http://www.jpl.nasa.gov/news/features.cfm?feature=2286
Jumping Asteroids
Jet Propulsion Laboratory
August 18, 2009
How our solar system was formed has fascinated scientists and laymen
alike for -- well, for a really, really long time. New research may have
answered a piece to the puzzle - how big were the first planetesimals?
For those of you scoring at home," planetesimals" were the first solid
objects in our newly minted solar system (also known as the
protoplanetary disk). They began life as small grains of dust orbiting
an infant sun. These grains would bump into each other, clump together
and gradually form larger grains of dust, which eventually became small
space rocks.
Now the theory goes that some of these small rock-sized planetesimals
aspired for greater things, and continued to gradually grow in size to
become asteroids, and that a few of those continued to grow beyond the
asteroid stage and become planets.
The problem with this tidy little theory is that when the burgeoning
space rocks grew to about one meter (3.3 feet) in size, orbital
mechanics tells us the gas comingling with them in the protoplanetary
disk should have acted like a brake, slowing their velocity appreciably.
Their orbital speed having been cut, these filing cabinet-sized space
rocks would have spiraled into the sun. Essentially, the gas would have
acted as a celestial "mini-vacuum." The problem is, there are asteroids
up there in space. Honest, ask any astronomer. So what happened?
Evidence is now mounting that these small space rocks quickly "jumped"
(or grew) in size from below one meter to multi-kilometer in size.
Planetesimals that big were big enough to plow through the drag created
by the gas in the protoplanetary disk without having their orbits
appreciably altered. Hence they did not spiral into the sun.
What data point to a jump in asteroid sizes? Simply, the asteroids
available for viewing in the night's sky. Telescopic surveys indicate
there is currently a plethora of asteroids less than one kilometer (.62
mile) wide but those over one kilometer drop considerably in number. The
authors used computer simulations in an attempt to mimic the impacts and
coagulation processes that took place over the millions of years between
when the asteroids formed and now. The only way they could arrive at the
current asteroid size distribution was to begin these simulations with
planetesimals that quickly morphed into asteroids hundreds of kilometers
in size. Once their growth spurt was over, these massive celestial
bodies began an epoch-sized game of demolition derby as they orbited the
sun. Over the eons, and with each extraterrestrial pileup, came fewer
and fewer large asteroids - a fragmentation process that continues to
this day. Despite the modest sizes of asteroids today, the paper's
authors conclude that asteroids must have been born big.
The paper, "Asteroids Were Born Big" is available now online from the
ScienceDirect website and will be available in a future edition of the
journal Icarus.
For more information about asteroids and other near-Earth objects please
visit: www.jpl.nasa.gov/asteroidwatch .
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