Asteroid Terrestrial-impact Last Alert System
|Survey type||Astronomical survey|
|Observatory code||T05 (ATLAS-HKO)|
The Asteroid Terrestrial-impact Last Alert System (ATLAS; Observatory codes T05 and T08) is a robotic astronomical survey and early-warning system optimized for detecting smaller near-Earth objects a few weeks to days before they impact Earth. Funded by NASA, and developed and operated by the University of Hawaii's Institute for Astronomy, the system's current two 0.5-meter telescopes are located at Haleakala (ATLAS-HKO) and Mauna Loa (ATLAS-MLO) observatories, on two of the Hawaiian Islands and 160 km apart. ATLAS began observations in 2015 and its two-telescopes version is fully operational since 2017. Each of the two telescopes surveys one quarter of the whole observable sky four times per clear night, for a four-fold coverage of the observable sky every two clear nights. The project has obtained NASA funding for two additional telescopes in the Southern hemisphere. Once those two telescopes will become operational, ATLAS's four-fold coverage of the observable sky will improve from every two clear nights to nightly.
Major astronomical impact events have significantly shaped Earth's history, having been implicated in the formation of the Earth–Moon system, the origin of water on Earth, the evolutionary history of life, and several mass extinctions. Notable prehistorical impact events include the Chicxulub impact, 66 million years ago, believed to be the cause of the Cretaceous–Paleogene extinction event. The 37 million years old asteroid impact that caused Mistastin crater generated temperatures exceeding 2,370 °C, the highest known to have naturally occurred on the surface of the Earth.
Throughout recorded history, hundreds of Earth impacts (and exploding bolides) have been reported, with some small fraction causing deaths, injuries, property damage, or other significant localised consequences. Stony asteroids with a diameter of 4 meters (13 ft) enter Earth's atmosphere approximately once per year. Asteroids with a diameter of 7 meters enter the atmosphere about every 5 years, with as much kinetic energy as the atomic bomb dropped on Hiroshima (approximately 16 kilotons of TNT), of which their air burst represents about one third or 5 kilotons. These relatively small asteroids ordinarily explode in the upper atmosphere and most or all of the solids are vaporized. Asteroids with a diameter of 20 m (66 ft) strike Earth approximately twice every century. One of the best-known recorded impacts in historical times is the 50 meter 1908 Tunguska event, which leveled several thousand square kilometers of forest in a very sparsely populated part of Siberia, Russia. Such an impact over a more populous region would have caused locally catastrophic damage. The 2013 Chelyabinsk meteor event is the only known impact in historical times to have resulted in a large number of injuries, with the possible exception of the possibly highly deadly but poorly documented 1490 Ch'ing-yang event in China. The approximately 20 meter Chelyabinsk meteor is the largest recorded object to have impacted a continent of the Earth since the Tunguska event.
Future impacts are bound to occur, with much higher odds for smaller regionally damaging asteroids than for larger globally damaging ones. The 2018 final book of physicist Stephen Hawking, Brief Answers to the Big Questions, considers a large asteroid collision the biggest threat to our planet. In April 2018, the B612 Foundation reported "It's a 100 per cent certainty we'll be hit [by a devastating asteroid], but we're not 100 per cent sure when." 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.
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.
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.
The Last Alert part of the system name acknowledges that ATLAS will find smaller asteroids years too late for potential deflection but would provide the days or weeks of warning needed to evacuate and otherwise prepare a target area. According to ATLAS project lead John Tonry, "that's enough time to evacuate the area of people, take measures to protect buildings and other infrastructure, and be alert to a tsunami danger generated by ocean impacts". Most of the more than 1 billion rubles damage and of the 1500 injuries caused by the 17-m Chelyabinsk meteor impact were from window glass broken by its shock wave. With even a few hours advance warning those losses and injuries could have been much reduced by actions as simple as propping all windows open before the impact and staying away from them.
