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Asteroids and Planetary Defense: How We Watch the Skies
A lecture handout
1. The Wake-Up Call: Asteroid 2024 YR4
- Discovered: December 2024 by the ATLAS survey.
- Early orbit prediction: ~1% chance of hitting Earth in December 2032.
- Size: ~40–90 meters (big enough to destroy a city).
- Result: Further tracking ruled out impact — but it showed how seriously scientists take even small probabilities.
2. How Scientists Discover Near-Earth Asteroids
Ground-Based Surveys
- Catalina Sky Survey (CSS), Pan-STARRS, ATLAS, ZTF.
- Scan the sky nightly, compare images, detect moving points of light.
- Rubin Observatory (Chile, opening soon): will map the whole sky every few nights with the world’s largest digital camera.
Space-Based Infrared Observatories
- NEOWISE (ended 2024): measured asteroid heat radiation, size, and reflectivity.
- NEO Surveyor (launch ~2027): will spot dark/sunward asteroids invisible to ground telescopes.
- ESA’s NEOMIR (early 2030s): similar mission at Sun–Earth L1.
Rapid Risk Assessment
- Minor Planet Center (MPC): collects all asteroid observations.
- NASA’s Scout system: instantly calculates early impact risk.
- CNEOS “Sentry” and ESA’s NEOCC “Aegis”: keep risk lists updated as orbits refine.
3. How We Observe and Characterize Them
- Radar: Goldstone radar maps shape, spin, distance to meter precision.
- Lightcurves: Brightness changes reveal rotation and shape.
- Spectroscopy & thermal IR: Tell us about composition (metal, rock, carbon).
- Stellar occultations: When an asteroid blocks a star, we measure its profile.
- New methods: Machine learning and synthetic tracking find faint/fast movers.
4. If One Were Headed Our Way — What Could We Do?
A.
Kinetic Impactor (Proven)
- Smash spacecraft into asteroid to nudge its orbit.
- Tested by NASA’s DART (2022): hit Dimorphos, shortened orbit by 33 minutes.
B.
Nuclear Standoff Explosion
- Detonate near asteroid to vaporize material and push it.
- Very powerful but politically sensitive and risky.
C.
Gravity Tractor
- Park a heavy spacecraft nearby; its gravity slowly pulls asteroid off course.
- Precise but very slow; requires decades of warning.
D.
Other Ideas
- Laser ablation: focus energy to vaporize asteroid material.
- Solar sails: attach reflective sails to use sunlight as propulsion.
- Mass drivers: eject asteroid’s own material for thrust.
5. Why Time is Everything
- 10+ years warning: Small nudge (like DART) can prevent disaster.
- 2 years warning: Options limited — might require nuclear device or just civil defense.
- No warning: Best we can do is evacuate, as with Chelyabinsk (2013).
6. Real Case Studies
- Apophis (2004 discovery): initially a 2.7% chance of impact in 2029.
- Later ruled out; will pass closer than satellites but harmless.
- Shows the value of decades of monitoring.
- 2024 YR4 (8 years warning): briefly alarming, quickly downgraded.
- DART (2022): first real planetary defense experiment — and a success.
7. Takeaway Message
- Discover early. Sky surveys are our first shield.
- Observe carefully. Radar, lightcurves, and spectroscopy refine the risks.
- Intervene if needed. Kinetic impactors are proven; others are in the toolbox.
- Time = Safety. The more years we have, the easier the solution.
📝 Closing Thought
We cannot stop earthquakes or volcanoes, but asteroid impacts are preventable — if we see them early enough. Planetary defense is not science fiction anymore: it is humanity’s first real plan to protect the planet from cosmic hazards.
