SPACE BULLETIN
More interplanetary updates, including colliding stars, minimoons, and cosmic lights that could point to an alien civilisation
BENNU SURPRISES
Last year, NASA’s OSIRIS-REx spacecraft was circling Bennu, a large asteroid that regularly passes close to Earth. OSIRISREx has been mapping the asteroid’s rocky surface using cameras and other instruments to help identify the best spot to land, later this year. The intention is to scoop up a sample of rock that will be brought back to Earth – in 2023. Bennu lacks a heated core necessary for geological activity, so is not technically ‘alive’, but researchers don’t consider the asteroid entirely ‘dead’ either, since it regularly emits small rocks. Some of these crash back down to Bennu’s surface, while others escape into deep space. But a third category is the rocks that wind up orbiting Bennu for a few days as mini-moons before returning to the surface.
This phenomenon is hitherto unprecedented and looks set to revolutionise our understanding of asteroids. “It wasn’t that long ago that the conventional wisdom was that asteroids are these dead bodies that didn’t change very much,” says Carl Hergenrother, the University of Arizona astronomer who
identified Bennu as a suitable target for OSIRIS-REx, who explains that as such behaviour could never have been seen from the ground, no one had seen an active asteroid up close in this way before. “So the question that is still on our minds,” he added, “is are we seeing a lower intensity process that is similar to what happens on other active asteroids, or is this something entirely different?”
The exploding rocks are not the first surprise yielded by Bennu. Last year, the OSIRISREx team observed 100cm (40in) sized particles exploding from the asteroid’s surface on three occasions; no one is yet sure why. The spacecraft’s cameras also detected a significant number of particles already orbiting Bennu, like a cloud of gnats. Scientists have previously seen massive ejections from around two dozen asteroids as they passed by Earth, but the mechanisms proposed to explain why these asteroids were ejecting material don’t apply to Bennu. The centrifugal force of an asteroid’s spin could eject material from the surface, but wouldn’t account for the range of particle orbits observed around
Bennu. Similarly, sublimation of ice water (the phenomenon that produces a comet’s tail) won’t suffice as an explanation because OSIRIS-REx detected particle ejections on certain parts of Bennu’s surface that are far too hot for ice.
The OSIRIS-REx team has identified two possible causes; firstly, Bennu’s extreme temperatures, which range from 240°F to -100°F (116°C to -73°C). A transition from boiling to freezing can result in particles cracking and flying apart. Secondly, Bennu’s bombardment by micrometeoroids, which, when they hit its surface, cause particles to be dislodged.
Unfortunately, OSIRIS-REx won’t remain in this location, observing Bennu, long enough to resolve the problem by itself, as the spacecraft has many other tasks to perform before returning to Earth. However, given how little we still know about asteroids, Hergenrother says there is now a strong case to be made for a dedicated mission to study the particle-ejection phenomenon on another asteroid in future. wired.com, 5 Dec 2019.
AUSTRALIAN ‘MINI-MOON’
Fireballs in the Earth’s atmosphere are not uncommon sights for skywatchers. A 2016 fireball that exploded over the Australian desert was at first deemed unremarkable, until images taken by a network of cameras that monitor the skies were examined. The Desert Fireball Network’s six cameras, spanning hundreds of miles across the Australian desert, managed to observe the 22 August 2016 fireball in great detail. The resulting camera images yielded data indicating the rock to be a temporarily captured orbiter, or, colloquially, a mini-moon. Its slow (6.8 miles/ 11km per second) velocity suggested it had probably been in orbit around Earth before crashing; a near-vertical trajectory ruled out satellite debris. The research team led by planetary scientist Patrick Shober of Australia’s Curtin
University calculated it 95 per cent probable the object was a temporarily captured orbiter.
There are so many rocks shooting past Earth, possibly millions, that it’s logical some will penetrate the atmosphere. Most end up as bolides (meteors that explode in mid-air before reaching the ground). But every now and then, one of these asteroids becomes captured in the Earth’s orbit for a time. It is very unusual; a 2012 computer simulation with 10 million virtual asteroids had just 18,000 caught in Earth’s orbit. sciencealert.com, 2 Dec 2019.
ON THE BLINK
A survey of astronomical observations made over the past 100 years led by Sweden’s Stockholm University is searching for the Milky Way’s weirdest events. The Vanishing & Appearing Sources during a Century of Observations (VASCO) project has found records of 100 separate sightings of red lights in the Earth’s skies over the past 70 years. Astrophysicists are hypothesising that these blinking red lights could be evidence of previously unseen astronomical phenomena, or of alien civilisations. Faced with examining over 600 million objects, the researchers whittled their search down to just 100 by focusing on observations of objects in space that either just disappear, or that have rapidly fluctuating brightness levels varying over a 70-year time period. The strangest objects flare up and dim across several orders of magnitude beyond the limits of what’s generally regarded as the norm for other types of highly variable objects, such as eclipsing binary systems or active galactic nuclei outbursts. A few even seem to have vanished.
Dying stars take millions or billions of years to transition to different evolutionary stages. Rapid changes over just 70 years are quite unprecedented, suggesting a phenomenon (or phenomena) not yet understood.
One explanation is that the historical sightings record R Coronae Borealis stars (a rare type of carbon-rich supergiant), whose brightnesses can fade up to nine magnitudes over irregular time intervals. Another is that the transient events are failed supernovae, thought to be the result of a gigantic star collapsing directly into a black hole with no sort of visible outburst. Failed supernovae have been theoretically predicted but not yet confirmed.
“The implications of finding such objects extend from traditional astrophysics fields to the more exotic searches for evidence of technologically advanced civilisations”, says a study published in The Astrophysical Journal. Since interstellar optical laser communication in the infrared range has been proposed as a potential indicator of advanced extraterrestrial civilisation, it has been suggested that some of the transient red blinking lights are indicative of just that. theregister. co.uk, 13 Dec 2019; cnet.com, 18 Dec 2019.
GOODBYE BETELGEUSE?
Betelgeuse, one of the brightest stars in the sky, may be set to explode. The red supergiant was formerly one of the easiest stars to identify on account of its brightness and its ‘location’ in
the constellation of Orion. But since October 2019, Betelgeuse has been dimming to such a point that it is now hard to spot with the naked eye. Astronomers believe this indicates a cosmic supernova is imminent. This rare event would mean that Betelgeuse would become as bright as the full Moon for a few months, casting its own shadow at night, before fading away. Because Betelgeuse is 600 light years away, the supernova, if it occurs, will already have happened just under 600 years ago. Any particles emitted from the explosion will eventually
reach our Solar System, but will take six million years to do so.
Betelgeuse has a mass 12 times that of our Sun, and has been burning so brightly that it is now dying after just eight million years. By contrast, our Sun is already five billion years old. So, Betelgeuse is a star that burns very brightly for a very (comparatively speaking) brief time. Don’t miss the show! D.Telegraph, 31 Dec 2019.
NEUTRON CRASH
New observations suggest that the collision, observed in 2017, of two neutron stars blasted a jet of charged particles into space. The discovery supports the theory that mysterious flashes of high-energy light (short gamma-ray bursts) are actually jets from neutron star collisions. sciencenews.org, 18 Dec 2019.