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STEREO Deputy Project Manager
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STEREO Project Scientist
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Junichi Maki from Niihama-City: What is the difference between the SOHO and STEREO missions?
Adams: I guess I'll start and then Mike will pick up. I think the primary difference between STEREO and SOHO is that STEREO is two satellites looking at the Earth-sun connection from the side, whereas SOHO is a single spacecraft looking straight on between the sun and the Earth.
Kaiser: That's true. SOHO is a considerably bigger spacecraft than STEREO. It's probably bigger than both STEREOs put together, and has a much larger complement of instruments, but basically measures the same type of things. SOHO has been up, they just had their 10th anniversary a few months ago. So it's been out there for a long time, and we hope it'll continue for a long time, because we will make use of it with STEREO.
Additional question: What is the main objective of the STEREO mission?
Kaiser: The main objective is to try to make better progress on predicting when space weather events will reach Earth. The sun gives off these tremendous storms called coronal mass ejections, or CMEs, and the ones that we're most interested in are the ones that are coming right at us. And with the SOHO spacecraft and others sitting at Earth, it's particularly hard to measure those, because you're looking right at them. And so what we're going to do with STEREO is place some spacecraft off to the side and basically triangulate these, so we'll get a much better handle on their speed and exactly when they will hit Earth. So we will be able to predict much better.
Andrew from Melbourne, Australia: By how many degrees per year will "ahead" and "behind" separate from Earth, and for how long could useful stereoscopic imaging of CMEs be obtained?
Adams: The STEREO ahead and behind spacecraft separate from the Earth each at 22-1/2 degrees per year, making a total of 45 degrees per year between the two satellites, which is what's important because you're imaging between the two satellites. And I believe that Mike can correct me, it's two years, right, Mike?
Kaiser: Right. After two years, the spacecraft are 90 degrees apart. But don't forget this SOHO spacecraft that we've already mentioned. If it stays around and keeps getting funded and still does its job, it'll sit in the middle and bisect that angle so we can do STEREO-type measurements for quite a long time, up to maybe four years.
Junichi Maki from Niihama-City: Can the STEREO mission predict and let the crew on board the ISS know of CMEs to keep them from being exposed to harmful X-rays, gamma rays and so on?
Adams: I think that the answer to that question is that NASA and NOAA are partners in doing space weather predictions, making space weather predictions. STEREO sees the large Earth-bound events and broadcasts them immediately to NOAA, where they're processed and distributed for everybody who's interested in space weather. At that point in time, ISS could pick up on an alert put out by NOAA in the event of a large CME.
Kaiser: Right. ISS, though, is probably not so particularly susceptible to these types of storms because it's relatively close to Earth and deep inside Earth's protective magnetic field. So a lot of the space weather events literally don't get to it.
Jesse from Kalispell: Why was the acronym STEREO chosen and how did you choose the acronym?
Kaiser: That one's easy. We actually started with the word "stereo," since we were going to have two spacecraft, and we tried to backtrack and think, now what can we use to make up "stereo," and it's Solar TErrestrial RElations Observatories. So, it is an acronym but we actually started with the word "stereo" and worked backwards.
Vlad from Montreal: What's the current estimate on public data availabity from the STEREO mission?
Kaiser: Well, there are two types of data. There's this space weather data that Jim mentioned, that's going to be broadcast almost immediately, and our goal is to have it up on the Web, available within five minutes of receipt on the ground. And then the higher-resolution data, the typical data, we basically get a download once a day, and that data will have to be processed, so typically, 24 to 48 hours after that data is taken, it'll become available on the Web site, stereo.gsfc.nasa.gov, and you can find everything there.
Junichi Maki from Niihama-City: Does the STEREO spacecraft observe and/or study the magnetic field of the sun and the Earth?
