In Defense of the Planet - Marianne Dyson - Analog Science Fiction
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Science Fact
In Defense
of the Planet
Marianne Dyson
Asteroids zip past Earth almost every day. Will one of them wipe out human civilization like
Chicxulub took out the dinosaurs 66 million years ago? Though an impact of that magnitude
may only happen every 50 to 100 million years, a smaller “city-killer” is almost guaranteed to tar-
get Earth within a few hundred years.
By the end of 2016, the population of Near Earth Objects (NEOs) stood at twenty-five thou-
sand with about half of them larger than 460 feet (140 m). Current surveys, tallied and tracked
by the International Astronomical Union’s Minor Planet Center ( www.minorpla netcenter.net),
are adding about five hundred more that size each year. None of these are calculated to impact
Earth within the next hundred years, though Bennu has a 1 in 2,700 chance of striking between
2175 and 2199. That’s good news, but scientists estimate that about 74% of NEOs that size re-
main to be found.
Thankfully, only a small number of asteroids are classified as Potentially Hazardous Asteroids
(PHA). To be designated a PHA, the object’s orbit must pass within 0.05 AU (about 4,650,000
mi/7,480,000 km, or about 20 lunar distances) of Earth, and it must be brighter than an absolute
magnitude (H) of 22. The smaller the magnitude, the brighter an object, and the larger its as-
sumed size. H of 22 translates to an object about five hundred feet (150m) in diameter.
* * *
The Defenders
If one of these objects is found to be headed toward a collision with Earth, an international
team of scientists and engineers will jump into action. These planetary defenders work for NASA
and other space agencies and academic institutions around the world. They are members of the
International Asteroid Warning Network (IAWN, pronounced “yawn”) formed by the United Na-
tions in 2013 specifically to track hazardous asteroids. They keep watch on the sky and share
the latest scientific information about asteroids and how to find and prevent them from hitting
Earth. They also test their methods and models through role-playing exercises held at planetary
defense conferences (PDC) every other year.
One such exercise began on March 6, 2017, when an asteroid was “discovered” by the Pan-
STARRS telescope in Hawaii and reported by threat (game) master, Paul Chodas, manager of
34 MARIANNE DYSONNOVEMBER/DECEMBER 2018
NASA’s Center for Near Earth Object Studies at the Jet Propulsion Laboratory. Designated 2017
PDC, initial calculations of its orbit and brightness resulted in it being classified as a Potentially
Hazardous Asteroid.
Within one day, NASA’s Jet Propulsion Lab Sentry automated impact monitoring system and
the European Space Agency’s CLOMON system determined the potential impact would be ten
years in the future, specifically, between 8:14 and 8:30 A.M. on July 21, 2027 in the northern
Trajectory of “pretend” Asteroid 2017 PDC. (NASA image from https://cneos.jpl.nasa.gov/pd/cs/pdc17/)
hemisphere on a track including nations from Japan to Ireland. This data was shared with other
members of the IAWN.
A month later, a simulated attempt was made to image the asteroid during a close approach
using the Arecibo Radio Telescope in Puerto Rico. Chodas reported that the object was not de-
tected, meaning its radar reflection was too faint. This put an upper limit on its size at about 984
feet (300 m). An object that size would not end civilization, but it could destroy a city and have
a global economic impact (pardon the pun).
The simulation resumed on May 15, 2017 at the Planetary Defense Conference in Tokyo,
Japan. On that day, a simulated press release from the IAWN announced that the probability of
impact by 2017 PDC had risen to 1%. This risk level is 4 (Yellow) on the Torino Scale (of 1 to
10), which ranks the severity of objects with a probability of impact within a hundred years. Yel-
low is for events meriting concern and automatically triggers scrutiny by international ob-
servers.
Conference participants divided by specialties into groups, which issued recommendations,
which were then discussed and approved by representatives of the leaders of pretend nations.
Similar to a Dungeons and Dragons game, the threat master then “rolled the dice” and reported
the success and failures of the recommended actions, and they “played” another round.
