Pain in the Assteroid

What follows is a point-by-point criticism of the astronomy, and, in some cases, the physics that was (ab)used in the movie. This list is not complete, mostly because there are too many to list, but also because some are too nitpicky (an example: in one shot, they showed stars in the night sky, and Polaris was obviously the brightest star in the view. This one did tickle me particularly, since I have a Bad Astronomy page about this very topic!). Incidentally, I don't expect this list will ever be used to help the accuracy of any future movies, but like all my pages, I simply hope it will help inform people about what is Good Astronomy, and what ain't. "Asteroid" ain't.

To be fair, they did get some things right. Asteroids do exist, and an impact by a large one would be a bad thing. They did mention Tunguska and Chixchulub, and, bless them, used a clip from a real documentary about asteroid impacts (that was the one showing the cartoon of dinosaurs getting blown away). But that's about all I can think of. Practically everything else in the movie was wildly inaccurate. Incidentally, the Pentagon has admitted that we currently have nothing --nothing-- that could possibly stop a big asteroid even if we had a couple of years notice. Sleep tight.

Good: Even in the thickest part of the asteroid belt, asteroids are few and far between, with an average distance of millions of miles between rocks of any appreciable size. The Solar System is big, and the asteroid belt has a very low density.

Good: As stated above, asteroids are very far apart. I can see a comet hitting a single asteroid, but two? No. The odds are incredibly, vanishingly small. You might try to say that the gravity from the comet knocked them loose, but the movie clearly depicted an impact, and the gravity of a comet is also very small, far too small to affect the orbit of an asteroid.

Good: They packed a lot of Bad Astronomy into this scene. First off, the rock hits in Montana days before the main asteroids impact the Earth. Follow closely here, because this is one of the worst mistakes they made: once an asteroid was bumped into its new orbit, it would stay on that orbit. All the time the asteroid is approaching the Earth, the Earth is orbiting the Sun on its own path. They would have us believe that a rock flung far ahead of the main asteroid would have its orbit intersect the Earth, as well as the main asteroid! The odds of this are unbelievably small. But then they go on and have the second asteroid on a collision course with Earth too! The Earth is a very small target, and space is very big. We are now talking odds so small that even someone who plays the lottery habitually would laugh at them.

Piling on top of that, later in the movie they have chunks of asteroid impacting all over the Earth. This means the asteroid would be coming in from different directions! Nice trick for a single asteroid. (Note added October 4, 1997: You might think I am being too unfair here. After all, the rotating Earth means that any point on the Earth could get hit. However, that still means the rocks would have to be spread out in space to get so spread out in time, and again their orbits would all have to intersect the Earth's. Not terribly likely, to say the absolute least).

Good: Once again, a high density of Bad Astronomy. First, you can't look through a telescope when the lights are on! You won't see anything. That's why observatories are located in dark sites, with no lights on in the actual room with the 'scope. Second, the room is a conference room. People walking around in the room would shake the 'scope, ruining any images. Real telescopes are isolated away from traffic. Third, they are looking through an eyepiece. No professional observatory telescope would have an eyepiece in it; they have electronic or photographic detectors attached, which are far more sensitive than the human eye. [Note added May 7, 1999: I should add that some smaller observatories-- many run by dedicated amateurs with little or no funding-- make important contributions to astronomy by scanning large areas of the heavens every clear night, something difficult for most professional observatories. Amateurs can keep track of lots of transient astronomical phenomena like comets, variable stars and double stars. Many of them actually do use eyepieces and the most sophisticated computer available: the human brain.] One last thing: there is an observatory in Boulder, Colorado, but it isn't much like the one depicted on TV. Bad Reader Dr. Marsha Allen let me know that there is a 24" set up there with a planetarium. I visited there once but didn't get a chance to see it. I imagine being just a few kilometers on the leeward side of the Rockies makes the atmosphere there pretty turbulent, which would make observing there difficult. Anyway, it isn't a "National" observatory, but I think we can let NBC slip that past. I wonder why they didn't simply have this take place at the Naval Observatory, or at Kitt Peak National Observatory? Anyone out there know?

