Space Access Update #130 3/4/13
Copyright 2013 by Space Access Society
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It has been a while since
we've put out an Update. In recent years
we've fallen into the habit of saying only what absolutely right-now must be
said (in part due to health issues, now largely
resolved.) Over the last year things
have been going remarkably well in this field, and we've been remarkably quiet.
But over the last few weeks,
we've had multiple reminders of one reason why our mission, promoting radically
cheaper space transportation, is not merely urgent, but essential.
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Warning Shots
It's one thing to know
intellectually that you live in the middle of a shooting gallery.
It's another to calmly
anticipate a long-spotted near-miss by a city-killer sized asteroid, then be
startled by a different rock coming in with no warning whatsoever, that, had it
held together another few seconds, would have killed a city.
And it's something else
entirely to spot a civilization-killer sized comet headed almost directly for
the planet next door, and to ask yourself: If it had been headed for our house
instead, could we have done anything about it?
We decided to take what's know
so far about Comet 2013-A1
(http://www.slate.com/blogs/bad_astronomy/2013/02/28/mars_impact_the_red_planet_may_get_hit_by_a_comet_in_october_2014.html)
and run the ballpark
numbers. If it was coming straight at us
(no more or less likely than it coming straight at Mars), and given our
existing space capabilities, could we do anything about it other than prepare
to die?
The short answer is: Maybe.
Now, "Maybe" is a
hugely better answer than the "No, it's all over" it would have been for
most of human history - but we think "Maybe" is still not good
enough. There are things we need to be
doing that we are not currently doing to change that "Maybe" to
"Sure, no problem". These
things can be relatively cheap if we do them right, and some of them will pay
for themselves many times over even if we never do need to steer an inbound
comet away from Earth.
The Ballpark Numbers
Ballparking a problem, doing a simplified estimate with
approximate numbers, can quickly tell you whether the problem is anywhere close
to solvable with available resources, and gives you an initial handle on how
important the different factors are. Some
of us also think it's fun... The rest of
you may want to skip to "Overall Feasibility" at the end of this
section.
(For those of you still with
us, we apologize in advance for avoiding getting down in the weeds on the
technical details, lest back-of-the-envelope turn into a book. Also, we mostly rule out developing your
favorite significant new technology, as too likely leading to fatal
delays. Minor modifications of, and
building some addition copies of, existing proven designs is probably the limit
of the practical in the likely time-frame.)
- Mass
Comet 2013-A1's diameter is
estimated between 15 and 50 kilometers.
Let's pick the middle of that range, 30 km diameter. Volume of a sphere is 4/3 Pi R Cubed, so
that's about 14,000 cubic kilometers of comet.
Comet average density is about 600 kg per cubic meter (the internet is
your friend!) so that's about 8.4 time ten to the 15th power kilograms of comet
- 8.4E15 kg - that we need to nudge hard enough so it misses.
- Time and Distance
The comet was spotted roughly
two years before impact. We'll guess
that if survival was on the line, we could slap together our best hope of
diverting it with (rearranged) existing hardware in a year, and have another
year for the hardware to coast out and meet it.
The comet is coming inward at
a few tens of kilometers per second, while our best rockets can send modest
payloads outward at a few kilometers per second. We'll guess that if we launch a year before
the comet arrives, we'll meet it a month before it arrives. There are about 2.6 million seconds in a
month.
Earth is about 12,700 km in
diameter. If we had perfect comet course
information, the minimum we'd need to deflect the comet in that month is just
over 1/2 Earth diameter, but we'll assume we'd like some margin for the various
likely sources of error. We'll aim to redirect
it by a full Earth diameter.
That's 12.7 million meters of
deflection in 2.6 million seconds, so we need to change the comet's velocity by
5 meters a second.
- Energy
The energy needed to
accelerate 8.4E15 kg by 5 m/s is 1/2 times mass in kg times (speed in meters
per second squared). It comes out to
1.05E17 joules (J) of energy.
The most compact,
transportable form of energy we currently have available is called a hydrogen
bomb. (Many other means of comet
diversion have been proposed; none are off-the-shelf.) H-bomb energy is typically measured in
"Megatons" (equivalent of TNT).
There are 4.184e15 J in one MT.
So, 1.05E17 divided by 4.184e15 gives us 25 MT.
25 megatons? That's barely a theater exchange!
Alas, it's not that
easy. Bomb energy needs to be
transformed to comet motion, and there will be losses. We can't afford to shatter the comet; our
best hope is to set the bomb(s) off nearby, so they heat one side of the comet
just enough to boil off volatiles and gently propel it sideways. At minimum we lose half of each bomb's energy
to open space, and the other half is unlikely to be converted to comet motion
with anything like 100% efficiency.
