Cohesive Forces that hold gravity-defying near-earth asteroid together revealed

An asteroid 1950 DA. Credit: NASA

Researchers at the University of Tennessee, Knoxville, have made a novel discovery that may potentially protect the world from future collisions with asteroids.

The team studied near-Earth asteroid 1950 DA and discovered that the body, which rotates so quickly it defies gravity, is held together by cohesive forces called van der Waals, never detected before on an asteroid.

The findings, published in this week's edition of the science journal Nature, have potential implications for defending our planet from a massive asteroid impact.

Previous research has shown that asteroids are loose piles of rubble held together by gravity and friction. However, the UT team found that 1950 DA is spinning so quickly that it defies these forces.

Ben Rozitis, a postdoctoral researcher; Eric MacLennan, a doctoral candidate; and Joshua Emery, an assistant professor in the Department of Earth and Planetary Sciences, wanted to know what keeps the body from breaking apart.

Looking at thermal images and orbital drift to calculate thermal inertia and bulk density, the team detected the action of cohesive forces in an environment with little gravity.

"We found that 1950 DA is rotating faster than the breakup limit for its density," said Rozitis. "So if just gravity were holding this rubble pile together, as is generally assumed, it would fly apart. Therefore, interparticle cohesive forces must be holding it together."

In fact, the rotation is so fast that at its equator, 1950 DA effectively experiences negative gravity. If an astronaut were to attempt to stand on this surface, he or she would fly off into space unless he or she were somehow anchored.

The presence of cohesive forces has been predicted in small asteroids, but definitive evidence has never been seen before.

The finding provides important information for efforts aimed at stopping an asteroid from crashing into Earth.

"Following the February 2013 asteroid impact in Chelyabinsk, Russia, there is renewed interest in figuring out how to deal with the potential hazard of an asteroid impact," said Rozitis.

"Understanding what holds these asteroids together can inform strategies to guard against future impacts."

This research reveals some potential techniques, such as a kinetic impactor which would deploy a massive object on a collision course with the asteroid, could exacerbate the impact's effects.

For example, this technique could destabilize the cohesive forces keeping the asteroid together, causing it to break apart into several threatening asteroids headed for Earth.

This may be what occurred with the asteroid P/2013 R3, which was caught by the Hubble Space Telescope in 2013 and 2014 coming undone, possibly due to a collision with a meteor.

"With such tenuous cohesive forces holding one of these asteroids together, a very small impulse may result in a complete disruption," said Rozitis.

The researchers' findings also have implications for space exploration. For example, the European Space Agency's Rosetta spacecraft landed on Comet 67P/Churyumov-Gerasimenko's surface last week and it may find a dusty surface dominated by such cohesive forces.

More information: Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA, Nature, dx.doi.org/10.1038/nature13632

Mars Curiosity Rover Captures 2 Mars Moons Together - NASA Video

A spectacular new video from NASA's Mars rover Curiosity shows the Red Planet's two tiny moons eclipsing each other in an otherworldly skywatching first.

Curiosity snapped 41 images of the Mars moons in the night sky on Aug. 1, with rover scientists then stitching them together to make the final 30-second video. It is the first time a view of the two Martian satellites — called Phobos and Deimos — eclipsing each other has been captured from the vantage point of the planet's surface, NASA officials said.

The new Curiosity video has plenty of scientific value in addition to its gee-whiz appeal, officials said. For example, researchers are studying the images to refine their knowledge of the orbits of Phobos and Deimos, both of which appear to be captured asteroids.

Mark Lemmon

"The ultimate goal is to improve orbit knowledge enough that we can improve the measurement of the tides Phobos raises on the Martian solid surface, giving knowledge of the Martian interior," Mark Lemmon of Texas A&M University said in a statement.

"We may also get data good enough to detect density variations within Phobos and to determine if Deimos' orbit is systematically changing," added Lemmon, who is a co-investigator for Curiosity's Mastcam instrument, which took the pictures using its telephoto lens.

Phobos' orbit is taking it closer to the surface of Mars very slowly, researchers said, while Deimos may gradually be getting farther and farther away from the planet.

Phobos is just 14 miles (22 kilometers) wide on average, while Deimos is even smaller. But Curiosity was able to spot both of them because they orbit quite close to the Red Planet's surface — 3,700 miles (6,000 km) in Phobos' case and 12,470 miles (20,070 km) for Deimos.

Earth's moon is gigantic compared to Phobos and Deimos, with a diameter of about 2,160 miles (3,475 km). But our planet's natural satellite orbits much farther away — its average distance is 239,000 miles (384,600 km) — so Phobos appears half as big in the sky to Curiosity as Earth's moon does to human skywatchers, NASA officials said.

This illustration provides a comparison for how big the moons of Mars appear to be, as seen from the surface of Mars, in relation to the size that Earth's moon appears to be when seen from the surface of Earth. 

Deimos, at far left, and Phobos, beside it, are shown together as they actually were photographed by the Mast Camera (Mastcam) NASA's Mars rover Curiosity on Aug. 1, 2013.

Credit: NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M Univ.

The International Space Station together with Space Shuttle Endeavour visible from Europe

The conditions in northern Europe are set for nice views of the ISS as it passes overhead up to four times a night this coming weekend. All you need is the timetable and a clear sky.

The International Space Station together with Space Shuttle Endeavour docked to it is so bright that spotting them with the naked eye is not at all difficult, provided you know where and when to look.

The ISS orbits Earth every 92 minutes only about 400 km above our heads and tilted about 52 degrees in relation to the equator. When the position of the Station is projected onto a conventional map of the world, it moves in a curve from south to north. The ISS passes over most regions on Earth every day, in some places several times a day.


For most locations in Europe the ISS can be seen easily only when the northern points of the orbit happen to coincide with good lighting conditions and night time. This is the case right now: the ISS enjoys very good visibility from the northern parts of Europe.

Normally the best time for ISS-gazing is just before dawn or just after sunset, when the observer is in the dark but the ISS is lit by the Sun. When visible, ISS is one of the brightest objects in the night sky, making it fairly easy to spot from when it rises above the horizon in a westerly direction until it sets towards the east.

How about photographing ISS as it glides through the night sky? Send your best images via twitpics to @esa or by email esabuzz@gmail.com