* * *
IN DEFENSE OF THE PLANET 35ANALOG
Let the Games Begin
The next thing the planetary defenders did was go after more data on the threat. Movies of-
ten condense the months or years it takes to gather this information into a microsecond
Google search. But in the real world, scientists must request time on space telescopes as well
as large ground telescopes when the target will be in range, and then repeat the observations
and analyze them. The defenders made these requests and also began the tedious process of
scanning through old sky surveys that might have serendipitously captured spectral and ther-
mal data for the asteroid.
Those in charge of deflecting the asteroid ran computer models to find trajectory options for
missions that could be launched in four to five years. They also checked already-planned mis-
sions to see what flight spares and imaging equipment such as spectrometers and cameras
might be “repurposed” as part of an asteroid flyby or rendezvous package. However, because of
the expense, no equipment was diverted, built, or launched. That would only happen if/when
the threat level increased.
The disaster planning group created a table showing the population, power, and port fa-
cilities in the impact zone. They estimated a one in ten chance that the impact would dis-
place more than ten million people because the track ran through heavily populated areas.
What would be expected at the impact site? Even this modest-sized asteroid would create a
crater 1.2 miles (1.91 km) in diameter. The atmospheric shock wave would collapse truss
bridges within 13 miles (21 km). Wood buildings would collapse within thirty miles (49 km),
and windows would shatter within seventy miles (112 km). An impact splashing into the Japan
Trench would create a tsunami 82 to 95 feet (25 to 29 m) tall at the shoreline. For comparison,
the 2011 Fukushima tsunami was about 32 feet (10 m) tall.
Rather than keeping the threat secret like in the movies, the leaders recommended educating
people in the path through local agencies, and helping those agencies prepare evacuation plans.
However, they did decide to withhold the detailed analysis of the impact’s effects from the pub-
lic since the potential event was still ten years in the future and would hopefully be prevented.
The threat master implemented the recommendations of the planetary defenders, moved the
calendar forward, and provided the data and results of their actions—which were somewhat
guaranteed to at least partially fail so that the “game” could continue.
* * *
The Danger Increases
What you don’t know might kill you. Unfortunately, even the best telescopes are blinded by
the Sun. So in any asteroid threat scenario, there will be weeks or months when no ground tele-
scope or Earth-orbiting assets can observe the asteroid. Thus the planetary defenders had to
wait 18 long months, until simulated November 2018 when the asteroid emerged from behind
the Sun, to learn what their recommended observations, data mining, and calculations had re-
vealed about the asteroid’s trajectory, size, and composition.
* * *
Where Is the Bullseye?
Observations by the Hubble Space Telescope and Japan’s eight-meter Subaru Telescope pro-
vided more accurate trajectory data. Usually, observations rule out an impact, but no one was
surprised that for this exercise, the probability of impact rose to 96%. The updated trajectory
also narrowed the impact target zone from Tokyo, across the Korean peninsula, to Beijing, Chi-
na. Dr. Catherine Plesko of Los Alamos National Labs, blogging about the exercise on Facebook,
said this scenario was, “very nearly a worse-case impact zone. To make matters worse, the ki-
netic impactor profiles are such that it’s easier to push the impact zone to the west (over China)
than to the east (over the Pacific). Truly a nasty, devious scenario to address.”
* * *
How Big Is Bad?
If the asteroid was found to be smaller than 328 feet (100 m), rather than spend billions and
risk failure, the team might decide to evacuate the area and let it hit. Based on brightness, the
initial size was reported to be between 328 to 820 feet (100–250 m) in diameter. But a large dark
36 MARIANNE DYSONNOVEMBER/DECEMBER 2018
object and a small white object can reflect the same amount of light. One way to distinguish be-
tween the two is to use the infrared heat signature since large dark objects absorb and then ra-
diate more heat than small white ones. Simulated infrared observations by the NEOWISE
spacecraft estimated 2017 PDC was between 650 and 900 feet (200–280 m) in diameter.
