Are Satellites Attacking the Earth? [Prof. Kung says…]

12 Oct

After hearing some news reports that yet another satellite was going to crash somewhere on our big blue planet, I had a few questions that only our very own Prof.  Kung could answer — I happen to know for a fact that there are satellites out there that send text messages to her (no joke), so I thought she might have the inside scoop.

I wanted to know why satellites were attacking the earth and what we could do about it.  Keep reading to find out the answers to these burning questions.

Why are satellites attacking the earth — why don’t they just stay in perpetual orbit?

Part of the issue here is that there is no hard “edge” to the Earth’s atmosphere. It just keeps getting thinner and thinner as you get higher and higher. We *define* the boundary of space to be at 100 km above sea level. However, there is still a significant amount of atmosphere above that threshold. Not enough to breath or to fly a plane through, but certainly enough to effect the orbit of various satellites.

The other part of this issue is that satellites are placed at different altitudes above the Earth, depending on many factors. Imagine you want to spy on another country or to take detailed measurements of weather/geology etc. Those types of satellites need to be at altitudes high enough to orbit the Earth, but at altitudes low enough to still have the ability to resolve features on the Earth’s surface. Those types of satellites are put into what is called “Low Earth Orbit” (LEO), less than 2000 km above sea level. Unfortunately, these satellites are still very much in Earth’s upper atmosphere. These satellites collide with the particles in the upper atmosphere, and by doing so lose some of their kinetic energy (this is called “atmospheric drag”). As drag slows down a satellite, its altitude will decrease (this is often called “orbital decay”). To avoid having these satellites drop out of the sky on a regular basis, such satellites carry fuel to allow them to occasionally reposition or “boast” their orbits to make up for atmospheric drag. Alas, this fuel eventually runs out and such a satellite will reach a low enough altitude that the atmospheric drag greatly increases and eventually sends the satellite plunging toward Earth.

Note that other satellites are placed in Geosynchronous or High Earth Orbit are at ~40,000 km altitude, so don’t encounter significant atmospheric drag. These satellites will hang out safely in orbit for a very long time without any intervention.

It seems like this is a big item in the news lately. Are more satellites crashing than normal, or is the media just picking up on it?

The media is definitely picking up on the “interesting/dramatic” aspect of satellites de-orbiting to fill the 24 hour news cycle! However, it is also possible that more large satellites have been coming down recently. I’m not actually sure of what the numbers are, but humans put a lot of satellites up in the 80’s and 90’s, and most of those satellites had no “end mission” plan. To reduce the clutter of “space debris” in orbit, international law now mandates that you must have a plan about what to do with your satellite once its mission ends, but such regulations have only been around for about a decade.

Nowadays, when you send a satellite up your “end of life” disposal options are: (1) carry controls/fuel that allow for a controlled re-entry or (2) carry controls/fuel so that you can boast your satellite to a higher orbit where it can hang out safely (e.g. (a) not plunge back to earth and (b) not get in the way of new satellites). Note that adding controls/fuel to your satellite always increases launch costs, so these are *not* trivial design changes.

Why don’t they just burn up in the atmosphere?

They mostly do, but some of the more solid chunks of satellite can survive re-entry.

Why don’t scientists know exactly where the satellites will land? Can’t it be calculated?

Too many variables! The drag the satellite will feel at any given time depends on the local atmospheric density. This varies with altitude and it also varies with the current solar activity and time of day, etc! While all these effects can be added to a mathematical/computer model, there are uncertainties in the magnitude of each effect. Taken together, those individual uncertainties result in a relatively large uncertainty in the exact trajectory of the satellite. LEO satellites orbit the Earth once every 90 minutes, so even an uncertainty of just a few minutes one way or the other results in a hugely different “landing point” for the satellite (e.g. North America vs. Europe!).

4 Responses to “Are Satellites Attacking the Earth? [Prof. Kung says…]”

  1. catherine October 12, 2011 at 12:44 pm #

    so cool!

  2. Holly October 12, 2011 at 12:54 pm #

    Good post, that was a really interesting read!

  3. RJS October 20, 2011 at 10:32 pm #

    So if satellites are not attacking the earth, might it be aliens?

  4. Mitch Langley October 22, 2011 at 2:01 pm #

    That’s so awesome! Another reason why I loved having Professor Cobb.

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