Earth's 10 Most Likely Hotspots to be Obliterated by an Asteroid --NASA Tracks Potentially Hazardous Objects
Observations from NASA's Wide-field Infrared Survey Explorer (WISE)have led to the best assessment yet of our solar system's population of potentially hazardousasteroids. The results reveal new information about their total numbers, origins and the possibledangers they may pose.
Potentially hazardous asteroids, or PHAs, are a subset of the larger group of near-Earth asteroids.The PHAs have the closest orbits to Earth's, coming within five million miles (about eightmillion kilometers), and they are big enough to survive passing through Earth's atmosphere andcause damage on a regional, or greater, scale.
While previous estimates of PHAs predicted similar numbers, they were rough approximations.NEOWISE has generated a more credible estimate of the objects' total numbers and sizes.
"The NEOWISE analysis shows us we've made a good start at finding those objects that trulyrepresent an impact hazard to Earth," said Lindley Johnson, program executive for the Near-Earth Object Observation Program at NASA Headquarters in Washington. "But we've many more to find, and it will take a concerted effort during the next couple of decades to find all ofthem that could do serious damage or be a mission destination in the future."
The new analysis also suggests that about twice as many PHAs as previously thought are likely to reside in "lower-inclination" orbits, which are more aligned with the plane of Earth's orbit. In addition, these lower-inclination objects appear to be somewhat brighter and smaller than theother near-Earth asteroids that spend more time far away from Earth. A possible explanation isthat many of the PHAs may have originated from a collision between two asteroids in the mainbelt lying between Mars and Jupiter. A larger body with a low-inclination orbit may have broken up in the main belt, causing some of the fragments to drift into orbits closer to Earth andeventually become PHAs.
Asteroids with lower-inclination orbits would be more likely to encounter Earth and would beeasier to reach. The results therefore suggest more near-Earth objects might be available forfuture robotic or human missions.
"NASA's NEOWISE project, which wasn't originally planned as part of WISE, has turned out tobe a huge bonus," said Amy Mainzer, NEOWISE principal investigator, at NASA's JetPropulsion Laboratory in Pasadena, Calif. "Everything we can learn about these objects helps us understand their origins and fate. Our team was surprised to find the overabundance of low-inclination PHAs. Because they will tend to make more close approaches to Earth, these target scans provide the best opportunities for the next generation of human and robotic exploration."
The discovery that many PHAs tend to be bright says something about their composition; they are more likely to be either stony, like granite, or metallic. This type of information is importantin assessing the space rocks' potential hazards to Earth. The composition of the bodies would affect how quickly they might burn up in our atmosphere if an encounter were to take place.
The WISE spacecraft scanned the sky twice in infrared light before entering hibernation mode inearly 2011. It catalogued hundreds of millions of objects, including super-luminous galaxies,stellar nurseries and closer-to-home asteroids. The NEOWISE project snapped images of about 600 near-Earth asteroids, about 135 of which were new discoveries. Because the telescope detected the infrared light, or heat, of asteroids, it was able to pick up both light and dark objects, resulting in a more representative look at the entire population. The infrared data allowed astronomers to make good measurements of the asteroids' diameters and, when combined with visible light observations, how much sunlight they reflect.
“The threat of the Earth being hit by an asteroid is increasingly being accepted as the single greatest natural disaster hazard faced by humanity,” according to Nick Bailey of the University of Southampton's School of Engineering Sciences team.
In 2009, Bailey and his colleagues used raw data from multiple impact simulations to rank each country based on the number of times and how severely they would be affected by each impact. The software, called NEOimpactor(from NASA's "NEO" or Near Earth Object program), was developed for measuring the impact of 'small' asteroids under one kilometer in diameter.
Results indicated that in terms of population lost, China, Indonesia, India, Japan and the United States face the greatest overall threat; while the United States, China, Sweden, Canada and Japan face the most severe economic effects due to the infrastructure destroyed.
The top ten countries most at risk are China, Indonesia, India, Japan, the United States, the Philippines, Italy, the United Kingdom, Brazil and Nigeria.
“The consequences for human populations and infrastructure as a result of an impact are enormous,” says Bailey. “Nearly one hundred years ago a remote region near the Tunguska River witnessed the largest asteroid impact event in living memory when a relatively small object (approximately 50 metres in diameter) exploded in mid-air. While it only flattened unpopulated forest, had it exploded over London it could have devastated everything within the M25. Our results highlight those countries that face the greatest risk from this most global of natural hazards and thus indicate which nations need to be involved in mitigating the threat.”
The team also examined how the consequences of an impact change with increasing impact energy. Initial results indicate that a 100 meter diameter asteroid will predominantly cause localized casualties and damage across a few countries when impacting on either land or ocean. However, the consequences of a 200 meter diameter asteroid hitting the ocean increase significantly, with the generated tsunamis reaching a global scale. At 500 meters in diameter, almost any ocean impact will generate significant casualties and economic cost across the world.
As Stephen Hawking says, the general consensus is that any comet or asteroid greater than 20 kilometers in diameter that strikes the Earth will result in the complete annihilation of complex life - animals and higher plants. (The asteroid Vesta, for example, one of the destinations of the Dawn Mission, is the size of Arizona).
Impact rates depend on how many comets and asteroids exist in a particular planetary system. In general there is one major impact every million years -a mere blink of the eye in geological time. It also depends on how often those objects are perturbed from safe orbits that parallel the Earth's orbit to new, Earth-crossing orbits that might, sooner or later, result in a catastrophic K/T or Permian-type mass extinction.
The asteroid that hit Vredefort located in the Free State Province of South Africa is one of the largest to ever impact Earth, estimated at over 10 km (6 miles) wide, although it is believed by many that the original size of the impact structure could have been 250 km in diameter, or possibly larger(though the Wilkes Land crater in Antarctica, if confirmed to have been the result of an impact event, is even larger at 500 kilometers across). The town of Vredefort is situated in the crater (image).
Dating back 2,023 million years, it is the oldest astrobleme found on earth so far, with a radius of 190km, it is also the most deeply eroded. Vredefort Dome Vredefort bears witness to the world’s greatest known single energy release event, which caused devastating global change, including, according to many scientists, major evolutionary changes.
Image at the top of the page shows the Asteroid Belt of our Solar System. Large white dots: are planets. The outermost large white dot is Jupiter. Green dots are main belt asteroids orbiting between Mars and Jupiter. Red dots: Asteroids that pass within Earth's orbit. Purple dots are comets. Small yellow dots: Trojan asteroids. They orbit 60 degrees ahead of or behind Jupiter, in the same orbit. They can drift around the average location, accounting for the broad scatter.