Credit: NASA

The Solar TErrestrial RElations Observatory (STEREO) is scheduled to launch aboard a Delta II launch vehicle from Cape Canaveral Air Force Station (CCAFS), Fla. The STEREO two-year mission consists of two nearly identical space-based observatories that will provide revolutionary views of the Sun-Earth system. STEREO is designed to:

• Understand the origin and consequences of coronal mass ejections (CMEs - enormous eruptions of matter from the sun).
  
  
• To determine the processes that control CME-driven disturbances from the sun to Earth's orbit.
  
  
• Discover the mechanisms and sites of solar energetic particle acceleration.
  
  
• Determine the 3-D structure and dynamics of coronal and interplanetary plasmas and magnetic fields

STEREO is also designed to provide unique alerts for Earth-directed CMEs and new information regarding their propagation and associated phenomena throughout the solar system. The mission will provide a new perspective on solar eruptions by imaging CMEs and background events from the two observatories simultaneously. The first spacecraft will lead and the second will fly behind the Earth in its orbit.

For the first three months after launch, the observatories will fly in an orbit from a point close to Earth to one that extends just beyond the moon. STEREO mission operations personnel at The Johns Hopkins University Applied Physics Laboratory (JHU/APL), in Laurel, Md., will synchronize spacecraft orbits so that about two months after launch they encounter the moon, at which time one of them is close enough to use the moon's gravity to redirect it to a position "behind" the Earth. Approximately one month later, the second observatory will encounter the moon again and be redirected to its orbit "ahead" of Earth. When combined with data from observatories on the ground or in low-Earth orbit, STEREO's data will allow scientists to track the buildup and lift-off of magnetic energy from the sun and the trajectory and magnetic field geometry of Earth-bound CMEs in 3-D.

NASA Goddard Space Flight Center's Solar Terrestrial Probes (STP) Program Office in Greenbelt, Md., manages the STEREO mission, instruments and its science center. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) designed, built, and will operate the twin observatories for NASA during the two-year mission.

What exactly is space weather?
"Space weather" describes changes in the solar system environment caused by variations in the sun and solar wind. These include transient events such as CMEs, solar flares, quasi-steady ambient solar wind and changes in the interplanetary magnetic field due to solar surface features like active regions and coronal holes. Solar flares and CMEs originate at the sun and can cause disturbances near Earth and throughout the solar system. Space weather can have a number of different effects. Dangerous radiation, especially in the form of high-speed particles can present a hazard to astronauts. Space weather can also lead to problems with spacecraft, communications, and power systems. In addition, interaction of space weather with the Earth's magnetic field causes the beautiful aurora (northern and southern lights).

What are coronal mass ejections (CMEs) and why study them?
CMEs are powerful eruptions that can eject a small part 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 miles per hour. They can create major disturbances in the interplanetary medium (dust, gas and plasma in the space between the planets), and if they reach Earth, trigger severe magnetic storms that affect satellites, communications, power grids and airlines. CME-driven shocks also play a significant role in accelerating solar energetic particles that can damage spacecraft and harm astronauts. Despite their importance, scientists don't fully understand the origin and evolution of CMEs. STEREO's unique 3-D measurements should help with those answers.

Understanding what causes CMEs and how they move through the solar system is one of the chief goals of the STEREO mission. The different telescopes in the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument suite image the CMEs. The actual material in CMEs is measured as they pass the spacecraft using the In situ Measurements of Particles and CME Transients (IMPACT) and PLAsma and Supra Thermal Ion and Composition (PLASTIC) instruments. The STEREO/WAVES (SWAVES) instruments observe radio signals produced by shock waves formed as the CMEs plow through the solar wind.

What are solar flares?
Solar flares are bright, explosive events that take place in the sun's lower corona (the sun's hot, thin outer atmosphere). They can be associated with CMEs, but are not the same thing. Scientists will use the SECCHI imaging instruments aboard STEREO to improve our understanding of how flares are related to CMEs.

Although most of what is called a solar flare occurs relatively low in the sun's atmosphere, flares do release charged particles that travel along the magnetic field lines of the interplanetary magnetic field (IMF). Electrons emitted this way by flares produce radio waves detected by the SWAVES instruments and allow researchers to map the IMF.

Sometimes these charged particles may be high enough in energy to qualify as solar energetic particles (SEPs). SEPs, along with the X-rays and gamma-rays produced by flares, can be very harmful to astronauts.

What will the 3-D images look like?
The 3D images will be constructed using data from the STEREO/SECCHI suite's Extreme Ultraviolet Imaging Telescope. Each of the two images the combine to form a 3-D image will be similar to images from the Extreme Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), but with significantly greater resolution (2k) and frequency (one image every few minutes). The EIT instrument takes images approximately every 12 minutes.

How does the solar cycle influence STEREO's mission?
The sun has an 11-year cycle of activity determined by the reversal of its magnetic poles. During the solar minimum, the sun may churn out a strong CME every two or three days; that's approximately 180 CMEs per year, though only about 10-15 CMEs are directed at Earth. During solar maximum, the sun averages five CMEs daily, and sends about 100 Earthward per year. The last solar maximum was approximately 2000-2001.

STEREO will track these disturbances from their onset at the sun's surface to beyond Earth's orbit, measure energetic particles generated by CMEs and flares, and sample fields and particles in the disturbances as they pass near Earth. The STEREO scientific program does not depend on the phase of the solar cycle because CMEs and other phenomena to be studied are common to all phases of the cycle. Although the CME rate varies with the solar cycle, assuming a CME rate consistent with the minimum of the solar magnetic activity cycle, STEREO expects to observe at least 60 CMEs in remote sensing instruments and at least 24 interplanetary events in-situ.