Space debris, also called space junk or orbital debris, can range in size from tiny chips of paint to pieces of broken rockets to entire defunct satellites. After decades of human spaceflight, lots of space debris has accumulated in orbit around Earth. Space debris poses a big hazard to spacecraft and astronauts because much of it is traveling at thousands of miles per hour. Even tiny particles can cause a catastrophic collision that can damage or destroy a spacecraft. Scientists can usually track larger objects in Earth's orbit and, when necessary, maneuver other spacecraft to avoid collisions. However, much space debris is too small to track, and there is a lot of it. Scientists estimate that there are over one hundred million pieces of space debris less than one centimeter wide! Micrometeoroids, small chunks of rock with a mass of less than one gram, pose a similar threat.
So, how do you protect a spacecraft from collisions with debris that is too small to see or track? Engineers design shielding to protect spacecraft from impacts. While they cannot protect spacecraft from collisions with large debris, the shields can help absorb and break up the impact from smaller debris. The ISS has over 100 different shields! There is a problem designing shields for spacecraft, however. It takes a lot of energy to launch things into space. Therefore, launching heavy materials into space requires a lot of fuel and is very expensive. That means that you cannot just cover an entire spacecraft in thick, heavy metal plating or armor. Engineers have to design shields that are effective at stopping debris but are also lightweight.
One such type of shield is called a Whipple shield, named after astronomer Fred Whipple. A Whipple shield consists of one or more "bumper" layers spaced apart from the spacecraft's outer walls (Figure 1). The goal of a Whipple shield is not to completely stop debris. Instead, when a piece of space debris hits a layer of the Whipple shield at an extremely high speed, it shatters and breaks into smaller pieces that spread out (Figure 2). This spreads the impact over a larger area and makes it less likely that the debris will puncture a hole in the spacecraft's outer wall. Since space debris is so small and space is so big, the odds that another piece of debris will hit the exact same spot on the bumper layer are very low. However, astronauts may eventually replace the layers of shielding.
