A drill or scoop pushes down into regolith to excavate it; by Newton's third law, the regolith pushes back on the spacecraft with an equal and opposite reaction force. On Earth that force is negligible next to the craft's weight. On a small asteroid, surface gravity is thousands of times weaker, so the same reaction force can exceed the local weight and fling the whole spacecraft off the surface.
Surface gravity: g = G·M / R², M = (4/3)π R³ ρ
Escape velocity: v_esc = √(2GM / R)
Net radial force: F_net = F_drill − m·g − T_anchor
With a 300 m, 2200 kg/m³ asteroid, g is only a few millimetres per second squared — a fraction of a percent of Earth gravity — and escape velocity is a slow walking pace. A modest 18 N drilling reaction is already far larger than the craft's weight (a few newtons), so without a harpoon anchor supplying counter-tension, the craft accelerates upward and can leave the surface entirely.
- Radius / density — set the asteroid's mass, which sets g and vesc through the formulas above.
- Drill reaction force — the continuous reaction thrust felt by the craft while drilling.
- Harpoon anchor — when engaged, a tether supplies tension once it goes taut, capping the craft's drift; when released, nothing but gravity opposes the drill.
- Ejected regolith — ejecta fast enough to exceed vesc never falls back, exactly as mission designers must account for on real bodies like Bennu or Ryugu.
This is a real engineering constraint on missions such as OSIRIS-REx and Hayabusa2, which used brief contact-and-thrust maneuvers rather than sustained drilling precisely to avoid pushing the spacecraft away from a body with almost no gravity to hold it down.