The Threat from Space
Earth orbits in a cosmic shooting gallery. Asteroids and comets constantly pass through the inner solar system, and on rare occasions, one strikes our planet. Most burn up in the atmosphere, but larger impacts can be catastrophic.
The most famous example is the impact 66 million years ago that created the Chicxulub crater in Mexico and contributed to the extinction of the dinosaurs. More recently, in 1908, an asteroid exploded above Siberia with the force of thousands of nuclear bombs, flattening forests across an area the size of a city.
The Planetary Defense Strategy
Planetary defense relies on three key steps:
- 1 Detection: Identify near-Earth asteroids and determine their trajectories. Are they on a collision course with Earth?
- 2 Characterization: Learn the asteroid's properties—size, composition, density, rotation, strength. This determines how it will respond to deflection attempts.
- 3 Deflection: If necessary, alter the asteroid's orbit slightly to miss Earth. Small changes made years in advance can prevent catastrophic impacts.
Deflection Strategies
Scientists have proposed several methods to deflect asteroids:
- • Kinetic Impact: Launch a spacecraft to collide with the asteroid, changing its velocity. This is the simplest method and has been demonstrated with NASA's DART mission (2022), which successfully impacted asteroid Dimorphos.
- • Gravity Tractor: A spacecraft hovers near the asteroid for an extended period. Its gravitational pull slowly changes the asteroid's orbit.
- • Ion Beam: A spacecraft directs a beam of charged particles at the asteroid's surface, slowly altering its trajectory.
- • Nuclear Deflection: As a last resort, a nuclear explosion near (not on) the asteroid could be used to deflect larger objects.
What Bennu Teaches Us
The OSIRIS-REx mission provided unprecedented data for planetary defense:
- → Detailed surface properties: High-resolution images reveal the asteroid's strength and how it would respond to impact.
- → Composition & density: Knowing what an asteroid is made of helps predict how it will behave when hit or heated.
- → Rotation & shape: These factors affect how an asteroid responds to forces and how momentum is transferred.
- → Rubble-pile structure: Bennu's loose, boulder-filled composition is typical of many near-Earth asteroids and influences deflection strategies.
The Path Forward
Thanks to missions like OSIRIS-REx, we now have a credible planetary defense strategy. We know how to detect asteroids, characterize them, and—if necessary—deflect them. The DART mission proved that kinetic impact works. OSIRIS-APEX's upcoming visit to asteroid Apophis will provide more data.
The key to planetary defense is early detection and early action. If we spot an asteroid decades before an impact, we can make small adjustments that—due to the laws of physics—compound over time. By the time the asteroid reaches Earth, it will have been pushed far off course.
Planetary defense is not science fiction; it is science in action. And it all begins with understanding asteroids like Bennu.