question: How has Earth not been hit by a life-ending asteroid yet?
comment: It has. The asteroid that killed the dinosaurs killed the majority of the species on the planet. But, do you mean something that would wipe out 100% of life? Part of the answer is pure chance. Space is big and the Earth is relatively tiny. The other part is that Jupiter actually acts to deflect a lot of incoming asteroids away from the inner planets.
passages: , is listed with the highest chance of impacting Earth, at 1 in 20 on September 5, 2095. At only across, the asteroid however is much too small to be considered a Potentially Hazardous Asteroid and poses no serious threat: the possible 2095 impact therefore rates only −3.32 on the Palermo Scale. Observations during the August 2022 close approach are expected to ascertain whether the asteroid will impact Earth in 2095.
  
  Section::::History of human awareness of NEOs.:Projects to minimize the threat.
   The next few times a known asteroid this large will come this close to Earth will be in 2028 when passes 0.65 LD from Earth, and in 2029 when the 325-meter 99942 Apophis comes even closer at just 0.10 LD.
  
  According to Jay Melosh, if an asteroid the size of (~400 m across) were to hit land, it would create a crater across, deep and generate a seven-magnitude-equivalent-earthquake. The chances of an actual collision with an asteroid like is about 1 percent in the next thousand years.
  
  Section::::Study.
   In March 2002, became the first asteroid with a positive rating on the Torino Scale, with about a 1 in 9,300 chance of an impact in 2049. Additional observations reduced the estimated risk to zero, and the asteroid was removed from the Sentry Risk Table in April 2002. It is now known that in the next two centuries, will pass the Earth at a safe closest distance (perigee) of on August 31, 2080.
   Larger asteroids can be detected even while far from the Earth, and their orbits can therefore be determined very precisely years in advance of any close approach. Thanks largely to Spaceguard cataloging initiated by a 2005 mandate of the United States Congress to NASA, the inventory of the Near Earth Objects with diameters above 1 kilometer is for instance now 97% complete, while the estimated completeness for 140 meter objects is around one third, and improving. Any impact by one of these known asteroids would be predicted decades to centuries in advance, long enough to consider deflecting them away from Earth. None of them will impact Earth for at least the next century. We are therefore largely safe from kilometer-size impacts for the mid-term future, but sub-km impacts remain a possibility at this point in time.
   Cumulatively among the asteroids listed below, there is a roughly 1 in 16,500 chance that any of the asteroids will impact Earth in 2019. Most of this comes from asteroid which has a 1 in 20,000 chance of impact on 9 September 2019. An impact from this asteroid is highly unlikely as there is only a 0.4% chance that the observations made of it in 2006 would fit to the 'impacting' orbit.
   This list does not include any of the hundreds of objects that collided with earth, which were not discovered in advance, but were recorded by sensors designed to detect detonation of nuclear devices. Of the objects so detected, 78 had an impact energy greater than that of a 1 kiloton device, including 11 which had an impact energy greater than that of a 10 kiloton device i.e. comparable to the atomic bombs used in the Second World War. .
   United States Representative George E. Brown, Jr. (D-CA) was quoted as voicing his support for planetary defense projects in "Air & Space Power Chronicles", saying "If some day in the future we discover well in advance that an asteroid that is big enough to cause a mass extinction is going to hit the Earth, and then we alter the course of that asteroid so that it does not hit us, it will be one of the most important accomplishments in all of human history."
   If a more severe impact is predicted, the response may require evacuation of the area, or with sufficient lead time available, an avoidance mission to repel the asteroid. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.
  
  Section::::Effectiveness of the current system.
   Cumulatively among the asteroids listed below, there is a roughly 1 in 98,700 chance that any of the asteroids will impact Earth in 2018. Most of this comes from asteroid 2008 US which is only ~2 meters in diameter and had a 1 in 240,000 chance of impact on 18 April 2018.
  
  Also included are asteroids discovered before impact (specifically, 2018 LA) and exceptionally massive fireballs with either an equivalent yield of more than 1 kiloton of TNT or an estimated size of more than 3 meters.
  
  Section::::See also.
  
