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Review: Deep Impact

ad for DIThe Astronomy of Deep Impact
The producers of DI had an ambitious plan: actually collect a team of scientists and ask them about the scientific plausibility of the movie. This has been done before (as I mentioned in my page about the movie "Asteroid"', NBC claims to have used consultants; I assume they simply ignored everything the scientists said), but this time, it appears that the producers listened. At least for the most part; there were plenty of small errors (many of which I will happily attribute to "artistic license", see below), and one mighty big one, but a lot of the movie was accurate. I get the impression that the producers of DI wanted to make an accurate movie, and used the consultant's info as much as they could, but I also get the impression they had a certain plot in mind and would not change parts of it no matter how inaccurate they were. The first and last few minutes of the movie imply this very strongly, and I'll describe that when I describe the scenes in question.

Now, on to the list of inaccuracies. I will try to indicate whether I think the bad astronomy was plot driven (in other words, kept because the plot demanded it) or simply a goof. Also, to be fair, if the movie got it right (and it very often did!) I will say so. Incidentally, the images of the movie shown below (and on the previous page are from the official Deep Impact Site. [NOTE (5/3/99): The site I originally got the images from has been removed, but the site linked looks exactly like it. I assume they simply changed domain names.] Also, at the bottom of this page is a list of links to other sites about comet impacts that you are welcome to browse. Take this chance to learn more about comets and their effects!

Bad: In the opening scene (why waste time? ;-) we see a group of young amateur astronomers stargazing. As we pan across the group, we see them studying maps with flashlights.

Good: The problem here is small: using a flashlight outside at night ruins your dark vision. The eye takes quite a bit of time to get adapted to the darkness; usually twenty minutes to get fully adjusted (less if you are in a light polluted area). Using a white flashlight destroys the very reason you're outside! However, the eye's dark adaption is not ruined by red light, so astronomers use red flashlights (usually modified by the high tech method of taping red cellophane over the business end of the flashlight). This may have been plot-driven; we'd want to be able to see the actor's faces.

The excellent website Intelligent Child (science for kids) has more info about night vision and red flashlights too.

image of Leo Bad: A moment later in this same scene, we come upon Our Hero, Leo Beiderman (Elijah Wood). He is observing the stars Mizar and Alcor through his telescope, situated right next to his (soon-to-be) girlfriend. Through the 'scope, we can clearly see Mizar, Alcor, and a third object at about the same brightness. This third object turns out to be (ominous chord please) A Comet.

Good: Mizar and Alcor are real stars. Mizar is the relatively bright star in the kink in the handle of the Big Dipper in Ursa Major. At about second magnitude, it is easy to find in even light polluted skies. Alcor is a dimmer companion to Mizar in the sky, and can be a bit tough to spot (it was once used as an eye test by ancient Arabians). The problem is, The Comet appears as bright as Mizar. If it were that bright, literally thousands of people would have spotted it much earlier. Comets do not move very quickly in the sky, so it would have taken quite a bit of time to get that bright. Someone would have spotted it months before. This is clearly plot-driven; Leo is the one to spot The Comet, and it perhaps would seem out of place for him to have a large enough 'scope to spot The Comet before the thousands of other people also looking for comets. I'll note here that many comets are spotted using relatively small 'scopes, but that is the exception. Most are discovered by very dedicated people with very good equipment.

Incidentally, there is a hidden piece of Bad Astronomy in this scene as well: the government would have no chance of hiding the existence of this comet for a year. Most comets are discovered when they are very faint, literally thousands of times fainter than Wolf-Beiderman when Leo found it. Long before the events in the film take place, probably even before the government itself would be notified, the comet would have been discovered, an orbit calculated, and people panicked. The whole premise of the first half of the movie is thus based on flawed logic.
My thanks to Bad Reader David Takemoto-Weerts for reminding me of this; I had thought of it while watching the movie but forgot about it when I wrote up this page originally!]

The movie did get a couple of points correct here though: most comets are indeed discovered by amateurs, and are named after the discoverers. That is why The Comet was named Wolf-Beiderman in the movie.