Kaiser: Yes, particularly the sun. There's a magnetometer aboard that measures the sun's magnetic field. Now this is the magnetic field at the spacecraft, so it's the solar wind's magnetic field. But during the early three months of the mission, before it even gets into orbit around the sun, it actually makes all these nice loops past the Earth-moon system, and the magnetometer will measure Earth's magnetic field at that time.
Ganesh from Pune: What is the propellant of the STEREO spacecraft?
Adams: The propellant that the STEREO spacecraft use is called hydrazine. It's a fairly common rocket fuel that most satellites use, and each of the observatories carries about 62 kilograms of hydrazine. Once we get on orbit, into the heliocentric orbit around the sun, we really only need about a kilogram and a half per observatory, per year, to maintain the mission life.
Ozkan Oz from Eskisehir, Turkey: Hi, I am from Turkey. I wonder if you could put a camera on one edge of the sun power panel for a wider angle of view?
Kaiser: Well, Jim and I looked at this question and both had completely different takes on it. So I'll give you my take. If you're asking the question, would this help with our STEREO viewing, the answer is no. With our eyes, which are about 65 millimeters apart or so, we have stereo vision out to about 100 meters. And so, if you work out the angle, that's about one minute of arc. So if you, if you want the two STEREO spacecraft to be one minute of arc apart relative to the sun, that works out to 80,000 kilometers, so putting one of the instruments out on the end of the solar panel a few meters away doesn't make any difference at all. But Jim's take on this question is completely different, and I'll let him explain it.
Adams: When we were designing the STEREO observatories, one of the first things that we did when we were locating the instruments was to identify the best places to put the instruments in order to maximize the view angle towards the sun. And if you look at the way the STEREO instruments are set up, we have an incredible wide angle field of view between the Earth and the sun, which is about 93 million miles. So we have the UVI instrument that's looking right at the surface of the sun. We have the Core 1 coronagraph that's looking at the inner corona. We have the Core 2 coronagraph that's looking at the outer corona. We have HI-1 that's looking at the region between the outer corona and a few million miles out from the sun. And then we have the HI-2 that looks the rest of the distance between HI-1 and the Earth itself. And so, this is actually an unprecedented angle of view between the Earth and the sun.
Additional question: What is a CME?
Kaiser: Well, CME -- personally, I think it's a very bad term. It should have been called solar mass ejections, but way back, historically, somebody called them coronal mass ejections, so that's what CME stands for. They're explosions that happen on the sun, and the explosions are usually caused by magnetic fields coming together and annihilating. And so they violently release a fair amount of gas, and often there are big solar flares associated with it. You've probably all seen pictures in the newspapers of these big bubbles of gas heading out from the sun, and sometimes these head towards Earth. And to move right into the question here, which has to do with what kind of damage they can cause, it's mostly electrical damage these days. All of our spacecraft that are up there all have micro-electronics parts, and they're very susceptible to small, small voltage and current changes, and these storms can induce a lot of that sort of thing and actually can cause spacecraft to shut down. So if you know one's coming ahead of time, you can take some preventive action. They've also been known on very big storms to actually trigger power outages on the Earth. There was a famous one in the late '80s in Canada that shut down the entire power grid; I think it was in Ontario, due to a solar storm. So there's been, this whole business of trying to predict when CMEs will arrive at Earth has been going on for a number of years. And as I said earlier, from a single spacecraft looking right at the sun, the ones that are coming at you are particularly difficult to measure. And so with STEREO, we're hoping to cut the error bar on our predicts, so to speak. Right now, we can probably only predict to plus or minus 12 hours. We're hoping to cut into that and do a much better job by triangulating from the sides.
Adams: You could also mention the communications outages and the airlines.
Kaiser: Right, communications outages. Airlines that fly polar routes are very interested in solar storms because there's one of the rules that you cannot ever be out of radio contact and during one of these big solar storms, it changes the ionosphere such that you could easily be out of contact up in the polar regions. Our GPS that we all depend on for aircraft navigation and even in our cars these days can be affected by changes in the ionosphere that are also affected by these solar storms. So people that depend on that like to know about solar storms. It's a whole field of people that are interested in this.
