Danger from the Skies: Space Junk!

Debris_objects_-_mostly_debris_-_in_low_Earth_orbit_LEO_-_view_over_the_equator-1-.jpg
70% of all catalogued objects are in low-Earth orbit (LEO), which extends to 2000 km (1200 miles) above the Earth's surface. To observe Earth, spacecraft must orbit at such a low altitude. The spatial density of objects increases at high latitudes.Note: The debris field shown in the image is an artist's impression based on actual data. However, the debris objects are shown at an exaggerated size to make them visible at the scale shown. European Space Agency

Earth doesn't orbit the Sun alone. Of course, there are the other planets, moons, asteroids, and comets. But, in near-Earth space, our planet is accompanied by a LOT of space junk left over from launched vehicles. Does it pose a danger? In some cases, it does.

In the movie Gravity, a group of astronauts finds out first-hand what it might be like for space explorers to run into an orbiting bit of space debris. The results are not good, although at least one astronaut makes it through safely. When it came out, the movie has generated a lot of discussion among space experts about its accuracy in some places, but the general story highlights a growing problem that we don't often think about here on Earth (and probably should): space junk returning home. 

What Goes Up often Comes Down

There's a cloud of space debris around Earth that planners have to deal with as they set schedules for rocket launches and low-Earth missions. Most of the material "out there" eventually comes back to Earth, such as the object WTF1190F. It was a piece of hardware likely dating back to the Apollo mission days. Its return to Earth on November 13, 2015, told scientists a lot about what happens as material plunges through our atmosphere (and "burns up" on the way down). Of course, spent satellites are often de-orbited, too, with similar results. The idea is that only little pieces make it back to the planet, and the bigger stuff is destroyed.

Knowing about space junk and where it is at any given moment is particularly important for people in the space launch business. This is because there are nearly 20,000 pieces of space debris up there. Most of it ranges from such small objects as gloves and cameras to pieces of rockets and artificial satellites. There's enough "stuff" up there to pose a real danger to observatories such as the Hubble Space Telescopeweather and communications satellites and the International Space Station. It also poses some risk to those of us on Earth. The good news is, the chances of something hitting us on land are fairly small. It's far more likely that a piece of space debris will fall into the oceans, or at least into an unhabitated part of a continent. 

To keep launch vehicles and orbiting satellites from running into these bits of space junk, organizations such as the North American Aerospace Defense Command (NORAD) observes and maintains a list of known objects orbiting Earth. Before every launch (and as satellites orbit the globe), the positions of all known debris must be known so that the launches and orbits can proceed without risk. The other good news is that most space debris burns up before it hits the planet.

The Atmosphere Can be a Drag (and That's Good!)

Pieces of junk in orbit can and do get caught up in our planet's atmosphere, just as meteoroids do. That slows them down, in a process called "atmospheric drag". If we're lucky, and a piece of orbital debris is small enough, it will likely vaporize as it falls to Earth under the tug of our planet's gravity. (This is exactly what happens to meteoroids when they encounter our atmosphere and the resulting flare of light we see as they vaporize is called a meteor. Earth regularly encounters streams of meteoroids, and when it does, we often see meteor showers.)  But, larger pieces of space junk can pose a threat to folks on Earth as well as get in the way or orbiting stations and satellites. 

Earth's atmosphere is not the same "size" all the time. At some times it stretches out much farther from the surface due to solar activity. So, scientists monitor the density of the atmosphere changes over time in the low-Earth orbit (LEO) zone. That's an area several hundred miles above the surface of our planet where most orbiting materials (including satellites and the International Space Station) exist. 

The Sun Plays a Role in Space Junk Re-entry

In addition to heating by the Sun (which helps "swell" our atmosphere), heat waves propagating from lower in the atmosphere can also have an effect. There are other events that affect our atmosphere and could have the effect of catapulting larger objects toward Earth's surface. Occasional solar storms cause the upper atmosphere to expand. These erratic solar storms (caused by coronal mass ejections)  can zip from the Sun toward Earth in less than two days, and they produce rapid changes in air density. 

Again, most space "junk" falling to Earth can and does vaporize on the way down. But, larger pieces can land and pose the potential for damage. Imagine being in the neighborhood if a large piece of a defunct satellite fell on your house! Or, imagine what would happen if a large solar storm resulted in enough atmospheric drag to pull a working satellite (or a space station) into a lower and more potentially dangerous orbit? It would not be good news for anybody in the path.

Predicting Re-Entry

The U.S. Air Force (which is involved with NORAD), and the U.S. National Center for Atmospheric Research (NCAR), the University of Colorado at Boulder, and the U.S. National Oceanic and Atmospheric Administration Space Weather Prediction Center work together to forecast space weather events and the effects they have on our atmosphere. Understanding those events will help us all in the long run by understanding the same effects on the orbits of space junk. Ultimately, the junk trackers will be able to forecast more accurate orbits and trajectories of space debris in near-Earth space. 

Fast Facts about Space Junk

  • Space junk is made up of objects left over from space flights, such as cameras, rocket pieces, and other small pieces of debris.
  • Occasionally space junk takes the form of a satellite directed to re-enter Earth's atmosphere. It's usually directed to impact Earth in the oceans or in uninhabited areas.
  • Agencies monitor thousands of pieces of space junk, charting the orbits of these objects. 
  • Much space junk vaporizes due to friction with Earth's atmosphere and never reaches the surface.