* * * *
A Case in Point
When first discovered in 2004, the asteroid now named Apophis 99942 was given a 2.7% probability of hitting Earth in 2029. In its longest dimension, Apophis is believed to measure about 220 meters—which, as Table 1 suggests, would make its impact unpleasant. Refined orbital estimates for 2029 now predict Apophis won't come any closer than 18,300 miles—from the Earth's center. That is, Apophis might pass inside the orbits of the geosynchronous communication satellites. (Because of the angle of approach, the satellites aren't at risk.)
* * * *
Table 1. Risks of, and from, asteroid/comet encounters.
* * * *
As for Apophis’ return visit in 2036, there remains an estimated 1-in-45,000 chance of a collision with Earth.
Why state collision risks as probabilities? First, orbit determinations are statistically derived from NEO sightings whose estimates of position and velocity unavoidably include a measure of uncertainty. Second, orbits can and do change over time, influenced by rarely known details like mass, spin, absorption and reemission of sunlight, and the gravitational influence of other asteroids. Even the influences of major planets are inexact since planetary orbits themselves are known only through estimates. The uncertainty in Apophis’ nearest approach in 2036 is millions of miles. The most likely separation that year is a comfortable 0.32 AU.[14]
* * * *
A Small Success
NASA's rock-tracking prowess recently had its fifteen minutes of fame. On October 6, 2008, the NASA-funded Catalina Sky Survey identified a small (few meters across) NEO dubbed 2008 TC 3 and predicted its atmospheric entry over Sudan the next day. The object did, in fact, burn up in the atmosphere over Sudan exactly as predicted, releasing an estimated one kiloton of energy.[15]
* * * *
When Push Comes to Shove
If an asteroid or comet is headed our way, there is really only one thing to do: Shove it into a slightly different orbit. The difficulty, of course, is that an object large enough to be dangerous carries a lot of momentum.
Let's consider the mass first. As one idealized asteroid, we'll assume a sphere[16] fifty meters in diameter, with the density of a typical stony meteoroid: about 3.5 grams/cc. This sphere masses about 229 million kilograms (or about 500 million pounds). As a second example, consider a 150-meter-diameter sphere—barely meeting the size standard for a PHO. This time we'll assume a composition like an iron meteoroid: 8 gm/cc. This sphere masses about 14.1 billion kilograms (or a bit over 31 billion pounds).
* * * *
Table 1. Representative collisions.
* * * *
Next, we'll consider velocity. Meteoroids typically reach Earth with velocities in the range of 10—70 kilometers/second (or about 6—43 miles/second). Earth's atmosphere slows most meteoroids down to terminal velocity, but would hardly affect anything as massive as our example asteroids.
Consider a mass M traveling at velocity V. (For simplicity we'll assume the object isn't spinning. Spin would contribute angular momentum and rotational kinetic energy.) Elementary mechanics gives us:
* * * *
Momentum = MV
Kinetic energy = 1/2 MV2
* * * *
The squared term in the kinetic-energy formula means the total energy goes up rapidly as velocity increases. Twice the velocity means four times the kinetic energy. When the asteroid comes to a halt—stopped in its tracks by the far more massive Earth—that kinetic energy turns to heat and shock waves.
Table 2 shows the momentum and kinetic energy involved if our sample asteroids strike with representative velocities. For comparison, there's a row for a fully loaded tractor-trailer traveling 65 miles/hour. The values for momentum and kinetic energy are shown in scientific notation—millions and billions hardly suffice. The kinetic energy of impact is also shown in equivalent megatons of explosives.
For reference, the largest atomic weapon ever tested had a yield of 50 megatons.
* * * *
When the Sky Really Is Falling
To deflect an imminently dangerous (per Table 2, read: massive and fast approaching) asteroid requires a large push. Conclusions at ADRS differed on many topics, but on one there was near unanimity: Nuclear explosives are the technically superior—and in many cases, the only currently practical—defense against an imminent PHO strike. Intuitively, that's reasonable. Shifting the orbit of megatons of rock or metal will take megatons of energy.
