In a Hollywood movie, a few brave astronauts can fly to an object bearing down on Earth to set off bombs and explode the menace before it hits this planet. In reality, however, huge asteroids like the ones that passed close to the planet in spring 2013 pose a potential hazard without a single solution, as well as an opportunity to study the elements that make up the planetary system.
The National Aeronautics and Space Administration oversees an ongoing project called The Near Earth Object Program, established in 1998. Near-Earth Objects, or NEOs for short, are bodies whose orbits around the Sun have been altered by the gravitational influence of other planets. Instead of being contained in the Main Asteroid Belt between Jupiter and Mars, these bodies are astronomical vagabonds, roaming the solar system.
In terms of space bodies, NEOs are classified as those bodies whose perihelion (closest point to the sun) is a distance of less than 1.3 AU, or astronomical unit. An astronomical unit is defined as the distance from the Earth to the Sun, or about 93 million miles (149,597,870.700 kilometers). In other words, if a comet or an asteroid comes within 124 million miles of the Sun, it's classified as a Near-Earth Object. Asteroids are further classified according to their perihelion, aphelion (farthest distance from the Sun) and their semi-major axes (how they rotate).
Based on these measurements, there are four categories of near-Earth asteroids, or NEAs. Atiras (named after asteroid 163693 Atira) are NEAs with orbits contained entirely with Earth's orbit. Atens (named after asteroid 2062 Aten) are NEAs with paths that cross Earth's orbit and rotations smaller than Earth's. Apollos (named after asteroid 1862 Apollo) also have paths that cross Earth's, and also have axes larger than Earth's. Amors (named after asteroid 1221 Amor) are bodies that approach Earth outside of its orbit but within the orbit of Mars.
Currently, the NEO program has identified 1,397 potentially hazardous asteroids, or PHAs. Whether an asteroid comes close enough to the Earth to cause any hazard often depends on the shape of its orbit. For example, asteroid 2013 LR6 flew past Earth at a distance of about 65,0000 miles at 12:42 EDT June 8. This asteroid actually flew between the Earth and its moon, but its trajectory was such that it posed no hazard. According to a NASA report, the "space rock" was about 30 feet in diameter and passed over the Southern Ocean south of Tasmania. NASA's Catalina Sky Survey discovered Asteroid 2013 LR6 on June 6.
Barely a week earlier, the 1998 QE2 asteroid passed by Earth at a distance of about 3.6 million miles (5.8 million kilometers) at 4:59 p.m. EDT, Friday, May 31. This pass was the 1.7-mile-wide asteroid's closest approach for probably the next 200 years, according to NASA's Asteroid Watch website. Using radar, scientists discovered that 1998 QE2 had acquired its own moon, a smaller piece of rock about 2,000 feet (600 meters) wide flying along with the larger body.
So what happens if an asteroid is found to be on a collision course with Earth? According to NASA's NEO Program, a robot spacecraft could be sent to nudge the trajectory enough to move the asteroid away from Earth – provided there's sufficient warning of "several years to a decade," says the Asteroid Watch website. In fact, this has already been done. A NASA spacecraft called Deep Impact (yes, called after the movie of the same name) intentionally struck Comet Tempel 1 on July 4, 2005, to study its elements.
A 2007 NASA report to the U.S. Congress, "Near-Earth Object Survey and Deflection Analysis of Alternatives" found that Hollywood's imagination wasn't that far off. Spacecraft impacts and nuclear explosions were found to be two of the more "relatively mature options" for deflecting space debris that might threaten Earth. A full copy of the report, including other options, is available online.
Aside from the potential collision hazard, skywatchers and astrophysicists alike are fascinated by NEOs because they're relatively unchanged debris from the solar system's formation some 4.6 billion years ago. In other words, today's asteroids are leftovers from the formation of the inner planets Mercury, Venus, Earth and Mars, just as comets are the remnants of the formation of the giant outer planets Jupiter, Saturn, Neptune and the dwarf planet Uranus. Together they give scientific insights into some of the chemical compositions that combined to form planets.