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Tracking Chandra

October 27, 1999 ::

CXC in orbit - artist illustration
An artist's illustration of the Chandra spacecraft in orbit.
Illustration: NGST
Chandra orbit path
Artist's conception of Chandra's orbit path & radiation belts.
Illustration: CXC/M.Weiss
Chandra reached its orbit in three steps. First, the Space Shuttle Columbia delivered it to a low Earth orbit. Then, the Inertial Upper Stage rocket boosted Chandra up to an altitude where a built-in propulsion system took Chandra to its present orbit. This elliptical orbit takes the spacecraft to an altitude of 133,000 km (82,646 mi) - more than a third of the distance to the moon - before returning to its closest approach to the Earth of 16,000 kilometers (9,942 miles). It takes approximately 64 hours and 18 minutes to complete an orbit.

The Chandra spacecraft spends approximately 85% of its orbit above the belts of charged particles that surround the Earth. This makes uninterrupted observations of as long as 55 hours possible, making the overall percentage of useful observing time much greater than the low Earth orbit of a few hundred kilometers used by most satellites.


Follow along with this tutorial to see for yourself just what the Chandra orbit looks like. Please note, your browser must be java-enabled.

In a browser window, bring up:

http://liftoff.msfc.nasa.gov/realtime/JTrack/3d/JTrack3d.html

Screen shot of Jtrack 3D
Screen shot of Jtrack 3D depicting the View, Satellite, and Options menubar as well as the Earth.
This loads the java-based J-Track 3D tool. You may get a screen telling you that this is not a secure transfer (it's a safe tool, don't worry about loading it) in which case click on Grant.

Allow some time for the tool and its database to load (it will warn that it is loading satellite data). After it loads, you should be viewing the Earth with numerous little dots around it. Above the Earth on the same screen is a set of menu bars that read View, Satellite, and Options. Follow these steps:

  1. Click on Satellite and choose Select. You will get a menu with a number of satellite names. Scroll down the list and choose Chandra. You should see the orbit track light up.
  2. Notice that the orbit is much bigger than the viewing screen. Click on View and select Zoom Out. Do this a few times until you can see the whole orbit.

    Note: This is a 3-d representation; the red part of the orbit is the part that is "out of the screen" and the grey part is "into the screen." The current position of the satellite is indicated by the position at which the name appears.
  3. Observe that there appears to be a line of dots across the Earth, along with more dots scattered in various places. Try the following:

    1. Place your cursor right at the center of the Earth and, holding down the left mouse button, slowly move the mouse straight down. The Earth (and everything else) tips down so that you are looking down on the northern hemisphere.
    2. Ah! The line of dots across the Earth is really a ring! What could that be? This is the ring of geostationary satellites - i.e. those satellites which just sit right at a fixed point above the equator. There is a particular altitude (about 40,000 km) at which the orbital period exactly matches the rotation period of the Earth. If a satellite orbits right along the equator at this altitude, it will just sit above a particular spot on the Earth. Some of these are weather satellites. Most are communications satellites. Yes, there is a lot of stuff up there!
    3. Click on any satellite to see it's name and orbit. (You can always get back to Chandra by repeating step "a"). If you look back at the Satellite options, under Select, you will see a button marked Sat. Info. Click on this, it will bring up information on the satellite you are looking at and display it in your browser window.

      Tracking Chandra In case you were wondering...
      In case you wondered if Chandra is really up there, take a look at these pictures shot by Gary Emerson with a 25 cm telescope at the E.E. Barnard Observatory. Gary used an astronomical program called "The Sky" to generate the positions of the spacecraft position. First he shot with a wide-angle camera and found Chandra, and then he homed in with the 25 cm telescope. Full Story
    4. Next, zoom back until the Earth takes up most of the window. You will witness the haze of low-altitude satellites which include things like Hubble, Mir, the Space Station, and many others. If you watch closely, you will see them move. (If you don't, go to Options and set the Update Rate to 1s or 5s.) You can find Hubble (HST), Mir, or the Space Station (Station) under Satellite/Select if you want to see where these are.

    5. Still having fun? If you have a good connection speed go to Options and choose Timing. Select X1000 and then go to Options under Update Rate and choose 1/4 second. Wait a while for this all to load. You should observe objects moving at 1000 times their actual rates. With a slower modem this may not work well.


(Many thanks to Dr. Pat Slane for contributing the step-by-step procedures.)


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