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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 as mentioned in this article. Cassini, in orbit around Saturn, fires its rocket engine “only at specific times” routinely. Voyager-1 and Voyager-2 have been firing theirs “only at specific times” ever since launch in 1977.

Dave Doody
Altadena, Calif. The menacing asteroid described in the article seems to have an unusual characteristic. It’s more than 13 times as dense as water, making it heavier than lead and more than 60 percent heavier than solid iron.

Stephen Curry
Dallas, Texas

Another possible approach to deflecting an asteroid would be shooting projectiles to change the body’s momentum in a carefully planned way. The mass being flung could be mined from the asteroid itself.

John D. Hoshor
Fort Myers, Fla.

I would think one could easily attach thrusting craft at either of the asteroid’s poles, where a continual thrust would be in only one direction. This should be no more difficult than positioning a gravitational tractor at an unvarying distance and direction from the asteroid.

Dan Lipp
Fort Collins, Colo.

A much more powerful and simpler-to-control asteroid tractor would use electric charge. Very small amounts of charge would generate more force than gravitation, allowing the spacecraft to be smaller. If the craft had an ion drive, the charged particles could be a by-product of the drive system.

Dave Blau
Cupertino, Calif.

The proposed space tractor could carry along a few thousand gallons of water, which would be poured onto the asteroid in such a way as to freeze on its surface. Once the solid is in place, there would be a location for a pushing or towing device.

Tom Harves
Greenbank, Wash.

NASA’s Stan Love replies that his team assumed an asteroid density of 2,000 kilograms per cubic meter, “appropriate for silicate rocks with internal porosity and void spaces caused by impact fracturing.” —R. Cowen