Asteroid impact prediction is the prediction of the dates and times of asteroids impacting Earth, along with the locations and severities of the impacts.
The process of impact prediction follows three major steps: Discovery of an asteroid and initial assessment of its orbit which is generally based on a short observation arc of less than 2 weeks. Follow-up observations to improve the orbit determination Calculating if, when and where the orbit may intersect with Earth at some point in the future. The usual purpose of predicting an impact is to direct an appropriate response.
Most asteroids are discovered by a camera on a telescope with a wide field of view. Image differencing software compares a recent image with earlier ones of the same part of the sky, detecting objects that have moved, brightened, or appeared. Those systems usually obtain a few observations per night, which can be linked up into a very preliminary orbit determination. This predicts approximate positions over the next few nights, and follow-ups can then be carried out by any telescope powerful enough to see the newly detected object. Orbit intersection calculations are then carried out by two independent systems, one (Sentry) run by NASA and the other (NEODyS) by ESA.
Current systems detect an arriving object only when several factors align, mainly its direction of approach relative to the Sun, the weather, and the Moon's phase. Smaller objects are less likely to be detected before impact.Smaller asteroids may be bright enough to observe only briefly. Larger asteroids remain visible longer, which can offset temporary obstacles such as bad weather or a bright Moon; however, proximity to the Sun can prevent asteroids of any size from being discovered. This is particularly true for Aten asteroids, which spend most of their time closer to the Sun than Earth and are therefore difficult to detect without a space-based system orbiting inside Earth's orbit. A few near misses by medium-size asteroids have been predicted years in advance, with a tiny chance of striking Earth, and a handful of small impactors have successfully been detected hours in advance. All of the latter struck wilderness or ocean, injuring no one. The majority of impacts are by small, undiscovered objects. They rarely hit a populated area, but can cause widespread damage when they do. Detection of smaller objects is improving as existing systems are upgraded and new ones come online, but all current systems have a blind spot around the Sun that can be overcome only by a dedicated space-based system or by discovering objects during an earlier approach to Earth, years before a potential impact.
History In 1992 a report to NASA recommended a coordinated survey (christened Spaceguard) to discover, verify and provide follow-up observations for Earth-crossing asteroids.Morrison, D., 25 January 1992, The Spaceguard Survey: Report of the NASA International Near-Earth-Object Detection Workshop , NASA, Washington, D.C. This survey was scaled to discover 90% of all objects larger than one kilometer within 25 years. Three years later, a further NASA report recommended search surveys that would discover 60–70% of the short-period, near-Earth objects larger than one kilometer within ten years and obtain 90% completeness within five more years.Shoemaker, E.M., 1995, Report of the Near-Earth Objects Survey Working Group, NASA Office of Space Science, Solar System Exploration Office
In 1998, NASA formally embraced the goal of finding and cataloging, by 2008, 90% of all near-Earth objects (NEOs) with diameters of 1 km or larger that could represent a collision risk to Earth. The 1 km diameter metric was chosen after considerable study indicated that an impact of an object smaller than 1 km could cause significant local or regional damage but is unlikely to cause a worldwide catastrophe. The impact of an object much larger than 1 km diameter could well result in worldwide damage up to, and potentially including, extinction of the human race. The NASA commitment has resulted in the funding of a number of NEO search efforts, which made considerable progress toward the 90% goal by the target date of 2008 and also produced the first ever successful prediction of an asteroid impact (the 4-meter was detected 19 hours before impact). However, the 2009 discovery of several NEOs approximately 2 to 3 kilometers in diameter (e.g. , , , and ) demonstrated there were still large objects to be detected.
Three years later, in 2012, the 40 meter diameter asteroid 367943 Duende was discovered and successfully predicted to be on close but non-colliding approach to Earth again just 11 months later. This was a landmark prediction as the object was only , and it was closely monitored as a result. On the day of its closest approach and by coincidence, a smaller asteroid was also approaching Earth, unpredicted and undetected, from a direction close to the Sun. Unlike 367943 Duende it was on a collision course and it impacted Earth 16 hours before 367943 Duende passed, becoming the Chelyabinsk meteor. It injured 1,500 people and damaged over 7,000 buildings, raising the profile of the dangers of even small asteroid impacts if they occur over populated areas. The asteroid is estimated to have been 18m () across.
