“Destination Earth?” –Stephen Hawking and RAS Astronomers Warn of Danger from Giant Comets





A decade ago, Stephen Hawking observed that one of the major factors in the possible scarcity of intelligent life in our galaxy is the high probability of an asteroid or comet colliding with inhabited planets. This week, a team of astronomers from Armagh Observatory and the University of Buckingham reported that the discovery of hundreds of giant comets in the outer planetary system over the last two decades means that these objects pose a much greater hazard to life than asteroids.

The enlargement above of a 1993 Hubble Space Telescope image above shows the brightest nuclei in a string of approximately 20 objects that comprised the comet Shoemaker-Levy 9 as it hurtled toward its July I994 collision with Jupiter.

The team, made up of Professors Bill Napier and Duncan Steel of the University of Buckingham, Professor Mark Bailey of Armagh Observatory, and Dr David Asher, also at Armagh, publish their review of recent research in the December issue of Astronomy & Geophysics (A&G), the journal of the Royal Astronomical Society.

Because they are so distant from the Earth, Centaurs appear as pinpricks of light in even the largest telescopes. Saturn's 200-km moon Phoebe, depicted in the image below, seems likely to be a Centaur that was captured by that planet's gravity at some time in the past. Until spacecraft are sent to visit other Centaurs, our best idea of what they look like comes from images like this one, obtained by the Cassini space probe orbiting Saturn. NASA’s New Horizons spacecraft, having flown past Pluto six months ago, has been targeted to conduct an approach to a 45-km wide trans-Neptunian object at the end of 2018. (Credit: NASA/JPL-Caltech/Space Science Institute).




Scientists had their first close-up look at Phoebe when Cassini began exploring the Saturn system in 2004. Using data from multiple spacecraft instruments and a computer model of the moon's chemistry, geophysics and geology, scientists found Phoebe was a so-called planetesimal, or remnant planetary building block.

Analysis suggest that Phoebe was born within the first 3 million years of the birth of the solar system, which occurred 4.5 billion years ago. The moon may originally have been porous but appears to have collapsed in on itself as it warmed up. Phoebe developed a density 40 percent higher than the average inner Saturnian moon.

Cassini images suggest Phoebe originated in the far-off Kuiper Belt, the region of ancient, icy, rocky bodies beyond Neptune's orbit. Data show Phoebe was spherical and hot early in its history, and has denser rock-rich material concentrated near its center. Its average density is about the same as Pluto, another object in the Kuiper Belt. Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

The giant comets, termed centaurs, move on unstable orbits crossing the paths of the massive outer planets Jupiter, Saturn, Uranus and Neptune. The planetary gravitational fields can occasionally deflect these objects in towards the Earth.

Centaurs are typically 50 to 100 kilometers across, or larger, and a single such body contains more mass than the entire population of Earth-crossing asteroids found to date. Calculations of the rate at which centaurs enter the inner solar system indicate that one will be deflected onto a path crossing the Earth’s orbit about once every 40,000 to 100,000 years. Whilst in near-Earth space they are expected to disintegrate into dust and larger fragments, flooding the inner solar system with cometary debris and making impacts on our planet inevitable.

Known severe upsets of the terrestrial environment and interruptions in the progress of ancient civilizations, together with our growing knowledge of interplanetary matter in near-Earth space, indicate the arrival of a centaur around 30,000 years ago. This giant comet would have strewn the inner planetary system with debris ranging in size from dust all the way up to lumps several kilometers across.

Specific episodes of environmental upheaval around 10,800 BCE and 2,300 BCE, identified by geologists and palaeontologists, are also consistent with this new understanding of cometary populations. Some of the greatest mass extinctions in the distant past, for example the death of the dinosaurs 65 million years ago, may similarly be associated with this giant comet hypothesis.

"In the last three decades we have invested a lot of effort in tracking and analyzing the risk of a collision between the Earth and an asteroid," says Napier. "Our work suggests we need to look beyond our immediate neighborhood too, and look out beyond the orbit of Jupiter to find centaurs. If we are right, then these distant comets could be a serious hazard, and it’s time to understand them better."

The researchers have also uncovered evidence from disparate fields of science in support of their model. For example, the ages of the sub-millimeter craters identified in lunar rocks returned in the Apollo program are almost all younger than 30,000 years, indicating a vast enhancement in the amount of dust in the inner Solar system since then.

The image below shows the outer solar system as we now recognize it. At the center of the map is the Sun, and close to it the tiny orbits of the terrestrial planets (Mercury, Venus, Earth and Mars). Moving outwards and shown in bright blue are the near-circular paths of the giant planets: Jupiter, Saturn, Uranus and Neptune. The orbit of Pluto is shown in white. Staying perpetually beyond Neptune are the trans-Neptunian objects (TNOs), in yellow: seventeen TNO orbits are shown here, with the total discovered population at present being over 1,500.



Shown in red are the orbits of 22 Centaurs (out of about 400 known objects), and these are essentially giant comets (most are 50-100 km in size, but some are several hundred km in diameter). Because the Centaurs cross the paths of the major planets, their orbits are unstable: some will eventually be ejected from the solar system, but others will be thrown onto trajectories bringing them inwards, therefore posing a danger to civilization and life on Earth. (Credit: Duncan Steel).

We have observed, as Stephen Hawking pointed out in Life in the Universe, the collision of a comet, Schumacher-Levi, with Jupiter (at top of page), which produced a series of enormous fireballs, plumes many thousands of kilometers high, hot "bubbles" of gas in the atmosphere, and large dark "scars" on the atmosphere which had lifetimes on the order of weeks.

Through Earth's history such collisions occur, on the average every one million year. If this figure is correct, it would mean that intelligent life on Earth has developed only because of the lucky chance that there have been no major collisions in the last 70 million years. Other planets in the galaxy, Hawking believes, on which life has developed, may not have had a long enough collision free period to evolve intelligent beings.

“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, who has developed a threat identifying program.

The team 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), has been specifically developed for measuring the impact of 'small' asteroids under one kilometer in diameter.

“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 meters 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.”

What would happen to the human species and life on Earth in general if an asteroid the size of the one that created the famous K/T Event of 65 million years ago at the end of the Mesozoic Era that resulted in the extinction of the dinosaurs impacted our planet.

As Stephen Hawking recognized, 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).

How many times in our galaxy alone has life finally evolved to the equivalent of our planets and animals on some far distant planet, only to be utterly destroyed by an impact? Galactic history suggests it might be a common occurrence.

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.

The Daily Galaxy via via RAS, University of Southampton, and http://www.rationalvedanta.net/node/131


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