Letters from the January 21, 2006, issue of Science News

Push, pull, zap, drench

I’m surprised that NASA envisions an absurdly massive, nuclear-powered “gravitational tug” to avoid “the biggest problem” of a contact-tug’s need to “fir[e] its rocket engine only at specific times” to compensate for an asteroid’s rotation (“Protecting Earth: Gravitational tractor could lure asteroids off course,” SN: 11/12/05, p.