Asteroid Terrestrial-impact Last Alert System

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.[3] 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.[4]

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.[5] Stony asteroids with a diameter of 4 meters (13 ft) enter Earth's atmosphere approximately once per year.[6] 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.[6] These relatively small asteroids ordinarily explode in the upper atmosphere and most or all of the solids are vaporized.[7] 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.[8] The 2013 Chelyabinsk meteor event is the only known impact in historical times to have resulted in a large number of injuries, with the potential 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.[9][10][11] 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."[12][13] 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.[14][15][16][17][18]

Annually discovered NEAs by survey since 1995

Large NEAs (at least 1 km in diameter) discovered each year

•   LINEAR
•   NEAT
•   Spacewatch
•   LONEOS
•   CSS
•   Pan-STARRS
•   NEOWISE
•   Others

Larger asteroids can be detected even while far from the Earth, and their orbits can therefore be determined very precisely many years in advance of any close approach. Thanks largely to Spaceguard cataloging initiated by a 2005 mandate of the United States Congress to NASA,[19] the inventory of the approximately one thousand Near Earth Objects with diameters above 1 kilometer was for instance 97% complete as of 2017.[20] The estimated completeness for 140 meter objects is around one third, and slowly 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, and we are therefore largely safe from globally civilisation-ending kilometer-size impacts for at least the mid-term future. Regionally catastrophic sub-km impacts, on the other hand, remain a possibility at this point in time.

Sub-150m impacting asteroids would not cause large scale damage but are still locally catastrophic. They are much more common and they can, by contrast to larger ones, only be detected when they come very close to the Earth. In most cases this only happens during their final approach. Those impacts therefore will always need a constant watch and they 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.[21] This time could be much reduced by pre-planning a ready to launch mission, but meeting the asteroid and then deflecting it by least the diameter of the Earth after its interception will always need several uncompressible 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".[22] Most of the more than 1 billion rubles damage[23] and of the 1500 injuries[24] caused by the 17-m Chelyabinsk meteor impact in 2013 were from window glass broken by its shock wave.[25] 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,[26] 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.[27] 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,[28][29] 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.[30] The full ATLAS concept consists of eight telescopes, spread over the globe for full-nightsky 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.[26] 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[31] and it therefore was in the blind spot of any Earth-based visible light warning system. A similar object arriving from a dark direction would now be detected by ATLAS a few days in advance.[32]

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,[33] supernovae,[26] non-impacting asteroids, comets, and dwarf planets.

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 160 km apart on Haleakala and Mauna Loa in the Hawaiian Islands, ATLAS1 and ATLAS2,[34] with the third telescope under construction at the South African Astronomical Observatory.[35][36] These telescopes are notable for their large 7.4° field of view — about 15 times the diameter of the full moon — of which their 10 500 × 10 500 CCD camera images the central 5.4° × 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 10 seconds for simultaneous camera readout and telescope repointing, each ATLAS unit can therefore scan the whole visible sky a little over once each night, with a median completeness limit at apparent magnitude 19.[37] 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. This allows to automatically link multiple observations of an asteroid into a preliminary orbit, with some robustness to the loss of one observation to overlap between the asteroid and a bright star, and to predict its approximate position on subsequent nights. Apparent magnitude 19 is classified as "respectably but not extremely faint", and is approximately 100 000 times too faint to be seen with a naked eye from a very dark location. It is 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.[26] This makes ATLAS better suited to finding small asteroids which can only be seen 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.[38] Further NASA grants fund continued operation of ATLAS until 2021[39] and the construction of two Southern telescopes.[28]

When completed in 2021, the new ATLAS site will restore the current lack of coverage in the Southern hemisphere (see Asteroid impact prediction). Sited around 120° (8 hours) east of existing surveys, the planned ATLAS South Africa and NEOSTEL telescopes will also provide warnings during the Hawaii and California daytimes. This mostly matters for small asteroids which become bright enough for detection for at most a day or two.

• SN 2018cow, a relatively bright supernova on 2018-06-16.
• 2018 AH, largest asteroid to pass so close to Earth since 1971 on 2018-01-02.
• A106fgF, a 2-5m asteroid which either passed extremely close or impacted Earth 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).[40]
• A10bMLz, unknown space debris, so-called "empty trash bag object" on 2019-01-25.[41]
• 2019 MO, an approximately 4 m asteroid which impacted the Caribbean Sea South of Puerto Rico in June 2019[42]
• C/2019 Y4 (ATLAS), comet
• 2020 VT4 a 5-10 m object which passed closer to Earth than any other known near-miss asteroid

• 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
• List of meteor air bursts
• The Spaceguard Foundation
• asteroid List and Updates

• Official website
• ATLAS: The Asteroid Terrestrial-impact Last Alert System
• Planetary Defense Conference 2017, (starts at 1h 10m 29s) on YouTube, Japan, 15 May 2017