Planetary Defense: Finding and Deflecting Asteroids

Discovery is the start, not the conclusion

Planetary defense begins with finding near-Earth objects and measuring their motion. Repeated positions constrain an orbit; size and physical properties help assess consequences and possible responses. A newly discovered object is not automatically a newly discovered threat.

Keep three labels separate. Near-Earth describes an orbital category. Potentially hazardous flags an object meeting orbital and estimated-size criteria worth monitoring. An impact probability is a calculation for a particular possible encounter. None can be substituted for the others in a careful headline.

Reference: NASA: Planetary Defense Explained

New observations change what we know

A short observation arc can fit many slightly different future trajectories. NASA JPL's Sentry system evaluates whether some trajectories compatible with observations could lead to an impact. Appearance on its monitoring list is not a prediction of a certain collision.

As observations narrow the range, an estimated probability can first rise and later disappear. The physical asteroid has not necessarily swerved; the assessment has improved. Removal from a risk list is often the expected result of better orbit knowledge, not evidence that the original uncertainty was imaginary.

Reference: NASA JPL CNEOS: Sentry Introduction

DART tested one way to change motion

In 2022, NASA's DART spacecraft struck Dimorphos, the small companion of Didymos. Measurements showed that its orbital period around Didymos changed. This demonstrated a kinetic impactor: a collision transfers momentum and can alter an asteroid's trajectory.

The target was not an Earth-impact threat. DART was a demonstration, not an emergency rescue, and it did not establish that one identical impact would work for every object. Structure, mass, impact geometry, and warning time influence what a future response could achieve.

Reference: NASA: DART

Check a worrying asteroid story in order

An old screenshot can outlive the assessment it quotes. This guide explains the process; it does not claim to give today's complete risk inventory.

1. Find the object's exact designation and the assessment date.
2. Separate a close-approach distance from an actual impact probability.
3. Consult the current official CNEOS record, including its uncertainty and observation history.
4. Use Astroboat's Asteroid Watch for browsing and context, not as a replacement for formal impact monitoring.