The ATLAS project was developed at the University of Hawaii with US$5 million funding from NASA, and its first element was deployed in 2015. This first telescope became fully operational at the end of 2015, and the second one in March 2017. Replacement of the initially substandard Schmidt corrector plates of both telescopes in June 2017 brought their image quality closer to its nominal 2 pixels (3.8") width and consequently improved their sensitivity by one magnitude. In August 2018, the project obtained US$3.8 million of additional NASA funding to install two telescopes in the Southern hemisphere, one of which will be hosted by the South African Astronomical Observatory, and the other most likely installed in Chile. This geographical expansion of ATLAS will provide visibility of the far Southern sky, more continuous coverage, better resilience to bad weather, and additional information on the asteroid orbit from the parallax effect. The full ATLAS concept consists of eight telescopes, spread over the globe for full-sky and 24h/24h coverage.
As long as their radiant is not too close to the Sun, and for the current Hawaii-based system not too far into the Southern hemisphere, the automated system provides a one-week warning for a 45 metres (150 ft) diameter asteroid, and a three-week warning for a 120 m (390 ft) one. By comparison, the February 2013 Chelyabinsk meteor impact was from an object estimated at 17 m (60 ft) diameter. Its arrival direction happened to be close to the Sun and it therefore was in the blind spot of any ATLAS-like system. A similar object arriving from a dark direction would be detected by ATLAS a couple of days in advance.
As a by-product of its main design goal, ATLAS can identify any moderately bright variable or moving object in the night sky. It therefore also looks for variable stars, supernovae, non-impacting asteroids, comets, and dwarf planets.
Design and operation
The full ATLAS concept consists of eight 50-centimeter diameter f/2 Wright-Schmidt telescopes, spread over the globe for full-night-sky and 24h/24h coverage, and each fitted with a 110 Megapixel CCD array camera. The current system consists of two such telescopes operating on Haleakala and Mauna Loa in the Hawaiian Islands, ATLAS1 and ATLAS2. A major feature of these telescopes is their large 7.4° field of view - about 15 times the diameter of the full moon- of which their 10 500 x 10 500 CCD camera images the central 5.4° x 5.4°. This system can image the whole night sky visible from Hawaii with about 1000 separate telescope pointings. At 30 seconds per exposure plus 5 seconds for readout and simultaneous repointing, each ATLAS telescope can therefore scan the whole visible sky a little over once each night, with a median completeness limit at apparent magnitude 19. Since the mission of the telescope is to identify moving objects, each telescope actually observes one quarter of the sky four times in a night at approximately 15-minute intervals, as needed to automatically link multiple observations of an asteroid into a preliminary orbit and predict its approximate position on subsequent nights. Apparent magnitude 19 is astronomer-speak for "respectably but not extremely faint", and it is the equivalent to the light of a match flame in New York viewed from San Francisco. ATLAS therefore scans the visible sky in much less depth, but much more quickly, than larger surveying telescope arrays such as University of Hawaii's Pan-STARRS. Pan-STARRS goes approximately 100 times deeper, but needs weeks instead of half a night to scan the whole sky just once. This makes ATLAS better suited to finding small asteroids during the just few days that they brighten dramatically when they happen to pass very close to the Earth.
NASA's Near Earth Observation Program initially provided a US$5 million grant, with $3.5 million covering the first three years of design, construction and software development, and the balance of the grant to fund the systems operation for two years following its entry into full operational service in late 2015. Further NASA grants fund continued operation of ATLAS until 2021 and the construction of two Southern telescopes.
- AT2018cow, possibly Supernova 2018cow on 2018-06-16.
- 2018 AH, largest asteroid to pass so close to Earth since 1972 on 2018-06-02.
- A106fgF, unconfirmed potential impactor on 2018-01-22.
- 2018 RC, near earth asteroid on 2018-09-03 (notable because it was discovered more than a day prior to closest approach on 2018-09-09).
- A10bMLz, unknown space debris, so-called "empty trash bag object" on 2019-01-25. 
- List of near-Earth object observation projects
- 1490 Ch'ing-yang event
- 99942 Apophis
- Asteroid impact prediction
- Asteroid impact avoidance
- B612 Foundation
- Framework Programmes for Research and Technological Development
- Gravity tractor
- List of meteor air bursts
- The Spaceguard Foundation
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