Asteroid Itokawa, sampled in 2005 by Japan’s Hayabusa spacecraft, is in the same size class as 2004 BL86 and its moon. JAXA
To date human beings have spotted about 300,000 asteroids. These range in scale from Ceres, the first discovered, way back on the first day of the 19th century (950 kilometers in diameter), to unnamed boulders a few meters wide. Little asteroids (say, the size of a bus or a house) far outnumber the big ones.
Only a few thousand asteroids follow paths about the Sun that bring them near Earth’s orbit. The largest of these is 1036 Ganymed, which measures about 33 kilometers across. It has a stony composition much like that of the second-largest near-Earth asteroid, banana-shaped 433 Eros, which measures 34 kilometers by 11 kilometers. Eros seldom comes nearer to Earth than about 27 million kilometers, or about 70 times the Earth-moon distance; Ganymed seldom comes nearer than 56 million kilometers. Eros is unique because a derelict American spacecraft called NEAR Shoemaker rests on its surface. Though designed as an orbiter, it landed on Eros on 12 February 2001, at the end of its mission, and continued to transmit for about two weeks.
A day or so ago, a 325-meter asteroid designated 2004 BL86 passed Earth. To get a sense of perspective, 325 meters, or roughly as wide as the Tour Eiffel is tall, is kind of big for a near-Earth asteroid. As asteroid flybys go, it was a close shave as such things go; it passed about 1.2 million kilometers from Earth. That distance is a bit more than three times the distance between the Earth and moon.
Any time an asteroid is due to pass Earth – even if it will pass more than a million kilometers away – the popular-audience space media kicks into inaccuracy overdrive. Adjectives I heard used to describe 2004 BL86 included “giant,” “huge,” “mountain-sized,” and “dangerous.” Phrases used to describes its minimum-approach distance included “so close you’ll be able to see it,” “very close,” and “a close encounter.” None of this language was accurate. One media source even called it the biggest asteroid to approach Earth in 200 years; in fact, this was the closest approach of this asteroid for 200 years.
The news media are not the only ones who commit such errors. Space educators who should know better also play up the “threat” from “killer” asteroids when a body like 2004 BL86 passes the Earth-moon system. They place objects like 2004 BL86 in the same category as the “dinokiller” that struck Earth 65 million years ago. That falls short in the reality department in at least a couple of ways: for one thing, the impactor that ended the Cretaceous with a mass extinction was an extraordinary object, on the same scale as Eros or Ganymed, and such bodies hit Earth only on time scales of tens of millions of years; for another, an object nearly as large the dinokiller struck Earth 35 million years ago where now Chesapeake Bay is found, and it caused no mass extinction.
Because of the poor quality of information they receive, many people have developed the mistaken notion that asteroids are frightening things. In fact, they are data-packed fossils of the formation of our Solar System. The appropriate emotion to feel when one passes by Earth is not fear; it is fascination. As proof of sheer nifty-ness of asteroids, I offer this: when 2004 BL86 passed Earth on 26-27 January, we scanned it with radar, and we found that it has a moon about 70 meters across. How cool is that?
I think by now you probably realize that I do not in any way endorse exploiting asteroids to scare people, no matter how slow a news day it might be. Just for grins, though, how about we imagine that 2004 BL86 had tried to live up to its fearsome adjectives and had actually struck the Earth yesterday?
The nice people at University College London and Purdue University have conspired to create a handy online impact modeling tool called “Impact: Earth!” I prefer the less graphics-intense version – to be found here – which is called, more prosaically, “Earth Impact Effects Program.” The latter operates faster and allows me use my imagination more.
The minds behind this modeling tool are careful to warn us that it might not be perfect. In fact, they warn that, if one enters “peculiar impact parameters,” they refuse to be responsible for what happens. It does, however, provide results in line with those arrived at in serious studies of impact effects, and the explanatory PDF it includes is convincing.
We know from spectral analysis that 2004 BL86 is another stony asteroid. We know that Earth is 75% covered in water. We know that, given the shape and tilt of its orbit about the Sun, 2004 BL86 is a bit more likely to intersect Earth near the equator rather than the poles. Now we know it has a moon, which should be also be considered when modeling impact effects.
So, first we choose an impact site. I spin my 16-inch globe – around and around she goes, and where she stops, nobody knows – and stop it with my finger. I look at the place I have picked; it’s in the Pacific just east of the Japanese island of Honshu. I do not like that place; after all, they are still picking up the pieces after the giant earthquake-tsunami-reactor meltdown disaster of 11 March 2011, and a nearby impact would be piling on. So, I’ll spin the globe again; this time my finger falls on the Atlantic Ocean about 300 kilometers east of the Bahamas. Well, they have to deal with killer hurricanes all the time, but if this experiment is to have meaning I have to be a little dispassionate. So, it’s east of the Bahamas (sorry, Bahamians and their neighbors).
