Asteroid impact avoidance
Asteroid impact avoidance encompasses the methods by which near-Earth objects on a potential collision course with Earth could be diverted, preventing destructive impact events. An impact by a sufficiently large asteroid or other NEOs would cause, depending on its impact location, massive tsunamis or multiple firestorms, and an impact winter caused by the sunlight-blocking effect of large quantities of pulverized rock dust and other debris placed into the stratosphere. A collision 66 million years ago between the Earth and an object approximately wide is thought to have produced the Chicxulub crater and triggered the Cretaceous–Paleogene extinction event that the scientific community understands to have caused the extinction of all non-avian dinosaurs.
While the chances of a major collision are low in the near term, it is a near-certainty that one will happen eventually unless defensive measures are taken. Astronomical events—such as the Shoemaker-Levy 9 impacts on Jupiter and the 2013 Chelyabinsk meteor, along with the growing number of near-Earth objects discovered and catalogued on the Sentry Risk Table—have drawn renewed attention to such threats. The popularity of the 2021 movie Don't Look Up helped to raise awareness of the possibility of avoiding NEOs. Awareness of the threat has grown rapidly during the past few decades, but much more needs to be accomplished before the human population can feel adequately protected from a potentially catastrophic asteroid impact.
In 2016, a NASA scientist warned that the Earth is unprepared for such an event. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit by a devastating asteroid, but we're not 100 percent 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.
Several ways of avoiding an asteroid impact have been described. There are two primary ways: to modify the trajectory of the object so that it does not collide with the Earth, or to modify the object by breaking it up so that the resulting fragments do not collide with the Earth or their
smaller size reduces the subsequent hazard posed to the Earth.
Nonetheless, in March 2019, scientists reported that asteroids may be much more difficult to destroy than thought earlier. An asteroid may reassemble itself due to gravity after being disrupted. In May 2021, NASA astronomers reported that 5 to 10 years of preparation may be needed to avoid a virtual impactor based on a simulated exercise conducted by the 2021 Planetary Defense Conference.
In 2022, NASA spacecraft DART impacted Dimorphos, reducing the minor-planet moon's orbital period by 32 minutes. This mission constitutes the first successful attempt at asteroid deflection. In 2027, China plans to launch a deflection mission to the near-Earth object 2015 XF261, with the impact estimated to occur in April 2029.
Deflection efforts
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. In June 2018, the US National Science and Technology Council warned that the United States was unprepared for an asteroid impact event, and developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision-avoidance project, as no known planetary defense hardware has yet been developed. It has been estimated that a velocity change of just is needed to successfully deflect a body on a direct collision trajectory. Thus for a large number of years before impact, much smaller velocity changes are needed. For example, it was estimated there was a high chance of 99942 Apophis swinging by Earth in 2029 with a 10−4 probability of returning on an impact trajectory in 2035 or 2036. It was then determined that a deflection from this potential return trajectory, several years before the swing-by, could be achieved with a velocity change on the order of 10−6 m/s.
NASA's Double Asteroid Redirection Test, the world's first full-scale mission to test technology for defending Earth against potential asteroid or comet hazards, launched on a SpaceX Falcon 9 rocket from Space Launch Complex 4 East at Vandenberg Space Force Base in California.
An impact by a asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the biosphere. There is also the threat from comets entering the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months. Impacts from objects as small as in diameter, which are far more common, are historically extremely destructive regionally.
Finding out the material composition of the object is also helpful before deciding which strategy is appropriate. Missions like the 2005 Deep Impact probe and the Rosetta spacecraft, have provided valuable information on what to expect. In October 2022, a method of mapping the insides of a potentially problematic asteroid in order to determine the best area for impact was proposed.
