Discover Dialogue: Planetary Geologist H. Jay Melosh

Just about every atom on Earth has been through a hypervelocity impact.

By Kathy A Svitil
Jul 24, 2005 5:00 AMNov 12, 2019 4:46 AM


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On July 4 an 820-pound copper bullet will smash into comet Tempel 1 at 23,000 miles per hour. The blast and the resulting crater should lift a veil on the mysterious innards of comets, believed to be pristine material from the beginning of the solar system. One of the most interested observers will be H. Jay Melosh, a professor at the University of Arizona’s Lunar and Planetary Laboratory and a member of NASA’s Deep Impact science team. He was the first scientist to suggest seriously that microorganisms could have traveled from one planet to another on meteors in the early days of the solar system.

Deep Impact sounds like caveman science: “Throw rock, make hole.”

M: You could characterize it that way, but an awful lot of science boils down to just that. You could use refined words about probing deeper, but we’ve had lots of luck explaining this mission to first graders because they understand perfectly what will happen.

What do you expect to see when it hits?

M: We expect to be surprised. We don’t know what the comet’s surface is made of. It could be hard, like concrete; it could be loose dust, like a bowl of cornflakes. The impactor is designed to make a big crater either way. We may see the spacecraft disappear into fluffy material and then blast back out later. We may see an ejecta curtain and a bright flash. We may make a big crater; we may make a small crater. We’ve had to consider all those different possibilities.

What’s the worst scenario?

M: We could miss. Even if that happens, we have at least a flyby, and we’ll get images of the comet at a higher resolution than anybody’s got before. But that would be a very disappointing outcome.

Isn’t this experiment only going to teach you about one comet?

M: We don’t know how different comets are. We’ve only imaged the nuclei of three—Halley, Borrelly, and Wild 2—at very low resolution. Until we start investigating some comet or another, we won’t know how any of them work. The short-period comets like Tempel 1 that have been around the sun many times are believed to have dust on the outside that is depleted of volatiles. We’re going to try to remove that from this comet. We’ll probably determine whether that dust is cemented together and is really hard or if it is loose with very low density. Deep inside, beneath the dust, are pristine ices—water ice, ammonia ice, CO2 ice, carbon monoxide ice, and others. The abundances of those tell us about the pressure, temperature, and composition conditions under which comets originate.

Any chance the impact could throw the comet onto a collision path with Earth?

M: No, no, no, no. You’ll probably see some Web sites that say that, but no, there’s no danger of that. The ratio of the mass of the impactor to the mass of the comet is about the same as the ratio of the mass of a mosquito to that of a fully loaded Boeing 767. It’s trivial.

What is your most controversial work?

M: It used to be the proposal that Martian meteorites could be ejected from Mars and come to Earth and that, given the conditions of the launch, it was possible that living organisms like bacterial spores could hitch a ride and transfer from planet to planet. I made that suggestion in a letter I sent to Nature in 1990 that was frankly played by them for laughs. Many of my colleagues thought I was absolutely nuts.

And now?

M: The idea is almost mainstream. A lot of people go around saying that it’s true, although at the moment there’s no evidence whatsoever that it happened.

How likely is it that microorganisms were transferred from one planet to another?

M: If there were microorganisms on Mars, it’s overwhelmingly likely that some would get to Earth by this process. Similarly, it’s likely that ejecta from Earth has carried microorganisms to Mars, where they had the opportunity to colonize. Now whether they succeeded in getting a foothold is much less certain.

Is there any way to tell?

M: Maybe, maybe not. It depends upon good preservation of rocks of that era. But certainly if we find the remnants of life on Mars, and it’s using the same genetic code, for example, as life on Earth, it is almost certain that the two forms of life are related.

Other than the obvious physical scars, how have impacts shaped Earth?

M: Just about every atom on Earth has been through the core of a hypervelocity impact. Things got to Earth by colliding with the growing planet. So in a sense, impacts have shaped everything.

Is our fate at the mercy of stuff flying around in space?