In principle, nuclear deflection is straightforward. A nuclear detonation near (or within) an asteroid releases a flood of gamma rays (very energetic photons) and neutrons. The radiation vaporizes part of the asteroid; the spewing gases create thrust. The radiation itself also delivers a push.
Alas, we know so little about asteroids, and that ignorance creates uncertainties about the effects of an explosion. The radiation absorption depends on the elemental composition of the asteroid. The asteroid's composition also influences what fraction of neutrons is blocked—absorbed by material already boiled off by the faster-moving photons. A blast might break an asteroid into pieces still headed our way, many still large enough to be dangerous. Or the fragments might coalesce over time, drawn together by their mutual gravitational attraction.
Delivering a nuclear device adds complication. The asteroid is a fast-moving target. Putting a spacecraft at precisely the correct spot and detonating the device at exactly the right instant as the spacecraft speeds past stretches the limits of navigational practicality.
The ideal detonation position is far from obvious. While ADRS presentations considered optimum detonation positions for idealized asteroids (and results varied), real asteroids won't be ideal. Optimization should take into account the target's shape and composition. An ill-timed detonation near an asymmetric object will waste energy by uselessly altering the object's spin.
Split-second timing for a flyby detonation is not the only approach. Another option is a rendezvous mission, whether to take up a position near the object, make a landing, or (with a multi-part payload) both. On the plus side, a rendezvous allows close-up study before making the targeting decision (or decisions, because the spacecraft might bring multiple devices). On the negative side, decelerating to orbit or land on the PHO expends more fuel and time. Carrying more fuel means bringing a smaller payload—hence, all other things equal, less ability to deflect the PHO. If the target is a comet, the coma is both a barrier to observation and a source of debris potentially hazardous to the spacecraft. Loitering near a comet might reduce the odds for mission success.
If a PHO on a threatening trajectory allows us more lead-time (i.e., if we discover the PHO and characterize its orbit early enough), the mission constraints are eased. A small orbital change made well ahead of time may suffice. In this case, the payload might simply be a high-energy impact device. Like bullet-hitting-a-bullet missile defense—and ABM programs deal with a closing rate of “only” about 8 km/sec—direct impact is a tough problem. A near miss with a nuke has some effect; a near miss by a kinetic impactor does nothing. Like a nuke, any major explosion or impact risks fragmenting the object.
* * * *
Nudge, Nudge
What if a PHO allows us lots of lead-time? Then more subtle methods may suffice to change the object's orbit. Even sunlight, given enough time, might do the trick—just paint the object white.[17]
Slightly faster-acting would be a structure that alters the sunlight pressure on the NEO. A huge solar sail, perhaps configured as a parabolic reflector (to concentrate the sunlight), is one approach. A solar sail kept between the object and the Sun becomes a solar shade; it reduces the incident light. Such structures must be huge to have much effect, even over long time spans. Huge here means hundreds of meters in diameter. The structure will be tricky to deploy and, however thin the material, massive. And while the science of solar sails is well established, solar-sail propulsion remains—after several attempts—an undemonstrated technology.
; One could install a propulsion device on an asteroid. It's unlikely we could deliver enough fuel to appreciably change an asteroid's orbit, but perhaps fuel could be produced in situ. Or perhaps solar cells can power an electromagnetic rail gun, using parts of the asteroid itself as reaction mass. Or a solar-powered laser on a standoff spacecraft could boil off asteroid material.
The gravity tractor is a particularly elegant concept. Place a small payload, the tractor, near the PHO. Gravity draws tractor and asteroid toward each other. Now, slooowly—so as not to overcome the weak attraction—move the tractor. The asteroid follows. The gravity tractor operates over a long period of time, so low-thrust, solar-powered propulsion is needed.
