Leonard David's INSIDE OUTER SPACE

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 kilometers in size, but some are several hundred kilometers 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

Giant comets could pose danger to life on Earth.

That’s the word from a team of astronomers from Armagh Observatory and the University of Buckingham.

They report 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.

Beware the centaurs

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.

The team consists of researchers Bill Napier and Duncan Steel of the University of Buckingham, Mark Bailey and David Asher of Armagh Observatory.

Their work is just out in the December issue of Astronomy & Geophysics (A&G), the journal of the Royal Astronomical Society (RAS).

Serious hazard

“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 in a RAS press statement.

Because they are so distant from the Earth, Centaurs appear as pinpricks of light in even the largest telescopes. Saturn’s 200-kilometers in size moon Phoebe, depicted in this image, 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-kilometers wide trans-Neptunian object at the end of 2018.
Credit: NASA/JPL-Caltech/Space Science Institute.

“Our work suggests we need to look beyond our immediate neighborhood too, and look out beyond the orbit of Jupiter to find centaurs,” Napier explains. “If we are right, then these distant comets could be a serious hazard, and it’s time to understand them better.”

Check out the full research paper at:

http://www.ras.org.uk/images/stories/press/Centaurs/Napier.Centaurs.revSB.pdf