* * *
Carbon, Stone, or Metal?
Asteroids come in three basic types: carbonaceous, stony, or metallic. Carbonaceous asteroids
make up about 75% of all asteroids. They are as dark as charcoal, requiring large telescopes to
see them—which is why they often slip by undetected. They have more gases, called volatiles,
than other asteroid types, and can therefore have their mass reduced by boiling them away via
an impact or explosion. Stony and metallic asteroids can be twice as dense as carbonaceous
types and thus require more energy to deflect.
Chodas reported that data mining successfully provided spectral measurements from a 2016
survey. 2017 PDC was identified as a carbonaceous asteroid with an albedo (reflectivity) of only
4–8%. Albedo plays a role not only in determining size (low albedo objects must be large to re-
flect enough light to be seen at a distance), but also offers a way to alter the orbit over time us-
ing the Yarkovsky Effect.
The Yarkovsky Effect
Photons of sunlight falling on a rotating object exert a force called the Yarkovsky Effect that
can slowly change their orbit around the Sun. The side facing the Sun heats up like the day
side of Earth. If the body spins counterclockwise, the “afternoon” quadrant becomes the
hottest. Without an atmosphere to soak up the energy, most of the radiation is soon re-radiat-
ed back into space, producing a tiny thrust outward from the “afternoon” area—pushing the
The orbit of the asteroid Bennu is being changed by the Yarkovsky Effect. It is the target of NASA’s OSIR-
IUS-Rex mission, which launched in 2016 and will arrive in 2019. (NASA image.)
IN DEFENSE OF THE PLANET 37ANALOG
object away from the Sun. (If the object is spinning clockwise, it will be pushed toward the
Sun.) The 1640-foot (500-meter) diameter asteroid Bennu moved 100 miles (160 km) over 12
years via the Yarkovsky Effect. Thus, asteroids might be “paintballed” with white or black
powder to gradually alter their orbits. [Reference: Nea l-Jones, Na ncy a nd Stolte, Da niel. “As-
ter oi d Nu dged by Su n li ght: Most P reci se Mea su rem en t of Ya r kov sk y E ffect.”
https:/ / www.n a sa .gov/ topi cs/ u n i ver se/ fea tu res/ ya rkosky-a steroi d.htm l Ma y
24, 2012.]
* * *
Choose Your Weapons
The job of the planetary defenders was to prevent 2017 PDC from striking the Earth.
But they still had no information about the shape, spin, or structure of the asteroid. Hitting an
elongated object off center might only result in spinning it, like trying to roll a pencil by pushing
on the eraser end. There is also a lot of difference between pushing a “rubble pile” and a solid rock
of the same size. And there might be some pesky satellites to consider.
The defenders therefore recommended quickly launching a flyby mission to get the shape and
satellite data. In case one should fail, they opted to launch a pair of spacecraft in October 2019
to fly past the asteroid in May 2020.
A second pair of spacecraft would be launched in June 2020 to rendezvous and go into orbit
around the asteroid about three years later, in May 2023. These two spacecraft would have
ground-penetrating radar that would reveal the inner structure of the asteroid. They would re-
main with the asteroid to observe the aftereffects of deflection attempts in 2024.
Two deflection options were identified: one to speed it up, and the other to slow it down.
Deflection
A deflection can avoid a collision between an asteroid and Earth in one of two ways. One way
is for the asteroid to be sped up so it crosses through the intersection of its orbit with Earth be-
fore Earth arrives. The other way is to slow it down enough for Earth to pass through before it
gets there.
Because the Earth is rotating, if the deflection is only partially successful, the impact point
shifts east or west of its original target. For PDC 2017, a partial slowing would shift the strike
west from Japan toward China, whereas a partial speed up would shift it east into the Pacific
Ocean.
Slowing down an asteroid only requires placing something in its path for it to run into. Speed-
ing up is considered harder because the spacecraft must go faster than the asteroid to give it a
push. Attaining that extra velocity (delta v) may require a powerful rocket or complicated or-
bital mechanics to arrange gravity assists from the Moon, Earth, or Venus.
