¶Planetary Defense. Earlier this month was the Planetary Defense Conference, which focuses on the threat to Earth posed by asteroids and comets, as well as their potential mitigations. Mitigations include nuclear weapons (for comets, since we won’t have much warning), kinetic impactors (being tested for the first time by the upcoming DART mission), and gravity tractors (which use a heavy spacecraft orbiting the asteroid while thrusting perpendicular to its orbit to slowly shift the combined gravitational system without any physical contact). Asteroid strikes are one of the few natural disasters that can potentially be prevented if we have early warning. One study presented at the conference, ADAM, found that the median required delta-v for redirection was 1.2 cm/s with 10 years warning, but only 0.55 cm/s with 20 years. (For comparison, DART will impart 0.2-0.3 cm/s.) Unfortunately, we have probably only identified one-third of the Near Earth Objects (NEOs) of 140m or larger. This is the size of an object that would cause regional, but not global, destruction. Projects currently active or on the horizon, including Pan-STARRS, the Catalina Sky Survey, ATLAS, and the LSST (which just received its primary mirror) should be able to catalog the majority of NEOs in this range. Identifying a significant percentage of smaller NEOs, still large enough to cause the Tunguska blast or create Meteor Crater in Arizona, will require new observatories. Resources: NASA’s Planetary Defense department, the International Asteroid Warning Network, and the B612 Foundation, started by Ed Lu, astronaut and co-inventor of the gravity tractor.