A fresh look at a nagging problem — asteroids moving in comet-like orbits — concludes that asteroids must make up about 4% of the vast, distant Oort Cloud of comets.
When a telescope atop Hawaii's Haleakala swept up a fast-moving object in August 1996, astronomers didn't know what to make of it. Designated 1996 PW, the little interloper had the highly elongated orbit of a comet that had ventured inward from the Oort Cloud, at the solar system's outermost fringe.
But it had no tail or coma — visually and spectroscopically, it looked like an asteroid.
At the time, dynamicists Paul Weissman (Jet Propulsion Laboratory) and Hal Levison (Southwest Research Institute) proposed that 1996 PW might actually be a rare hybrid: an asteroid from the Oort Cloud. Their suggestion ran completely counter to the consensus notion that only comets existed in that vast, distant reservoir. But Weissman and Levison had run the numbers: they calculated that, along with a trillion or so comets, roughly 8 billion asteroids could have been flung out into the Oort Cloud by close planetary encounters early in solar-system history.
When other researchers suggested that 1996 PW was probably just an "extinct" comet, having depleted the volatile ices that create a coma or tail, the notion of asteroids in the Oort Cloud got shelved — but not completely forgotten.
In the decades since, dynamicists have radically altered their views of the early system. Now it's widely believed that the orbits of Jupiter, Saturn, Uranus, and Neptune must have moved around — and maybe a lot. In one scenario, dubbed the "Grand Tack," Jupiter dove inward to about where Mars orbits today before retreating (thanks to a resonance with Saturn) to its current location.
In any case, this gravitational chaos must have flung small bodies everywhere — into each other, into planets or the Sun, and out of the solar system entirely. The ones that just fell short of escaping into interstellar space ended up in the Oort Cloud.
Now dynamcists led by Andrew Shannon (University of Cambridge) have taken a fresh look at what-went-where in the early solar system. Their computer simulations, published October 29th in Monthly Notices of the Royal Astronomical Society, confirm that lots of rocky bodies originally within 2½ astronomical units of the Sun should be lurking among the Oort Cloud's half trillion comets. It's a tricky calculation, because astronomers can only guess the assorted sizes of those distant bodies. (Observers now have data on three of them: C/2013 A1, which skirted by Mars last month; C/2013 P2; and C/2014 S3.)
Shannon and his colleagues find that Oort Cloud asteroids are a minority, perhaps 4% of all the bodies out there. But that's still 8 billion objects (eerily matching the Weissman-Levison estimate), totaling perhaps a third of Earth's mass. "The Oort Cloud has more asteroids than the asteroid belt does!" they point out. In fact, it's even possible that chunks of debris from the postulated Moon-forming impact might have been hurled out there.
So how come interlopers like 1996 PW aren't more common? They're small, likely dark, and lack a typical comet's large, bright coma, so they're too faint to see. Even the powerful Large Synoptic Survey Telescope, which might be ready by 2019, will be challenged to spot them. Shannon's team estimates that the LSST might sweep up a dozen Oort Cloud asteroids over a decade.
Keep tabs on observable asteroids and comets with the 2015 edition of the RASC's Observer's Handbook.