Presentation on theme: "Death from Space David Jewitt, UCLA planets.ucla.edu Objective: a broad, accessible overview of modern solar system science."— Presentation transcript:
Death from Space David Jewitt, UCLA planets.ucla.edu Objective: a broad, accessible overview of modern solar system science.
About 40,000 tons of debris hit Earth each year - roughly 1 kg per sec. Most is in the form of dust The effects depend first on the impactor size and second on impactor strength Bodies smaller than a few meters are completely stopped by the atmosphere unless they are iron and very strong. Bodies larger than ~few 100 m cannot be stopped, even if they are strengthless Impacts vs. Impactor Size
Brownlee Particles (micron sized) Very tiny (10 µm = 0.01 mm and smaller) particles are heated by friction with the air but do not melt because they are small enough to radiate their heat away. The unmelted dust floats down to Earth and can be collected high in the atmosphere by airplanes equipped with sticky pads. Brownlee particles probably come from comets. They are aggregates of even tinier grains that, in some cases, may be the original interstellar grains that were part of the protoplanetary disk. BrownleeOne of his particles
Meteors (mm sized) ("meteor" = "shooting star" in popular language) Larger grains cannot radiate their energy away fast enough and burn up from friction with air. They are the meteors....lasting about 1 second. Most meteors are destroyed about 90 km high. Most are random but some occur in "streams" or "showers" that can usually be associated with the Earth passing through the orbit of a comet. Comet Halley, for example, produces the "Orionid" and "Aquarid" showers. Meteors in showers share a common direction across the sky.
Impact speed ~ 30 km/s Some "drop stones" (i.e. pieces hit the ground) Large Meteors are called “Bolides” (Size ~ 1 meter)
If they remain coherent, 100-m bodies pass through atmosphere at cosmic velocity. But many break up due to "ram pressure" P = ρ v 2, where ρ = density of air, v = speed. e.g. ρ = 1 kg m -3, v = 30 km/s = 3x10 4 m/s then P = 10 9 N m -2 but 1 atmosphere = 1 bar = 10 5 N m -2, so, P = 10 4 bar This like being hit with a hammer, and the body breaks if not strong. Then, the subsequent interaction is between air and the fragments, which can fragment again and again, until the particles are so small they can be stopped by friction. Result: atmospheric explosion 100 meter sized bodies
20 MT asteroid (Arizona, ground explosion) 20 MT comet (Tunguska, air burst)
Tunguska, Siberia, 1908 Pine forest flattened over 2000 sq. km E ~ 20 MT
Tunguska - Felled Trees
Airburst 10 km
Tunguska Facts E ~ 20 MT (10^17 J) TNT Height ~ 10 km (one scale height) Brighter than the Sun Trees are flash-burned on one side Sound “deafening” at 500 km (& “loud” at 1000 km) Barometric pressure wave recorded in England Bright twilights (stratospheric dust) for weeks afterwards Orbit and strength consistent with comet Rate probably ~1/1000 yr...could have next one at any time
Tektites Are glassy rocks found spread over wide areas of Earth in distinct families or “fields” each with slightly different composition. They appear to be melted Earth rocks that have been blasted upwards and fell back through the atmosphere at high speed. Glass results from rapid cooling (during flight, probably). Many have aerodynamic shapes, showing that they were hot in flight. Typically, though, there is no known associated crater. The details of their formation remain unclear.
