Impacts of Space Objects

LATEST UPDATE:

POSTED: 10 June 2020

• Meteors and Asteroids - Some Nomenclature

Space objects that collide with Earth, have primarily come from fragmented asteroids and comets. As the amount of anthropogenic space debris is increasing, so is the chance increasing that some of human-made objects re-enter Earth's atmosphere. Most objects are small enough that they burn up completely, but some larger fragments can hit Earth's surface (e.g. the Columbia Space Shuttle October 28, 2003). Meteoroids enter Earth's atmosphere every day at speed of more than 10km/s. Most burn up in the atmosphere and never reach the ground. Events such as the Tunguska event (see below) happen much more rarely, perhaps once every 300 years ².
• meteoroid: fragments of asteroids and comets that orbit sun; may intersect Earth's orbit and collide; a stony meteoride of 10m diameter can produce an explosion of around 20kilotons (Hiroshima bomb)
• meteor: shooting star; the light phenomenon when a meteoroid enters Earth's atmosphere and burns up
• metorite: the fragment that survives the travel through the atmosphere and hits the ground;
• meteor showers: unusually frequent occurrence of shooting stars; happens a few times a year but spectacular "events" are Perseids on Aug 12, the Leonids on Nov 17 and the Geminids on Decc 14 of each year; the meteors are caused by small dust particles (most smaller than a sand grains) buring up in Earth's atmosphere; the dust particles were left behind when comets intersected Earth's orbit, except for the Geminids that are caused by the remaining trail of an asteroid
• bolide: either an extraterrestrial body that collides with Earth or an exceptionally bright, fireball-like meteor



• Meteorites

• iron(metallic): thought to resemble Earth's core
• stony (rocky): thought to resemble Earth's mantle; less commonly found because less likely to survive journey through the atmosphere; once on the ground, they weather more quickly than iron meteorites; less easily recognized as meteorites than iron meteorites (mistaken for rocks)
• an estimates 500 meteorites reach Earth's surface each year only 5-6 of which are recovered and made known to scientists²
• most iron meteorites are thought to come from asteroids that were initially large enough to differentiate (see Lecture 3)
• tektites: glassy objects, typically a few cm across; formed when a large meteorite hit Earth's surface and melts surrounding rock



• Some Example of Impact Craters by Iron Meteorites²

• Barringer Meteor Crater: near Flagstaff, AZ; 1200m diameter, 170m deep, surrounded by 45m high rim of rock; created 50,000 yrs ago; meteorite 50m across; largest recovered fragment 640kg;l impact speed 10-20 km/s; impact produced explosion equivalent to at least 2.5 megatons TNT (150 time Hiroshima); produce EQ with estimated magnitude of 5.5 or higher; all life died instantaneously within 3-4km; shockwave leveled erverything within 20km; did not seriously impact Earth's climate; recolonization of flora and fauna likely within 100 yrs
• Odessa Meteor Crater: TX; southwestern Ector County; largest of several craters in the area; craters may have come from fragmented asteroid; created 50,000 yrs ago; 168m diameter; originally 30m deep; currently 5m deep (filled due to erosion and sedimentation); largest recovered piece 300 pounds
• Wabar Craters: in Rub' al Khali desert in Saudi Arabia; three prominent roughly circular craters, 116, 64 and 11m wide; age estimated at 140 yrs (Earth Impact Database); meterorite mass 3500 tonnes; largest fragment recovered 2.2 tonnes; impact speed 11 - 15 km/s; biggest piece struck with force of Hiroshima bomb
• Wolfe Creek Crater: Western Australia; 875m diameter; less than 300,000 yrs old; 50,000kg meteorite
• Tunguska event: Central Siberia; explosion in atmosphere 5-10km above surface, no crater; 1908; tress felled by blast in atmosphere; blast estimated at 10-20 metatons TNT (equivalent to most powerful ever nuclear bomb detonated by U.S. Castle Bravo/Bikini Atoll); bright bluish light noticed at Lake Baikal, 200km away; associated shockwave knocked people off their feet several 100km away; pressure fluctuation large enough to disturb barometers in Britain; in U.S. decrease of atmospheric transparency for several months; association of glass spheres found in area to event uncertain as area is a volcanic region that could produce them; whether source was an asteroid or comet is under debate; a comet could disintegrate more easily in the atmosphere and could produce a skyglow; a later discovered meteor shower seems consistent with the notion that the Tunguska body was a piece of Comet Encke though others argue a comet could not have survived as long as the Tunguska body apparently did; others suggest that the arrived from the direction of the asteroid belt



