1996 PW has a highly elongated orbit just like a comet from the Oort Cloud – except it’s an asteroid. Astronomers now think they know how it got there. 1996 PW is about 5-10 miles (8-16 km) across. Credit: NASA/JPL-Horizons
Our view of the solar system will forever be incomplete. While frustrating at first blush, it means that fresh discoveries are always just around the corner. Case in point. On August 9, 1996 astronomers atop Mt. Haleakala in Maui, Hawaii discovered a most peculiar asteroid. 1996 PW has a highly elongated that looks like a Frisbee seen from the side and takes 5,900 years to make one trip around the Sun.
When farthest, 1996 PW is 48.8 billion miles away or 104 times more distant than Pluto. That places it among the billions of icy comets that comprise the Oort Cloud, a roughly spherical cocoon centered on the Sun and extending up to a light year from it in all directions.
Like moths around the solar flame, some 500 billion comets and perhaps 8 billion asteroids occupy a vast region of space called the Oort Cloud. The Kuiper Belt is a second asteroid belt that lies beyond the orbit of Neptune.
Odd thing was, 1996 PW was an asteroid – it never exhibited a fuzzy coma or tail typical of a comet and appeared spectroscopically to be made of rock. No dust or gas of any kind was detected even when the object was closest to the Sun. So what was it doing so far from home?
Some astronomers thought it may have been an active comet long ago but depleted its ices to where it’s now unrecognizable from an asteroid. Maybe.
A new study by Andrew Shannon (University of Cambridge), based on simulations of the rolling-stone-ways of the giant planets early in the solar system’s history, points to 1996 PW once being much closer to the Sun.
The planets haven’t always been in their present day orbits. In particular, Jupiter, the largest and most gravitationally potent planet, roamed inward to the orbit of Mars before backing out to its present orbit. Gravitational interactions with the dusty disk of material around the Sun called the solar nebula pulled the planet in. Later, interaction with Saturn yanked it back out. Scientists dub the back-and-forth shimmy the “Grand Tack”.
Interaction between the dusty-gassy solar nebula surrounding the Sun the young solar system and Saturn caused Jupiter to migrate first inward and then outward, scattering the hapless asteroids as it came and went. Credit: NASA
“We refer to Jupiter’s path as the Grand Tack, because the big theme in this work is Jupiter migrating toward the sun and then stopping, turning around, and migrating back outward,” writes Kevin Walsh of the Southwest Research Institute in Boulder, Colorado in a 2011 paper in Nature. “This change in direction is like the course that a sailboat takes when it tacks around a buoy.”
Jupiter’s gravitational might profoundly affected the asteroid belt at the time. Based on Shannon’s computer simulations, the giant planet’s do-si-do created chaos, with some asteroids kicked toward the Sun, others moved to a newly-created main belt and still others booted right out of the solar system.
Many were also flung to the icy realm of the Oort just short of leaving the Sun’s domain altogether. Shannon estimates that 4% or 8 billion rocky asteroids that once orbited within 2.5 times Earth’s distance from the Sun now mingle among the cloud’s half-trillion comets. Heck, that’s more asteroids than populate the main asteroid belt!
Very few “Oort asteroids” have been discovered and you can guess why. They’re small, generally dark and incredibly far away. A comet gives itself away with a bright coma and tail. Not these guys.They’re lurkers. To find them we’ll need dedicated, large telescope surveys like the upcoming Large Synoptic Survey Telescope with its 8-meter mirror slated for “first light” in 2019. But even that great eye will be challenged – Shannon predicts only a dozen discoveries a decade with the wide-field survey telescope.
One interesting sidelight about Oort Cloud asteroids. Like comets, they do drop in on the inner solar system from time to time. 1996 PW comes within just 232 million miles (373 million km) of the Sun. If one ever did have Earth in its sights, it would be hard to spot in advance and more difficult to divert because its much faster speed. One the bright side, Shannon and team estimate an impact would occur only once every billion years. I guess I can handle those odds and drag myself to work another day.