The space junk hazard - Skywatching

Back in the late 1950s and early '60s, when our first artificial satellites were put into orbit, we didn't pay much attention to what went with them. 

The last stage of the launcher, various adapter rings, clamps, fairings and other bits were just allowed to float away. Actually in this case, “float” really meant moving at around 30,000 km/h. Then, eventually the satellite itself went dead, and became another contribution to what we now call “space junk.” 

Fortunately, objects in very low orbits experience a small amount of atmospheric drag. Their orbits shrink until they burn up in the atmosphere. However, space junk in high orbits will remain there for centuries or maybe even indefinitely. 

In those early days, nobody really thought of near-Earth space being choked with satellites, so space junk was not a problem. Now it is, especially with the launching of thousands of new satellites to provide global 5G Internet access and other communication services. 

In addition, we have orbiting observatories, and are looking forward to observing with the James Webb Space Telescope without worrying that something will hit it. There are tens of thousands of bits of space junk up there, ranging from dead satellites to nuts, bolts and flecks of paint, and there is a growing realization we really need to do something before it is too late; but what? Hypersonic collisions are not like low-speed ones. 

So what does happen when a piece of space junk hits a spacecraft?

Imagine a baseball approaching a bat. When it hits, the part of the ball in contact with the bat starts to move with the bat. However, the other side of the ball is still moving at the speed it was thrown. The ball starts to flatten against the bat and its kinetic energy is stored in the distortion of the ball. Then, when whole ball is moving with the speed of the bat, the distortion relaxes; the ball becomes spherical again and bounces off the bat. The result is the ball moves off with the speed of the bat plus a good fraction of speed with which it was thrown. 

The whole process takes place smoothly because the forces shaping the ball are transmitted through its fabric at the speed of sound, while the ball is moving much more slowly. Hypersonic impacts are very different. 

Here is the story of a typical impact experiment aimed at helping us better understand high speed impacts. An aluminum sphere 1 cm in diameter, fired from a special gun, moving at around 30,000 km/h, a typical speed for an object in orbit, hits a slab of aluminum around 10 cm thick. The speed of sound in aluminum is around 22,500 km/h, so the sphere is moving faster than sound and the material in the sphere never has a chance to distort, accommodating the impact. 

The result is that in about a few millionths of a second the kinetic energy is converted to heat, heating the sphere and part of the material it hit to tens of thousands of degrees. Solid aluminum becomes highly compressed aluminum vapour. As the force of the impact relaxes, the compression ceases, and that hot, dense ball of aluminum vapour explodes. The impact and explosion send shock waves through the slab, blowing off a layer of metal from the other side. The explosion blows a hole in the slab several centimetres wide and several centimetres deep. 

Since we cannot afford to give spacecraft walls several or more centimetres thick, we cannot live with an increasing volume of space junk; we have to come up with another way to deal with it.

Since there is no sort of catcher's mitt that will stop something moving at 30,000 km/h, the only way we know at the moment to deal with space junk is to rendezvous with each bit. We have then to match speeds with it, catch it, and then move on the next. This process is tedious and will require a lot of fuel. Innovative new solutions are needed.

Venus shines brightly in the southwest after sunset and Mars rises in the early hours, Jupiter a bit later, with Saturn very low in the dawn glow. 

The moon will be full on the 9th.