Update for Saturday, Jan 12, 2008

The increased solar wind density that we had been hoping for (and expecting) is here! The funny thing is that, although it means that the number of electrons and ions in the solar wind is 3 times more than yesterday, it is still just 3 (!) per cubic centimeter. That means if you hold your thumb up (the size of the tip of your thumb is something like a cubic centimeter), there would only be 3 electrons and 3 protons in that space. This is not much, I agree, but keep in mind that the total number of particles in the solar wind is huge and the speed they are traveling at is also huge (about 500 km/s right now, or 300 miles per second or more than 1 million miles per hour), so the total amount of energy is, well, huge!

Well, anyway, that's the good news. The bad news is that the magnetic field in the solar wind was far weaker than we expected and the solar wind speed was somewhat less than expected. We had gotten a preview yesterday and the day before (by looking at STEREO data) and had every reason to think that we would get a similar dose of good solar wind, but it did not happen. At the same time, there was good aurora well to the north of us. We could see it in the allsky cameras, but it was too far north to be able to "probe" it with the radar. A disappointing day, but that's the way it goes, sometimes. We'll try again tomorrow!!!

---------------------------------------------------------------
So far, I have talked about a range of topics, but have not said much about how satellites get built. It turns out that a very interesting satellite will be launched next summer, called the Interstellar Boundary Explorer (IBEX) satellite. Its mission is to explore the very outer regions of the solar wind. Remember, the solar wind blows past all of the planets, so when I say IBEX will explore its outer regions, I mean that it will "look" at a region of the solar wind that is much further than the furthest planet in our solar system. I don't mean that the satellite will actually go there, though. It will make the measurements from an orbit near Earth.


This is the IBEX satellite.

Prof. Eberhard Moebius at UNH is providing part of an instrument for IBEX, one that will measure what are called "energetic neutral particles". I've mostly been talking about electrically charged particles so far. In this case, though, the particles that this instrument will measure were electrically charged at one point, but managed to adopt an electron and, in so doing, became neutral. This means that they are free to travel in straight lines and do not have to follow magnetic field lines. So, basically, the instrument will be measuring particles that have drifted halfway across out solar system!!

So how do you build these things? Well, there are lots of different types of parts that go on a satellite, depending on the mission. Most of the stuff that I've been talking about has been related to research in space. Of course, satellites are also used for communication, weather forecasting and many other things. Whatever the objective is, satellites nearly always have the same basic "systems" and an important part of putting a satellite together is to make sure that all of these systems work together.

Some examples:

Electrical system: Every satellite has an electrical system that consists of solar panels, batteries and lots of electronics that control the power from the solar panels to charge the batteries, to power the instruments and to power the transmitters, etc.

Mechanical system: This includes the nuts and bolts that hold the satellite together and also includes the "deployment" mechanisms. When a satellite gets launched, it is generally packaged as small as possible. Once it has been separated from the launch vehicle, things like antennas, solar panels and other gismos need to be released so that they pop into place - this is what deployment is all about.


This is an older GOES weather satellite. These satellites use momentum wheels to keep cameras constantly aimed at skies over the US. There are always two of these operating, one over the eastern US and one over the western US, to give us photos of clouds that are used to forecast weather.

Attitude Control System (ACS): this is the stuff that keeps the satellite pointed correctly. Well, I should clarify - sometimes a satellite is intended to spin constantly so that it is stable (like a football). Other times, though, a satellite is pointed with a very high degree of precision. Hubble, for example, can point to any point in the sky and stay pointed to that spot to within a small fraction of a degree. So, and ACS system can be complicated and often uses things called "momentum wheels", or "magnetic torquers", etc.

Thermal system: When a satellite is in space, there is no easy way to control its temperature. When it is in the sun, it tends to get very, very hot, since there is no way to get rid of the heat (because the solar wind density is so low - or because there is so little stuf in the solar wind to carry the heat away). When it is behind Earth, there is no source of heat and the satellite cools dramatically (like the air does in a desert, for example). In order to make sure that the onboard temperatures stay within a certain range, engineers use blankets and come up with all sorts of tricks to control the heat.


This is the Cluster satellite (actually, one of the four Cluster satellites, showing thermal blankets and solar panels.

In the end, a satellite will only work if all of these systems work together correctly. For example, any time a transmitter is used, the electrical system will generate heat (which the thermal system has to deal with). Or, if something gets too hot, it might warp something like a mirror used in a camera, so the mechanical design has to be able to handle that. Likewise, things that are bolted together will conduct heat, so the thermal design needs to account for that. And so it goes, on and on... Actually, it's not much different than an orchestra: the wind, string and percussion sections all need to blend together to create the magic of a symphony!!