Lunar gravity assists for asteroids
Some asteroid enthusiasts humorously see the Moon mainly as an object to offer gravity assists, not to mine the Moon.
A "gravity assist" entails using a fly-by with the Moon to divert the trajectory of a payload and to impart delta-v, saving large amounts of fuel. Almost all NASA probes to other planets have depended on gravity assists, e.g., passing by the Moon and the Earth one or more times on their way out, and sometimes other planets as well for the purpose of gravity assists. For example, Voyager more than doubled its speed when it passed Jupiter.
One or more lunar gravity assists, sometimes in concert with an Earth gravity assist, will be used to:
A single lunar gravity assist is illustrated conceptually below.
The maximum braking the Moon can provide is about 2.2 km/sec, using a "double lunar gravity assist", whereby the asteroid passes by the Moon coming in, then past the Earth, then past the Moon again going back out. This would divert the asteroid by almost 90 degrees from its original path, and capture it into a highly elliptical Earth orbit. Subsequent gravity assists would insert it into a more circular orbit around Earth after which it would perform final small thrusting maneuvers to achieve its desired destination orbit.
Many asteroids require a delta-v of much less than 2.2 km/sec, and require only a single lunar gravity assist (not an Earth gravity assist) to be captured, and optionally additional lunar gravity assists to divert the asteroid into a more circular orbit.
Gravity assists improve the economics of retrieving asteroid payloads, as well as outbound missions, and greatly broadens the number of attractive asteroids.
(In this game of "orbital billiards", we are tapping a gravitational energy source as asteroid payloads exchange orbital momentum with the Moon and the Earth -- the asteroid slows down while the Moon speeds up. Because asteroids are so small compared to the Earth and Moon, the effects on the Moon and Earth are so small as to be immeasurable. It would take millions of captured asteroids to cause any detectable changes in the Moon's or Earth's orbits. It's like measuring the effects of mosquitoes hitting the Empire State Building -- significant to the mosquito, but not to the building.)
We probably would not want to bring a complete asteroid in, but instead a series of small cargo containers which are more easily maneuvered and pose no significant threat to Earth. Trajectories are something we know very precisely, well in advance, and there's no need to get too close to Earth. The abovementioned 2.265 km/sec gravity assist maneuver was based on approaching no closer than several thousand miles (kilometers) of Earth's surface in order to allay such concerns. (Some people have proposed using the Earth's atmosphere for "aerobraking", but that's not at all what we are talking about here. We won't ever need to alert any emergency rendezvous team for pure gravity assist maneuvers.) One would expect that a quick response rendezvous team would be set up to protect Earth in the long run against both man-made objects and naturally occurring asteroids and big rocks that pass by Earth. Already, military and civilian telescopes have detected big rocks and sizeable asteroids passing very close to Earth, including skimming the upper atmosphere. If any of these naturally occurring objects had hit Earth, it would cause a natural disaster, possibly to the entire planet, not a man-made disaster. Man-made capabilities can prevent natural planetary damage.