08 August 2011

Earth MORE likely to get hit now! & Europe's Plans

Earth Impacts: More Likely in the Past or Present?by Staff WritersMunich, Germany (SPX) Aug 04, 2011

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Is the Earth more likely or less likely to be hit by an asteroid or comet now as compared to, say, 20 million years ago? Several studies have claimed to have found periodic variations, with the probability of giant impacts increasing and decreasing in a regular pattern.
Now a new analysis by Coryn Bailer-Jones from the Max Planck Institute for Astronomy (MPIA), set to be published in the Monthly Notes of the Royal Astronomical Society, shows those simple periodic patterns to be statistical artifacts. His results indicate either that the Earth is as likely to suffer a major impact now as it was in the past, or that there has been a slight increase in impact rate events over the past 250 million years.
Giant impacts by comets or asteroids have been linked to several mass extinction events on Earth, most famously to the demise of the dinosaurs 65 million years ago. Nearly 200 identifiable craters on the Earth's surface, some of them hundreds of kilometers in diameter, bear witness to these catastrophic collisions.
Understanding the way impact rates might have varied over time is not just an academic question. It is an important ingredient when scientists estimate the risk Earth currently faces from catastrophic cosmic impacts.
Since the mid-1980s, a number of authors have claimed to have identified periodic variations in the impact rate. Using crater data, notably the age estimates for the different craters, they derive a regular pattern where, every so-and-so-many million years (values vary between 13 and 50 million years), an era with fewer impacts is followed by an era with increased impact activity, and so on.
One proposed mechanism for these variations is the periodic motion of our solar system relative to the main plane of the Milky Way Galaxy.
This could lead to differences in the way that the minute gravitational influence of nearby stars tugs on the objects in the Oort cloud, a giant repository of comets that forms a shell around the outer solar system, nearly a light-year away from the Sun, leading to episodes in which more comets than usual leave the Oort cloud to make their way into the inner solar system - and, potentially, towards a collision with the Earth.
A more spectacular proposal posits the existence of an as-yet undetected companion star to the Sun, dubbed "Nemesis". Its highly elongated orbit, the reasoning goes, would periodically bring Nemesis closer to the Oort cloud, again triggering an increase in the number of comets setting course for Earth.
For MPIA's Coryn-Bailer-Jones, these results are evidence not of undiscovered cosmic phenomena, but of subtle pitfalls of traditional ("frequentist") statistical reasoning. Bailer-Jones: "There is a tendency for people to find patterns in nature that do not exist. Unfortunately, in certain situations traditional statistics plays to that particular weakness."
That is why, for his analysis, Bailer-Jones chose an alternative way of evaluating probabilities ("Bayesian statistics"), which avoids many of the pitfalls that hamper the traditional analysis of impact crater data. He found that simple periodic variations can be confidently ruled out.
Instead, there is a general trend: From about 250 million years ago to the present, the impact rate, as judged by the number of craters of different ages, increases steadily.
There are two possible explanations for this trend. Smaller craters erode more easily, and older craters have had more time to erode away. The trend could simply reflect the fact that larger, younger craters are easier for us to find than smaller, older ones.
"If we look only at craters larger than 35 km and younger than 400 million years, which are less affected by erosion and infilling, we find no such trend," Bailer-Jones explains.
On the other hand, at least part of the increasing impact rate could be real. In fact, there are analyses of impact craters on the Moon, where there are no natural geological processes leading to infilling and erosion of craters, that point towards just such a trend.
Whatever the reason for the trend, simple periodic variations such as those caused by Nemesis are laid to rest by Bailer-Jones' results. "From the crater record there is no evidence for Nemesis. What remains is the intriguing question of whether or not impacts have become ever more frequent over the past 250 million years," he concludes.
The work described here is set to be published as C. A. L. Bailer-Jones, "Bayesian time series analysis of terrestrial impact cratering", in the Monthly Notices of the Royal Astronomical Society.


From: http://www.technologyreview.com/blog/arxiv/27028/

Europe's Plan to Move An Asteroid

If we ever need to prevent an asteroid hitting Earth, we'll have to deflect it somehow. To practice, the European Space Agency is planning a game of asteroid billiards
If astronomers ever spot an asteroid coming our way, the world will have to decide pretty quickly what to do.
One idea is to knock it off course. But although that sounds pretty straightforward, it hides a multitude of challenges. The most obvious of these is to determine how big an impact would be necessary and how this could be delivered.
In 2002, the European Space Agency began a program called Don Quijote to find out how best to perform such a deflection.
Don Quijote involves sending two spacecraft to a near Earth asteroid; one to smash into it and the other to watch while in orbit above the impact crater. The goal is to change the asteroid's semimajor axis by more than 100 metres and to measure the change with an accuracy greater than 1 per cent.
But the question is how best to monitor what's going on in a way that is relevant to other asteroids. After all, the ultimate plan is to use the information from this mission to move some other asteroid with our name on it.
Now, Stephen Wolters at the Open University in the UK and a few friends have published a new analysis of the mission saying that measuring the change in orbit is not enough. Instead, the spacecraft needs to characterise the impact in detail, determining the density of the material near the asteroid's surface, the size of the surface grains as well as the mass and speed distribution of the impact ejecta.
Only with this information will it be possible to work out exactly how the momentum from the impactor was transferred to the asteroid.
That significantly changes the mission. In addition to an on-board radio transmitter that will allow space scientists back on Earth to work out its distance exactly, the spacecraft will need a sophisticated imaging suite capable of photographing the damage and carrying out infrared spectroscopy to determine the asteroid's mineral content.
The thermal cameras will also pick up any changes in temperature. That's important because of the small but potentially significant force from the way the asteroid emits heat.
If the asteroid emits thermal photons equally in all directions, their force cancels out. But if it emits more in some directions than others, then this force will slowly push an asteroid. That can happen if the asteroid tends to cool down at a rate that matches its speed of rotation--so that all the thermal photons emitted after the surface has been in the Sun, travel in the same direction.
This force, called the Yarkovsky effect, could make the difference between whether an impact with Earth occurs or not. So knowing its value is crucial, particularly if the impactor changes the asteroid's rate of rotation.
So that should make Don Quijote a better mission but it also makes it a more expensive one. And that raises a question mark over whether the European Space Agency will want to pay for it on its own.
But then again, why should it? There's a good argument that the international community as a whole should shoulder the burden and the cost of planetary protection.
The only question then is how badly we need Don Quijote to keep our planet safe.
Ref: arxiv.org/abs/1107.4229: Measurement Requirements For A Near-Earth Asteroid Impact Mitigation Demonstration Mission

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