http://www.theglobeandmail.com/news/technology/science/rock-out-way-out/article1467307/
Although an asteroid mission might take humans 20 times farther from Earth than the moon, it would cost less than a lunar mission, which requires a separate, fuelled-up lander that can escape the moon's gravity when it's time to depart. Small asteroids have almost no gravity to speak of so a close encounter would be not so much a landing as a docking.
Astronauts would be tethered to their mother ship and might travel hand over hand across the asteroid's surface like nimble rock climbers scaling a cliff. There would be no blast-off at the end of the visit - just a cast-off and an about-face home.
However, Mr. Landis is quick to point out that an asteroid mission would not be easy. It would require far longer travel times than a moon mission, exposing astronauts to much higher doses of solar and cosmic radiation. It would also be far enough away to prohibit easy communication with Earth. The two-way signal delay between spacecraft and mission control could stretch to nearly a minute, leaving astronauts effectively on their own when it comes to manoeuvring their craft and responding to emergencies.
