Avoiding large asteroids is easy, but spacecraft still need some shielding
If an asteroid hitting the Earth can cause extinction, or at least break a lot of windows, it doesn’t take a lot of imagination to picture what would happen to a spaceship colliding with one. And yet, even with a whole swarm of asteroids lurking just beyond Mars, NASA mission planners apparently don’t sweat sending probes out past Jupiter and Saturn. Does this mean Voyager I had more fancy maneuvers than the Millenium Falcon, or is there another dimension to the issue of avoiding being damaged by floating rocks in space?
Since our local asteroid belt is presumably around 4.6 billion years old, those rocks have had plenty of time to sort themselves out. Sizeable asteroids would be caught in each other’s gravity, spiraling in until they collided. After that, they’d either be tiny pieces or slowly clump together, like a ball of space gravel. So what’s left is widely, if unevenly, dispersed in their orbit around the sun. Calculations not made by a protocol droid found that there are maybe a billion asteroids larger than 300 feet across. To not have smashed into each other by now, they’re like spread out enough that you’d find just one in a 33 quadrillion cubic mile patch of space. A lot more space than your average spacecraft, or even 120,000 Earths, need to pass by.
Protection from pellets
This isn’t to say that an asteroid needs to be at least 300 feet across to be of concern. Closer to Earth, there’s actually a fair amount of concern about things hitting our functional satellites and the International Space Station (ISS), largely because it’s actually a much smaller amount of space to have to share. Aside from more “natural” objects, there’s decades of man-made debris from our various orbital activities. Many of the medium- to larger-sized objects are constantly tracked as they orbit the planet so that we can try to steer the ISS out of their path. For the the tinier pieces, spacecraft rely on Whipple shielding.
Whipple shielding is a relatively thin layer of Kevlar or Nextel, surrounding a spacecraft. It doesn’t fit snugly though, as a gap between the “bumper” and the hull is critical to its design. Anything in Earth’s orbit is likely moving close to 15,600 miles per hour, which can pack a lot of energy even into a tiny, .3 inch fragment of aluminum. When a pellet that size hits a Whipple shield at full speed, the outer layer of Kevlar can easily have a hole punched through it. However, that impact will also liquefy and disperse the projectile, so that what remains hits the hull of the spacecraft with much less force in any single location, reducing the chance of a catastrophic breach. It’s more airbag than deflector shield, but for now it does the trick.
Source: You Could Actually Snooze Your Way Through an Asteroid Belt by Kenneth Chang, The New York Times