Both options are limited by how much mass current rockets can deliver to deep space. Most
cases require multiple rocket launches.
The spacecraft that the rockets launch varies according to the method chosen to change the
asteroid’s velocity. Options might include solar sails, gravity tractors, and mass drivers (as well as
Bruce Willis placing nuclear charges). But most of these technologies remain untested and may
not work as envisioned. (OSIRIUS-Rex will provide a first test of a gravity tractor in 2019.) So for
near-term threats, kinetic impactors, with or without nuclear devices, launched via conven-
tional rockets are the best understood, most available, and therefore the most likely choice to
be approved by world leaders.
The most efficient time to change an orbit is when the object is closest to (perihelion) or far-
thest from (aphelion) the Sun. If launch windows are missed, it may be necessary to launch
more impactors or increase their fuel or payload capacity (at great expense) to impart the nec-
essary velocity change.
Composition and structure also determine the number and type of impactors. If the asteroid
38 MARIANNE DYSONNOVEMBER/DECEMBER 2018
is a rubble pile, the impactors might pass through it like bullets through a pillow. Or the im-
pactors may simply break a large mass into smaller pieces that still hit the Earth. This adds un-
certainty to evacuation plans plus may cause long-term damage to the upper atmosphere.
Would a nuke do a better job? Exploding a nuclear device above the surface has the advan-
tage of transferring some kinetic energy directly and vaporizing the surface material whose
plume would act like a rocket engine. Boiling off frozen gases also reduces the asteroid’s mass.
But if the deflection fails or is only partially successful, there’s a risk that some debris may be ra-
dioactive, contaminating the atmosphere and complicating ground operations.
Considering the high stakes and lack of time to prepare, test, and launch equipment, the de-
fenders assumed a 50% probability of failure for each spacecraft. They estimated that three im-
pactors would be required to def lect the asteroid away from Earth, though one
nuclear-equipped device could probably do the job. But it wasn’t clear if any nation would be
willing to provide the nuclear device and also assume the “blame” or deal with the contamina-
tion or political fallout if something went wrong with the launch or operations.
Calculations showed that each impactor striking the asteroid would move the risk corridor
285 miles (460 km) east. Because the blast radius was 31–93 miles (50–150 km), the impact
point had to be shifted at least 155 miles (250 km) offshore of Japan, requiring three impactors.
So it was decided to launch six spacecraft in March 2020 to crash into the asteroid four years
later in February 2024. Success would deflect the impact point eastward toward the Pacific
Ocean, and hopefully enough to miss the Earth altogether. Unfortunately, the launch window in
March was two months before the flyby spacecraft would reveal the asteroid’s shape, satellite,
and spin information in May 2020.
If the “speed up/eastward” impactors failed, the rendezvous spacecraft could be used to slow
down the asteroid by letting the asteroid run into them. But that would deflect the asteroid’s tar-
get westward away from Tokyo toward China or Europe if it wasn’t sufficient to slow it down
enough to miss the Earth. To make sure these rendezvous spacecraft could serve as a real back-
up, the defenders recommended that they be equipped with nuclear devices.
The group estimated the cost of the two rendezvous spacecraft at three billion dollars if nu-
clear devices were included, and two billion dollars if not. The six kinetic impactors would cost
an additional six billion dollars. It wasn’t clear to the participants how these costs would be cov-
ered.
Would the data from the flyby mission change any of these plans?
* * *
Double the Trouble
For the next round, in simulated May of 2020, the threat master reported the good news first:
that the flyby missions were successful. The bad news was in what they revealed: asteroid 2017
PDC was a binary! The primary body was about nine hundred feet (270 m) and the moon was
roughly 330 feet (100 m). The moon’s orbital radius was 0.6 to 1.2 miles (1–2 km) with a period
of a few days. The orbital parameters raised the mass of the primary by about 20% over pre-fly-
by estimates.