History of US government mandates
Efforts in asteroid impact prediction have concentrated on the survey method. The 1992 NASA-sponsored Near-Earth-Object Interception Workshop hosted by Los Alamos National Laboratory evaluated issues involved in intercepting celestial objects that could hit Earth. In a 1992 report to NASA, a coordinated Spaceguard Survey was recommended to discover, verify and provide follow-up observations for Earth-crossing asteroids. This survey was expected to discover 90% of these objects larger than one kilometer within 25 years. Three years later, another NASA report recommended search surveys that would discover 60–70% of short-period, near-Earth objects larger than one kilometer within ten years and obtain 90% completeness within five more years.In 1998, NASA formally embraced the goal of finding and cataloging, by 2008, 90% of all near-Earth objects 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 species. The NASA commitment has resulted in the funding of a number of NEO search efforts, which made considerable progress toward the 90% goal by 2008. However the 2009 discovery of several NEOs approximately 2 to 3 kilometers in diameter demonstrated there were still large objects to be detected.
United States Representative George E. Brown Jr. 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."
Because of Congressman Brown's long-standing commitment to planetary defense, a U.S. House of Representatives' bill, H.R. 1022, was named in his honor: The George E. Brown Jr. Near-Earth Object Survey Act. This bill "to provide for a Near-Earth Object Survey program to detect, track, catalogue, and characterize certain near-Earth asteroids and comets" was introduced in March 2005 by Rep. Dana Rohrabacher. It was eventually rolled into S.1281, the NASA Authorization Act of 2005, passed by Congress on December 22, 2005, subsequently signed by the President, and stating in part:
The result of this directive was a report presented to Congress in early March 2007. This was an Analysis of Alternatives study led by NASA's Program Analysis and Evaluation office with support from outside consultants, the Aerospace Corporation, NASA Langley Research Center, and SAIC.
Ongoing projects
The Minor Planet Center in Cambridge, Massachusetts has been cataloging the orbits of asteroids and comets since 1947. It has recently been joined by surveys that specialize in locating the near-Earth objects, many funded by NASA's Near Earth Object program office as part of their Spaceguard program. One of the best-known is LINEAR that began in 1996. By 2004 LINEAR was discovering tens of thousands of objects each year and accounting for 65% of all new asteroid detections. LINEAR uses two one-meter telescopes and one half-meter telescope based in New Mexico.The Catalina Sky Survey is conducted at the Steward Observatory's Catalina Station, located near Tucson, Arizona, in the United States. It uses two telescopes, a f/2 telescope on the peak of Mount Lemmon, and a f/1.7 Schmidt telescope near Mount Bigelow. In 2005, CSS became the most prolific NEO survey surpassing Lincoln Near-Earth Asteroid Research in total number of NEOs and potentially hazardous asteroids discovered each year since. CSS discovered 310 NEOs in 2005, 396 in 2006, 466 in 2007, and in 2008 564 NEOs were found.
Spacewatch, which uses a telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis equipment to search the skies for intruders, was set up in 1980 by Tom Gehrels and Robert S. McMillan of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by McMillan. The Spacewatch project has acquired a telescope, also at Kitt Peak, to hunt for NEOs, and has provided the old 90-centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability.
Other near-Earth object tracking programs include Near-Earth Asteroid Tracking, Lowell Observatory Near-Earth-Object Search, Campo Imperatore Near-Earth Object Survey, Japanese Spaceguard Association, and Asiago-DLR Asteroid Survey. Pan-STARRS completed telescope construction in 2010, and it is now actively observing.
The Asteroid Terrestrial-impact Last Alert System, in operation since 2015, conducts almost nightly scans of the night sky with a view to later-stage detection on the collision stretch of the asteroid orbit. Those would be much too late for deflection, but still in time for evacuation and preparation of the affected Earth region.
Another project, supported by the European Union, is NEOShield, which analyses realistic options for preventing the collision of a NEO with Earth. Their aim is to provide test mission designs for feasible NEO mitigation concepts. The project particularly emphasises on two aspects.
- The first one is the focus on technological development on essential techniques and instruments needed for guidance, navigation and control in close vicinity of asteroids and comets. This will, for example, allow hitting such bodies with a high-velocity kinetic impactor spacecraft and observing them before, during and after a mitigation attempt, e.g., for orbit determination and monitoring.
- The second one focuses on refining Near Earth Object characterisation. Moreover, NEOShield-2 will carry out astronomical observations of NEOs, to improve the understanding of their physical properties, concentrating on the smaller sizes of most concern for mitigation purposes, and to identify further objects suitable for missions for physical characterisation and NEO deflection demonstration.
NEODyS is an online database of known NEOs.