Near-Earth objects (NEOs) are solar system asteroids and comets which come closer than 1.3 astronomical units (AUs) from the Sun. Anything which comes closer than 1.0 AU from the Sun and is along the plane of the solar system will cross Earth's orbit. Thus, it is important to calculate the orbits of NEOs to know if they present any imminent or future collision risk to future space missions, or even to Earth itself.
Potentially hazardous asteroids
NEOs which are at least 325 feet across and have a minimum orbit intersection distance of less than 0.05 AUs from Earth are reclassified as potentially hazardous asteroids (PHAs). This size of space object is large enough to cause serious regional devastation on Earth. Historically, an impact of this size can be expected roughly once every 10,000 years.
It was only recently in human history that objects of this size could be tracked in advance at all. Modern optical and infrared telescopes can make out most objects of this size, but they have to be looking in the right direction. A telescope can see only a very limited section of sky at a time.
NASA has a standing congressional mandate to find, chart, and catalog all NEOs which have a diameter of at least 0.6 miles. The search will continue as long as there is a reasonable likelihood of undiscovered NEOs. As of November 2012, there are 1,348 known PHAs.
Even more recently, technology has advanced to the point that early intervention could avert a disastrous collision by a PHA by reducing its size or changing its orbit. Advancements in the same technology could also potentially be used to mine a NEO.
Many asteroids are extremely high in iron and nickel, and may also contain other metals which are constantly in demand on Earth. Seeing an opportunity, Planetary Resources Inc. has developed a long-term strategic plan to mine suitable NEOs for their metals. The plan includes development and deployment of any necessary technologies over the next couple of decades.
The plan begins with an optical survey and detailed orbit calculation of all NEO asteroids, using purpose-built space telescopes. Survey robots will be sent to likely NEOs for sampling and deep scanning. Suitable NEOs would then be brought to lunar orbit and mined robotically.
If successful, the same technology can also protect the Earth from future collisions with PHAs. Through robotic mining, an NHA which is on a collision course with Earth could be reduced to small rubble which will burn up harmlessly in the atmosphere, and any useful materials could be extracted at the same time. If it works, it would be a win-win situation for Earth.