The tractor's propulsion could come from a solar sail or ion thrusters powered by an array of solar cells.[18] Using solar-sail propulsion, there is only one position for the gravity tractor: where light-pressure thrust on the sail balances the gravitational drag between asteroid and tractor. That positional restriction makes this sort of gravity tractor a single point of failure. If anything should go wrong, we lose our defense against the object that was being towed. A single ion-thruster-propelled gravity tractor is also a single point of failure. A constellation of solar-cell-equipped, ion-propelled gravity tractors, however, would have redundancy if single units failed.
So a dramatic intervention or (time permitting) something subtle? On the one hand, non-contact methods work very slowly. The mechanism must work flawlessly—or be deployed with sufficient redundancy to cope with failures—for many years. On the other hand, non-contact methods avoid the risk and uncertainties of fragmenting the PHO. Given how little we know about specific asteroids and comets, that is a major plus.
With so much relevant technology yet to be proven, and so few asteroids and comets yet explored, it's premature to commit to one or two defensive measures. We cannot yet know in detail how space objects would respond to nuclear detonations or kinetic impacts, or whether in situ resources can be used to change space objects’ orbits gradually.
* * * *
Progress
Enough dwelling on what we don't know. We've learned a great deal about asteroids and comets in the past few years ago, thanks to:
* NEAR-Shoemaker: rendezvous and landing mission to asteroid Eros.
* Deep Impact: rendezvous and collision with Comet Tempel 1.
* Stardust: sample return mission to Comet Wild 2.
And we'll learn even more about asteroids and comets in the next few years:
* Dawn: mission to asteroid Vesta and (now promoted to dwarf planet) Ceres, arriving at the former in 2011 and the latter in 2015.
* Rosetta: orbiter/lander mission to Comet 67P/Churyumov-Gerasimenko, arriving in 2014, after two asteroid flybys (European Space Agency).
* * * *
Getting There Is Half the Fun
All the mitigation options make a common assumption: that we can deliver a significant payload to the PHO. For an object closing on Earth at 70 km/sec, that's no easy task. Almost certainly, chemical rockets alone won't suffice for missions to such fast-moving rocks. For reference, the Saturn V rockets used in the Apollo program cut off at 11.2 km/sec.
Over time, spacecraft with ion thrusters can build up enormous velocities. In principle, so can solar sails. The catch is, time permitting.
Gravitational slingshots are another way to gain velocity. The Rosetta mission, for example, will have swung around Earth three times before it reaches its destination comet. Exploiting Earth's (or another body's) gravitational energy in this way costs maneuvering time, and requires that some planet be in the right place at the right time.
* * * *
The Real World Intrudes
It's worth remembering: For lead times of years (or perhaps even a few decades), standoff nuclear devices may offer our only chance of deflecting kilometer-scale objects.
The Outer Space Treaty[19], in force since 1967, prohibits the stationing of nuclear weapons in space or on any celestial object. The treaty has no loophole to permit use against a PHO. (One ADRS attendee argued unpersuasively that deflecting a space object was “propulsion,” not a weapon, and thus does not come within the purview of the treaty.)
Even more restrictive for planetary defense, the treaty expressly forbids “the testing of any type of weapon” in space. I'm no space lawyer, but kinetic impactors—guided projectiles with closing speeds of kilometers/second—seem like weapons to me. If they are to become a part of our planetary defenses, I would hope we test them before needing one to work.
Could the treaty be amended? Clarified? Surely countries would rather update (or ignore) the treaty than face a major asteroid or comet strike. Whether the Outer Space Treaty should be amended in anticipation—say, to allow a demonstration test against a harmless asteroid—seems more contentious.
As with most international treaties, signatories can withdraw upon one year's notice.
Judging from ADRS discussion of planetary-defense policy—and here we debated as citizens, not technical experts—only the questions are clear. Would countries agree to subcontract Earth's defense to a particular space-capable nation(s)? What is to stop a country from acting unilaterally? While an international planetary-defense organization might be preferred by some nations, other countries would be loath to see an international organization controlling missiles and nukes. Will the public trust an international entity to maintain control over its nukes, or not to preempt its nukes for other purposes? If a NEO intervention is only partially successful—the asteroid, or a fragment thereof, smacks Country B instead of Country A—who is responsible? What if a defensive launch fails and spreads radioactive materials? Groups have taken NASA to court (so far, without success) to stop launches of spacecraft using radioisotope power generation. Might someone sue an international planetary-defense group, perhaps in the World Court, to prohibit launches of nuclear devices?
