From AIAA’s “Daily Launch” email newsletter for 8-26-2021 (yellow highlight mine):
Asteroid Deflection May Require Multiple Attempts
The New York Times (8/25) reports that researchers presented findings on asteroid deflection research at the 84th annual meeting of the Meteoritical Society this month. The researchers found that kinetic impact deflection is a feasible and potentially effective means of sending an asteroid out of the way of Earth. Researchers found that carbon-rich meteorites were more likely to shatter when hit with high-velocity aluminum spheres.
Bear in mind that the carbon-rich asteroids and meteorites are the most numerous type. Examine Figure 4 below, for why this is an important finding.
Asteroid 2020 SW was discovered 9-18-2020, and made its closest approach on 9-24-2020. The warning time was thus 6 days. It was estimated to be 5-10 m in size. Closest approach distance was 22,000 km.
Using the somewhat-arbitrary density of 2.5 g/cc = 2500 kg/cu.m, that corresponds to a mass between 49.7 and 398 metric tons. This definitely falls in the city-buster range for speeds between 10 and 20 km/s, if it were to impact the surface, or explode close to it.
Given 6 days notice, it might have been possible to evacuate a threatened city. This one could have been a "success" story, rare among those listed in the article.
In 2009 I attended a meeting held in Spain about defending Earth from threatening asteroid impacts. At that time, we had been able to locate most of the large (extinction-event) objects, we were starting efforts to locate the smaller “city-buster” objects, and we had some ideas about how to deal with them.
I'm sorry to report that not much has changed since then. We are now beginning to find some of the smaller "city-busters", but that's about it.
Recent public news accounts:
Asteroid 2020QG passed ~1830 miles from Earth on Sunday 8-23-20. It was not seen until some 6 hours after it passed. This object was 10-20 feet (3-6 m) in diameter (for those unfamiliar with metric, a meter is about 10% longer than a yard). Its speed past the Earth was ~27,600 mph. An on-line animation in Wikipedia shows ~12.3 km/s at ~9300 km center-to-center.
Asteroid 2018VP1 is predicted to pass Earth Monday 11-2-20 (the day before election day in the US), give or take ~2 days, depending upon whose estimate you believe. It is said to be 6.5 feet in size. It is listed in Wikipedia as 2-4 m size, with an animation that shows a fairly-slow 9.7 km/s pass, about 419,000 km center-to-center away (not far outside the moon's orbit).
A somewhat-recent event, only a few years ago:
The Chelyabinsk object was about 20 m dia, with 19.16 km/s velocity, estimated at 12,000-13,000 metric tons. It exploded in the atmosphere, with a yield estimated at 400-500 KT-equivalent to a nuclear weapon (where 1 KT = 4.184 GJ). This thing fell 2-15-2013, and was completely undetected before its entry. It injured around 1000 people, and blew out most of the windows in that city.
Here are the data as listed in Wikipedia, for the previous 2 years, plus this one so far. To this I added the Chelyabinsk object. It would take more than a day to evacuate a city that was threatened by such an event, which is why the measure of warning time is important. That list is in Figure 1.
So, note the warning times color-coded in the list. Only the single green one might have afforded sufficient time to evacuate. Note also that a center to center distance under 6.37 thousands of km is a direct impact, because that figure is the radius of the Earth.
Figure 1 – List of Small Asteroids Recently
Conclusion: our track record seeing the small ones before they pass is NOT good at all, whether or not we include the Chelyabinsk object!
What’s still coming that we know of:
Here are the predictions for some selected future encounters, per Wikipedia, as Figure 2. To this I added the recently-reported "election day" object. The notation "LD" refers to lunar distance. If "yes", the predicted encounter leads to a pass within the orbit of our moon. Bear in mind that both measurement inaccuracies and any sort of disturbance can throw these close-pass center-to-center distances off.
Figure 2 -- Known Future Threats
The point here is that we already know of some larger objects that will pass uncomfortably close in the coming years. Smaller objects are more numerous, and mostly as-yet undetected. That means there are a lot of “city busters” out there, some of which we can expect to hit us. This threat is quite real!
Figuring the explosive yield range of air bursts and impacts:
Chelyabinsk object: 20 m dia sphere has 4189 cu.m volume. Using middle-of-the-road mass 12,500 tons = 12.5 E6 kg, density is mass/volume = 2984 kg/cu.m. Fresh water is 1000 kg/cu.m, so the specific gravity is about 2.98. How consolidated that object was, is unknown. But it's the best data available to me. Most minerals are in this range of density.
