1Astro 202Spring 2008COMETS and ASTEROIDSSmall bodies in the solar systemImpacts on Earth and other planetsThe NEO threat to EarthLecture 3Don CampbellPositions of all known asteroids projected onto the plane of the Earth’s orbit on Jan 20, 2008Red ones (Apollo and Alten) cross the orbit of Earth so are potentially hazardous. Yellow ones (Amors) don’t cross Earth’s orbit now but could in the future if their orbits are disturbed. NEAR-EARTH ASTEROIDSComet/Asteroid Impact Hazard in Perspective• To “scientists” the fact that asteroids and comets strike the Earth is obvious• Greatest natural disasters on Earth are caused by the impacts of large asteroids and comets• They are so infrequent that they are normally disregarded• At present, only one known object has even a small likihoodof impacting Earth soon (Apophis).• SO WHAT IS THE URGENCY?• We now have the technical capability to find and, possibly, divert/destroy any small asteroid that threatens Earth• SHOULD WE USE RESOURCES FOR THIS PURPOSE?How Well Do we Know?• Level of the hazard Well • Energy released in a collision Very well• Physical and chemical side effects Some• Biological and social side effects Not much2NASA Near Earth Object web site at JPL:http://neo.jpl.nasa.govNASA/Ames NEO Impact Hazard page:http://impact.arc.nasa.govSpacewatch Project:http://www.lpl.arizona.edu/spacewatch/Torino Scale:http://impact.arc.nasa.gov/torino/index.htmlSome useful NEO/Impact Hazard Websites:• Smallest asteroid known: 1993KA2 4-9 meters• Closest approach by an asteroid: Apophis on April 13, 20029•~250 m sized object, will pass inside orbits of geosynchronous sats.The B612 Foundation"Our goal is to significantly alter the orbit of an asteroid in a controlled manner by 2015"Asteroid and comet impacts have both destroyed and shaped life on Earth since it formed. The Earth orbits the Sun in a vast swarm of near Earth asteroids (NEAs). The probability of an unacceptable collision in this century is ~2%. We now have the capability to anticipate an impact and to prevent it. * B612 is the asteroid home of the Little Prince in Antoine de Saint-Exupery's child's story The Little Prince. http://www.b612foundation.orgArticle Posted: October 31, 2005By: David MorrisonDuring its close passage by the Earth on April 13 2029, it is possible (but improbable) that asteroid Apophis will pass through a keyhole leading to a collision in 2036. Recent concern has centered on how to evaluate the possibility of a collision in 2036 and take action, if necessary, to deflect the asteroid. Asteroid. Image credit: U.S. Geological Survey Apophis trajectory by Earth on April 13, 202931 km diameter ~ 105MT4Morrison et al. “Hazards” p. 59 (1994)Reference: Hazards Due to Comets and Asteroids,”T. Gehrels, ed. University of Arizona Press (1994).Add 1 to eachGlobal effectBlast wave:Severe effects locallyEven K-T event affected only a few % of the Earth’s surfaceToon et al., Hazards” p. 791 (1994Toon et al., Hazards” p. 791 (1994)What are the effects that cause the most widespread “destruction”?5TSUNAMIS – Tidal wavesOceans cover 70% of EarthHigh probability of impactWave height depends on impact energy and depth of ocean1m to 10m waves very destructiveToon et al., Hazards” p. 791 (1994)2004 Sumatran Earthquake and Tsunami – 9.3 on Richter scale – hugh!Energy released was (only!)~26 MTToon et al., Hazards” p. 791 (1994)Dust and smoke6Dust loading - K-T event deposited ~1 cm of fine powder over whole EarthExpect dust cloud to cover Earth in a few weeks or lessRemoved by fallout or by washout by rain within ~6 monthsWhat are the climatic effects?About ~ 5x106MTToon et al., Harzards” p. 791 (1994)Ignition Threshold for Impact Generated FiresDurda & Kring (2004). J. Geoph. Res., Vol 2 109, p. 8004.On Earth it takesCrater > 85 km for Continental Scale FiresCrater > 135 km for Global Scale FiresChicxulub Crater (K-T event) is 170 km!Caused by heating of the atmosphere as ejecta from the impact falls back into the atmosphere7Toon et al., Hazards” p. 791 (1994)How serious can an asteroid impact be?8Species ExtinctionsSpecies Extinctions9Topics:• Finding them – How? How do we tell their size?• Tracking and predicting orbit• Characterizing population and specific objects• Mitigation options• How is mitigation decision made?How do we detect NEAsStars are at fixed distances and shine because they are very hot – e.g. our SunIf we know its surface temperature – can tell from studying the spectrum of the lightThen: Apparent brightness of the star proportional to size / distance2Asteroids shine by reflected sunlight – their surfaces are coldBrightness determined by:How big the areHow reflective their surfaces are - typically they reflect about 10%of the incident sunlightHow far they are from the SunHow far they are from EarthBrightness proportional to size x albedo / DSun2x DEarth2Both Ds vary with time as asteroid moves in its orbitNot interested in all NEAs, only ones that threaten Earth – PHAsi.e. Apollo and Aten NEAs. But have to find all to determine which are PHAsSpaceguard Survey• Initiated ~ 1992• Aim: Find 90% of NEA’s bigger than 1 km by 2009• How many expected? – Shoemaker et al. ~1000– Bottke et al. ~ 900• By November 2006 ~ 700 found• Total number (all sizes found) ~ 4400In 2005 Congress requested NASA study how to:Detect objects . 140 meters – regional damageFind 90% by 2020, determine which are potentially hazardousInvestigate ways to mitigate the threatReport due in early 200710The blue area shows all near-Earth asteroids while the red areashows only large near-Earth asteroids. In this context, "large" is defined as an asteroid having an absolute magnitude (H)of 18.0 or brighter which roughly corresponds to diameters of 1 km or larger. Not surprisingly, those discovery teams who search the largest amount of sky each month will have the most success in finding new NEOs. How much sky each telescope covers per month will depend upon a number of factors including the number of clear nights available for observing, the sensitivity and efficiency of the CCD detector, and the field of view of the telescope. Wide field of view telescopes can cover more sky per given time than telescopes with narrower fields of view. It is also important for search teams to extend their