Junichi Maki from Niihama-City: How long do CME events last? In addition, how long does it take CMEs to get to Earth?
Kaiser: The actual explosion itself is fairly brief. I mean, this question has lots of answers. If you look at the actual explosion and the flare associated with it, you're talking about a few minutes when most of the energy is released. Within this CME itself, this big bubble of gas is moving outward at, say, 1,000 kilometers per second or so, and can take three or four days to actually reach Earth. So some of the energetic particles reach Earth in an hour or so, but the main bulk of the gas that was expelled might take three or four days. So I don't know what you want for an answer here; maybe minutes to days is the answer.
Junichi Maki from Niihama-City: Why is the temperature of coronal mass ejections so high -- several million Kelvins -- while on the other hand, the temperature of the sun is only 6,000 Kelvins?
Kaiser: One of the big enigmas of our time. In fact, surprisingly, one of the coolest places on the sun is the visible surface, the so-called photosphere, which is, indeed, about only 6,000 degrees, which sounds like a lot. If you barrel into the sun and go down into the center, it's about 35 million degrees. That's where the nuclear furnace really happens. But also, if you go up into the sun's atmosphere, into the corona, surprisingly, the temperature goes up to a couple of million degrees, and there's been a lot of argument on how this happens. And the current thinking is that all the magnetic field of the sun, all the little bubbling, magnetic bubbles on the sun, all propagate upwards into the solar atmosphere and carry heat with them, and heat up the sun. So the coronal mass ejections, indeed, when they're near the sun are a few million degrees. Now by the time they get to Earth, they've cooled off considerably. So we don't have to worry about burning up, but it's one of the ongoing problems in solar physics right now.
Junichi Maki from Niihama-City: The sun has the 11-year solar cycle. Do CMEs tend to occur when the sun is at its peak? When is the next peak?
Kaiser: Yes. The CME rate also follows the solar cycle. Right now, when we're very near a solar minimum, the CME rate is about one every couple of days. When we reach solar max, which is about six and a half years from now, so, you know, 2012 or so, we'll probably get up to about five or six CMEs per day. So there's about a 10 to one variation in the rate of occurrence of CMEs.
Junichi Maki from Niihama-City: How will the two STEREO spacecraft be protected from exposures to harmful CMEs?
Adams: The satellites have been carefully analyzed for the impact of radiation on all of our sensitive electronics, as well as all of the surfaces have been coated with conductive coating that allows the satellite to not charge up differentially. So, if you think about the fact that coronal mass ejections, the solar wind and the photo-electric effect could cause the satellites to charge in one spot to kilovolts relative to another spot, then you could, just like you get electrostatic discharges at home when you walk across the carpet, you could get a discharge on the satellite, and that could cause an eruption in our activity. So we've taken great pains to engineer that out, by creating conductive surfaces all over the satellite that will equalize the voltages that build up.
Kaiser: And just having the electronic boxes physically inside the spacecraft helps a lot, because you've got protective covering of all that metal on the outside.
Junichi Maki from Niihama-City: When STEREO catches sight of CMEs, how soon can the spacecraft give us "alert" messages?
Kaiser: Well, with the real-time data which comes down, and it's basically a snapshot image every five minutes, and sort of one minute averages the sum of the other parameters, that's sent down continuously, can be processed and turned around on the ground in another five minutes. So probably from the time one is first observed to the time, you know, the alert goes out, can be as short as 10 minutes. Pretty good.
Junichi Maki from Niihama-City: How will the STEREO spacecraft take 3-D pictures of CMEs?
