The Vertical Impact Facility, commonly known as the "Vertical Gun," is distinctive in that its target chamber can be heated or refrigerated. As far as we know, it's the only gun in the world with such a chamber, and it's this ability that makes feasible the study of impact into ice. We can install barrels on this gun with bores (the diameter of the hole that the projectile travels through) ranging from 5.56 to 20 mm in diameter. (These numbers are no coincidence. Because we're part of the US government, much of our equipment can be called "spinoff" from the US military. Most of the rifles used by the Army, Marines, and other branches of the armed services use 5.56-mm ammunition. The standard gun on most US fighter aircraft is a 20-mm cannon.) We alter the velocity of the projectile by adjusting the amount and type of the gunpowder used; this gun can be used reliably to launch projectiles at velocities from 0.7 to 3 km s-1. For comparison, the speed of a bullet fired from a typical rifle is less than 1 km s-1. The Vertical Gun can be fired much more frequently than the light-gas gun, and this is why. It's simple! The long barrel (about 5 cm in diameter) is attached to the top of the velocity chamber near the floor, while the small, electrically activated breech is screwed onto the other end of the barrel. The entire assembly is mounted rigidly to the blue I-beam which, in turn, is welded to the I-beams supporting the floor. A disadvantage inherent to this gun is the short distance between the muzzle and the target. Both the Flat Plate Accelerator and the Light-Gas Gun have a chamber between the barrel and the impact chamber that allows gases from the burning gunpowder to expand and provides a harmless place for residue to fall. Firing one of these guns is a pretty messy event, to be honest about it. There's unburned powder and the remains of the burned powder coming out of the barrel, and the sabot splatters itself all over the interior of the chamber, since it's stopped at very high velocity by a husky piece of steel. When the condensed gases from the burned powder are included, the result inside the chamber is something that isn't pretty to see. In order to circumvent this problem in the Vertical Gun, we often place a target chamber inside the impact chamber. This secondary chamber has a small opening in its top through which the projectile should travel on its way to the target, which is usually located on or near the chamber's floor. Since the opening in the top is so small, most of the gases from the gunpowder will flow around the outside of the inner chamber, and interaction of these gases with the cratering events will thus be minimized. Most of the action during a Vertical-Gun experiment happens in here, the impact chamber. The bucket for the sand target is in the middle of the chamber, just below the plumb bob (the thing that looks like a bomb, just suspended there by a thin cord) used to center the target. The laser, cylindrical lens, and mirror are protected from ejecta and gun debris by the lexan box to the right of the target. There's a hole in the top of the chamber (just visible behind the hemispherical glass light housing) through which the projectile passes; just below that hole, between it and the target bucket, is the simple but effective blast deflector that Gerry rigged up. The target bucket is 27 cm across. This is an example of a crater formed in resin-coated sand. The original target was circular, but we "baked" the entire bucket of sand after the experiment. This caused the resin to melt and harden, so we could cut the crater in half. This allows us to study what happens below the surface of the target when an impact crater is formed. This gun is particularly well-suited to the study of impacts in which the target consists of sand or other non-coherent materials; with the gun pointing straight down, it's a simple matter to fill a container and put it in the chamber. (It will be left as an exercise for the reader to determine what should happen if we tried to shoot something like sand with the Light-Gas Gun or the Flat-Plate Accelerator.) In a recent series of experiments for two of our colleagues at Brown University, we used targets consisting of layers of colored sand which have been mixed with small amounts of epoxy. After the impact, we put the whole target into an oven for a few hours, and the epoxy mixed with the sand melts, converting the entire target to a solid block. That allows us to cut it in half so we can actually look at a cross-section of the crater. We can then study the movement of the layers caused by the impact, measure the amount of material excavated from the crater, and inspect other features of the crater that would otherwise be invisible below the surface of the target.