STEREO on the Sun
Published on Oct 26, 2006 at 7:41 pm.
3 Comments.
Filed under NASA, Sun.
Last night, October 25, a Delta II rocket lifted off from Pad 17B at the Cape Canaveral Air Force Station.  It was carrying twin NASA spacecraft on a mission to study our nearest star, the Sun.  The acronym selected for this mision is STEREO (standing for Solar Terrestrial Relations Observatory), and it is quite aprospos because these two craft will study the Sun from slightly different positions, permiting stereoscopic, or 3-D, studies of our star, the Sun. The spacecraft will achieve their stereoscopic views of the Sun by looking at it from two slightly different angles. Both will orbit the Sun about one Astronomical Unit out, close to Earth’s orbit. One will be ahead of the Earth along its orbit, and one trailing behind. To place them both into separate positions using one rocket, NASA fired them on a path that will eventually take them towards the Moon. They will be separate long before they get there, and the Moon will then sling one ahead of the Earth (Spacecraft A) and the other (Spacecraft B) trailing the Earth in our planet’s orbit around the Sun. This will give them the parallax needed to create three dimensional images of coronal mass ejections as they are hurled outward from the Sun.  The lunar flyby will be in about two months from now.
STEREO is the third of a series of NASA missions to study the Sun and its effect on Earth. The first was TIMED (Thermosphere Ionosphere Mesosphere Energetic and Dynamics Mission) which focused on the response of Earth’s upper atmosphere to solar radiation, and the second was a joint mission with the Japanese Aerospace Exploration Agency (JAXA) called Hinode launched just last month from Japan. And, now we have the STEREO mission.
Each spacecraft has 16 instruments to image and measure the Sun and the solar-terrestrial environment. Each spacecraft has a mass of about 620 kilograms and is about the size of a couch (a little smaller than 4 feet by 4 feet by 7 feet) with the solar panels folded up for launch. When the solar panels are deployed, though, the dimensions swell to about 4 feet by 7 feet by 22 feet. The mission has so far cost about $550 million, and is expected to last two years. The spacecraft themselves are likely to last longer than that;  however, their orbits will eventually carry them too far apart to act together to study the Sun. This is because they are not going to be in exactly the same orbit, with Spacecraft A in an orbit slightly faster than Earth’s and Spacecraft B in an orbit slightly slower. The end result is that the spacecraft will appear to separate by about 45 degrees per year. Eventually, they will be on opposite sides of the Sun. Now, that doesn’t mean that the spacecraft will necessarily quit sending back useful data. The instruments will still be able to gather data, though. However, stereographic representations of that data will no longer be possible.
So, why do we care to study the Sun in the first place? Well, the reasons are far too many to go into here in one blog entry. But, I can give some examples. The Sun is very active, with peaks in its activity occuring about every 11 years or so. In fact, we are soon going to be entering another period of activity in a few years. When active, the Sun’s magnetic field wraps up and builds up enormous potential energies. Every now and then, this magnetic energy is suddenly released in a solar flare, which is a titanic explosion that totally boggles the mind as to its intensity (think every hydrogen bomb every built detonated simultaneously, and then multiply by 100). When this happens, a chunk of the Sun’s corona can be blasted out into space. The coronal mass ejection (CME) then acts like a giant blog of plasma traveling outward from the Sun. When this blog slams into Earth, all sorts of things can happen that we generically call a geomagnetic storm. During such a storm, satellites can be knocked offline, or even permanently damaged, pipelines and power lines can suddenly develop massive unexpected current surges, radio communication can become spotty, or even impossible, and all sorts of other things. Also, during such an event, airline passengers and crew can receive significant doses of radiation, which I blogged about previously. And, radiation exposure is even worse for astronauts, a concern if we truly want to further lunar exploration and perhaps one day send a manned mission to Mars. But, the problem is that we don’t really understand the process well enough to predict these events. Even when we detect one going on, we don’t know exactly what the effect on Earth will be until the plasma blast hits our outermost satellites that serve as sentinels against such events, giving us only minutes or hours warning to prepare. So, STEREO is supposed to help us understand these events.
By observing from two directions, and putting the images together to form a three dimensional map of what is going on, we hope to understand the formation and structure of CMEs. That is the role of the SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) instruments aboard the STEREO craft. Other instruments are designed to measure the composition and energies of the particles spewed out in CMEs. It is hoped that this information can help us to predict CMEs before they happen, and together with data from the other missions, to predict the effect that these CMEs will have once they reach us.Â
-Astroprof
(Image Credit:Â NASA)
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Zavatar on October 26, 2006 at 7:53 pm: 1
Great post about a great mission. i do however, have some questions:
- Will the data collected from the mission be used to create 3D images (like the ones you use those funny glasses to see)? That would be so cool, to be able to actually see the sun in 3D
- How do scientists calculate the amount of energy released in a CME? I have no problem with he numbers you show (after all, you know this stuff), but how do you actually calculate them?
Cheers
Astroprof on October 26, 2006 at 8:19 pm: 2
Yes, they will be producing 3D images. Often you get far more data by other means, but NASA always tries to get at least some impressive pictures, since that is what the public understands best. So, there should be some very impressive images coming out of this mission.
As to measuring the energy released in a CME, we mainly guess. We can measure changes in magnetic field, and infer energy changes. We can measure the velocity and estimate the mass of the CME, and that gives us a measure of the kinetic energy. We can measure the energies of the particles when they hit the satellites, and that is part of it. And, we can measure the radiation emited by the flare. Add all that together, and you get a lower limit to the amount of energy released. The figures that I normally give are very conservative, and most solar physicists cite numbers much larger for the energy released. It is hoped that the fleet of spacecraft now studying the Sun will be able to give us more accurate measurements. We know that we are not measuring all of the energy released. We know that our energy measurements are too small, but we don’t know how much too small.
A Ler…-- Rastos de Luz on October 27, 2006 at 5:12 pm: 3
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