Kaiser: There are actually several questions that are sort of along that line. The next one is why will 3-D pictures help us learn more about CMEs, and another one of what are the advantages of taking 3-D pictures of the CMEs in detail, so, you know, kind of one answer can cover all of these. The two spacecraft will be put in orbit around the sun and they're almost in Earth's orbit, but not quite. There's one spacecraft which we cleverly called "Ahead," because it moves ahead of Earth, and it's going to be a little bit closer to the sun than Earth's orbit, so that instead of going around every 365 days, one year, it actually goes around in 345. The other spacecraft, also cleverly called "Behind," because it's the behind spacecraft, is further from the sun than Earth, slightly, and it goes around in 385 days. So the two of them together move away from Earth, as Jim said earlier, at about 45 degrees a year. And so, when these spacecraft are separated, they're taking different images which can be combined in a 3-D fashion, much like our eyes combine to make 3-D images. It's actually also triangulation for some of the instruments. They don't actually make instruments, but they actually can triangulate from, from two different locations. And this will, this will allow us to measure exactly where these Earth-directed CMEs are headed exactly...which way they're headed and the velocity. So we'll be able to get a much better prediction on that. As far as some of the details in the images, you know, these, these cameras have tremendous resolution, so they'll be able to look at some of the tiny little magnetic loops and actually observe those in 3-D for the first time. This is sort of indirectly related to space weather, but the solar physics community is very excited about that aspect of STEREO.
Arun from Cupertino: Are CMEs the same as solar flares?
Kaiser: We get asked this question a lot. I like to think of the difference between them as sort of a hurricane versus a tornado. Solar flares are very often associated with CMEs, especially, you know, when the CME first explodes, when you have this first magnetic annihilation, there are often violent releases that look like solar flares, that release a lot of energetic particles, as well as this big bubble of gas. And so they're very much related and they can have comparable amounts of energy. But on the other hand, there are solar flares that are not accompanied by CMEs. So solar flares can happen without CMEs, but they're often related, just like tornados are often related with hurricanes, but tornados can also occur all by themselves.
Junichi Maki from Niihama-City: How often do CMEs occur in a year on average? When was the biggest CME that has ever occurred?
Kaiser: Well, my answer before, having to do with the solar cycle -- let's see. About one CME every two days, at solar minimum right now, so what would that work out to 180 a year. And then about five, about 10 times that many during solar max. So maybe the order of 1,500 (or) 1,800 a year during solar max. The biggest CME depends on who you ask -- is that the most energetic, the one that's the widest angle, the one that has the most mass? Certainly, we've had some very big ones lately. There was one in January of last year that was the biggest in some respects and had the most energetic protons associated with it, and it also was the very fastest, I think, ever observed. But then a couple years earlier, there was another one that went up and rang the bell as far as some of the indexes that are used to measure these things, had the most energy and the flares associated with it. So nailing down exactly which one's the biggest is not so easy because it depends on how you measure it.
David from Tuscaloosa, Ala.: How close can a space vehicle approach the sun and how do we know this?
Adams: The STEREO observatories are not going to get any closer to the sun, realistically speaking, than the Earth already is. They're in the same orbit around the sun as the Earth, roughly. As Mike pointed out, the Ahead goes ahead, goes around the sun just a little faster than the Earth and the Behind satellite goes around the sun, I should say, just a little faster than the Earth, and the Behind satellite goes around the sun just a little slower than the Earth. So they're at roughly the same distance from the sun as the Earth is. Certainly, NASA has sent missions closer into the sun. We have one on its way to Mercury right now called MESSENGER, and I really don't have any idea how close we can put a satellite to the sun.
Kaiser: There's a mission that's been under study for some time called Solar Probe, which will actually come within four solar radii of the surface, so much, much closer than Mercury. And of course, one of the big things you worry about is the heat, because as you get close to the sun it gets pretty, pretty toasty. So there's a big heat shield out in front of this thing that in itself is quite a challenge to design, and a lot of the instruments kind of hide behind the heat shield and peek out. So this mission hasn't flown yet, but in principle we should be able to go in that close to the sun.
Adams: So they would go to the sun at night, essentially.
Kaiser: (Joking) Yeah, go into the dark side of the sun, that'll help.
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