A "typical" small asteroid is 2-6 m in size. Treating that as the diameter of a sphere, "typical" volumes are 4.19-113 cu.m. Using the Chelyabinsk object estimate of 2980 kg/cu.m density, the "typical" masses are 12,500 kg = 12.5 metric tons, up to 337,000 kg = 337 metric tons.
The actual astronomers have better data to use in such calculations than I have, but their methods and formulas, and mine, are the same. This stuff is just not as certain as it often appears to the public.
About the lowest velocity ever seen is near 10 km/s, and about the highest velocity to be expected is nearer 20 km/s, at least as a good guess. Kinetic energy is 0.5 mass x velocity squared, where for mass in kg and velocity in m/s, you get energy in Joules: kg m^2/s^2 = N-m = J. Every 4.184 billion Joules is a kiloton (KT) equivalent to a nuclear weapon.
I used these data to estimate the results in Figure 3.
Figure 4 – Kinetic Energy As Equivalent Nuclear Weapon Yield
These figures say that these 2-6 m size "small asteroid" objects will have energies comparable to nuclear warheads in the 150 KT to 16 MT range. They are very definitely "big-city-busters"!
For comparison, the Hiroshima and Nagasaki bombs were about 15 and 20 KT in yield. Many of our military warheads are now in the 200 KT range. The giant "Tsar Bomb" test, of October 1961 in the Russian arctic, was around 66 MT, the largest nuclear explosion ever seen.
Not only are these real city-buster threats, we also have a poor track record of seeing them until it is too late! That is precisely because they are small, dark, and often approach from a more sunward direction, where our ground-based telescopes are blind.
There are differences with nuclear weapon effects:
The kinetic energy of these things is not the whole story, unlike nuclear weapons. There are also the tremendous interplanetary speeds, and the downward angle of the entry trajectory.
Unlike a nuclear weapon, the fireball associated with an air burst is still traveling along the trajectory at great speed for a while. If during that interval, it gets close to (or impacts) the surface, it will incinerate everything in the vicinity.
Downward angle plays a strong role, too. The Chelyabinsk object struck at a very shallow angle across the sky, so that its air burst happended dozens of miles up in the air. Had this been steeper downward, the burst would have been much closer to the city, utterly destroying it, and killing its people.
A little steeper still, and the object might have struck the surface without bursting in the air. That just releases even more blast and heat right at the impact point.
What we need:
(1) We need to be able to detect these things, in time to actually do something about them!
(2) We need to obtain real ground-truth data directly from them, so that we can figure out exactly what to do, given the chance.
(3) We need to implement those means of doing something, which includes both the means to get there fast enough, and the actual means to deflect or destroy them.
The B612 Foundation at one time proposed an asteroid-defense satellite to be placed in an orbit nearer that of Venus than Earth, so that it could see threats approaching Earth from sunward, by being even more sunward, but looking outward from the sun.
The sensor of this proposed satellite was not based on visible light or radar, but infrared. These objects are brighter and easier to see in the infrared than in visible light (or radar), although they are still dim.
We still need that satellite, and preferably 2 or 3 of them at any one time! We do not have any of them! What we do have (mostly telescopes based on Earth), is having great difficulty seeing things that are only 1-100 m in size.
That is because (1) these telescopes are not located sunward of Earth looking outward at dark, cold deep space, and (2) they does not use infrared (asteroids, being somewhat near the sun, are simply warmer than the cold of deep space).
Knowing how to deflect or destroy:
There seem to be three general types of these small objects. (1) By far the most numerous are those that seem to be made of a mix of carbonaceous and stony particles, not very consolidated (and often with significant void spaces internally), and actually rather weak structurally.
These are essentially flying sand-and-gravel-and-boulder piles, which would fly completely apart if you actually pushed against one. These are the ones that tend to explode up in the atmosphere, from the suddenly-crushing drag forces of encountering air while moving at interplanetary speeds.
The small solid particles of post-explosion debris, if larger than about a quarter inch diameter, actually do not burn up entirely, and hit the ground at modest speeds.