Bad: Leo reports his discovery to the local professional astronomer, who programs his software to plot a course for The Comet. It displays a three-dimensional graphic which shows The Comet headed right for Earth. In his panic to report the discovery, he wrecks his car. Kaboom!

Good: Leo should have reported the discovery to Brian Marsden at the International Astronomical Union's Minor Planet Center. We can forgive him this breach of professional etiquette since Leo wanted to report to his club's sponsor first. The real problem is that it takes several observations over many nights to get a good orbital calculation for comets, especially one that's a year away from impact. It's like looking at a snapshot of a baseball and being asked when it will hit the ground. What direction is it heading? How fast is it moving? One observation doesn't really tell you anything; you need to see it move. The 3D graphic was a bit over the top too. While not really impossible, it's more of a Hollywood silliness. It's there simply to drive the point home that we're in for a bit of trouble. And I have a plot review here too: killing the pro was just plain silly, and was telegraphed for way too long. Was anyone really surprised when his car crashed (and exploded??? Puhleease!)?

Bad: The professional astronomer, when we first see him, is in an observatory. He is sitting in the same room as the telescope, with lights, computers, etc. running.

Good: This is the same mistake made in the NBC TV movie "Asteroid" and the Family Channel's miserable "Doomsday Rock". Basically, you need to keep a telescope in a dark, stable area, and not have bright lights nearby and people walking around! For more details, I'll simply say to look over the links to those other two movies.

Bad: We see the group of young astronauts sitting at a bar grousing about having the older more experienced astronaut on their team.

Good: This is my own opinion here, but what I have seen of astronauts is a lot of respect for the ones that are veterans of many flights. I got the impression that Robert Duvall's character was based loosely on Story Musgrave, a Shuttle astronaut of almost unbelievable record. He holds seven degrees (including a doctorate in medicine), and has been on six Shuttle flights, including the first Hubble Space Telescope servicing mission. He's a legend at NASA, and I have a hard time picturing a group of young astronauts sitting around saying anything about him except that they wish they could do half as well. I didn't like that scene in DI, and it was obviously only placed in there for character conflict.

Bad: The President says we don't have any spacecraft that can catch up to The Comet, so they need an Orion type engine.

Good: This scene is mostly good, and the Badness is an omission. We really don't have any spacecraft that could rendezvous with an object moving at 40 or more kilometers per second (note that there were probes sent to Comet Halley in 1985, but they were fly-by's, not landers). The Orion, in turn, is a real idea: you build a ship on top of a massive shield, and (hold on to your seats) blow up nuclear bombs under the shield. The explosion propels the ship (imagine huge shock absorbers). This idea was shelved a long time ago because a treaty was signed by many nations not to blow up nuclear weapons in space. Still, the concept is actually real. My problem with the movie is that it is never explained; it would only take a few lines to say what it was, and they never did. My point is that Orion is real, and if they dropped a line or two about its history they might have inspired some kids to look it up. A great opportunity to teach was lost (although maybe in my own small way I can try to revive it).

Bad: We send astronauts to land on The Comet to blow it up.

Good: This is a difficult subject. No one is really quite sure just what the best way would be to prevent a comet from impacting the Earth. One of the most important factors is time. If you have a long lead time, like say twenty years, then you would take a different approach then if you had a month or a year. In general a comet is a big lump of ice orbiting the Sun. If you give it a little nudge, it might only move a little bit, but that movement never stops; the comet gets more and more "off-course" as time goes on. Imagine it's originally aimed at Earth, and we give it a small nudge. If enough time can elapse between our nudge and the time it gets near the Earth, it may have moved enough from the original path to completely miss the Earth! So if you have enough time, even a small nudge can save you. The closer it is, the less time you have, and the larger a nudge you have to give it.

A back-of-the-envelope calculation shows (if you have help from Bad Reader Mati Meron, who is in no way responsible for any errors I might be making here!) that you need about 150 or so megatons to push the comet hard enough to have it miss the Earth entirely, given a year's warning (half that if it's two years) with the mass I find for Wolf-Beiderman. Unless I have made some large error in my math, it seems to me we need never send astronauts to The Comet at all; just set up a series of bombs to blow up near it and let Newton do the driving.

