26 November/December 2020
Space
7leagues mapped out various def lection scenarios
for Bennu that unfolded over the course of 300
years. They tested different mass ratios for the
smaller asteroid (1/1,000th the size of Bennu and
1/10,000th the size of Bennu), evaluated three dif-
ferent tether lengths (621, 1,242, and 1,864 miles
long), and assessed possible angles at which the
tethers could be attached to the asteroid.
Their simulations revealed that asteroids as
small as 1/10,000th the mass of the main object
would be sufficient enough to pull Bennu into a
different, safer orbit—roughly the equivalent of
tethering a No. 2 pencil to an orbiting astronaut.
“The neat trick to this technique is that it changes
the position of the original asteroid rather than its
velocity, because attaching the two asteroids makes
them into a single object, with the center of mass
in a different place than the original asteroid, pre-
tether,” Rivkin notes. “And because the center of
mass (and so, its position) is different but the veloc-
ity stays the same, the orbit changes.”
An asteroid 1/10,000th the mass of Bennu, with
a tether 1,864 miles long attached along an equa-
torial orbit, pulled Bennu off of its trajectory by
almost 600,000 miles, or 150 times the radius of
Earth, according to the paper. However, an aster-
oid 1/1,000th the mass of Bennu, attached via the
same tether, pulled Bennu off course by as much as6 million miles (1,500 times Earth’s radius). In the
latter scenario, the team found attaching the tether
at a 45-degree angle resulted in a shift in trajectory
of about 20 million miles. At a 90-degree angle, the
def lection was about 23.6 million miles.
The lasso plan dramatically lowers the risk of
peppering Earth with asteroid fragments, but a lot
needs to be worked out before we send robotic space
cowboys across the solar system. Actually corral-
ling and attaching two asteroids would be, to put
it mildly, complex. “The first step is always to ana-
lyze the physics of the method,” Venditti says. “If
the math works, then the logistics and engineer-
ing side of the method needs to be developed.” To
organize the logistics, astronomers would need to
spot the potential threat far in advance. Bennu’s
next close approach to Earth will happen in 2060.
According to Venditti’s team, a mission launched
by 2035 would give us enough time to def lect it if
needed. (But don’t worry. At its closest, Bennu will
still be 500,000 miles away.)
When it comes to defending Earth from maraud-
ing asteroids, Rivkin argues that creativity is key:
“Anything is possible, I suppose, if you’re willing
to invest enough resources into making it happen.
Planetary defense is a great topic for thinking about
out-of-the-box solutions.” This is the perfect time
to experiment.WIND/
SHOCKWAVE
A simulation of
asteroid impacts
on land revealed
around 60 percent
of fatalities
would result from
organ-rupturing
shockwaves and
wind gusts some
call “hypercanes,”
which reach 500-
mph speeds.FIREBALL
Thermal radiation
generated during
some asteroid
impacts can reach
500 degrees
Fahrenheit, hot
enough to melt
your skin. Overall,
the study showed
exposure to this
heat would cause
about 30 percent
of deaths.TSUNAMI
An asteroid
crashing into
the ocean could
create 400-foot
waves, according
to a 2003 study
from UC–Santa
Cruz. Overall, the
2017 simulations
indicated such
tsunamis would
only cause one in
five deaths.FALLING
DEBRIS
When an asteroid
slams into Earth,
it could send
a torrent of
rocks showering
down upon the
surrounding region.
Stand back—the
simulations showed
these showers
caused about 1
percent of deaths.CRATERS
The risk of falling
into a giant hole in
the Earth’s surface
in the wake of an
asteroid impact is
only 0.17 percent,
but stay frosty.
Arizona’s Meteor
Crater is almost
4,000 feet across,
but its asteroid
was just 160 feet in
diameter.MASS
EXTINCTION,
SIMULATED
A 2017 study in
Geophysical Research
Letters analyzed the
causes of death from
50,000 computer-
simulated asteroid
impacts both on land
and at sea. The results
were not pretty.