In April 2018, the B612 Foundation stated "It's 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.
Discovery of near-Earth asteroids The first step in predicting impacts is detecting asteroids and determining their orbits. Finding faint near-Earth objects against the much more numerous background stars is very much a needle in a haystack search. It is achieved by sky surveys that are designed to discover near-Earth asteroids. Unlike the majority of telescopes that have a narrow field of view and high magnification, survey telescopes have a wide field of view to scan the entire sky in a reasonable amount of time with enough sensitivity to pick up the faint near-Earth objects they are searching for.
NEO focused surveys revisit the same area of sky several times in succession. Movement can then be detected using image differencing techniques. Anything that moves from image to image against the background of stars is compared to a catalogue of all known objects, and if it is not already known is reported as a new discovery along with its precise position and the observation time. This then allows other observers to confirm and add to the data about the newly discovered object.
Cataloging vs warning surveys Asteroid surveys can be broadly classified as either cataloging surveys, which use larger telescopes to mostly identify larger asteroids well before they come notably close to Earth, or warning surveys, which use smaller telescopes to mostly look for smaller asteroids within several million kilometers of Earth. Cataloging systems focus on finding larger asteroids years in advance and they scan the sky slowly (of the order of once per month), but deeply. Warning systems focus on scanning the sky relatively quickly (of the order of once per night). They typically cannot detect objects that are as faint as cataloging systems but they will not miss an asteroid that dramatically brightens for just a few days when it passes very close to Earth. Some systems compromise and scan the sky approximately once per week.
Cataloging systems For larger asteroids (> 100 m to 1 km across), prediction is based on cataloging the asteroid, years to centuries before it could impact. This technique is possible as their size makes them bright enough to be seen from a long distance. Their orbits therefore can be measured and any future impacts predicted long before they are on an impact approach to Earth. This long period of warning is important as an impact from a 1 km object would cause worldwide damage and a minimum of around a decade of lead time would be needed to deflect it away from Earth. As of 2018, the inventory is nearly complete for the kilometer-size objects (around 900) which would cause global damage, and approximately one third complete for 140 meter objects (around 8500) which would cause major regional damage.Incompleteness refers to the fraction of undiscovered asteroids, not the amount of time remaining to achieve completeness. The asteroids remaining to be discovered are the ones which are hardest to find.The exact percentage of objects discovered is uncertain but is estimated using statistical techniques. 2018 estimates for objects at least 1 km in size put the figure somewhere between 89% and 99%, with an expected value of 94%. This matches the figure from a 2017 NASA report which was estimated independently using a different technique The effectiveness of the cataloging is somewhat limited by the fact that some proportion of the objects have been lost since their discovery, due to insufficient observations to accurately determine their orbits.
Warning systems Smaller near-Earth objects number into millions and therefore impact Earth much more often, though obviously with much less damage. The vast majority remain undiscovered. They seldom pass close enough to Earth that they become bright enough to observe, and so most can only be observed when within a few million kilometers of Earth. They therefore cannot usually be catalogued well in advance and can only be warned about, a few weeks to days in advance.
Current mechanisms for detecting asteroids on approach rely on ground based visible-light telescopes with wide fields of view. Those currently can monitor the sky at most every night, and therefore miss most of the smaller asteroids which are bright enough to detect for less than a day. Such very small asteroids much more commonly impact Earth than larger ones, but cause 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 will miss even large ones. These and other problems mean very few impacts are successfully predicted .
Asteroids detected by warning systems are much too close to their time of potential impact to deflect them away from Earth, but there is still enough time to mitigate the consequences of the impact by evacuating and otherwise preparing the affected area. Warning systems can also detect asteroids which have been successfully catalogued as existing, but whose orbit was insufficiently well determined to allow a prediction of where they are now.
Surveys
The main NEO-focused surveys are listed below, along with future telescopes that are already funded.