  BULLET::::- List of asteroid close approaches to Earth
   Sub-150m impacting asteroids would not cause global damage but are still locally catastrophic. They can, by contrast to larger ones, only be detected when they come very close to the Earth, which in most cases only happens during their final approach. Those impacts therefore will always need a constant watch and typically cannot be identified earlier than a few weeks in advance, far too late for interception. According to expert testimony in the United States Congress in 2013, NASA would presently require at least five years of preparation before a mission to intercept an asteroid could be launched. Meeting the asteroid and deflecting it by least the diameter of the Earth after its interception would then needs several additional years.
   From the lists in the first section, these are the largest known asteroids per year that approach Earth within one LD. (More than one asteroid per year may be listed if its size is or more.) For comparison, the 1908 Tunguska event was caused by an object about in size, while the 2013 Chelyabinsk meteor which injured thousands of people and buildings when it generated a large airburst over Russia was estimated to be just across.
   Current mechanisms for detecting asteroids on final approach rely on ground based telescopes with wide fields of view. Those currently can monitor the sky at most every second night, and therefore miss most of the smaller asteroids which are bright enough to detect for less than two days. Such very small asteroids much more commonly impact Earth than larger ones, but they make little damage. Missing them therefore has limited consequences. Much more importantly, ground-based telescopes are blind to most of the asteroids which impact the day side of the planet and would miss even large ones. These and other problems mean very few impacts are successfully predicted (see §Effectiveness of the current system and §Improving impact prediction).
   BULLET::::- Powell, Corey S. "How to Deflect a Killer Asteroid: Researchers Come Up With Contingency Plans That Could Help Our Planet Dodge A Cosmic Bullet", "Discover", September 18, 2013, pp. 58–60 (subscription).
  
  BULLET::::- Schweickart, Lu, Hut and Chapman. "The Asteroid Tugboat: To Prevent An Asteroid From Hitting Earth, A Space Tug Equipped With Plasma Engines Could Give It A Push", October 13, 2003, "Scientific American"
  
  BULLET::::- Steel, Duncan. "Rogue Asteroids and Doomsday Comets: The Search for the Million Megaton Menace That Threatens Life on Earth", Wiley & Sons, 1995, [1997], , .
  
  Section::::External links.
  
  BULLET::::- B612 Foundation: early website homepage (archived)
   With an observation arc of only 4 days, the asteroid has a poorly determined orbit. It is already known that there is no risk of an Earth impact before 2098. The power of such an air burst would be somewhere between the Chelyabinsk meteor and the Tunguska event depending on the actual size of the asteroid.
   The October 2012 observations have reduced the orbit uncertainties by more than a factor of 60, meaning that the Earth's position in February 2040 no longer falls within the range of possible future paths for the asteroid. On 4 February 2040 the asteroid will pass no closer than (~2.3 LD) from Earth. Until 21 December 2012 it was listed on the Sentry Risk Table with a rating on the Torino Scale of Level 1. A Torino rating of 1 is a routine discovery in which a pass near the Earth is predicted that poses no unusual level of danger. It is estimated that an impact would produce the equivalent of 100 megatons of TNT, roughly twice that of the most powerful nuclear weapon ever detonated (Tsar Bomba). This is powerful enough to damage a region at least a hundred miles wide.
   Several asteroids have collided with earth in recent geological history. The Chicxulub asteroid, for example, was around 6 miles in diameter and is theorized to have caused the extinction of the non-avian dinosaurs 66 million years ago at the end of the Cretaceous. No sufficiently large asteroid currently exists in an Earth-crossing orbit; however, a comet of sufficient size to cause human extinction could impact the Earth, though the annual probability may be less than 10. Geoscientist Brian Toon estimates that while a few people, such as "some fishermen in Costa Rica", could plausibly survive a 6-mile meteorite, a 60-mile meteorite would be large enough to "incinerate everybody". Asteroids with around a 1 km diameter have impacted the Earth on average once every 500,000 years; these are probably too small to pose an extinction risk, but might kill billions of people. Larger asteroids are less common. Small near-Earth asteroids are regularly observed and can impact anywhere on the Earth injuring local populations. As of 2013, Spaceguard estimates it has identified 95% of all NEOs over 1 km in size.
   A few near misses have been predicted, years in advance, with a tiny chance of actually striking Earth. A handful of actual impactors have been detected hours in advance. All were small, struck wilderness or ocean, and hurt nobody. Current systems only detect an arriving object when several factors are just right, mainly the direction of approach relative to the Sun, weather, and phase of the Moon. The result is a low rate of success. Performance is improving as existing systems are upgraded and new ones come on line, but the blind spot issue which the current systems face around the Sun can only be overcome by a dedicated space based system.
   2009 DD45
  
  BBC News Online cites the minimum distance as (about 1/5 lunar distances). passed farther away (40 thousand miles versus 4 thousand miles) but was substantially larger than 2004 FU162, a small asteroid about 6 m (20 ft) across which came within about in 2004, and is more similar in size to 2004 FH. With an uncertainty parameter of 3, the asteroid is predicted to make its next close encounters with Earth on 2056-Feb-29 and 2067-Mar-03.
  