The new trajectory data also showed (big surprise) that both bodies, unless diverted, would
hit and destroy Tokyo on July 21, 2027.
Six spacecraft were built and launched in March 2020 as planned, but one of them failed—so
five spacecraft were on their way to reach the bodies in February 2024. The new higher mass of
the primary would require all five spacecraft to score a hit, versus the original three, to move
the asteroid enough to avoid a collision with Earth.
Two more spacecraft were ready to be launched as planned in June to rendezvous with the
asteroid in 2023. Decision makers had decided not to install nuclear devices on these spacecraft,
though the spacecraft had been designed to carry them.
The clock was ticking. What did the defenders want to do?
IN DEFENSE OF THE PLANET 39ANALOG
* * *
The Nuclear Option
They first reaffirmed the need for the rendezvous spacecraft to determine the internal struc-
ture of the bodies. After the encounter with the “eastward” impactors, they would also be need-
ed to track the moon and resulting fragments and determine which ones remained a threat to
Earth.
They determined that if the binary system remained intact, the secondary object would not
cause any damage beyond what the primary already caused since they would hit the same loca-
tion. However, if the primary was deflected and only the secondary hit, the location would be
uncertain, and the risk corridor widened. The best solution would be to time the kinetic im-
pactors to keep the binary together while shifting both so the impact moved into the Pacific far
enough (about 500 miles/800 km) to avoid a tsunami hazard. This would require all five of the
impactors to be successful.
Should they consider the “slow-down/westward” deflection launch option available in July
2023? The change in velocity required to move the asteroid westward enough to miss the Earth
was considered too large, and the attempt might simply move the target from one vulnerable pop-
ulation (Tokyo) to another (Beijing).
The key recommendation coming from all the groups was to put nuclear devices on the ren-
dezvous spacecraft despite only having one month to get it done and overcome their leaders’ ob-
jections. This could effectively remove the threat from both the primary and secondary bodies,
because the primary would be deflected and the secondary destroyed by the blast.
If the nukes could not be added because of timing or political reasons, then they suggested
launching two more kinetic impactors and preparing additional flyby missions, with nuclear de-
vices if possible, to deflect or destroy the secondary body.
The communications team noted that 75% of the public was opposed to the use of nuclear
devices. They proposed “rebranding” them as Atomic Deflection Devices (ADD) to reduce fear.
They recommended releasing plans and timelines for moving business and services and people
out of harm’s way to reduce anxiety. They emphasized the message that with planned reloca-
tions, no one dies.
With catastrophe looming, the pretend national leaders approved adding ADD to the ren-
dezvous missions at the last minute. They also agreed to additional missions, including two more
flybys with ADD.
The threat master rolled the dice, and the simulation jumped ahead another 18 months.
* * *
Rendezvous Reveal
On May 15, 2020, the defenders learned that sadly, only one of the nuclear-equipped ren-
dezvous spacecraft had made it to asteroid 2017 PDC. The other had experienced an unrecov-
erable reaction-wheel malfunction a year after launch. Five kinetic impactors were still on
course, with the first to arrive on February 24, 2024 with the others striking one after another
in the days following.
Data from the surviving rendezvous spacecraft revealed that the secondary object was 295 feet
(90 m) in diameter. Its orbit around the primary was eccentric with a periapsis of 0.6 mile (1 km),
and apoapsis of 1.8 miles (3 km), though it was only loosely bound to the primary. The densities of
both bodies were determined to be 1.9 g/cm3, more than water (1g/cm3) but less than baking
soda (2.2 g/cm3).
What would happen in February 2024? Would the deflection missions succeed fully, partially,
or fail? Would the leaders have to detonate the nuclear device on the surviving rendezvous
spacecraft? If that also failed, how would they deal with evacuating tens of millions of people,
moving/protecting historic artifacts, and preparing to deal with environmental, economic, med-
ical, and political fallout for years afterward?