Some simulations suggest that several small nuclear detonations would deflect an object better than one big blast. The organization(s) entrusted with Earth's defense might therefore want to develop or stockpile miniature nuclear devices: “suitcase nukes.” The associated proliferation risk is not insignificant, and one terrorist attack using a stolen suitcase nuke would produce a casualty count comparable to a fairly hefty asteroid.
Maybe we won't seriously prepare for an asteroid strike. The basic data are in Table 1: While fatalities could be cataclysmic, the expected fatalities in a century—fatalities weighted by the likelihood of an impact—are quite small. Other challenges, like diseases and climate change and the current recession, have more severe expected short-term consequences and are apt to consume mindshare and scarce threat-mitigation resources. Like not buying insurance against a “once-in-a-century” flood risk, skimping on planetary defense may look prudent—until it doesn't.
If we knew a big rock would hit Earth, our attention would likely increase. We could chose to wait until we spot such a rock and hope the world will then respond wisely—but to wait guarantees we'll have to respond to the emergency from a state of ignorance and unpreparedness.
* * * *
What Now?
Let's think proactively. What preparations might we take for planetary defense? At a high level, there are three tasks to undertake:
* Study—and tag with a radio beacon, for more accurate tracking—a few asteroids chosen for their threat potential. (If they are of scientific interest, so much the better.) Without detailed knowledge of the mass, shape, and composition of an object, any deflection plan will necessarily be subject to major errors.
* Test—on non-threatening asteroids—candidate deflection technologies.
* Decide who has what planetary-defense authority, and amend the Outer Space Treaty to allow for planetary defense.
We almost certainly have decades before a NEO endangers us, but most mitigation strategies will also need a long time to deploy and operate. It's risky to ignore the NEO hazard until a collision is near-certain.
<
br /> If you consider planetary defense worthy of investment, what can you do? Option 1: Lobby NASA and Congress. It may help to point out that PHO-related missions will also advance the main areas of NASA focus: technology demonstration and science. Option 2: Convince a billionaire. Wealthy citizens are massively underwriting such worthy research projects as the Large Synoptic Survey Telescope Project (which has many uses, including asteroid surveys) and the Allen Telescope Array (for SETI purposes).
For years, NASA has had a fairly stable budget (well below that of its Apollo heyday), and existing programs have their constituencies. Synergizing with current NASA initiatives may be more a realistic goal than competing for funds. Crewed spaceflight programs are among NASA's biggest efforts, and can contribute.
A near-Earth-asteroid mission is in some ways—notably the fuel payload needed for target rendezvous and return to Earth—easier than a moon mission. Announcing a NEO destination for the Constellation program (NASA's latest human-spaceflight initiative)[20] rather than a lunar return mission might spare the U.S. from an embarrassing space-race loss to China. A mission to Phobos or Deimos, both likely asteroids captured by Mars, would be easier than a mission to Mars itself. From a base on either Martian moon astronauts could prove most of the technology necessary for a Mars base. They could also teleoperate robot explorers[21] without contaminating the pre-biotic conditions—or perhaps, even, alien life—on Mars itself.
And trying to be positive, in 2008 Congress tasked NASA to recommend “the optimal approach to developing a deflection capability.” Now if Congress would only increase the $4 million NASA spends annually on asteroid surveys and deflection studies ... [22]
* * * *
Catch a Falling Star
Asteroids are potential resources, too.
A few near-Earth objects of suitable composition, nudged into safe and convenient orbits, could revolutionize the economics of spaceflight and space colonization. Water, of course, from comets. Oxygen from the water. Metals refined in space using plentiful solar power. Silicon processed in space to build mass quantities of solar cells.
Analog SFF, November 2009 Page 10