 |
| Thinkstock |
April 27, 2015 - UNITED STATES - In 2013, a small asteroid exploded in the atmosphere over
Chelyabinsk, Russia. The sonic boom from the event sent more than a
thousand people to the hospital, mostly from flying glass from shattered
windows. The Chelyabinsk meteor was a relatively small chunk of space
rock—asteroid researchers think it was probably about 20 meters (66
feet) across—but exploding over a city made it a noteworthy event. It's
probable many similar asteroids hit Earth on a regular basis, but most
don't happen to fly over metropolitan areas; they fall into the ocean or
over lightly populated regions.
However, Earth has played target in the cosmic darts tournament
before. Meteor Crater in Arizona, the Tunguska impact in Siberia in
1908, and most famously the Chicxulub asteroid in Mexico (which played a
part in the extinction of the dinosaurs) are just three of many known
examples. That's why many people are looking at viable options for
planetary defense: destroying or turning asteroids aside before they can
hit Earth. And planetary defense is one reason the United States'
National Nuclear Safety Administration (NNSA)
has given for not destroying some of its surplus nuclear warheads.
It's easy to be cynical about American nuclear weapons policy,
especially now that we're decades since the end of the Cold War. Debates
over nuclear winter, mutually assured destruction, and the like feel
very distant. So reports that the US wasn't following the stated
schedule for decommissioning nukes in the name of planetary defense
triggered the skeptical radar, not least since
The Atlantic,
The Wall Street Journal, and other sources made it sound like the plan was to blow asteroids to smithereens.
 |
Careful with that thing! We may need it to nuke an asteroid.
Share America |
There are many good reasons to doubt the wisdom of such a strategy,
but it turns out this initial impression—and the impression given by
many published articles—was wrong. The real plan is a lot less
problematic than trying to obliterate an asteroid.
As a result, there's reason to be less cynical about the prospect of
nuking asteroids, though there are still some open questions and fierce
debate over planetary defense. To see the fuller picture, it's
necessary to look at the risks of asteroid impacts, what we know about
asteroids themselves, and what that means for the prospect of pushing
them around. So let's examine whether the stated goals of stockpiling
nukes are consistent with asteroid mitigation; sadly Bruce Willis will
not be involved.
Death from above
Most asteroids and the smaller chunks of rock we call meteoroids are
Chelyabinsk-scale threats, but a significant number are bigger—and not
all of them are far away. While many big asteroids reside in the Main
Belt (sometimes simply called the "asteroid belt") between Mars and
Jupiter, researchers have identified
a large number of near-Earth asteroids (NEAs),
of which about 1,563 are deemed "potentially hazardous." NEAs are
asteroids that orbit in a range a little closer to us than Mars.
However, while a number of these NEAs cross Earth's orbit frequently—the
"Apollo" and "Aten" groups of asteroids—very few large specimens come
anywhere close to us. And none of them present an immediate danger to
us. After all, the Solar System is a big place and Earth is relatively
small.
For that reason, experts are divided on how much we should worry
about asteroid impacts right now. The risk is low for the next few
decades, but the potential damage is sufficiently high even for small
impacts that some think we should focus a lot of effort on mitigation.
The dinosaur-killing Chicxulub asteroid may have been 10 kilometers (six
miles) across, but we don't need one that big to wreak serious havoc.
 |
The potato-shaped near-Earth asteroid Eros may look harmless, but it's nearly 35km long.
|
So if we want to take the long view, there is a persistent danger.
Gravity from Jupiter and (to a lesser degree) Saturn can kick asteroids
out of the Main Belt. Some of those are ejected from the Solar System
entirely or fall into the Sun, but others end up in shallower orbits,
where they might become new NEAs.
The rate at which the gas giants
are creating NEAs may be slow, but we're well advised to keep watch
anyway. If we spotted an asteroid today that could be on a collision
course with Earth, say within the next 30 years, it gives us time to
prepare
now rather than closer to the time of potential disaster.
"The dinosaurs didn't have a space program, much less telescopes, so
it didn't end very well for the dinosaurs," says Alessondra Springmann, a
planetary scientist who studies asteroids at the University of Arizona.
"Hopefully we can find asteroids before they find us."