10 km body would hit the ground with cosmic speed (maybe 30 km/s) and vaporize itself and maybe 1000x its own mass of rock. Ejecta would punch hole in atmosphere - some material ejected back to space through the hole. A lot of ejected material travels sub-escape speed and falls back, around the Earth. Infalling debris hits at >> km/s and burns up, like wall-to-wall meteors. Result: toaster oven in which surface is cooked. Animals on and near the surface immediately die. Animals able to burrow and those beneath water survive the roast. Dust and sulphur compounds would pollute the stratosphere, taking months to years to clear. Result, diminished sunlight causes photosynthetic shutdown, destroying the food chain from the bottom. This is a global extinction like the one 65Myr ago that wiped out dinosaurs. Effects of Giant Impact
CHANCE OF DEATH BY IMPACT IS ROUGHLY 1/20,000 - SAME AS DEATH PROBABILITY IN AIRPLANE ACCIDENT
Energy in Megatons TNT Objects not yet found
Air pollution2,000,000 Murder (US only)15,000 Tobacco5,000,000 Shark Attack 5 Asteroid Impact90 Murder (World**)2,000,000 HIV/AIDS2,100,000 Traffic Accident1,200,000 Earthquake36,000 Malaria1,000,000 ** - democide = murder by governments, 20th Century Chance of Avoidable Death (#deaths per year)
Air pollution2,000,000 Murder (US only)15,000 Tobacco5,000,000 Shark Attack 5 Asteroid Impact90 Murder (World)2,000,000 HIV/AIDS2,100,000 Traffic Accident1,200,000 Earthquake36,000 Malaria1,000,000
Shark Attack 5 Asteroid Impact90 Murder (US only)15,000 Earthquake36,000 Malaria1,000,000 Traffic Accident1,200,000 Air pollution2,000,000 Murder (World)2,000,000 HIV/AIDS2,100,000 Tobacco5,000,000 * - USA murders only ** - democide = murder by governments, 20th Century Chance of Avoidable Death (#deaths per year)
What is the "cost of a life"? This subject is hotly debated by insurance agencies and federal agencies because when citizens die, somebody has to pay. Included are lost earnings, loss of family support, property, legal costs etc. The number is needed for cost-benefit analyses. Discussion The current value of a US life is put at $6M - $9M. Given the small number of asteroid-caused deaths, and given the fact that NOBODY we know of has been killed by impact, should we forget about it? Probably not - most impact deaths are from big events, where are large number are killed in one instant. It only seems placid in between these killer events.
Sudden re-setting of the environment by impact or other catastrophe acts to re-set the competition between various life forms. This is the idea behind "punctuated equilibrium" for the evolution of species. Rise of the mammals is a good example. Before the KT event, we were little furry animals hiding in holes and the giant lizards ruled the Earth. But no more. Evolution - Punctuated Evolution
Detection/Deflection Strategies Detection: Currently we search for impactors in a haphazard way. Soon, we might have special telescopes to search for impactors. But people are reactive, not proactive (especially politicians), and so money for this has not yet been obtained. Deflection: If the impactor is small, it will be probably be nearby and we will have only days. Only option is city evacuation. Bigger objects can be detected further away and will have longer lead times. We currently possess no method to deflect them. There are discussions involving bombs, rockets, mirrors and gravity tractors.
Gravity Tractor: a spacecraft with nuclear rockets hovers near the asteroid and, to avoid being pulled in by asteroid gravity, continuously fires its rockets, keeping a fixed distance from the surface. The effect is for the tractor rocket to pull the asteroid to one side.
Push-rocket: would stand on the surface and fires, to deflect the asteroid. Problem: can it in fact stand on the surface?
Bruce Willis: Bruce Willis is not powerful enough to deflect an asteroid. Neither do we possess any bombs powerful enough to do so. Worse, bombs might just fragment the object without changing its direction, making things worse. Thanks, Bruce.
Asteroid: “star like” Comet: “hairy star”
Asteroid: “star like” Comet: “hairy star”
Any Questions About Death from Space?
B612 Sentinel D = 50 cm 5 to 10 µm Thermal IR 165 sq. deg/hr Launch 2017?
Large Synoptic Survey Telescope D = 8 m 3.2 Gpixel CCD
The ATLAS Project (John Tonry) D = 25 cm 20,000 sq. deg/day (x2) 2k x 4k CCD (x2, x4)) Triangulation from 2 sites Deathplunge objects
Bottom Line We remain unprepared for the next impactor. We will probably not see it coming (because nobody wants to pay to search (but see Project Atlas)). If we see it coming, there is essentially nothing we can do about it other than attempt an evacuation of the impact site (but big-city evacuation has never been attempted). BUT, it’s not hopeless. We can build telescopes to find potential impactors and, given sufficient threat and sufficient time, try to figure out avoidance strategies (LSST, B612 Sentinel, ATLAS).