Cosmic Warning Shots
by the Space Access Society editorial team
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 known so far about Comet 2013-A1
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.
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.
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.
• – 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.
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.
This article is excerpted from the Space Access Society newsletter: Space Access Update #130