Asteroid researchers would also like to see them spotted because they
want to study the structure of NEAs. "Planets are formed out of the
same things the asteroids are formed out of," says Springmann. "But
planets have been heated up, they've melted, the planets have surface
properties, and Venus, Earth, Mars, the outer planets all have
atmospheres." Asteroids, by contrast, are relatively pristine: "They're a
whole source of information about Solar System as it formed." NEAs are
particularly nice for the practical reason that they're close to us. We
don't need to fly space probes (or presumably humans) out past the orbit
of Mars to study them.
 |
The asteroid Ida has a small moon (right) named Dactyl.
|
One fascinating discovery has been made by early NEA studies. "About
15 percent of near-Earth asteroids larger than 200 meters [656 feet]
have moons," says Springmann. By measuring the orbits of those moons,
researchers can find the mass of the asteroids they orbit using Kepler's
Third Law (the same way astronomers measured the mass of the Sun or
Jupiter). Combining the mass with radar measurements of the size of
NEAs, astronomers have obtained the overall density of the asteroids.
The result: many asteroids are "rubble piles" (more formally known as
"gravitational aggregates"), collections of smaller rocks and grains
held together by gravity and molecular forces derived from static
electricity. With some densities just greater than water, "they're more
like conglomerations of styrofoam rather than a big type of boulder,"
says Springmann.
The aggregate nature has some interesting consequences. According to
our theoretical understanding of these bodies, sunlight can heat one
side of these asteroids slightly more than the other, increasing their
spin until they literally fragment.
At least one seems to have broken up entirely,
but in less drastic cases, a smaller chunk falls off to make one of the
moons astronomers have observed. And we might be able to exploit the
effect sunlight uses to steer asteroids away from Earth.
Asteroid variety
Cristina Thomas of Goddard Space Flight Center notes that the sheer
variation in asteroid properties makes classification hard. Asteroids
range from very dark—blacker than a chalkboard—to light-colored,
reflecting as much as 50 percent of sunlight back. A major way to
catalog asteroid diversity is by looking at the mineralogy of
meteorites, which are smaller chunks that have fallen to Earth, but
which are fragments of the same types of rock constituting their larger
asteroid cousins.
That's why all of this information is incredibly relevant for
planetary defense. Rubble pile asteroids would require different
mitigation techniques than solid, rocky, monolithic asteroids, but just
because they're loose aggregates doesn't mean they can be easily broken
up. "If you smacked an asteroid and it fell apart, it's likely that
given enough time, it would re-accrete unless you had a very, very large
offense," says Cristina Thomas of NASA's Goddard Space Flight Center.
The tool we need to reduce the threat is more a metaphorical shovel than
a hammer.
Of course, even if we successfully broke an asteroid into pieces,
we'd need to make sure those pieces don't all hit us and cause as much
destruction as the original asteroid. From the science fiction films
Armageddon and
Deep Impact, you might get the idea that
we shouldn't close our eyes or fall asleep
the best strategy is to use nuclear weapons to blow the threatening
space rock to smithereens. This idea isn't off the table completely, but
it must be kept as a last resort, something to be considered only if
there isn't enough time to put together something less risky to Earth.
And, somewhat surprisingly, there's
another possible use of nuclear weapons that is less risky.
Planetary defense
Megan Bruck Syal is one of the people looking into more sensible
asteroid mitigation strategies: using some kind of heavy mass called a
"kinetic impactor" or nuclear explosion to gently nudge the asteroid off
course. The idea is similar for both. Since Earth presents a relatively
small target compared with the vastness of empty space in the Solar
System, even a small change in an asteroid's orbit would cause it to
miss us, provided we intervened early enough.
 |
Simulation of a 10-ton mass impacting Asteroid
Golevka (about 500 m across) at 10 km/s, using the Spheral code (arrow
denotes direction of impact). With sufficient warning
time, kinetic
impactors can be used to deflect hazardous asteroids, preventing future
Earth impacts. Asteroid shape affects the deflection outcome;
simulations
are necessary to quantify uncertainty in asteroid response. Megan Bruck Syal (LLNL), J. Michael Owen (LLNL) |
The
Deep Impact probe
(named before the movie came out) performed an early version of this
experiment when it hit Comet Tempel 1 with a 370 kilogram (820 pound)
copper slug equipped with sensors. This comet is significantly larger
than most dangerous NEAs, so the nudge from the slug was smaller than a
more deliberate deflection attempt would be.
