Small, untrackable space debris is, according to NASA, the number one threat to human spaceflight, and according to the USAF, it’s also a major threat to national security — and these days, it’s not just a threat to nations with space assets. Every nation uses space in some way; even if they don’t have satellites of their own, every nation relies on commercial and government-operated satellites from other nations. Weather forecasting, telecommunications, and navigation have benefitted so massively from satellites that we now depend on them.
So how much damage can be done? Well, the infamous collision between Iridium-33 and the defunct Cosmos-2251 is the most obvious example, as the first known hypervelocity unintentional collision between two artificial satellites. It destroyed Iridium-33, of course, and littered the shared orbital space with debris. At least twice, the ISS has had to respond to near passes of some of the shrapnel, much of which remains in orbit today, and continues to threaten spacecraft in low Earth orbit. The orbits are unstable, and so the debris is continually sinking, and eventually it will no longer be a problem. But a great deal more debris remains, and many old satellites were not built to safe themselves after a mission; their tanks may still hold propellant, and their batteries may still be able to charge off of the solar panels. Without heaters to maintain propellant, and without a computer to govern voltage in the batteries, this turns such dead satellites into time bombs.
But the worst fear for agencies like NASA is that much of the debris is too small to be tracked. Pieces a few inches or larger can be tracked, which allows the ISS crew to either relocate the station or seek shelter in their Soyuz lifeboats until the danger has passed. Smaller pieces are still deadly, but can strike with no warning because their positions are unknown.
Europe’s Sentinel 1A radar imaging satellite recently drove that point home, when ground controls noticed the power output of one of its solar arrays abruptly drop. It didn’t drop much, not enough to affect operations in any way, but it was concerning. Soon, they noticed something else: the spacecraft’s orbit had changed ever so slightly. Something had imparted kinetic energy to it. Unusually for satellites, Sentinel 1B is equipped with cameras that can see its solar arrays; they were installed for engineering purposes, to monitor deployment of the solar arrays. After the unexplained drop in power output, engineers turned the cameras back on and saw a divot about 40 cm across. Something struck the spacecraft. By the nature of the damage, controllers estimate the impactor was less than 5 mm in size. Perhaps a paint fleck; there are a lot of those up there. It could also have been a natural object; many meteors are that size or smaller. But it drives home the seriousness of the threat:
This obviously isn’t the first time an impact crater has been found on a spacecraft. Most spacecraft are never seen again after launch, with one notable exception: crewed spacecraft. The Space Shuttles bear many scars from micrometeoroids. This one was found on a window of the Space Shuttle Challenger after STS-7:
This much larger hole was found on Endeavour after STS-118; this hole is much bigger, having punched straight through Endeavour’s radiator panel, and one wonders how much more harm it would have done had it struck a window:
Atlantis also suffered an impact like this; luckily, in both cases the impactor missed the critical Freon loops inside the panel; such an impact would trigger an immediate mission abort as the Shuttle would be forced to switch to its flash evaporation system to keep the computers cool and alive, and that has only a limited supply of coolant as it’s only meant to keep things going during launch and entry, when the payload bay doors are closed and the radiators stowed.
ISS has had MMOD strikes on itself too, of course. Here’s one in the Cupola, with a pen for size comparison:
And here’s one on the Hubble Space Telescope, on its main antenna dish:
This problem is very, very real, and right now, we don’t have many solutions. Shuttle mitigated the danger a little by, whenever possible, orienting the Orbiter so that its massive main engines were facing into the direction of travel. While those engines are absolutely critical, at that point in the mission they have done their job and are just dead weight; if they have to get hit, it would be better to be hit there. But when docked to ISS, Shuttle did not have this option; as they were mainly using the forward PMA port, when docked Shuttle was forced to be traveling with the delicate and critical heat shield facing forwards. This is why, after Columbia, NASA mandated a heat shield inspection — immediately after launch, to find damage due to the foam shedding from the ET, and again right before undocking in order to find MMOD damage. But few spacecraft have this luxury. They must point wherever they must point in order to carry out their missions. And so, the threat will persist.