  Section::::External links.
  
  BULLET::::- Universe Today article on 2009 DD45
  
  BULLET::::- 100 meter asteroid will pass Earth Monday!, Phil Plait's Bad Astronomy blog
   The probability of a mid-sized asteroid (similar to the one that destroyed the Tunguska River area of Russia in 1908) hitting Earth during the 21st century has been estimated at about 30%. Since the Earth is currently more populated than in previous eras, there is a greater risk of large casualties arising from a mid-sized asteroid impact. However, as of the early 2010s, only about a half of one per cent of Tunguska-type NEOs had been located by astronomers using ground-based telescope surveys.
   In April 2018, the B612 Foundation reported "It's a 100 per cent certain we'll be hit [by a devastating asteroid], but we're not 100 per cent sure when."Also in 2018, physicist Stephen Hawking, in his final book "Brief Answers to the Big Questions", considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the ""National Near-Earth Object Preparedness Strategy Action Plan"" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.
   BULLET::::- "Defenders of the Planet" (2001), a three-part British TV mini-series discussing the individuals and organizations working to defend the Earth against killer asteroids and other extraterrestrial threats; broadcast on The Learning Channel.
  
  BULLET::::- "Danny Phantom": In the series finale episodes "Phantom Planet" an asteroid is on a collision course with Earth. Danny convinces Earth's ghosts to turn the Earth intangible, avoiding disaster.
  
  BULLET::::- "The Sarah Jane Adventures": In "Whatever Happened to Sarah Jane?" (2007), a meteor on a collision course with the Earth is ultimately deflected back into space by Sarah Jane's alien computer, Mr. Smith.
   Section::::Passed by outside of atmosphere.:Objects  50 meters.
  
  Asteroids larger than about .
  
  Section::::Predicted encounters.
  
  Incomplete list of asteroids larger than about predicted to pass close to Earth (see also Asteroid impact prediction and Sentry Risk Table):
  
  A list of predicted NEO approaches at larger distances is maintained as a database by the NASA Near Earth Object Program.
  
  Section::::Earth grazers.
    has a notably low minimum orbital intersection distance with Earth of , or 0.06 LD. The asteroid's next encounter with Earth will be on 26 February 2019 at a much larger distance of . It closest future approach is predicted to occur on 28 February 2064, then at a nominal distance of .
  
  Section::::Physical characteristics.
   In April 2018, the B612 Foundation reported "It’s 100 per cent certain we’ll be hit [by a devastating asteroid], but we’re not 100 per cent certain when." Also in 2018, physicist Stephen Hawking, in his final book "Brief Answers to the Big Questions", considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the ""National Near-Earth Object Preparedness Strategy Action Plan"" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.
   At the 2014 NASA Innovative Advanced Concepts (NIAC) conference, Wie and his colleagues stated that "we have the solution, using our baseline concept, to be able to mitigate the asteroid-impact threat, with any range of warning." For example, according to their computer models, with a warning time of 30 days, a asteroid would be neutralized by using a single HAIV, with less than 0.1% of the destroyed object's mass potentially striking Earth, which by comparison would be more than acceptable.
  
presuppositions: The Earth has not been hit by a life-ending asteroid. Earth has not been hit by a life-ending asteroid.
corrections: The asteroid that killed the dinosaurs killed the majority of the species on the planet. Earth was hit by a life-ending asteroid that killed the dinosaurs and the majority of the species on the planet.
raw_presuppositions: The Earth has not been hit by a life-ending asteroid. Earth has not been hit by a life-ending asteroid.
raw_corrections: The asteroid that killed the dinosaurs killed the majority of the species on the planet. Earth was hit by a life-ending asteroid that killed the dinosaurs and the majority of the species on the planet.