The amount of junk in Earth’s immediate vicinity is increasing every day. According to reports by NASA and the European Space Agency, there are currently more than 17,000 identifiable pieces of artificial debris; and more than 170 million undetectable debris in the path of our satellites. That’s a lot. This collection of debris is ricocheting in orbit at speeds exceeding 17000 mph. Just for reference, the average bullet travels at a pace that is ten times less than the speed of a piece of junk in orbit. In this post, we will try to understand how the ISS protects itself from space debris and junk.
Space debris collectively refers to micrometeoroids, paint flakes, satellite parts, and dead satellite coolants. Artificial debris is usually in orbit around Earth, along with satellites. Natural debris like micrometeoroids orbits the Sun.
So if a piece of this debris was to strike a satellite, it could seriously jeopardize the satellite. If it hits the pressurized container of the International Space Station, it could endanger the lives of the astronauts on board, along with damaging expensive equipment. If it strikes an astronaut during extra-vehicular activity, it could puncture their suit and prove to be fatal.
Chris Hadfield’s tweet is an excellent example that we can refer to, to visually ascertain the kind of damage a small piece of debris could inflict. We’ll explain the image later in this post. Here are a few more pictures that can testify to the need to have better protection for all our stuff in space.
How does the ISS avoid a collision with space debris?
Obviously, space agencies across the world have devised plans to prevent any form of physical interaction with a foreign object in space. The current strategy is three-fold.
Prevention is better than cure. Hence, the first step towards protection from space debris is to avoid it. Completely. An imaginary box is drawn with the spacecraft at the center of it. The box, known as a pizza box because of its shape, is 1.5km deep, 50km wide, and 50km long. If a trackable piece of space junk enters this region, the ISS performs a Debris Avoidance Maneuver (DAM). In this course of action, the International Space Station is slightly shifted using onboard thrusters and removed from the path of the incoming debris.
A coalition of ground support stations continuously tracks this identifiable space debris. The Department of Defence’s Joint Space Operations Center has a Space Surveillance Network that tracks debris quite accurately. Mission control in Houston and Russia handles the maneuvers to avoid impact in the case of a threat.
Here’s a video that will give you an idea of the activity that goes on during a DAM
What happens in the case of an emergency?
In the event of an emergency when a collision with the debris is imminent, the crew’s safety precedes everything else. The team is instructed to evacuate to the re-entry capsule of the docked Soyuz spacecraft. The ground control plans their trip back to Earth.
If, somehow, an untracked piece of debris makes its way towards the ISS, the International Space Station is capable of offering a physical layer of protection. The ISS is mostly made up of aluminum. Additionally, it has debris protection blankets made up of Kevlar and Nextel fibers. To address Chris Hadfield’s tweet, the ISS also uses a special kind of metal armor known as a Whipple shield. The shield shown in Chris’ tweet is a monolithic shield and is used for demonstrating the effect of a projectile had that kind of shield been used. Let’s take a look at the image again:
The half oblong hollow part of the bottom of the monolithic shield is the result of spallation. Spallation is what happens when there is no other path for energy transfer to take place. So it shatters the side opposite to the point of impact.
A Whipple shield (pictured above) is a multi-layer shield explicitly designed for this purpose. It has several layers with gaps between them. The task of a Whipple shield is to disperse any debris that might hit the shield and to provide a path for the energy from the collision to disperse. In essence, it protects the spacecraft from bearing the direct force of the impact.
In the event of an impact with debris, pressurized units of the ISS can start leaking. If the depressurization is not rapid, astronauts are supposed to get a reading on the current pressure, contact mission control, and prep the Soyuz capsule for re-entry. If the crew has sufficient time to isolate the leak, they can use the patch kits allocated to them to fix it. Or they can close off the module. Repair work is then usually performed by the next mission crew.
Here’s a video recounting the events that unfolded during one of the worst collisions in space. Collision at 04:22.