• Asteroids

• asteroid belt between Mars and Jupiter: small (under 1000km) rocky, metallic and icy masses
• orbit where a 10th planet should be
• gravitational pull of Jupiter may have prevented planet formation
• Kirkwood Gaps: asteroid belt has gaps which distances relate to orbit time of Jupiter
• most are too small for gravitational forces to form a spherical body
• some asteroids may actually be more than one body, held together loosely by gravitational forces
• the three largest (Ceres, Pallas, Vesta) make up about half of the total mass in the asteroid belt
• all asteroids together would make a body with 1500km diameter, less than half the size of the Moon
• a group of asteroid (Apollos) have Earth-crossing orbits and another (Amors) have Mars-crossing orbits and come close to Earth's orbit
• Apollos and Amors provide asteroids that impact on Earth



Size of Asteroids

Name	number of bodies	diameter/km
Ceres	1	933
Pallas	1	523
Vesta	1	501
	200	>100
	1000	>30
	1,000,000	>1




• Comets

• have extremely elongated orbits around sun
• "dirty snowballs" consisting of ice and rocky debris; core and tail; tail forms when sunlight and solar wind heat and ionize material; comet then ejects gases and trapped dust that appear as the tail; most have cores less than 15km across; orbit may be changed by pull of planets when it enters the solar system; when comets cross Earth's orbit, the debris left behind by the tail is the source for sometimes spectacular meteor showers
• contain carbon compounds, N, H, O; some scientists think that comets are a major source for organic material and water on Earth
• divided into short-term and long-term comets, referring to their re-occurrence
• short-period comets: orbits in less than 200 yrs; come from Kuiper Belt; probably remains from when outer icy planets formed; some examples: Halley (75 yr period; core 15km across; only naked-eye comet certain to return within human lifetime), Encke (a few yrs; core 1-3km across; might be piece of a larger Bronce-age comet that destroyed Fertile Crescent/produced crater in Iraq); Biela (6yrs); Tuttle (13.5 yrs; responsible for Ursids meteor shower in late December)
• long-period comets: most comets are long-period; come from far beyond Solar System; originate from Oort cloud; some examples: Hale-Bopp (1997, discovered in 1995; core 40km across); Hyakutake (1996, dicovered in Jan, passed very close to Earth in Mar; one of the closest cometary approaches in 200 yrs; first-time discovery of X-ray emission; longest tail yet known for a comet; used to have period of 15,000 yrs but now has 72,000 yrs after passage through solar system; core 2km across)
• Kuiper Belt: flattened disk beyond Neptune out to 50 AU; source of short-period comets; outer edge thought to be defined by interaction with Neptune; Pluto and the larger Eris, and Neptune's Moon Triton are considered Kuiper Belt Objects (see also Lecture 3)
• AU: astronomical unit; Earth-Sun distance; 150 Mio km (93 Mio mi)
• Oort cloud: cloud about 50,000-100,000 AU from sun; source of long-period comets; no direct observation of cloud but dimension and existence estimated from orbits of comets; thought to be remnant of original nebula from 5 billion years ago



• Large Impacts

• thought to be rare events: e.g. a 1m impactor happens once a year but a 100m impactor happens once every 10,000 yrs and a 10km diameter impactor happens once in 100 Mio yrs
• large impact cause mass extinctions
• some consequences of large impacts:
  • large crater
  • gigantic earthquakes and aftershocks; e.g. the K-T impact was an estimated magnitude 11.3
  • uncontrollable wildfires; e.g. the K-T impact caused comsumption of much of the vegetation in N. America, India, equatorial region
  • huge amount of NO₂ in atmosphere would cause acid rain
  • dust and aerosols in atmosphere would block sunlight, increase albedo, cooling climate and inhibiting photosynthesis for weeks to months
  • increased CO₂ in atmosphere would lead to global warming for years, after dust settled
  • up to 300m high tsunami, if asteroid hit oceans
  • huge amount of steam carrying debris high into atmosphere, if asteroid hit oceans
  • additional CO₂ into atmosphere, if asteroid hit coral reefs in shallow tropical ocean

