"North Korea has launched a ..." is a sentence that rarely ends well, and is usually punctuated by the sound of wind being sucked through teeth. This week the hermit kingdom's National Aerospace Development Administration (that's right, NADA) set off international alarm bells by sending a satellite---the Kwangmyongsong-4---into orbit.
North Korea gave the International Maritime Organization a few days notice of the launch, but still managed to anger allies and enemies alike because of fears surrounding the rogue nation's intercontinental ballistic missile ambitions. Kwangmyongsong-4 probably isn't a nuke, but that doesn't necessarily mean it's safe. Yesterday, US officials reported that the satellite was tumbling in its orbit. It's stable now, but that still left people worried, considering there are more than 1,700 other satellites---and hundreds of thousands more bits of debris---orbiting the Earth.
In the worst case scenario, a collision between orbiting sats can lead to the so-called Kessler Syndrome---one satellite hits another, creating a debris cloud that in turn takes out another satellite, creating a debris cloud... and so on until the space surrounding Earth is an orbital mosh pit. Think Gravity, except it's not just the International Space Station that gets taken out. Depending on the altitude, humanity could lose its access to weather forecasting, GPS, and more than 700 channels of premium television. And not just temporarily: Space would be a no-go zone until somebody figured out how to clear out all that high velocity debris.
The odds of such a scenario are slim---but not impossible. Exactly how un-impossible depends upon the Kwangmyongsong-4's orbit. Unfortunately, official channels wouldn't comment on the collision risk. In a series of phone calls, NORAD, NASA, the office of the Secretary of Defense, and US Strategic Command played a game of public relationsSpikeball---each shooting me down while passing me off to someone else.
On the bright side, government agencies publicly publish their tracking data on the aptly-named website Space-Track.org. On the not so bright side, that tracking data looks like this:
Relax, that's called a two-line element, and it's actually not that hard to decode. All you need is the format key (Wikipedia's is the most well organized) and a little tenacity. Oh, and it helps to have an astrodynamicist holding your hand. "Typically TLEs are pretty much the only thing the public has to get an idea where things in space are located," says Moriba Jah, director of University of Arizona's Space Object Behavioral Sciences.
With enough math know-how, you could technically take all the information in the TLE to trace a rough estimate of Kwangmyongsong's orbit around your personal globe. Oh, you don't have a globe? You can also download an app to do it for you. I used JSatTrak, a Java-based tool. Once you download it, click the Utilities menu, select Update Satellite TLE Data. Then click Windows and open the Satellite Browser. In the window that opens, click the + sign to open Special Interest satellites, and then Last 30 Days' Launches. Scroll down until you see KMS-4, then drag it over to your Object List. At this point your JSatTrack window should look something like this:
From there you can overlay data from more than 1,700 other satellites---and see if their trajectories overlap. But if you find a probe that intersects the Kwangmyongsong-4, you still have to figure out altitude to know if the two objects collide. TLE doesn't tell you this directly, but it does tell you how many times the satellite orbits the Earth in a day---astrodynamicists call this figure mean motion. What's important to realize is that this is basically a measurement of distance over time, in other words, velocity. And because of what Johannes Kepler taught us about orbital mechanics, you can use this value to calculate orbital height.
The TLE tells you that Kwangmyongsong-4 circles the earth about 15.27 times a day. Or put another way, once every hour and a half. "But what we really want is that number in terms of seconds," says Jah. That's because you're going to plug that number into a handy little equation devised by Johannes Kepler and let classical physics (and Moriba Jah) do the work for you.
Simple, right? Right. Which puts Kwangmyongsong-4 at around 475 kilometers (or 295 miles) above Earth, or roughly 45 miles higher than the International Space Station. But not all the time. Satellites wobble, jiggle, bounce. "Earth's gravity field is lumpy, and the Earth is not a perfect sphere," says Jah. "Satellites get large accelerations over the Himalayas, and slow down over the oceans. Above and beyond you have effects from sun and moon that cause big perturbations." And TLEs aren't sensitive enough to capture all those perturbations. Not to mention the fact that no orbit is perfectly circular.
Of course, JSatTrack also lets you visualize satellite tracks in 3D, so you can see whether Kwangmyongsong-4's track is above, below, or upon any other satellite's orbit.
And even if Kwangmyongsong-4 is well above the ISS or other objects in Low Earth Orbit, that doesn't mean it will stay that way. "That thing is not going to stay up there forever," says Jah. He thinks Kwangmyongsong-4's orbit was supposed to be sun synchronous---meaning it would stay constantly on the Earth’s day side---but the satellite is too low to stay sunny. "This thing is probably a failed attempt to get higher up, and is going to re-enter soon." If he's right, and NADA lets the probe's orbit decay, it could drift into another satellite's right of way. What's a neurotic satellite-head to do? Refresh your TLE data often, cross your fingers, and, if you can, try to have some faith in the North Koreans' ability to do orbital calculations.
