Fun with Killer Asteroids

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. 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; 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 describe 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 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 as the dinokiller struck Earth 35 million years ago where now Chesapeake Bay is located, and it caused no mass extinction.

Because of the poor quality of information they receive, many people with only a casual interest in space 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 of these objects passes by Earth is not fear; it is fascination. As proof of the 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 a previously unknown moon about 70 meters across. How cool is that?

I think by now you probably realize that I do not 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 the fearsome adjectives used to describe it and had actually struck the Earth yesterday?

The nice people at University College London (UCL) and Purdue University have conspired to create a handy online impact modeling tool called “Impact: Earth!” I prefer the less graphics-intensive 2010 version – to be found here – which is called, more prosaically, “Earth Impact Effects Program.” The latter operates faster and allows me to 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 academic studies of impact effects, and the explanatory document that accompanies it is convincing.

We know from spectral analysis that 2004 BL86 is another stony asteroid; they are quite common. 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 than near the poles. Now we know it has a moon, which should be considered when modeling impact effects.

So, first we choose an impact site. I spin my 16-inch globe – around and around it goes, and where it stops, nobody knows – and stop it with my finger. I look at the place I have picked; it is in the Pacific just east of the Japanese island of Honshu. I do not like that choice: after all, the poor folks there 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. They have to deal with killer hurricanes all of the time there, but if this experiment is to have meaning I have to be a little bit dispassionate. So, it’s east of the Bahamas (sorry, Bahamanians 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 by itself (I will add the newly found moon later). 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 second 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” of 1000 kilograms per cubic meter and a compromise depth of 3000 meters.

OK. All set. Here comes our asteroid. I click on the “calculate effects” button.

2004 BL86 would have had a kinetic energy equal to 341,000 megatons of TNT before it entered our atmosphere. Pretty impressive. According to the model, 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 asteroid would have begun to disintegrate 59 kilometers above the ocean. It would have shattered into many small pieces by the time it hit the water. The pieces would have splashed down in an ellipse measuring about 0.9-by-0.6 kilometers wide. This would have produced a “crater” – a splash, really – about 7.87 kilometers wide. Fragments would have reached the sea floor, forming a submerged crater field. The largest crater in the field would have measured 194 meters across and 68.5 meters deep.

The impact fireball would have remained below the northern horizon as viewed from Puerto Rico, so I would have felt no wave of heat from the impact. If the impact had taken place at night, I would have seen a brilliant flash on the horizon. The seismic effects at the impact site would have resembled a magnitude 3.6 earthquake. Three hundred kilometers away, in Puerto Rico, nothing would have been felt.

For people used to hurricanes, the atmospheric effects of the impact would have been a walk in the park. The roar of the impact would have been about as noisy as loud traffic. The wind blowing from the impact site would have reached San Juan traveling at a speed of 7.61 meters per second. That is 17 miles per hour, for those who have trouble doing the conversion.

The tsunami the impact caused would have been damaging. It would have reached the north side of Puerto Rico 35 minutes after the impact. The wave might have reached a height of 14.4 meters.

Based on the UCL/Purdue 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 would have become involved in search-and-rescue efforts; the sea lanes where the asteroid would have impacted are among the busiest in the western hemisphere, and almost certainly many ships in that corner of the Atlantic would have been sunk. Also, the low-lying Bahamas would probably 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 have barely reached the ocean surface, creating no splash and barely any wind. Its effects would have been 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. I will repeat that the modeling software is not perfect. Though I would defend my inputs as plausible, GI/GO applies. The point is, however, that it seems probable that a body the size of 2004 BL86 does not have much affect on the Earth when it strikes. No mass extinction occurs, the climate does not shift to some new harsh state, and the effect on human lives even a short distance away from the impact site are akin only to those that people have long felt from volcanoes, hurricanes, tornadoes, earthquakes, and warfare.

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 them away from Earth. As we do these things, we can study them to learn more about our Solar System. Perhaps we can even develop techniques that will make mining them profitable or allow us to convert 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, useful minerals, or living space for future generations.