You have probably seen pictures of comets with their long tails of dust and ice sweeping majestically behind them. That tail is one of the defining characteristics of comets. Small objects orbiting the sun that do not sprout a tail have long been considered to be asteroids. While astronomers have found some “dead” comets, those with no tail being pushed off by the sun, they have become asteroids.
As
the ice melts from the comet by the sun’s heat, it pushes off the comet with a
small force. Think of a pot of water evaporating. Even though you can’t feel
the “wind” caused by this water vapor leaving the water, it’s there, and it
pushes back on the water with a tiny but persistent force. With comets, this force
of the vaporized ice leaving the comet acts like a tiny rocket motor. It can
push the comet in its orbit in a way that astronomers can measure. The orbit of
a comet isn’t controlled solely by the gravity of the sun, planets, and other
objects in our solar system, but also due to this outgassing. Asteroids don’t
behave this way.
Or
so astronomers thought.
In
2016, astronomer David Farnocchia, with NASA’s Center for Near-Earth Object
Studies, discovered something quite puzzling. He found an asteroid, 2003 RM, whose
motion through the solar system couldn’t be fully accounted for by the gravity
of other bodies. It behaved like a comet shooting water vapor into space,
except that he could find no evidence of that.
There
is another force, known as the Yarkovsky effect, which can alter the orbit of
small bodies around the sun. As photons from the sun impact a space rock, they
deliver an almost imperceptible push on the rock. Though minuscule, it can,
over time, affect the rock’s orbital path. Also, the sunward side heats up from
the sun’s thermal energy. But, as the asteroid rotates, the heated side rolls
into the dark and radiates the excess heat into space, again providing a tiny
bit of thrust. Farnocchia took all of that into account for 2003 RM, yet it
still couldn’t explain the small deviation of the asteroid’s orbit.
Comet Tsuchinshan-ATLAS credit Wladimir Bulgar - Science Photo Library via Getty Images
With
all comets, as the ice is vaporized and outgassed from the sun’s heat, it also
releases dust trapped in the ice. That beautiful tail we see coming from a
comet is actually composed of dust. But even using our biggest and best
telescopes, no one has found any dust trailing 2003RM. It behaved as if it had
a tiny rocket engine attached to it. Eventually, Farnocchia and other
astronomers found thirteen more such objects. They dubbed them “dark comets.”
This
is more than just a curiosity for astronomers. Farnocchia’s job is to search
for asteroids that have a possibility of hitting Earth. “My job,” he says, “is
to predict how things move in space. So if there’s something novel or
unexpected, that’s where the advance in the field lies for us.”
Current
telescopes can easily see any dust that might be coming off of a comet, but
none has been found to date. New telescopes planned or already under
construction will be able to detect any tiny bits of water vapor that may be
puffing off these dark comets. And since some are close enough that they might
have a chance of collision with Earth down the road, being able to predict
their position and movement may become more crucial in the future. For now, he
will monitor them and track their movements, just in case one wanders too close
to us.