Absent better data, we'll assume 20% of that 50% gets converted to comet
motion, for an overall efficiency of
10%. (Even that may not be trivial to
achieve.) Given that assumption, we need
to deliver 250 MT of H-bombs.
- Hardware
If we have a year, we're
pretty much going with space launch boosters already planned for that
year. We'll assume that we can probably
build some extra upper stages to deep-space adapt the otherwise-suitable boosters
intended for LEO launches, but we probably can't significantly increase the
overall number of boosters - the hardware in general is long-lead, and the
launch facilities are limited.
A rough rule-of-thumb is that
a given booster's payload to Earth-escape plus several km/s will be between a
fifth and a tenth of its LEO payload. By
this rule of thumb, and assuming upper-stage upgrades as needed, there were
roughly 60 launches in 2012 that might have sent from one ton to three tons on
a useful comet-intercept trajectory. So,
very roughly, that's 120 tons, give or take quite a bit, that we could place at
the comet a month before Earth arrival.
Modern H-bombs in the 1 to 5
MT range tend to weigh around 250 to 300 kg per MT, while the best older larger
designs got down near 200 kg/MT. We'll
assume 250 kg/MT, so 250 MT of bombs will mass roughly 62 tons.
That leaves us 58 tons for
everything else - sensors, guidance, communications, steering, and of course
shielding against the dust and gravel that will abound near the comet. Divided into some number of payloads, that's
an average of 50% bomb and 50% everything else per payload, which seems
achievable.
Overall Feasibility?
The short answer is, maybe we
could now defend ourselves against an inbound comet spotted two years in
advance. Maybe.
We almost certainly can
deliver sufficient energy to the comet's neighborhood in time, barring political/bureaucratic
delays and design screwups. These possibilities
are strong programmatic arguments for a widely distributed effort with lots of
small payloads. Putting all our eggs in
one basket on this one would be a REALLY bad idea.
However, depending on the
technical details of how best to move the comet (some of which is still
classified weapons physics) a smaller number of larger payloads might be
necessary, which currently would complicate the transportation end of the
problem considerably.
Whether we can effectively
apply that energy to successfully divert the comet, we just don't know. The problem has been studied a fair amount,
and the answers vary. Nobody's actually
tested it. We would, under the
circumstances, have little choice but to try.
Conclusions
We might not see an
equivalent comet headed for Earth for thousands of years. Or, it could be spotted next week. Meanwhile panic, crash programs, or
investment in standing armies don't make sense.
Prudent investment in useful capabilities does.
Useful Capabilities:
- Better Observation
Invest in the instruments and
organizations to spot new comets somewhat farther out, and also to spot and
track Earth-crossing asteroids far more comprehensively.
- Better Propulsion
Invest in improved high-energy
deep-space propulsion R&D to let us make timely intercepts of deep-space
objects at ten or more kilometers per second, allowing us to meet them farther
out (and thus farther ahead of time) where it's easier to divert them.
- Better Knowledge
Send more probes out to
asteroids and the more accessible comets, and learn more about their
composition, especially interior composition.
Send manned expeditions where it might be useful. And start experimenting with actually moving
space objects. The first time we
field-test the means should NOT be on an object headed anywhere near us.
(Arranging international
political support for such high-energy space tests will be difficult, but will
also be a good first step toward organizing emergency use of the range of
boosters needed. Only a dozen or so of
those ~60 annual useful launches are US
boosters.)
- Better Organization
Have plans in place and ready
to go if an incoming object is spotted.
Do NOT give the job to an existing hidebound bureaucracy. Do not form a new bureaucracy; it'll most
likely be hidebound by the time it's ever needed. Make a plan to quickly pull together the
capabilities and talents needed into an ad hoc organization should the need
arise. Pre-arrange contingency access to
the resources needed, both national and to the extent practical
international. Do a dry-run exercise of
the plan every few years, update it as needed, and fix problems as they are
found.
- Better Access
Lower cost, shorter-leadtime,
higher-flightrate, more reliable space transportation will help with most aspects
of this problem. In particular, robust
in-space propellant-transfer capabilities and stockpiles will greatly increase
options as to what size payloads can be sent out, how quickly. Even if we never do need to divert a comet
from Earth, these things will give us access to the resources of the inner
Solar System and make us as a species orders-of-magnitude richer.
Closing Thought
Whether or not Comet 2013-A1
provides us with a visible-from-Earth Mars impact lightshow come October of
next year, this is a wakeup call. We
have work to do. Let's get to it.
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Space Access Society's sole
purpose is to promote radical reductions in the cost of reaching space.
You may redistribute this Update in any medium you choose, as long as you do it
unedited in its entirety. You may reproduce selected portions of this Update if
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Space Access Society
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"Reach low orbit and you're halfway to anywhere in the Solar System"
- Robert A. Heinlein