Solar TErrestrial RElations Observatory

stereo_logoSTEREO (Solar TErrestrial RElations Observatory) is the third mission in NASA’s Solar Terrestrial Probes program (STP). The mission, launched in October 2006, has provided a unique and revolutionary view of the Sun-Earth System. The two nearly identical observatories – one ahead of Earth in its orbit, the other trailing behind – have traced the flow of energy and matter from the Sun to Earth.

Starting in February 2011, and continuing on for the next eight years, mankind now has its first ever 360 degree view of a star – our own Sun. By combining images from NASA’s Solar Terrestrial Relations Observatory (STEREO) Ahead and Behind spacecraft, together with images from NASA’s Solar Dynamic Observatory (SDO) satellite, a complete map of the solar globe can be formed.

Previous to the STEREO mission, astronomers could only see the side of the Sun facing Earth, and had little knowledge of what happened to solar features after they rotated out of view. Would active regions grow larger, and affect the space weather environment when they rotated back again two weeks later, or would they decay away? What about new active regions forming on the far side of the Sun, waiting to surprise us? With STEREO’s 360 degree view of the entire Sun, that will no longer happen.

STEREO has revealed the 3D structure of coronal mass ejections; violent eruptions of matter from the sun that can disrupt satellites and power grids, and help us understand why they happen. STEREO is a key addition to the fleet of space weather detection satellites by providing more accurate alerts for the arrival time of Earth-directed solar ejections with its unique side-viewing perspective.

STEREO’s scientific objectives are to:

  1. Understand the causes and mechanisms of coronal mass ejection (CME) initiation.
  2. Characterize the propagation of CMEs through the heliosphere.
  3. Discover the mechanisms and sites of energetic particle acceleration in the low corona and the interplanetary medium.
  4. Improve the determination of the structure of the ambient solar wind.

STEREO orbital diagram

Although, as seen from Earth, the two spacecraft seem to be going in opposite directions, they’re really going in the same direction, just at different speeds. Note that the orbits of the two STEREO spacecraft differ not only in their orbital distances, but also by how much that distance varies over the orbit. This property is described by a parameter known as the orbital eccentricity, denoted with the symbol e. A perfectly circular orbit would have e=0. STEREO-Ahead’s orbit is very close to circular, with e=0.006, while that of STEREO-Behind is more eccentric (e=0.042). The eccentricity of Earth’s orbit falls somewhere in between (e=0.017).

STEREO Orbital Insertion

Getting the STEREO spacecraft into orbit around the Sun was not simple. It involved using the Moon’s gravity to “slingshot” the spacecraft in their proper orbits. Both spacecraft were originally launched together on a single Delta II rocket on 26 October 2006. Immediately after launch they are placed into highly elliptical orbits that range from just a few hundred kilometers above Earth’s surface out to a little beyond the distance of the Moon. Over the next few weeks the two spacecraft slowly separated from each other, and the Mission Operations carefully adjusted the orbits of each to line them up for when both flew by the Moon a few minutes apart on 15 December 2006. The Moon’s gravity grabbed both spacecraft, and flung STEREO-Ahead completely away from Earth into its orbit about the Sun. STEREO-Behind was also flung out, but not completely, and came back to swing by the Moon again on 21 January 2007, when it was then completely flung away in the opposite direction into its own orbit about the Sun.

EUV Imagers (SECCHI/EUVI)

he Extreme Ultraviolet Imager (EUVI) is part of the SECCHI instrument suite currently being developed for the NASA STEREO mission. As an integral element of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), the EUVI plays a critical role in addressing the SECCHI science objectives, in particular: 1) Investigate the initiation of Coronal Mass Ejections (CMEs): How flux systems interact during the CME initiation, the role of reconnection, and coronal dimming 2) Investigate the physical evolution of CMEs: Their 3-dimensional structure, how they are accelerated, and the response of the low corona 3) Investigate the 3-dimensional structure of Active Regions

The EUVI telescope was developed at (LMSAL). The EUVI mirrors were figured and coated at (IOTA) and calibrated at (IAS), the focal plane assembly was developed at NRL and the University of Birmingham, the camera electronics were developed at RAL, and the aperture door was supplied by (MPS). The EUVI observes the chromosphere and low corona in four different EUV emission lines between 17.1 and 30.4 nm. It is a small, normal-incidence telescope with thin metal filters, multilayer coated mirrors, and a back- thinned CCD detector. EUV radiation enters the telescope through a thin metal film filter of 150 nm of aluminum. This filter suppresses most of the UV, visible, and IR radiation and keeps the solar heat out of the telescope.

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