The impact flux (hazard?) on Earth The young Earth and Moon suffered the same heavy bombardment early in the Solar System Only the Moon preserves the record of this The lunar record indicates roughly constant crater formation during the last 3 billion years. Same for Earth?
Earth cratering rate: last 3x10 yr 9 3 estimates of the rate: 1) Look at lunar record and adjust for Earth's larger size (bigger target!) and gravity. 2) Look at recent terrestrial craters and correct for erosion to get rate 3) Estimate cratering rate from studies of near Earth objects (Apollos and Atens) hitting Earth All agree to factor of 2. Implies rate has been ~constant
At what rate do objects strike Earth? Is this something to worry about? Should we spend 100 M$ for some telescopes to find the impactors?
Should money be spent? A) Yes, we should do this to protect society. B) No, if the heavens decide our time is up, so be it. C) No, we should spend the money on other more urgent social programs (eg, health care, housing) D) No, this is just astronomers trying to get money
Proceed from smallest to largest Interplanetary Dust Paticles (IDPs) Several tons of dust (1 100 µm) hit Earth per day Dust collected in the stratosphere by old spy planes! Many IDPs are of cometary origin
Meteors ~0.1 mm to cm sized bodies streaking through atmosphere at 12 60 km/second! Light is emitted as particle 'burns up' high in atmosphere Some times of year are especially active for meteor activity: meteor showers:why?
Meteor showers are linked to comets Comet leaves debris behind in path On the day Earth intersects comet's orbit, showers occur (every year)
Meteorites (cm to 10 meters) Meteorites are smallest objects which penetrate atmosphere and reach ground (but atmosphere slows!) Many collected in Antarctica Some 'falls' observed; sometimes spectacular fireballs No confirmed human deaths
The Peekskill fireball October 9, 1992 Crossed northeastern U.S.; was heavily videotaped (football!). Film shown in class. Allowed orbit determination (asteroidal orbit) Hit a car!
10 300 meters Last objects for which Earth's atmosphere is an obstacle (slows them down before ground) But kinetic energy very large (>10 megatons) Explosion in atmosphere (10 30 km up) One happened in Tunguska, Siberia in June 1908 In London: Explosion HEARD Barometers moved Nights glowed for days
Tunguska effects First expeditions >20 years later Bolide (meteor) was sighted 500 km away Witnesses knocked off porches 50 km Witnesses found herds of burnt reindeer
Radial treefall pattern for >10 km Similar to atmospheric nuclear explosion 1930 (above) Today (below)
At what rate do 'Tunguskas' happen? Every few hundred years Human recorded history only a few thousand years...
In your lifetime: Shoemaker Levy 9 hit Jupiter! A roughly 1 km comet was tidally disrupted while passing Jupiter and then struck the planet in 1994.
Biggies: The KT event The dinosaurs suddenly disappeared 65 Myr ago after dominating the planet for >100 Myr The impact of an asteroid is believed to be responsible. What evidence??
Evidence (1): Iridium layer Iridium is 'iron loving' element, almost all sank to Earth's core But there is a layer of iridium rich sediment all over the Earth at an age corresponding to 65 Myr ago ASTEROIDS (at least non differentiated ones) are rich in iridium
Measurements of 'iridium spike' Concentration of iridium as a function of height in geologic strata (left is older strata, right is younger) Where could a massive input of iridium come from? Not volcanoes. Also evidence for global fires (ash layer)
(2) Crater found! Chicxulub crater identified in Mexico, just off Yucatan peninsula. Crater date: ~65 Myr ago Shocked rocks and impact glasses prove this is an impact crater After effects are what caused extinctions: caused 'global winter'for several years while dust settled
Chicxulub is a multi ringed impact basin Gravity profile maps show the details of the partially submerged structure, 180 km diam. From this size and iridium content, impactor estimated to be 10 km diameter How often do such events happen?
Again, bigger = rarer At left, asteroid diameter (meters) versus frequency K/T ~ 100 Myr Tunguskas ~300 yrs Intermediate (like Berringer crater, left) sizes every ~100 kyr
The impact threat (?) Objects with 1 km or greater have globally catastrophic consequences.
Impact consequences On any given year, there is a roughly 1 in 100,000 chance of a >1 km impactor hitting This scale of disaster has never happened in recorded history. Should we do something? Consequence of 1 km impactor : 1 1000 million fatalities (depending on where it hits)
Impact insurance? We spend hundreds of millions of $/year for airline safety, but 'only' a few hundred people die per year. Airline death rate: (100 people/year) 1 km impactor only occurs every 10 years but fatalities are 100 million 6 Impact death rate is therefore also 100 /year But airplane accidents can NEVER kill everyone... Societal risk vs personal.
'Personal' risks Estimated risk for an American over a 50 year period. Risk of death from botulism 1 in 2,000,000 Risk of death from fireworks 1 in 1,000,000 Risk of death from tornados 1 in 50,000 Risk of death from airplane crash 1 in 20,000 Risk of death from asteroid impact 1 in 20,000 Risk of death from electrocution Risk of death from firearms accident 1 in 2,000 Risk of death from homicide Risk of death from automobile accident 1 in 100 1 in 5,000 1 in 300
Societal risks Impacts are the only risk that can threaten all of humanity. Should we spend money trying to find potentially impacting asteroids??
Surveying the near Earth objects There are several small telescopes that search for Earth crossing asteroids and comets Would take a hundred years (a human lifetime) to find all such 'NEOs' Should ~$100 million be spent to find all in 10 years? (< annual budget of U.S. FAA for airlines)
Should money be spent? A) Yes, we should do this to protect society. B) No, if the heavens decide our time is up, so be it. C) No, we should spend the money on other more urgent social programs (eg, health care, housing) D) No, this is just astronomers trying to get money
Pan Starrs is being built! A network of telescopes which will search the entire sky several times per month. 1.4 gigapixel camera on 1.8m telescopes see: pan starrs.ifa.hawaii.edu/