Blowing up The Comet is a different story. To start with, blowing it up might be a very bad thing to do. But if you really want to blow it up, you could. I have heard people suggest simply throwing rocks, literally, at an incoming comet, but that may not pack quite enough punch; a one ton rock slamming into a comet would only provide about one-ten-thousandth the energy of a one megaton bomb. The idea of getting bombs inside the comet is a good one; if you can disrupt the comet enough, perhaps jet effects will also drive it off course (or, given enough time, the expanding debris will get big enough that most of it misses the Earth). I have my doubts about actually being able to disrupt the comet, but I don't know enough about explosive yields and comet tensile strength to even guess if we can actually blow one up (there are some indications, however, that the comet that slammed into Jupiter in 1994 was only very loosely held together; it was likened to a pile of gravel). Anyway, there are ways to get bombs inside a comet without needing people there; for example, drill heads attached to the missiles, or very hard tipped bombs that would actually drive themselves into the surface upon impact (some probes to Mars are actually using something similar to this to sample the subsurface region). If we wanted to drill bombs under the surface, then I would be happier if we had some real live astronauts up there, capable of thinking their way out of a problem. They could be used as a sort of backup in case the machines themselves don't work.

So in DI my problem is manyfold: we don't want to blow up The Comet, we want to push it out of the way. We had the time (two years, as stated in the movie). And if we did want to blow it up, sending astronauts is a good idea as a backup in case of automation failure, not as a primary defense.

Bad: The comet surface is mostly white or gray.

Good: Comets are surprisingly dark, among the darkest objects in the solar system. When the European spacecraft named Giotto went to Comet Halley back in 1985, it found that the nucleus of the comet was darker than coal, reflecting only 3% of the light that hits it (for comparison, the Earth reflects about 39%). Oddly though, I give the producers credit here: they admitted to the astronomical consultants that filming a black comet against a black sky was impossible, so they had to make the surface seeable! Oh well. Some things have to be changed in the interest of being able to see what's going on.

Bad: The astronauts are worried while they are at The Comet because if they dally too long, the Sun will rise over their landing spot and cause The Comet to vent. Sure enough, they are delayed, and as the sunlight hits the nearby surface of The Comet, vents immediately open up.

Good: Comets do vent like that. Comets are made up of frozen gasses, and warm sunlight can vaporize the gas. If the comet rotates, part of it is in sunlight while part is in shadow (just like here on Earth). Usually, the more volatile gasses bleed away from the surface while the comet is far from the Sun, since they have a lower melting point, but deep down they are insulated by the ice above. A deep fissure or crack in the surface may expose some of those gasses to the warm light, creating a vent. As the gas evaporates, it can actually create jets that shoot out of the surface. This happened quite dramatically with the last big comet, Hale-Bopp. (That link will also show you a very cool picture of Hale-Bopp's jet as well, check it out!) The problem is, the sunlight would have to get down the fissure to warm the gas, which would only happen when the sun was relatively high in the sky. Those jets in the movie started as soon as sunlight hit the surface, which has had lots of time to bleed off the nasty gas. In reality, the astronauts would have had more time. I realize this is a minor point, but comet vents are pretty cool and I wanted the chance to talk about them. ;-)

Bad: One of the away team astronauts is blinded by the sunlight.

Good: I suspect this may not be totally inaccurate; I am not sure how close The Comet was to the Sun when this happened. I would expect NASA to give them sun visors, however, or build in a UV block to the visor. They were on the dark side of The Comet, after all, so they would not have had their sun visors down, and he was admittedly too panicked to think of snapping it down. Still, a moment's exposure should not have so badly burned his face. That was added for drama.

image from DI showing comet Bad: While Jenny Lerner (Tea Leoni) is memorializing the lost astronaut, an MSNBC graphic shows his face next to an image of The Comet. I was luckily able to find the very picture in question (shown to the left). You can see the comet in the picture above the "EA" in the word "EARTH".

Good: The comet image was actually a Hubble picture of Comet Hyakutake, the bright comet that graced our skies shortly before Hale-Bopp landed on the scene. This isn't Bad, but I just thought I should point it out. The original HST image is shown to the right.

Bad: We see the astronauts bouncing around in the light gravity of The Comet.