(2) There are a few asteroids that are really more-or-less solid rocks, made of a mix of different minerals, and even some metal content. These tend to hang together fairly well, and would likely not fly apart out in space, if you pushed against one, at least fairly gently. How internally fractured they really are, is an open question.
These bodies tend to make it to the Earth's surface, if larger than about a quarter inch. Not being debris of an atmospheric explosion, they are moving very much faster when they hit, causing their own explosions and craters.
(3) There are a very, very few that are actually solid chunks of metal, mostly iron. If larger than about a quarter inch, they make it through the atmosphere to strike at very high velocities as one solid object.
This produces really big explosions and craters. Meteor Crater in Arizona was created about 50,000 years ago by one of these, just about the size of the objects we are interested in detecting and deflecting-or-destroying.
Some of these we could push against (in some way) and deflect into a miss, if we could get there in time to do it. But not the most-numerous unconsolidated bodies. Those will fly apart if pushed, creating a storm of debris.
If that disruption of the threatening object happens close to the Earth, then you just turned a damaging bullet strike into an even more damaging shotgun blast! That strategy is viable only if you can do it far enough from the Earth that most of the debris storm you have created will miss. See Figure 4.
Figure 4 – Bullet Strike vs Shotgun Blast, with Timing
Deflection / destruction techniques:
Space is a vacuum. There is nothing out there to propagate a blast wave. Nuclear explosions create great heat as radiant energy, but no blast out there. You won't have time to drill (and we don't know how to drill into these things anyway) to emplace a nuclear bomb inside one of these bodies.
What you do is detonate your nuclear device alongside the threat. The immense amount of radiant heat will vaporize and spall-off significant mass from it, hopefully without completely disrupting it. The resulting momentum "kick" in the opposite direction will subtly alter the body's trajectory. Do this "right", and "soon enough", and you can cause it to miss.
The same sort of thing can be done with impactors instead of nuclear weapons. You hit the thing from the right direction, and "soon enough", and the impact creates the spall-off, just on a smaller scale than a nuclear explosion. The momentum reaction changes the body's course slightly, provided that it is not disrupted. See Figure 5.
Figure 5 – How Spallation Creates a Force
The gentlest technique is the so-called "gravity tractor". This takes the longest time to have effect, so you must know (and go) years in advance of the impact threat to Earth. You send a spacecraft to rendezvous, and hover, alongside the body.
Its propulsion prevents its falling onto the body. But the tiny force of gravity between spacecraft and body forces the body to "follow the spacecraft". This is a very tiny effect, but it is real. See Figure 6.
Figure 6 -- Gravity Tractor
To summarize – see Figure 7.
Figure 7 -- Actions We Can Take, with Timing
What to do about this:
Now look carefully at the indicated timing required in Figure 7, and compare that to the track record seeing these threats in advance of their approach in Figure 1. It is NOT GOOD! We should be working on this, but are mostly not. THAT is why we need the detection satellites.
Getting there soon enough to do any good:
For an Earth-crossing asteroid, there is a short window of time to get there from here, and a very demanding propulsion requirement to do the mission, even if a one-way trip with an unmanned spacecraft. The perihelion velocity of the asteroid is substantially higher than the Earth in its orbit about the sun, by at least 2-3 km/s.
What you have to do to get there is wait until the Earth is in the "right place" in its orbit about the sun, relative to where the asteroid is. You cannot go "just any time you want", like we can to the moon. Then you escape from the Earth, and wait in an Earth-like orbit about the sun for the asteroid to catch up with you. Then you burn a second time to catch up with the faster asteroid.
The cost of all this (14+ km/s) is more than a direct one-way shot to Mars at optimum orbital positions (about 12 km/s). See Figure 8.
Figure 8 – Traveling to an Earth-Crossing Asteroid
If you send astronauts, then they must have a way home. Before it is too late (days to weeks), they must depart the asteroid into an Earth-like orbit, waiting for the Earth to catch up with them. Then you might do a free entry, or you might do a propulsive deceleration into Earth orbit, to bring them home.
At the very least this is 17 km/s velocity change required. Maybe over 20 km/s. At present, we have no rocket vehicles capable of flying this fast. We will need nuclear propulsion, or else very huge chemical rockets pushing very small probes and space capsules.
Why humans need to go, sooner or later:
A robot can only deal with what its programmers anticipated. It just cannot adapt to the unexpected or the unforeseen, without direct human intervention. That is simply just one of the truths of our time.
