High-Speed Photos of Low-Speed Impacts
What it's all about
Craters like the ones you can see on the Moon or the famous Arizona crater were made by asteroids crashing at high speeds of thousands of kilometers per second into the soil. In contrast, consider a little steel ball dropping down from about a meter into sand and creating a little crater there. The latter is called a low-speed impact and that's what we're about here. The main difference between is that for high-speed impacts the pressure is so immense that sand and stone essentially behave like liquids during the crash (and the whole action can be imagined like an explosion) while for low-speed impacts, none of the involved paticles get plastically deformed.
The images presented here resulted from a 10-day S3 (SSS, Summer School of Science) course led by me in 2005. The photo series was actually taken by the participants Quinn Burke and Slaven Glumac who operated all the devices.
There is a separate page for technical details describing more in-depth how these photos were made.
3cm steel ball dropping into sand
A steel ball (density about 7.5g/cm3) with a diameter of 3cm was dropped from a height of 88cm into sand which served as "granular media". The crater forms within less than 200ms so that the human eye cannot see the details of crater formation. However, with an exposure of better than 1/10,000 second (<100us) one can freeze the moment.
Note that the timings displayed below suffer some error because the image series is made with one impact per frame and reqires re-leveling of the sand each time.
Interested? But that's just the first 35ms, so let's look at the whole crater formation now. We just need to zoom a bit back for that:
Especially note the shock wave propagation around 50ms. After 125ms, there are still some sand particles in the air but the formationis essentially complete.