Good: A comet, even a large one, is still small by planetary standards. Wolf-Beiderman was said to be 7 miles across, or about 11 kilometers. It looked roughly spherical to me, so its mass would be about 1018 grams, or in more palatable units, about 1000 trillion tons. Yes, you read that right. Remember, The Comet is bigger than Mt. Everest! That sounds like a lot, but it is only a tiny fraction of the Earth's mass. The gravity on the surface of The Comet would be about 0.0001 times that on the Earth. If you tip the scales at, say, 80 kilograms (which is my weight, about 175 pounds), you would weigh only 12 grams on The Comet. That's less than half an ounce (yes, I know I am mixing weight and mass here, which are two different beasts... but that's a different Bad story). The escape velocity would be about 4 or so meters per second. It would be hard to jump that fast, though if you start running that's a good sprint. You could actually launch yourself into space if you gave yourself a running start! No wonder one of the astronauts was lost! In a turn of Good Astronomy, the spacecraft didn't so much land on The Comet as it got near enough to send out tethers. These were shot into the surface to hold the ship down. It turns out that in the original script, the writers had the astronauts landing on the comet, but were told by the consultants that the gravity was too light. The writers then changed it so that the ship docked instead of landed. A blow struck by good science!

Bad: At one point when the President is speaking we see a crowd watching the broadcast in New York City's Times Square. A long shot from above shows the crowd in the streets, and a nearby theater's marquee shows that the theater is playing "Fire In the Sky".

Good: This is not Bad, but actually a wonderful inside joke. "A Fire In the Sky" is the title of a movie from 1978 about a comet that hits Phoenix. I looked the title up, and noticed something that is bad: the title of the movie, notice, is "A Fire In the Sky", while the marquee read simply "Fire In the Sky". That is the name of a movie about the supposedly true story of a guy that was abducted by aliens. I may have misread the marquee (it was only up for a moment) but I don't think so. Kinda funny nonetheless.

Good: This one is simply good. During one scene, we see The Comet glide silently by in space. You need a medium for sound to propagate, and space is a vacuum, therefore, no sounds.

Bad: Fourteen hours from impact, we see young Leo riding in the back of a pickup, going home to return to his wife. He looks up at the twin comets, which are suspended in a sky full of stars.

Good: That close to impact, The Comet(s) would have been very very bright; so bright that you wouldn't see many stars in the sky. Next time the Moon is full go outside and see how many stars you can find compared to a night when the Moon is not up and you'll see how that works. The stars get washed out by bright light, which is exactly why we cannot see them during the day!

Bad: image of comet across water
The first comet roars slowly across the sky, trailing smoke and fire. Thousands watch and listen as it heads towards its ocean impact.

Good: The Comet was moving at something like 40 kilometers per second. It was shown moving at a very low angle across the sky, but even so, at that speed it would appear to cross the sky in something like ten seconds. It was moving too slowly in the movie. I am willing to give them the smoke and fire bit; comets are dark, so as it ablates it will give off what looks like smoke. The comet entry would have been much brighter, too, blindingly bright. When I was a kid I saw a meteor burn up that was so bright it lit up the sky and left an afterimage on my eye for several minutes, and that meteor was probably smaller than a pea. Wolf-Beiderman was a bit bigger than that! Something like that gives off a lot of energy. The people watching it would have been blinded. But then, not for long...

image of comet impact/shockwave The Comet was also moving much faster than the speed of sound as well. This means it would generate a tremendous shock wave, the force of which would have smashed flat everyone watching within hundreds of kilometers. They would never have heard The Comet; they would have been squished into jelly before the sound could reach them. Yuck. In the image to the right, you can see the shock wave as a circular front with the impact in the middle. The impact graphics were very good.

