An asteroid came within sixty miles of wiping out Rome in the fifth century.

Just keep whistling past that graveyard, folks...

Obviously, global warming is the much bigger threat...

I'm not a geologist or cosmologist by any means, so I have a question. Don't we have records of the fall of Rome written by scribes and historians of that era? How is it that no record of this event, make that huge event has ever been found? I don't doubt the fact of the event but, can we question the date of the event? Could there be something in the samples that would scue the carbon dating? Just trying to spur comment and also trying to learn something about the accuracy of carbon dating.

Coorect me if I'm wrong, but if this is true, doesn't it support the argument that more money should be spent on exploring the solar system as opposed to the space tourism and colonization goal you advocate.

If asteroids do represent a substantial threat, then we need to spend resources on studying them and on ways to divert Potentially Hazardous Asteroids (PHAs)-- a program that does not require manned intervetion.

Which is it, space tourism or PHAs?

> doesn't it support the argument that more money should be spent on exploring the solar system as opposed to the space tourism and colonization goal you advocate.

In a different world, maybe so.

In this world, you'll get more money for space exploration by piggybacking on tourism and colonization than you will for space exploration as a stand-alone enterprise.

At this point, you might even find that there are a lot of space enthusiasts who are willing to starve space exploration until/unless we get serious about tourism and colonization.

Also, how do you know that humans aren't required, or preferred, for managing asteroids? The same technologies that allow us to harvest asteroids for their materials will allow us to divert them from dangerous trajectories.

Also, the dramatic reduction in costs that will result from the increased scale of activities (caused by a large tourism industry) will make asteroid management (and space exploration in general) much more affordable.

Another great catch, Rand. I'm jealous. Good questions here also, to which I would reply:

1. If it happened at night, or in bad weather, or both, it could have passed unnoticed except to those within a few kilometers of the impact point. It would be interesting to know the estimate of population density for that immediate area in the early 5th century, and the probability that even a single literate person could have observed the strike (and survived).

2. PHAs are likely to be detected by a distributed network of amateur astronomers on Earth rather than any activity in space, manned or otherwise. Now if we have to actually move one, that's another story, of course.

Okay, the article describes this as probably being around 10 metres across, and equivalent in terms of the size of the explosion to what you would get from a 200 kiloton weapon. Siberia 1908 was 50 metres across, and how much bigger was the explosion? Can we get some sort of rough relation between the size of the asteroid and the size of the explosion?

As Heinlein once said (bad paraphrase here), the Earth is too small a basket for all of our (humanity's) eggs to be in.

As for the presence of a literate person, not only would they have to be literate, but also capable of writing down their observation in a preservable form. Given the tumultuous happenings near Rome, I think people might have been a little more interested in their own personal survival (remember, late Rome had tremendous problems feeding itself) than in recording a flash/blast.

Explosion "size," which I will take to be equivalent yield in TNT, is determined by several factors, including:

Diameter of impactor -- all other things being equal, yield would scale with the cube of the radius, so a 50-meter object would create an explosion 125 times as big as a 10-meter object.

Density of impactor -- again, all other things being equal, yield would scale with density, so that in the case of two impactors the same size, one of nickel-iron (~5 g/cc) would create twice as large an explosion as one made of silicates (~2.5 g/cc). All other things are not equal, however, as atmospheric penetration varies widely by composition. Icy bodies disintegrate and vaporize at high altitudes; a large silicate body would blow up in the lower atmosphere (as at Tunguska), and a big nickel-iron meteorite would reach the surface.

Velocity of impactor -- kinetic energy and therefore explosive yield scales with the square of velocity. Incoming relative velocities (before atmospheric braking) may range from near Earth's escape velocity (~11 km/sec) up to 1 + the square root of 2 times Earth's velocity around the Sun (~72 km/sec), for something in a near-parabolic orbit hitting us head-on. Difference in kinetic energy for impactors of identical size and composition may therefore vary by a factor of ~50.

Impact site -- coupling of explosive energy is much greater underground or underwater, but an airblast (Tunguska) can affect a much wider area than one at the surface. On land, that is. Most destructive impact would probably be one into the ocean near a heavily-populated coastline.

Hope this helps. I have a somewhat-related comment here.

"Also, how do you know that humans aren't required, or preferred, for managing asteroids?"

Well, that's the nub, isn't it? First you determine the nature and extent of a problem and then you determine how to handle it.

Giving manned spaceflight a higher priority than science and exploration is to put the cart before the horse and would explain the state of near stasis of the space program for the past three decades.

Maybe using manned missions is the best way to handle a PHA problem. But assuming that manned missions are the solution and then looking for problems to solve is getting everything backward.

I don't know what "giving manned spaceflight the priority" means. If by "manned spaceflight" you mean NASA's disastrous Shuttle and station programs, then it has been given much too high a priority. But if you mean "opening up space to humanity," then it's gotten short shrift since the dawn of the space age.

Manned vs unmanned is a false choice. My point is that if we have a policy that encourages large-scale human activities, we'll get that, and a much more exploration and science program as well. This could have been done for the same amount of money that we've squandered on space pork at the major NASA centers, but it was never the national goal.

> Giving manned spaceflight a higher priority than science and exploration is to put the cart before the horse

You just don't get it.

You're not going to get any money for "science and exploration".

If you'd like some science and exploration to happen, you'll have to piggyback on something that people will pay for.