‘Cometas escuros’ podem ser ameaça à Terra

 
O planeta Terra pode estar sob a ameaça de ser atingido por milhares de cometas que circulam nos arredores do sistema solar e não podem ser detectados pelos cientistas, afirma uma reportagem publicada na edição desta semana da revista britânica New Scientist.
A revista entrevistou dois astrônomos britânicos que afirmam que, apesar de todo trabalho de monitoramento desses corpos celestes feitos por agências espaciais, muitos deles não poderiam ser detectados por serem o que eles chamam de “cometas escuros”.
Segundo Bill Napier, da Universidade de Cardiff, no País de Gales, e David Asher, do Observatório de Armagh, na Irlanda do Norte, estes cometas escuros podem ser uma ameaça à Terra.
“Cometas escuros, dormentes, são uma significativa, mas muitas vezes invisível, ameaça ao planeta”, disse Napier à revista.
Segundo os cientistas, pelos cálculos sobre a entrada de cometas no sistema solar, é possível que haja pelo menos 3 mil desse corpos celestes próximos à região, mas apenas 25 deles são conhecidos.
Napier e Asher afirmam que muitos desses cometas não podem ser vistos “simplesmente porque são muito escuros”.
Isto acontece quando o gelo de um cometa “ativo” – que reflete a luz do sol – se evapora, deixando para trás somente uma crosta que reflete apenas uma fração de luz.
Os cientistas citam como exemplo o cometa IRAS-Araki-Alcock, que passou a uma distância de 5 milhões de quilômetros da Terra em 1983 – a menor registrada em 200 anos.
Segundo eles, o cometa foi detectado apenas duas semanas antes de sua aproximação.
“Ele tinha apenas 1% de suas superfície ativa”, diz Napier.
De acordo com os pesquisadores, quando uma sonda da Nasa pousou no cometa Borrelly, em 2001, também teria registrado várias manchas “negras” em sua superfície.
Outro cientista entrevistado pela revista, no entanto, se mostrou mais cético sobre a ameaça.
Segundo Clark Chapman, do Southwest Research Institute, no Colorado, Estados Unidos, “estes cometas absorveriam bem a luz do sol, então poderiam ser detectados pelo calor que emitiriam”.

‘Dark’ Comets Could Hit Earth Without Warning

Hazardous comets and asteroids are tracked by various space agencies, but British-based astronomers say there are many dangerous ones still going undetected, New Scientist magazine reported.

Previous research suggests comets nudge in the Earth’s direction when the solar system periodically passes through the galactic plane.

These comets appear to coincide with the dates of ancient impact craters found on Earth — suggesting they were caused by comets and not asteroids.

“There is a case to be made that dark, dormant comets are a significant but largely unseen hazard,” said Bill Napier at Cardiff University.

Napier and David Asher, at Armagh Observatory, say dark comets may still be in the solar system, and could come near Earth.

Dark comets happen when a comet’s reflective water ice evaporates away, and leaves behind an organic crust.

Since the water’s gone, they don’t reflect light, making it hard for astronomers to see them until they’re right on top of us.

‘Dark’ comets may pose threat to Earth

SWATHES of dark comets may be prowling the solar system, posing a deadly threat to Earth.

Hazardous comets and asteroids are monitored by various space agencies under an umbrella effort known as Spaceguard. The vast majority of objects found so far are rocky asteroids. Yet UK-based astronomers Bill Napier at Cardiff University and David Asher at Armagh Observatory in Northern Ireland claim that many comets could be going undetected. “There is a case to be made that dark, dormant comets are a significant but largely unseen hazard,” says Napier.

In previous work, Napier and Janaki Wickramasinghe, also at Cardiff, have suggested that when the solar system periodically passes through the galactic plane, it nudges comets in our direction (New Scientist, 19 April 2008, p 10).

These periodic comet showers appear to correlate with the dates of ancient impact craters found on Earth, which would suggest that most impactors in the past were comets, not asteroids.

Now Napier and Asher warn that some of these comets may still be zipping around the solar system. Other observations support their case. The rate that bright comets enter the solar system implies there should be around 3000 of them buzzing around, and yet only 25 are known.

We may not see them, say the pair, simply because they are too dark. (Astronomy & Geophysics, DOI: 10.1111/j.1468-4004.2009.50118.x).

Such dark comets are not unheard of. They occur when an “active” comet’s reflective water ice has evaporated away, leaving behind an organic crust that only reflects a small fraction of light.

In 1983, Comet IRAS-Araki-Alcock passed by Earth at a distance of 5 million kilometres, the closest known pass by any known comet for 200 years. It was spotted only two weeks ahead of its closest approach. “It had only 1 per cent of its surface active,” says Napier. Comet Borrelly, visited by NASA’s Deep Space 1 probe in 2001, was found to have extremely dark patches over much of its surface.

“There may be merit to this idea,” says Steve Larson of the University of Arizona’s Catalina Sky Survey in Tucson, one of the main contributors to Spaceguard.

Clark Chapman at the Southwest Research Institute in Boulder, Colorado, is sceptical, but points out that such dark comets “would absorb sunlight very well” and so could be detected by the heat they would emit.

Anúncios

Sobre Baldaci

A Haggai Alummi M606, An Amateur astronomer, A husband, father and a visionary baptist pastor working hard to Plant a Church in Cuiaba, Brazil and to establish the first Cyber Theater and Planetarium in my state(http://on.fb.me/hkCGff) Unique pastor and astronomer in Brazil. Working using astronomy to reach poor childrens and against drugs. http://bit.ly/eu0zgB Member of SOC-JPL-NASA (http://bit.ly/eoyow6) and Astronomer without borders (http://bit.ly/dTCOJG) Host of program “Eyes on skies” TV Assembléia Legislativa(http://on.fb.me/gmF5LE) e Ombudsman of Education State Secretary of Mato Grosso with assistance to 711 schools. About me at Haggai´s web site: http://bit.ly/hAfceX
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