Bad: Moments after impact, a wall of water rises up. The water near the beach recedes drastically, supposedly because it is being drawn up into the wave. We hear the huge roar of the water as it approaches landfall.

image of tsunami Good: As the President said in his speech, the water displaced from the impact would be moving faster than sound. What that means is that water far away from the impact would have no warning when the wave hits it; therefore the water would not have receded. As far as the water near the beach is concerned, nothing unusual is happening, until the wave is literally on top of the beach. It was a cool graphic, but not realistic.
[Note: I have been informed after posting this that the speed of the wave is supersonic in the air, but not in water. Sound travels faster in water, so it is possible that the wave has slowed enough that water on the beach is aware of the tsunami, before the tsunami actually hits. I am still not clear on why the beach water would recede in this case, but I have also been told that in earthquake induced tsunamis the water in front of the wave actually does recede. Hmmm.]

Also, the sound from the water is moving only marginally faster than the water itself. The water initially starts out moving faster than sound, but slows as it approaches landfall (the movie got this part correct). The sound from the wave would be in front of the wave itself by only a few seconds. You would see the wave approaching silently for a few seconds before hearing it (discounting the fact that sound travels faster through solid ground, so you might feel it through the ground beneath your feet).
[Note again: The wave does slow as it moves towards land. It is possible that is has slowed enough, especially traveling the couple of hundred kilometers from the ocean to DC, that it lacked a lot of its initial punch by the time it hit the Capitol.]

Bad: Minutes before final impact, the astronauts blow up the second comet, and we are treated to a spectacular light show.

Good: Aaaaarrgg! This was the Biggest Baddest Astronomy in the movie. Blowing up a comet does no good at all, and might even make matters worse. Just because the pieces are smaller doesn't mean you have changed anything. If every piece still impacts the Earth (by that I mean actually is stopped by the Earth or its atmosphere) you are still dumping all the kinetic energy of The Comet into the Earth's atmosphere! That's a HUGE amount of energy, dumped in practically all at once. It would still create a massive explosion, dwarfing all of our nuclear bombs combined. Even if you could somehow soften the blow, all that heat would wreak havoc with our weather. Some people actually think it might be better to simply let a big one hit rather than blow it up, because the Earth itself can absorb the energy of impact better than the atmosphere can. This is still argued, though. I'd prefer not to try any experiments!

Bad: In the very final scene, we see a bedraggled but salvageable Capitol Building.

Good: I live near DC, and am well aware of how close it is to the ocean (and to many rivers as well). DC would have been scoured clean off the face of the Earth by the tsunami (as would New York City have been, but we see semi-intact buildings under water at one point). Think of it this way: a cubic meter of water weighs one ton. A wall of water the height and width of a building would comfortably outweigh the building itself. Mind you, the tsunami was moving at about 1000 kilometers an hour, or about as fast as a jet plane. Do you really think anything would be left standing? [Oops! (February 22, 1999): Actually, the tsunami would be moving less than that speed. Still, it would travel at several hundred kilometers an hour. I'd prefer to be far away, like on another planet.] This was simply a feel-good ending, tacked on and in my opinion inappropriate. Now, we do not know if that was simply a rebuilding of the Capitol, or if it was even in DC. However, the speech at the end strongly implied that this took place in DC ("... and the waters receded.") As I have said before, in movies such as this, the problem is usually not one of exaggeration, but of actually hugely underestimating the power and fury of such a catastrophic event.

That's it (for now). I'll reiterate that I liked the movie (my wife did too). I thought the critics were too harsh when they said the subplots were boring (though I could have done with less about Leo), and the rest of the movie was fine. Moments before the first impact, I wanted to stick my head between my knees and hyperventilate; that drama was very thick, and I have very vivid nightmares about big impacts. Yegads, that sequence was amazing! Anyway, by the time most of you read this the movie will have gone from the theaters, and it will not look nearly as good on television if you rent it. Still, all in all it was a good flick, and worth watching.

Some related comet links

I would be remiss not to mention a few other web sites where you can get more info about comets and impacts.

One last note: comet impacts and the like are old hat to science fiction fans. Deep Impact was in some ways similar to the classic novel "Lucifer's Hammer", written by Larry Niven and Jerry Pournelle. This book sparks a bit of controversy among fans about its quality, but I liked it quite a bit. The descriptions of the comet impact are wonderful (if terrifying) and the aftermath is in my opinion pretty accurate. The characters are also fun and interesting. I recommend it! A good library or bookstore will carry it. A web search at Yahoo! will also yield some web sites with reviews of the book.
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