Bad Reader Chris Schelin informs me that an episode of the TV series ``Lois and Clark'' made this same mistake in yet another storyline about an asteroid threatening the Earth. This stuff is like a virus!

Good: An image taken through the 'scope would be in black and white (technically, "greyscale"), unless they use false color imaging. In that case, the image would still not look like what they displayed-- the colors would be more garish and psychedelic looking. Images are commonly displayed with weird colors to help enhance faint details. Next, they imply that the comet is being observed for the first time here (note: they implied this, not stated it, so I may be being too tough here). If it has a period of 4000 years, when exactly did Fletcher discover it? I'm guessing Fletcher was not a common name in 2000 B.C. Sounds like they threw in the period to make it sound more dramatic. Also, the comet was shown to be very bright, which implies it was close. However, comets take months to get from the distance of the asteroid belt to Earth's orbit. The asteroids themselves made it in record time as well. At 60,000 mph (the speed stated in the movie) it would take about a half a year to get from there to here. Lastly, if it were that bright that far away, it would have been discovered much earlier, like months before. Hale-Bopp, an extremely bright comet currently visible (in February 1997) in the northern hemisphere, was discovered out past Jupiter years before closest approach. Incidentally (minor nitpick here): astronomers refer to comets as "Comet So-and-so", not "So-and-so's Comet". It is officially "Comet Halley", not "Halley's Comet". So they should have called it "Comet Fletcher".

Good: Real astronomers use kilometers. Hardcore astronomers use centimeters.

Good: It is about right to have a PhD by 29 (I was 29 when I got mine), but then to be appointed head of a national observatory? Nope. She would have to go through years of postdoctoral research, then get an assistant professorship, tenure track, and then get her appointment. And when exactly did she have time to give birth to and raise a son? Realistically, she should have been about 40 years old, or else she got her PhD at age 11.

Good: At 60,000 miles per hour, that rock will go through the atmosphere in a few seconds, unless it is at a very shallow angle. At an angle that small it would skip off the atmosphere like a rock on water.

This brings up another point. When small meteors hit the ground, the impact velocity is actually very small!. Only large (like, hundreds of meters across) meteors will actually hit the ground with a large enough force to cause an explosion. Small ones slow down drastically when they hit the atmosphere, and reach a speed of only a few hundred kilometers an hour while they are still many kilometers in the air. They reach terminal velocity, and go no faster. Note that a meteorite hit a car in New York, and while the car was ruined, there was no explosion. It was like dropping a chunk of metal from a few hundred meters up. That's how slowly these things move.

So the original small rock that hit near the truck in Montana would not have exploded, but simply plopped to the ground. It wouldn't have even been hot! It would have plenty of time to cool off on its way down.

Good: No telescope on the planet could track an object moving that quickly at that distance! The asteroid would be moving extremely quickly, crossing the sky in only a few seconds, yet they show the telescope sitting perfectly still.

Good: Predicting the orbits of asteroids is a relatively easy task, especially for big bright ones where you can make lots of observations. There was an especially dumb line that says something like "the influence of the Sun, the Moon, the Earth.. we can't predict what it'll do!" Of course it's predictable! The basic math was worked out hundreds of years ago.

Good: Eros is a real asteroid, but it never gets as far out as the asteroid belt. The farthest point in its orbit is near Mars, and the nearest point to the Sun is just outside Earth's orbit.

Good: This one had me laughing out loud. The rock is said to be the same size as the one that wiped out the dinosaurs, or about 10 kilometers across. This is the size of large mountain (it would mass about a trillion tons, by the way). Now picture two lasers on board an aircraft trying to vaporize Mt. Everest. Right. The Secretary of Defense then says that the lasers are less effectual in space than in air, which is exactly backwards: air absorbs the laser, making it less effective than in a vacuum.