The modeling software allows me to select my distance from the impact point. Of course, I am tempted to put myself far enough away that I could conceivably be in Paris, but I will instead suck it up and put myself in harm’s way. I’ll imagine that I am in Puerto Rico, about 300 kilometers south of the impact point.
Next, I will enter the impactor’s size, starting with 2004 BL86 (I will add the newly found moon later). That would be 325 meters. Now I need to decide on its density. I select “dense rock” with a mass of 3000 kilograms per cubic meter.
The average asteroid impact velocity is 17 kilometers per second, but I will ramp it up a bit to 23 kilometers per seconds because of the shape of 2004 BL86’s orbit about the Sun. The most probable impact angle is 45°, so I’ll go with that. I want to avoid “peculiar impact parameters,” after all.
Almost done. The last step is to define the target density. Three hundred kilometers east of the Bahamas is deep ocean. In fact, the deepest part of the Atlantic, the Puerto Rico Trench, is close by. I enter a target density for “water or ice” of 1000 kilograms per cubic meter.
OK. All set. Here comes our asteroid. I click on the “calculate effects” button.
The impactor carries as much kinetic energy as 341,000 megatons of TNT before it enters the atmosphere. Pretty impressive. Such an event occurs – can this be right? – about every 84,000 years. That seems rather often – but it is 10 times longer than recorded human history.
The rest of the results provide an answer to this question. The impactor begins to disintegrate 59 kilometers above the ocean. It is shattered into many small pieces when it hits with a total energy of 312,000 megatons. The pieces splash down in an ellipse measuring about 0.9-by-0.6 kilometers wide. This produces a “crater” – a splash, really – about 6.7 kilometers wide by 2.47 kilometers deep. But the modeling software seems a tad confused, for it treats the water as solid, giving “final crater” dimensions of 5.36 kilometers wide by 566 meters deep. We will just assume that the sea bottom is churned up some, perhaps forming a recognizable crater, perhaps not. After all, in that deep part of the ocean, it isn’t as if anyone would trip over it.
The impact fireball occurs over the horizon, so we feel no wave of heat as a result. The seismic effects are more profound; they are like a magnitude 6.9 earthquake at the impact site. Three hundred kilometers away, in Puerto Rico, some dishes and windows are broken and parked cars rock. Buildings creak and doors might move.
Ejecta – stuff blasted skyward by the impact – would arrive 4.24 minutes after the impact. There isn’t much of it. Perhaps a layer as thick as the dust that accumulates on your furniture while you are away on vacation. I am, however, suspicious of that number, since the impact was in water, not rock. It also does not model tsunami effects. I expect that a big splash 300 kilometers away would produce a wave at least equal to a hurricane storm surge in San Juan harbor.
Speaking of hurricanes – for people used to tropical storms and hurricanes, the atmospheric effects of the impact would be a walk in the park. The roar of the impact would be about as loud as loud traffic. The wind blowing from the impact site would reach a speed of 7.61 meters per second. That is 17 miles per hour, for those who have trouble doing the conversion.
Based on the model, I expect that a lot of people in Puerto Rico would not have noticed if 2004 BL86 had splashed into the Atlantic east of the Bahamas. Of course, they would soon have found out about it and become involved in search-and-rescue efforts; the sea lanes where the impact took place are among the busiest in the western hemisphere, and almost certainly many ships closer to the impact site would have been sunk. The low-lying Bahamas might also have suffered more than Puerto Rico from the effects of the splash.
What about 2004 BL86’s 70-meter-diameter moon? I leave all the parameters the same except the impactor diameter and click the button. The moon would barely reach the ocean surface, creating no crater and barely any wind. Its effects would be lost among those of 2004 BL86 itself. Lone impactors its size hit Earth every 2200 years; given that our recorded history is not pocked with accounts of such impacts, it would seem that when such objects do strike Earth, they are not much noticed.
These results are suggestive, not definitive. The modeling software is not perfect, and though I would defend my inputs as plausible, GI/GO applies. The point is, however, that it seems highly probable that a body the size of 2004 BL86 does not much affect the Earth when it strikes. No mass extinction occurs, the climate does not shift to some new state, and the impacts on humans even a short distance away are akin only to those that humans have felt from volcanoes, hurricanes, tornadoes, earthquakes, and warfare all through our history.
Do I argue here that we should ignore asteroids as a non-threat? Of course not. We should find all of them. We have the technology to do that. We should test techniques for deflecting the ones that pose even a local threat. As we do these things, we can study all those fascinating space rocks. Perhaps we can even develop techniques that will make mining them profitable or hollow some out, converting them into habitats or interplanetary transports.
If we can believe that every asteroid is a killer, then we can certainly believe that every asteroid could be a source of fascinating data on the early Solar System and useful minerals, or serve as a space habitat for future generations.
