Potentially Hazardous Asteroids

1. Potentially HazardousAsteroids Philip W. Sharp Department of Mathematics University of Auckland

2. What is a PHA? • Size not used. Instead, use absolute magnitude, must be at least 22.0. • The body comes within 0.05 AU of Earth. 1142

3. Chicxulub Impact • “Chic-shoo-loob” • 65 Mya • Crater is about 180 km in dia, buried beneath the Yucatan Peninsula, Mexico. • Impactor is thought to have been 10 km across, weighed 3 trillion tonnes, hit at 20 kps, and had the energy equivalent to 60 -100 Tt.

6. Quiz • What were the possible physical effects of the Chicxulub impact? • What caused the Tunguska Event in 1908? • How many different ways can “Chicxulub” be pronounced?

7. Effects • Earthquakes (11 – 12.4) • Volcanic eruptions • Tsunamis: primary (1km) and secondary • White-hot debris fell back to earth a) wild fires b) smoke mixed with rain clouds, created acid rain • Dust high in the atmosphere

9. Tunguska Thought to have been caused by an air burst of a large meteoroid or comet at an altitude of 5-10km. Impactor is thought to have been a few tens of metres across. 10-15 Mt. Flattened 80M trees over 2,150 square kms. Photograph taken by 1927 expedition.

10. In December, 2002, a posting to the dinosaur@usc.edu forum listed 13 possible pronunciations of the word Chicxulub: SHICK-shah-lube, Chicks-ooh-lub, Cheek-hoo-loob, Chick-shoe-lube, chik-shooloob, tchik-ksooloob, CHICK-shoo-loob, Cheekshooloob, Chich-a-lube, Chicks-a-lub Chicks-a-loob, CHAI-shoo-loob, Chikjulub

11. Meteor Crater

12. 50 thousand years ago • Impactor • 50 m in diameter • 300 thousand tonnes • 12.8 kps • Crater • 1200 m in diameter • 170 m deep • Rim 45 m high

14. Tzar Bomba • 30 October, 1961 • Detonated about 4km above the surface • Design yield was 100 Mt • Yield reduced to 50 Mt to stop fall out • reaching mainland Russia. 50 Mt is about • ¼ of the yield of the 1883 Krakatoa eruption • 210 petaJ in 39 ns = 5.4 yottawatts • (1.4% of the Sun’s power)

16. Mathematical Modelling • Real Solar System • Model of the real Solar System • Input to the model • Solution of the model for the given input

17. Modelling the orbit 1. Detection Several programs have been set up * Spacewatch – U of Arizona * LINEAR – MIT (USAF, NASA) * Spaceguard Foundation - private

18. 2. Initial orbit (Keplerian) Assume the body moves in an elliptical orbit about the Sun, ignore other bodies.

19. Angular information only • Celebrated method of Gauss • Requires three observations Distance and velocity information as well • Distance obtained using radar • Velocity using Doppler • Require just two observations

20. Observational uncertainty • Can lead to large errors in the initial orbit • Several techniques to reduce the uncertainty

21. 3. Improved orbit Use least squares and the best model for the orbital motion of the large bodies in the Solar System.

23. Components of the best model • The mass of the asteroids. • General relativity. • Earth-Moon interactions: • the oblateness of Earth and the Moon; • tides raised on Earth by the Moon and vice versa; • internal structure of Earth.

24. Yarkovsky Effect

25. 4. Estimating the probability of hitting Earth • Generate the initial r and v of 1M artificial small bodies. The r and v are chosen from Gaussian distributions (law of errors) centred on the best values for the real body. • Integrate the motion of the bodies for a few hundred years.

27. Apophis • Discovered in 2004. • Exaggerated headlines. • No chance of hitting April 13, 2029. • 1 in 250,000 chance of hitting April 13, 2036. • 3 in one million chance of hitting in 2068. • About 320 metres in diameter. • 510 Mt.