M: Yes, to some extent. Impacts have happened in the past. They’ll happen in the future. We’re doing what we can right now to find all the objects that are greater than a kilometer in diameter. We know that there are probably not 2,000 such objects out there, as we’d previously thought, but more like 1,100, and we’ve discovered more than half. None of them at the moment have our name on them, but that doesn’t mean that in the future these orbits won’t change, putting something on a collision course with Earth. We are in a unique position to be able to do something about it.

You’ve come up with your own scheme?

M: Oh, yes, right. I have to give you a little background. In 1992 there was a meeting at Los Alamos National Laboratory to look at the consequences of large asteroid impacts on Earth. The Los Alamos response was to blow the asteroids up with nuclear weapons. I thought the nukes were more of a danger to Earth than the asteroid itself.

How so?

M: Kilometer-size asteroids, the ones that could cause global problems, would demand up to 100 gigaton weapons—not megatons. That’s larger than anything ever developed. The bomb boys at the meeting were eager to start developing these superbombs. But I was afraid of governments launching gigaton nuclear weapons on spacecraft, as was a Russian colleague who had participated in the Soviet weapons program and had some wonderful stories about nuclear tests that went wrong. So we conspired to find a plausible way to deflect an asteroid without using nuclear weapons. We came up with a solar collector.

How would it work?

M: The idea is to use a big, flimsy, slow—and therefore easily destroyed—solar collector to focus sunlight on a spot on an asteroid and get it hot enough that it forms something like a jet on a comet. If that jet expands for a long time, it will create enough of a directed force on the asteroid to actually push it out of a collision course with Earth.

Could you do it with large asteroids?

M: Yes, with a large enough solar collector. A kilometer-size asteroid could be deflected in a year with a solar collector about a kilometer in diameter—which, I subsequently learned, exists. There’s a company in Southern California that contacted me and told me that they’d built it. Then they contacted me again and said, “Oh, no, we didn’t do any such thing.” They did fly one on a shuttle mission that was 14 meters in diameter. And let’s say that their engineers know very well how to build a kilometer-diameter thing.

It sounds more complicated than just blowing up the asteroid.

M: It is more complicated. With a nuclear weapon, you just fly a warhead out there and detonate it. But you’ve also got radioactive pieces that are going to fall on Earth. With the solar collector, we go out there and orbit the asteroid, but the whole deflection scenario has the advantage of being slow, big, fragile, and unlikely to be misused as a weapon. That’s my contribution to a nonnuclear world.

Why did you develop an impact-effects calculator and put it on the Web?

M: Every time impacts are in the news, I get phone calls from people in the media who want to know, for example, “What would happen here in Charlottesville if Atlanta was struck?” And every time I get such a call, I spend a couple of hours doing the same darned thing, getting out the nuclear-weapons-effects manuals and the seismic manuals and figuring out the answer. My idea was that this would defuse a lot of concerns about the effects of small impacts, although an awful lot of people put big objects in the calculator and put themselves close to it and frighten themselves. We intentionally avoided gory things, but there are graphic descriptions.

Such as?

M: The three major effects—thermal radiation, which arrives first, then seismic shaking and ejecta, and finally air blasts. So the calculator will say at this sound intensity that your eardrums will rupture, your nose will bleed, your eyes will rupture, and so forth.

If an asteroid was going to strike Earth, where could it hit and do the least amount of damage?

M: In the middle of the Sahara, where it would just go boom and nobody would notice. The oceans would not be terribly bad.

What’s the worst place?

M: Obviously, if a 100-meter object came down over New York, London, San Francisco, or any other major city, it would be quite bad.

Could an impact trigger other effects?

M: Suppose a city destroyed by an impact was in a country armed with nuclear weapons, and they thought they were under attack. Say Moscow was suddenly obliterated by a huge 20-megaton explosion, which is about what a 100-meter object would provide. If they didn’t realize that it was a meteorite and thought it was a hostile act by the United States, it could start a nuclear war. The Russians have let their missile network decline to the point that they cannot tell the difference between an impact and an explosion. The U.S. defense network is the only one that can. If a big boom took place, and we said it was really a meteorite impact, would we be believed? That’s my biggest concern.

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