Hi vipulgoyal,

Verbs that end in "-ing" are sometimes referred to as "timeless" verbs; these verbs are used when referring to an activity that occurs now, occurred yesterday and will probably occur tomorrow. In this SC, there's a reference to an asteroid "orbiting" the sun - this makes sense because the asteroid was orbiting the sun yesterday, it's orbiting the sun now and will probably still be orbiting the sun tomorrow. The verb "orbits" implies present-tense only, which isn't correct in this context.

GMAT assassins aren't born, they're made,
Rich