Complex sentence: Maya speculates that He must be countering the gravitational force of the Moon--which is much closer and larger in relation to this planet than it was to Earth--otherwise, the planet would break apart under the strain.
Split into simpler sentences: Maya speculates that He must be countering the gravitational force of the Moon, which is much closer and larger in relation to this planet than it was to Earth. Otherwise, the planet would break apart under the strain.