* * *
Happy Endings
40 MARIANNE DYSONAs information about Asteroid 2017 PDC became available, the threat and responses evolved over time.
The original impact was predicted to occur in July 2027.
*At the time of the exercise, the James Webb Space Telescope was to be launched in 2019. It has since
been delayed to 2021.
41ANALOG
On the last day of the conference, the threat master surprised everyone with a happy ending
by two different scenarios.
The first way to win was to detonate the nuclear device on the surviving rendezvous spacecraft
at perihelion on February 6, 2024, before the impactors arrived and possibly disrupted the binary
system. He had determined that an explosion at 0.6 miles (1 km) from the primary would totally
destroy the secondary and impart a delta v of 0.03 fps (1 cm/s) to the primary. This push was
enough for the asteroid to shoot through the orbital crossroads ahead of the Earth by about 621
miles (1000 km).
The second way to win was to let the kinetic impactors ram into the primary and speed it up.
Chodas admitted that when he began the exercise, he hadn’t thought that it was possible to do
an eastward deflection with kinetic impactors, but the defenders had found a possible trajecto-
ry. So with each impactor delivering about 0.026 fps (0.08 cm/s) to the velocity, after four
strikes f ive days apart, the primary, with the secondary still orbiting it, would be sped up
enough to miss the Earth by the same 621 miles (1,000 km) that the nuclear device would
achieve.
As Plesko posted on Facebook, “Just this once! Everybody lives!”
* * *
Possible Sequels
An asteroid passing close to Earth has its orbit altered by Earth’s gravity. If it zips by at just the
right distance, referred to as a gravitational resonance keyhole, then a future collision might be
set up. This opens the door for a 2017 PDC sequel, though the impact would be reduced along
with the “body” count since the secondary was destroyed!
If the approaching object is as large as the Chicxulub impactor, unless the defenders have
plenty of advanced warning to prepare, or some faster and more powerful launch capabilities
and deflection options become available, not even Bruce Willis could save us from Apocalypse.
But with the help of modern observatories, computers, and networks, the odds of a large as-
teroid going undetected until it is within months of impact are increasingly slim, though, to the
morbid delight of science fiction writers, still possible.
Stories and role-playing exercises about asteroid impacts offer a way to share ideas and help
uncover and address problems that might be encountered. One small example was someone on
Facebook suggesting that for an impact in water, sonic alarms be used to scare dolphins, whales,
and fish from the area.
Perhaps the greatest value of asteroid stories is that by raising awareness of the threat and po-
tential consequences, the knowledge, tools, and political support will already be in place so real-
world heroes can successfully defend our planet.
* * *
References/For Further Study
“The Probability of Collisions with Earth.” NASA. Accessed October 12, 2017.
https://www2.jpl.na sa .gov/sl9/ba ck2.html
Minor Planet Center: http://www.minorpla netcenter.net/
Planetary Defense Conference Exercise—2017. NASA JPL Center for Near Earth Object Studies.
https://cneos.jpl.na sa .gov/pd/cs/pdc17/ YouTube presentations.
https://www.youtube.com/wa tch?v=pbUvDqFIgqg&list=PL11KdwQJKIqFBvzQY4a -
0K4wGdPtBdwc8
“Planetary Defense Frequently Asked Questions.” NASA. Aug. 29, 2017.
https://www.nasa.gov/planetarydefense/faq
Ma ria nne J. Dyson wa s inspired by science fiction a nd the Apollo Progra m to become one of
42 MARIANNE DYSONNOVEMBER/DECEMBER 2018 NASA’s first fema le flight controllers. She is now a n awa rd-winning children’s a uthor, educa - tiona l spea ker, a nd freela nce science writer. Her most recent books include her memoir, A Pa ssion for Spa ce, her science fiction collection, Fly Me to the Moon, a nd a children’s pop-up book co-a uthored with Buzz Aldrin, To the Moon a nd Ba ck. To lea rn more, plea se visit www.mDyson.com. IN DEFENSE OF THE PLANET 43
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