What is the primary reason for the mass loss of asteroid 6478 Gault?

• A. Rotational Instability
• B. Solar energy-driven ice vaporization at perihelion
• C. Tidal disruption by a planet
• D. Gravitational shedding due to a binary companion

Answer: (A)

Mass loss on 6478 Gault is best explained by rotational instability from spin-up due to the YORP effect. Over time, sunlight torques can steadily increase an asteroid’s rotation rate. For a small, loosely bound rubble-pile like Gault, reaching a high spin rate means centrifugal forces at the equator can exceed gravity and the material’s weak cohesion. When that threshold is crossed, surface material can be shed and form dust tails that drift away under the influence of solar radiation pressure and gravity. This pattern—episodic dust emission without sustained gas activity—fits what’s observed for Gault.

Other scenarios don’t match the evidence as well. Sublimation of ice would require volatiles and would typically produce signs of gas-driven activity, which aren’t present for this main-belt object. Tidal disruption by a planet would need a close planetary encounter and a distinctive fragmentation signature that isn’t seen. Gravitational shedding from a binary companion would show different dynamical behavior and signatures not observed here. The rapid rotation near the spin limit provides the clean, consistent explanation for the observed mass loss.