How frequently do near Earth asteroids (NEAs) closely approach Earth? Usually, NEAs only show up in the news media if they are very close and very large, or they impact a populated area. There are, however, many more that make close approaches, but safely pass by the Earth. Some of these may be candidates for possible mining.
If we want to select asteroids for possible mining, then the ideal candidates are those that come close to Earth slowly, but don’t hit, and are sizable to allow for extensive mining. How frequently do candidate asteroids show up?
JPL maintains a table of NEO close approaches (NEO = near Earth object). The table describes close approaches for the next 60 days. I abstracted from the table today (August 23, 2013) to see how frequently these occur.
Here are some sizable asteroids that have velocity < 10.0 km/s relative to Earth, and < 20 LD (lunar distances) at close approach.
|2007 CN26||Aug 28, 2013||11.9||6.85||170 – 380|
|2008 PW4||Aug 30, 2013||14.3||6.77||90 – 200|
|2010 CD55||Aug 31, 2013||17.9||6.13||64 – 140|
|2010 CF19||Sep 9, 2013||14.9||8.37||130 – 290|
|2002 NV16||Sep 30, 2013||13.9||4.27||140 – 310|
As a point of comparison, the velocity needed to reach low Earth orbit is about 7.9 km/s. Escape velocity is 11.2 km/s at the surface of the Earth. What this means is that a probe from Earth to one of these NEAs would spend the majority of its velocity change (delta V) just leaving the Earth.
The uncertainty of diameter is substantial, which means we don’t have a good handle on mass. Rendezvous/loiter near these objects would help determine mass and shed light on their make-up.
The relatively low close approach speeds means that these orbits are not highly elliptical. They are likely to be generally in the Venus-Earth-Mars system. Faster speeds would indicate more energetic orbits which might take them to the outer planets (or at least to the main asteroid belt).
There are a lot of asteroids which are farther or passing by much faster. Those would take more energy to reach than these, and thus I did not include them. There are also many that are smaller, and thus not much mining to be done. So, given the table for the next 60 days, these are the ones that might make prime candidates for rendezvous missions. Extrapolating to over a year’s time, there might be 50-60 asteroids each year that might be worth visiting as mining candidates.