Syal says, "Typically if you have a 10-year warning period, the guess
is that you'd need to change its velocity on the order of a centimeter
per second." While that's a small number, the difficulty in achieving it
increases with the size of the asteroid. "It sometimes becomes
necessary to use a nuclear device simply because the current launch
vehicles aren't capable of transporting enough mass to bring a kinetic
impactor that would be able to accomplish the deflection."
On Earth, much of the damage from a nuclear explosion comes from
shock waves and heat traveling through the air, but that doesn't work in
space. Instead, Syal and colleagues propose an almost gentle-sounding
process called "nuclear ablation." This involves exploding a nuke
several hundred meters away from an asteroid. With no air to carry a
shockwave, the products of the explosion are high-energy gamma rays and
neutrons, which will pummel the surface material and heat it up. The
energetic particles are sufficient to vaporize some material and strip
electrons off atoms, creating a hot plasma that will in turn blast some
particles back out into space. If everything works as planned, the
plasma will act like a rocket thruster, altering both the asteroid orbit
and rotation.
To help ensure the nuclear ablation strategy works, planetary defense
researchers need to know everything about the surfaces, interiors, and
rotation rates of asteroids. The inside of rubble pile asteroids may be
like styrofoam, but their surfaces are often coated in even
finer-grained material known as regolith. The lunar regolith is made of
very small particles (which is why Apollo mission spacesuits and
equipment returned to Earth absolutely coated in the stuff), but we
don't know a lot about general asteroid regolith properties yet.
Characterizing it is a mission goal of several upcoming space probes,
including the
Hayabusa 2 mission to the poetically named 1999 JU3 and
the OSIRIS-REx mission to the near-Earth object called Bennu—an object that has a small chance of smashing into Earth in about 200 years.
OSIRIS-REx will provide a lot of data on how sunlight affects the
surface of an asteroid—essential information for knowing how a nuclear
blast could steer one—as well as return a sample back to Earth.
But
we'll get even more useful information out of Hayabusa 2. Cristina
Thomas says, "Hayabusa 2 is carrying an explosive payload that they're
going to detonate a bit above the surface to see how its target 1999 JU3
reacts." It won't be a nudge big enough to steer it out of its orbit,
but Hayabusa 2 will still provide data on how an asteroid reacts to an
explosion. "I'm super excited about Hayabusa 2 trying to blow up that
asteroid for science," Thomas says. And so say we all.
Other strategies
Researchers and other concerned citizens—including an organization called the B612 Foundation—have
proposed many more ideas than the ones focused on here. Among these is a
kind of gravitational tractor: a massive spaceship to provide just
enough gravitational force on an asteroid to move it out of its present
orbit, something that will be tested as part of NASA's asteroid return mission.
Another more science-fictionish idea uses powerful lasers to heat up
the surface material, creating jets similar to those in the nuclear
ablation concept.
Politics versus science
The nuclear ablation side of asteroid mitigation is relatively small.
Syal is the only full-time researcher working on it at LLNL, and even
her research also includes using ballistic impactors to knock asteroids
off course. Syal's colleague Paul Miller leads the nuclear ablation
effort, working in conjunction with the
NNSA as well as with NASA and a number of universities, but the collaboration involves relatively few people, and the research primarily involves computer simulations.
The arguments in favor of nuclear ablation asteroid mitigation are
fairly clear. Not least of those arguments: we already have the rockets
to carry them, in the form of repurposed ballistic missiles. The
Minotaur V rocket, which carried the
Lunar Atmospheric and Dust Environment Experiment (LADEE) to the Moon, was based on such a nuclear missile, a nice metaphorical transformation of a sword into a plowshare.
Yet there are still limits to this approach, one rather large one
being the politics of nuclear weapons. Presently we can't even test
nuclear ablation directly, thanks to the
1963 Limited Test Ban Treaty,
which forbids exploding nuclear devices in space. Though it's likely
the nations of the world would allow an exception if a killer asteroid
were inbound, other countries might rightfully wonder why the United
States is retaining a stockpile of warheads for
untested use against an uncertain future threat.
While nuclear ablation is a potentially realistic strategy, thanks to
international politics, hidden agendas, and the strange history of the
Cold War, it may not be possible to separate the politics of nukes from
the real science of asteroid mitigation. That doesn't mean we shouldn't
try to disentangle them, if only to determine whether there's any
scientific reasons to keep excess warheads around in the post-Cold War
world. -
ARS Technica.
