a strong affect the impact dynamics (see @royer11).

We use a small spherical projectile. The glass marble projectile we used for most experiments has a mass of 5.57 g, a diameter of 16.15 mm and a density of 2.5 g cm$^{-3}$. This density is within the scatter of different types of quartz and glasses. A small size in the lab is convenient as a $R_p \sim 1$ cm radius projectile at an impact speed of $v_{impact} = 3$ m/s has dimensionless Froude number $Fr \equiv v_{impact}/\sqrt{g R_p} \sim 10$ that is similar to that of a meter radius projectile at an impact speed similar to the escape velocity $\sim 20 $ cm/s on a small asteroid like Bennu with effective gravity $\sim 10^{-4}$ g. The Froude number is related to the dimensionless number, $\pi_2$, used to described impact crater scaling relations [@housen03]; $\pi_2 = 3.22 g R_p/v^2 = 3.22 Fr^{-2}$.

We desire a way to launch projectiles that minimizes initial spin, allows us to adjust impact velocity and impact angle and gives reproducible craters and outcomes (ricochet or not). After trying a rail gun, we settled on a pendulum launcher because it gave us low projectile spin and more repeatable 