New research on Tunguska finds such events happen less often than we thought

New research on Tunguska finds such events happen less often than we thought

Enlarge / The famed Tunguska event of 1908 scorched a five-mile swath of trees and caused many to fall away from the center of the blast in a distinct radial pattern. (credit: Photo12/Universal Images Group via Getty Image)

Last month marked the 111th anniversary of the Tunguska event, a blast that flattened trees across half a million acres of Siberian forest on June 20, 1908. Scientists have been puzzling over the details ever since. We now have fresh evidence about what transpired back then, in the form of new data gleaned from a well-documented rare meteor burst near Chelyabinsk, Russia, in February 2013. That data shores up the hypothesis that the Tunguska event was most likely due to an asteroid impact. The findings are described in a series of scholarly papers commemorating the event, published in a special July 15 issue of the journal Icarus.

Seismometers all over

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No exomoons yet, but we may have spotted a disk that will form them

No exomoons yet, but we may have spotted a disk that will form them

Enlarge / There’s a lot going on near a young star, with at least two planets forming around it.

Up until the last few decades, our picture of what might reside around distant stars was shaped entirely by the planets, moons, asteroids, and other bodies in our own Solar System. But the discovery of thousands of exoplanets has dramatically improved our picture of what’s out there in terms of large bodies. Comets and asteroids, by contrast, are well below our ability to image for the indefinite future.

Moons, however, are awkwardly in between. It should be possible to image them indirectly, as their gravitational influence will alter the timing with which their planets orbit the star. And we might get a more direct indication of their presence as they will sometimes add to the shadow cast as transiting planets pass in front of their host star. We’ve searched for these

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Alternative theory of gravity makes a nearly testable prediction

Alternative theory of gravity makes a nearly testable prediction

Enlarge / Galaxy clusters generated by the Universe simulator IllustrisTNG. (credit: TNG Collaboration)

From our current perspective, the Universe seems to be dominated by two things we find frustratingly difficult to understand. One of these is dark matter, which describes the fact that everything from galaxies on up behaves as if it has more mass than we can detect. While that has spawned extensive searches for particles that could account for the visual discrepancy, it’s also triggered the development of alternative theories of gravity, ones that can replace relativity while accounting for the discrepancies in apparent mass.

So far, these proposals have fallen well short of replacing general relativity. And they say nothing about the other big mystery, dark energy, which appears to be accelerating the expansion of the Universe. Instead, researchers have developed an entirely separate class of theories that could modify gravity in a way that eliminates

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Space-based gravitational-wave detector may detect strange exoplanets

Space-based gravitational-wave detector may detect strange exoplanets

Enlarge / A depiction of one of the trio of satellites that will form the Laser Interferometer Space Antenna. (credit: NASA)

The first detection of gravitational waves came via an instrument that has to strain to overcome the constant background of vibrations and jolts that occur on Earth. Its success has helped push for the pursuit of a project that would rise above all that noise. LISA—the Laser Interferometer Space Antenna—would detect gravitational waves using the same technique as LIGO but place its hardware in space, free of any ground-based vibrations. Preliminary tests of prototype hardware has found that the idea should work.

LISA isn’t expected to be put in place until the 2030s, but that hasn’t stopped astronomers and physicists from contemplating the things that it might possibly detect. Two of these astronomers, Nicola Tamanini and Camilla Danielski, are now suggesting that LISA could be used to identify

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Sonic black holes produce “Hawking radiation,” may confirm famous theory

Sonic black holes produce “Hawking radiation,” may confirm famous theory

Enlarge / Simulated view of a black hole in front of the Large Magellanic Cloud. (credit: Wikimedia Commons/Alain r)

Israeli physicists think they have confirmed one of the late Stephen Hawking’s most famous predictions by creating the sonic equivalent of a black hole out of an exotic superfluid of ultra-cold atoms. Jeff Steinhauer and colleagues at the Israel Institute of Technology (Technion) described these intriguing experimental results in a new paper in Nature.

The standard description of a black hole is an object with such a strong gravitational force that light can’t even escape once it moves behind a point of no return known as the event horizon. But in the 1970s, Hawking demonstrated that—theoretically, at least—black holes should emit tiny amounts of radiation and gradually evaporate over time.

Blame the intricacies of quantum mechanics for this Hawking radiation. From a quantum perspective, the vacuum of space

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Impact that formed the Moon might have splashed into Earth’s magma ocean

Impact that formed the Moon might have splashed into Earth’s magma ocean

Enlarge / An early version of the collision model, showing a head-on impact.

The Earth and its moon are unique in our Solar System. Earth is the only rocky planet with a large moon, and only the dwarf planet Pluto has a moon that’s so similar in size to its host planet. The Moon is also remarkably similar to the Earth in terms of its composition, suggesting they formed from the same pool of material instead of the Moon forming elsewhere and having been captured.

This collection of properties led to a number of ideas about how the Moon formed, all of which failed to fit the data in various ways. Eventually, however, scientists came up with an idea that seemed to get most of the big picture right: a collision between the Earth and a Mars-sized object happened early in the Solar System’s history, creating a cloud of debris

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