Across the distances needed, it is just not feasible to remotely-operate a probe, in anything resembling a timely fashion. Light just does not cross those distances that fast. Another truth of our time.
What that really means is this: sooner or later in the process, humans simply must go to these asteroids to investigate their real "ground truth" properties. There is just no other way at this time in history to find out "what is" versus "what is not".
This isn't like going to the moon, which was a mission about a week, to at most two weeks, long. We are talking about weeks to months in space for the Earth-grazing asteroids. For the non-Earth-grazing asteroids, and for most comets, we are really talking about years in space. And we just don't know yet how to do that, without killing our crews.
Bottom line: we need more capable rockets, and we need a vehicle for crews that provides artificial gravity and radiation protection. These voyages could range from days to years in duration, and a small space capsule is only "good" psychologically for a couple of weeks. These living spaces must be large.
There is no better reason for both unmanned, and manned, space program efforts than protecting the Earth against asteroid impacts. The massive extinction event 66 million years ago that extinguished the dinosaurs in favor of the mammals, is just one example of what damage a 6-10 km size object can do.
This is not a zero-sum game (despite what politicians insist); this is something that needs to be ongoing, at whatever level we think we can afford, in any given year. Spend more when you think you can afford more, simple as that. But NEVER zero! Some outcomes are just not tolerable, no matter how unlikely!
I recommend the following things be done:
(1) Deploy a constellation of at least 3 asteroid-detection satellites, using infrared detection devices supplemented by visible and radar, in orbits about the sun approximately at the orbit of Venus.
This is just not that expensive a thing to do, and it will identify the vast bulk of the small (2-6 m) threats to the Earth. Such would make evacuation of a threatened city possible for the first time.
(2) Develop a means to send unmanned probes, followed by manned missions, to several of the Earth-crossing asteroids, to find out real "ground truth" about their characteristics. The manned voyages will probably require enhanced propulsion, possibly nuclear. Crewed vehicles will be quite large.
The place to test nuclear propulsion safely is on the moon, where there are no neighbors to annoy, and where there is no air and water to pollute. There is no better reason than this, for a return to the moon! There are other reasons to go back to the moon, but none are better than for helping to provide protection of the Earth against asteroid threats.
(3) Develop the rockets and the spacecraft needed to send humans to the Earth-crossing asteroids as soon as possible. This requires both propulsion upgrades, and it requires built-in protection against space radiation and microgravity disease. (If you can go there, you can go to Mars, or pretty much anywhere else in the inner solar system, including out to the main asteroid belt.)
What that really means is artificial spin gravity, and about a meter or more of low molecular weight insulation materials to double as radiation shielding. And plenty of living space for long missions.
These were also my conclusions in 2009 at that meeting. They have NOT changed since!
If your elected and appointed officials are not addressing these issues, you may safely conclude that they are lying to you, whether by omission or commission. In any event, they are not protecting the public safety, which is their sworn duty. And that deserves your attention at election times.
4-21-09 On Asteroid Defense and a Good Reason for Having National Space Programs
7-22-09 On the Future of the US Manned Space Program
9-6-09 Space Program Public Support
10-31-09 The Future of NASA Manned Space
3-10-10 About Old "Project Orion" -- the Nuclear Explosion Drive
4-17-10 Space Recommendations
1-21-11 Fundamental Design Criteria for Alternative Space Suit Approaches
8-2-11 What Should the Government's Manned Space Exploration Strategy Be?
5-2-12 Space Travel Radiation Risks
12-31-12 On Long-Term Sustainable Interplanetary Travel
1-5-13 Using Nuclear Rockets Safely for Manned Space Travel
2-15-13 On the Two Dangers From Space Friday 2-15-13
10-2-13 Budget Moon Missions
11-17-13 Rocks From Space
11-17-14 Space Suit and Habitat Atmospheres
1-17-15 Stagnation in Space
4-11-15 Radiation Risks for Mars Trip
1-15-16 Astronaut Facing Drowning Points Out Need for Better Space Suit
2-15-16 Suits and Atmospheres for Space
3-3-16 Effects of Microgravity Demand Artificial Gravity
3-16-18 Suit and Habitat Atmospheres 2018
10-5-18 Space Radiation Risks: GCR vs SFE
7-14-19 Just Mooning Around