the Solar Terrestrial Probes (STP) Program which offers a continuous sequence of flexible, cost-capped missions designed to systematically study the sun-Earth system. Solar Terrestrial Probes missions focus on studying the sun and Earth as an integrated system using a blend of in situ and remote-sensing observations, often from multiple platforms. STEREO contributes to all three of the Heliophysics Division's scientific objectives: (1) understanding the changing flow of energy and matter throughout the sun, heliosphere and planetary environments; (2) exploring the fundamental physical processes of space plasma systems; and (3) defining the origins and societal impacts of variability in the sun-Earth connection.

STEREO is the third mission within the Solar Terrestrial Probes Program.

8. How many and what types of instruments are onboard the observatories?

Each twin STEREO observatory carries two instruments and two instrument suites. This combination provides a total of 16 instruments per observatory. The Johns Hopkins University Applied Physics Laboratory, in Laurel, Md., designed and built the spacecraft platform that houses the instruments.

INSTRUMENT SUITES

The Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) is a suite of remote-sensing instruments consisting of an extreme ultraviolet imager, two white-light coronagraphs, and a heliospheric imager. These instruments study the 3-D evolution of coronal mass ejections--the most energetic eruptions on the sun and primary cause of major geomagnetic storms--from their origin at the sun's surface through the corona and interplanetary medium to their eventual impact at Earth. The principal investigator for the SECCHI instrument suite is Dr. Brian Wood of the Naval Research Laboratory in Washington, D.C.

The In situ Measurements of PArticles and CME Transients (IMPACT) investigation provides measurements of the solar wind electrons, interplanetary magnetic fields, and solar energetic particles. IMPACT comprises seven instruments including a solar wind electron analyzer, a magnetometer, and an array of particle detectors measuring the energetic ions and electrons accelerated in coronal mass ejection (CME) shocks and in solar flares. IMPACT's principal investigator is Dr. Janet Luhmann of the University of California, Berkeley.

INSTRUMENTS

The PLAsma and SupraThermal Ion Composition (PLASTIC) instrument studies coronal-solar wind and solar wind-heliospheric processes. PLASTIC provides in situ plasma characteristics of protons, alpha particles and heavy ions. It supplies key diagnostic measurements of mass and charge state composition of heavy ions and characterizes the coronal mass ejection plasma from ambient solar wind plasma. The University of New Hampshire's Dr. Antoinette Galvin is the principal investigator for PLASTIC.

STEREO/WAVES (S/WAVES) is an interplanetary radio burst tracker that traces the generation and evolution of traveling radio disturbances from the sun to Earth's orbit. A radio and plasma wave receiver, S/WAVES is both a remote-sensing and an in-situ instrument. Dr. Milan Maksimovic, of the Paris Observatory, Meudon, is the principal investigator.

9. What are the characteristics of the STEREO spacecraft (size, mass, etc.)?

Key characteristics of the twin STEREO observatories:

Mass: 1,364 pounds (620 kilograms)
Dimensions
3.75 feet (1.14 meters) high
4.00 feet (1.22 meters) wide (launch configuration)
21.24 feet (6.47 meters) wide (solar arrays deployed)
6.62 feet (2.03 meters) deep
Average power consumption per orbit: 475 watts
Data downlink: 720 kilobits per second
Memory: 1 gigabyte
Attitude:
Control -- within 7 arc seconds (0.0019 degrees)
Knowledge -- within 0.1 arc second (0.000028 degrees)

1 arc second = 1/3,600 of a degree.

10. When and from where were the observatories launched?

The twin STEREO observatories were launched aboard a Delta II 7925-10L rocket from Cape Canaveral, Fla., on Oct. 25, 2006.

11. How long will the mission last?

STEREO's prime mission lifetime was two years. However, because it is still returning valuable data it is continuing to operate. NASA does periodic reviews to determine if the mission should be extended.

Payload operations will be conducted remotely from each of the instruments' facilities:

SECCHI -- Naval Research Laboratory
S/WAVES -- Paris Observatory, Meudon/University of Minnesota
IMPACT -- University of California, Berkeley
PLASTIC -- University of New Hampshire

12. What are the mission costs?

The total cost for STEREO is approximately $550 million for the spacecraft, its instruments, the launch vehicle, as well as ground operations, mission operations and data analysis.

13. Who is sponsoring and managing the mission? Who is building and operating the spacecraft?

STEREO is sponsored by NASA Headquarters Office of Space Science, Washington, D.C. NASA Goddard Space Flight Center's Solar Terrestrial Probes Program Office, in Greenbelt, Md., manages the mission, instruments and science center. The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., designed, built and operates the twin observatories for NASA during the 2-year mission.

14. Why are the SECCHI/EUVI images different colors and what do they mean?

All of the SECCHI/EUVI images are actually produced by extreme ultraviolet (EUV) light from the Sun. This is light that is between ultraviolet light and x-ray light in the electromagnetic spectrum and is not visible to our eyes directly.

EUVI images are taken at four different wavelengths and four colors in order of wavelength (bluer=shorter wavelength, redder=longer) were assigned to represent each of them. Just as the human eye is capable of discriminating different colors in the visible, so EUVI's four bandpasses discriminate among four "colors" in the extreme ultraviolet. In addition, each color table was carefully constructed to bring out typical features of its particular wavelength.

The red images have a wavelength of 304 Angstroms, the yellow are 284 Angstroms, the green are 195 Angstroms, and the blue are 171 Angstroms. So, just like in the visible spectrum where red is the longest wavelength and blue is the shortest with yellow and green in between, the EUVI image colors were chosen so that the longest wavelength is reddish and the shortest is bluish. This is the same color scheme used for the SOHO EIT images which cover the same set of wavelengths.

For more information on EUVI images see our EUV Sun page.

15. What is that odd looking thing I see in this STEREO image?

Not everything visible in STEREO images is related to the Sun or the solar atmosphere. Please see our web page explaining the various things we are asked about in our images:

/artifacts/artifacts.shtml

16. What's the latest on STEREO-B?

See our mission status page at https://stereo-ssc.nascom.nasa.gov/status.shtml

Last Revised: Tuesday, 26-Aug-2025 19:34:42 UTC
Responsible NASA Official: [email address: Therese.A.Kucera<at>nasa<dot>gov]
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