New Scientist - 30 June 2010
Early-Stage Ejecta Velocity Distribution for Vertical Hypervelocity Impacts into SandVideo: Crater formation
They move too fast for human eyes to see, but a camera has tracked individual sand particles spraying from an impact site in the first moments of crater formation.
Such movies could help us piece together the objects that created the craters which pock the surfaces of the moon and other celestial bodies. They could also help to predict the effects of impacts – to determine the risk flying debris poses to astronauts, for example.
Brendan Hermalyn and Peter Schultz of Brown University in Providence, Rhode Island, used a gun at NASA's Ames Vertical Gun Range to fire projectiles into sand at speeds of up to 5.6 kilometres per second.
The resulting craters form in 100 milliseconds or less. By using high-speed video cameras that can capture up to 15,000 frames per second, the team could track individual sand particles as they were ejected from the impact.
Most of the material thrown up by the impact took to the air during the impact's "main stage", landing within a few crater radii of where the projectile hit. But the high-speed video also revealed particles kicked up in a relatively unstudied early stage.
During this phase of crater formation material is flung fastest and farthest, for example creating the long rays of the moon's Tycho crater and, potentially, launching a number of meteoroids into space from the surface of the moon and Mars.
The new experiments show the material released during this early stage is ejected faster and at a lower angle than the main stage.
- Icarus (in press), doi: 10.1016/j.icarus.2010.05.025