Friday was the first sunny day in Boston for what seems like weeks. As luck had it, some sixth-grade students were visiting the Clay Center Observatory, and I had a chance to show them the Sun though white-light and hydrogen-alpha filters.

STEREO views of the entire Sun

This 360°-view of the entire solar disk is based on high-resolution images taken by the STEREO spacecraft at the extreme-ultraviolet wavelength of 304 angstroms on February 2, 2011. The twin probes are now positioned on opposite sides of the Sun and together can record the entire disk at any time.


The disk, sadly, was pretty bland, but I told the kids that maybe some sunspots on the back side would soon rotate into view. Full disclosure: I was guessing.

Now I'll no longer have to guess, thanks to twin NASA spacecraft called STEREO (short for Solar Terrestrial Relations Observatory). Launched some 4½ years ago into heliocentric orbits, STEREO A ("Ahead") and B ("Behind") have been gradually separating from each other. As of today, they've reached their final observing locations on opposite sides of the Sun. For the first time, solar scientists can scrutinize our star's entire disk — its front and back — at once.

STEREO records the Sun with a suite of imagers collectively called the Sun Earth Connection Coronal and Heliospheric Investigation, or SECCHI. (Why the convoluted acronym? It's to honor Angelo Secchi (1818-78), an astrophysicist and Jesuit priest who was among the first to photograph solar eclipses.)

STEREO spacecraft

An artist's concept of STEREO. Each of the twin spacecraft carries a quartet of instrument packages to study the Sun and the heliosphere.


This multiple-wavelength camera cluster is designed to capture powerful solar eruptions called coronal mass ejections (CMEs) from their trigger points on the Sun's surface, as they travel upward through the corona, and escape into the interplanetary medium.

Armed with STEREO's instrumentation and its 360° view, solar physicists hope to understand CMEs as never before. This is a big deal, since CMEs can pack a huge electromagnetic wallop when they strike Earth's magnetosphere. They can damage satellites, endanger Space Station astronauts, and wreak havoc with power-distribution grids on the ground.

You likely haven't heard much about STEREO and its mission — but, trust me, you will in the months and years ahead. The Sun is just rousing itself from a deep and prolonged hibernation in its 11-year-long activity cycle. But once activity ramps up in a year or two, CMEs will occur at least every couple of weeks.

By then, I suspect there'll be plenty of sunspots to show my students.


Image of Mike Emmert

Mike Emmert

February 11, 2011 at 2:37 pm

I think SECCHI is more clever than convoluted. 😀

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Image of Harlan E Clark, PhD

Harlan E Clark, PhD

February 13, 2011 at 8:17 pm

How stable are the orbits of the two spacecraft? Are they locked in place relative to each other, or is it possible that over long periods of time they can drift slightly and thus not be exactly opposite the sun?

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Image of John


February 15, 2011 at 8:57 am

@Harlan: Kelly's description of these as the "final" position for these orbiters was incorrect. "A"'s orbit will stay inside earth's orbit, so it will continue to move "ahead", and B's orbit is outside earth's orbit, so it will continue to fall behind, relative to the earth's position around the sun.

At the link in the article
showing the path from launch until now, click video #10 showing the path from now through 2019.

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