Good: This one was close, but no good. There are basically two types of asteroids, rocky and metallic. Metallic asteroids could easily survive the heat of atmospheric entry (as stated above, they would impact quite slowly unless they were very big). However, rocky ones tend to explode very high in the atmosphere due to pressure effects-- the shock of passing through the atmosphere at high velocity breaks the rock up, which gives it more surface area, which makes it break up faster, until KABOOM! It explodes. This is what probably happened over Tunguska, Siberia, in 1908, which flattened trees for kilometers around, and it blew up 15 kilometers in the air!

So we are now talking about an asteroid that has some metal, and some rock. Not too likely, but given that, we still have the problem that small chunks either explode way up in the air (if rocky) or fall slowly to the ground (if metallic). The scenario they discuss-- with some exploding meters off the ground, while others impact at high velocity-- is simply wrong.

Good: Another example of lots of Bad Astronomy in one place. One: comets do not go screaming across the sky like that one did. It appeared to be moving several degrees per second, which implies a distance of only a few hundred or thousand kilometers away. If it were that close, it would fill the sky and you would have been hearing a lot about it. Comets, unlike asteroids, do not move on easily predictable paths because gas can be emitted in jets from the core, pushing the comet in random directions. If it came that close, it could easily have hit the Earth. Two: the tail of the comet did not point away from the Sun. There are two tails in a comet. One is formed by the solar wind pushing the material that comes out of the nucleus and so is always pointing away from the Sun. The other is dust and neutral gas from the comet that evaporates off the comet. This dust and gas separates from the comet head as it warms, and left to itself would follow pretty much the same orbit as the comet. But the pressure of light from the Sun changes the velocity of the dust and therefore the orbit. The velocity change is small compared to the comet's velocity, so the tail curves away gently. Both types of tail point more or less away from the Sun, but the movie did not show this. Three: How did the comet survive the impact that sent the two asteroids careening towards the Earth? Comets, in general, are much more fragile than asteroids. The impact would have disintegrated the comet. Yet, at the movie's end, we see Comet Fletcher whole and hearty.

[Note added March 1, 2001: originally, I had said about the dust tail: ``The other [tail] is dust and junk from the comet that is basically left behind as the comet moves.'' I can't believe I wrote that. It's wrong! I fixed the copy above to reflect a better description of the effect. My thanks to Bad Reader Jorg Schmitt for pointing out my error!]

A better line, which would also have been more accurate, would have been, "We have confirmation from 30 independent teams of astronomers from all over the world. I'm pretty damn sure." I like that better, and it shows, unlike what they present in the movie, that she is not the only astronomer on the planet.

That is just an example of one of hundreds of problems with the movie that could have been improved by taking even a small amount of advice from a real, breathing astronomer. One of these days I'll have the guts to call the networks and offer my services as an advisor. By the time the next big asteroid strikes, we may actually get a good, scientific movie.

Speaking of which, the movie "Contact", based on the book by Carl Sagan, is due out soon. With Dr. Sagan's advice on the screenplay, I can actually have hope of seeing a good, accurate movie. I fear that after his untimely death things may be changed, but I can still hope.

A note: The Discovery Channel, of which I am a big fan, had a tie-in show called "3 Minutes to Impact" which aired the day before"Asteroid". In it, they interviewed veteran comet observer Gene Shoemaker, who commented that the $1 million budget dedicated to finding possible impactors was too small, and should be tripled. This execrable movie cost $19 million to make. That money (which did not seem to help the special effects much, in my opinion) could actually go a long way to finding and helping divert asteroids. Ironic, eh?

1. One of many Near-Earth Asteroid Rendezvous pages, with lots of links and info about the mission to visit and investigate asteroids.

2. Of course, I must include a link to The Nine Planets.

3. We already have some closeup images of asteroids, taken by the Galileo Spacecraft.

From any of the above sites are more links to more information. Or, do a net search using keywords like "asteroid", "Eros", or whatever else you can think of!