The rear Masuka drive had failed and the drive techs didn't know when it'd be up again. Bente ordered the mass driver to full power, which wasn't much more than it normally gave.
The rock was going to miss Earth orbit if she didn't act quickly and correctly.
She yawed the rock, which moved too slowly, and accelerated toward the Earth. This gave her some breathing space. She used the computer to project a braking maneuver using the Earth's gravity.