• Torino Scale

Shows potential threat and magnitude of hazard by approaching comets and asteroids. The scale goes from 0 (no collision hazard) to 10 (collision will cause global catastrophe). Currently the is no known object to have a value higher than 0 for the next 100 years check at NASA. However, some impactors may not be discovered until a short time before intersection.
• Levy-Shoemaker comet: plunged into Jupiter's atmosphere between July 16 and July 22, 1994; was unknown before March 24, 1993 when Carolyn and Eugene M. Shoemaker and David Levy discovered it with a telescope of the Mount Palomar Observatory. Impact speeds where estimated at 60km/s. The comet was orbiting Jupiter rather than the sun, with an orbital period of about 2 yrs. Prior to its capture by Jupiter, it is assumed to have been a short-period comet, with an aphelion just inside Jupiter's orbit and a perihelion in the asteroid belt. It is believed that it disintergrated into several pieces during a close encounter with Jupiter in July 1992, the largest piece being 2km across.



• Space Age and Space Debris

The space age and related launches of ever growing numbers of rockets increases the threat of impacts by human-made space objects. Many burn up in the atmosphere but some may make into to the surface.
• impacts by failed launches: launches are hazardous because rockets are full to capacity with fuels; some have nuclear reactors on board, so pose particular threat;
some examples:
• NASA/ESA launch in 1990 of unmanned solar probe Ulysses was controversial because it has a nuclear power generator
• Russia launch of Mars space craft Mars-96 from Baikomur/Kazakhstan crashed into the Pacific Ocean between Easter Island and Chile during its third revolution around Earth after the third stage of the launch rocket failed; the spacecraft had a nuclear reactor on board
• 1998 Russia launch of 12 American (Globalstar) communications satellites in Baikonur/Kazakhstan failed 4.5min into launch; the rocket and satellites are believed to have crashed in the Khakasskaya region north of the Altai mountain range in Siberia
• Russian launch of a solar sail space craft from Mir space station in 1999 jammed and burnt up in atmosphere
• Russian launch of a solar sail space craft in 2001 burnt up in atmosphere due to failure of booster
• Russian military communications satellite "lost" on June 21, 2005, six minutes into the launch; according to experts, the satellite either "fell into a forest with the rocket's third stage or self-destructed"
• joint Russian/U.S. launch of a solar sail space vehicle crashed back to Earth on June 22, 2005, two minutes into the launch; the rocket was launched from a nuclear submarine in the Barents Sea
• Russian launch of European telecommunication satellite in Baikonur/Kazakhstan crashed on July 26, 2006, when the Dnepr rocket crashed shortly after blastoff. Kazakhstan banned further launches of Dneprs until the cause of the crash was determined, out of concerns over pollution. The next launch (successful) followed only 10 days later

DID YOU KNOW? 27,680: The number of objects sent into space between 1957-2002; 18,669: number of those burnt up (in atmosphere); 9,011: objects still in orbit; of these, 6,115 are space debris only 10% of all objects launched into space are still functioning 1






Number of space launches by country (1957-2003) ¹

Country	Number of Launches
USSR	2289
Russia	354
USA	1261
France	161
China	68
Japan	57
India	11
ESA*	6
Israel	4
UK	1
*ESA: European Space Agency

• Related Web Sites

• NASA: National Aeronautics and Space Administration
• ESA: European Space Agency
• Earth Impact Database



• References

• ⁽¹⁾"The Penguin State of the World Atlas" by Dan Smith, 2003. Penguin Books, ISBN: 0-14.200318-2
• ⁽²⁾Wikipedia site