ROBERT SIEGEL, host:
A little over a week ago, an amateur Australian astronomer scoped the universe. He got wind of a cosmic collision. A comet had slammed into the planet Jupiter, leaving a scar the size of the Pacific Ocean. Now, the surface of Jupiter is a lot bigger than Earth's, but as scars go, that is a big one. And what misses Jupiter could conceivably hit us.
Planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colorado, says Jupiter is both helping and hurting us in the comet and asteroid impact department, and joins us from Boulder right now. How is Jupiter helping us?
Dr. HAL LEVISON (Planetary Scientist, Southwest Research Institute): Jupiter is helping us by protecting us from a class of comets that come from a spherical distribution of objects that stretch 10 percent of the way to the nearest star called the Oort cloud.
SIEGEL: Called the Oort cloud?
Dr. LEVISON: Yes.
SIEGEL: And it's attracting them to it by gravitation?
Dr. LEVISON: No, it's playing the role of a homerun hitter. And as these things try to penetrate into the inner part of the solar system, occasionally Jupiter will knock them out of the solar system and hit a homerun, protecting the Earth from potential impact from these guys.
SIEGEL: So those comets aren't going to get anywhere near Earth. First they got to get past this big slugger Jupiter, so it's a help in that.
Dr. LEVISON: That's right. And that's true for most comets.
SIEGEL: How is Jupiter hurting us?
Dr. LEVISON: Well, Jupiter is hurting us because between Mars and Jupiter is this belt of rocky objects we call the asteroid belt. And the gravitational tugs of Jupiter slowly force these things out of the asteroid belt and onto orbits that cross the orbit of Mars. And then Mars can knock them around gravitationally until they become onto orbits that cross the orbit of Earth, where they can hit us. And by far, that's the largest hazard that we face.
SIEGEL: You mean these asteroids that would otherwise stay safely in their belt and far from us can be yanked out of it because of Jupiter's gravitational pull?
Dr. LEVISON: Exactly right.
SIEGEL: So, on balance, is Jupiter helping us or hurting us more?
Dr. LEVISON: On balance, Jupiter is hurting us more because the biggest hazard we face is the asteroids from the asteroid belt.
SIEGEL: Because they're bigger than comets or more dangerous?
Dr. LEVISON: No, because there are more of them that can get down into Earth-crossing orbits. I mean, it's a funny balance. If you try to imagine a solar system, let's say, without Jupiter and Saturn, it's not clear really what would happen. In that situation, we'd probably be faced with the situation where most of the impactors on the Earth were from comets. There wouldn't be an asteroid belt probably, but the Oort cloud would be much more dangerous.
SIEGEL: Now, what if that thing had hit the Earth? What would've happened?
Dr. LEVISON: If that thing had hit us, we would've been probably annihilated. It may happen yet.
SIEGEL: I gather Jupiter is frequently hit by comets because it's so big and because it's attracting them?
Dr. LEVISON: And it's protecting us. That's right. There are three basic source of impactors on the Earth. There's this asteroid belt, there's a region beyond the orbit of Neptune, which we call the Kuiper belt, which is made of icy objects. And then there's the Oort cloud. And Jupiter is being pelted all the time by objects from this Kuiper belt.
SIEGEL: How is it that an event, which I gather in the scale of the entire universe isn't huge to do, but how is it that it escapes the notice of larger observatories and the like and is witnessed by an Australian stargazer?
Dr. LEVISON: The amazing thing about astronomy, in my view, is how many amateur astronomers there are. I started my life as an amateur astronomer. And there are only a few really large telescopes. They're hard to get time on, and so they're dedicated to particular projects. And the amateurs really are the only ones that have time just to monitor things to see what's happening, and that's exactly what this guy does. My understanding is he's an expert on just taking images of Jupiter and looking how it changes over time.
SIEGEL: Part of the neighborhood watch looking out the front door.
Dr. LEVISON: Yeah, that's a good way of putting it. That's a great analogy.
SIEGEL: Well, Hal Levison, thank you very much for talking with us about this.
Dr. LEVISON: It was a pleasure talking to you.
SIEGEL: And Dr. Levison is a planetary scientist at the Southwest Research Institute in Boulder, Colorado. We were talking about the comet that hit Jupiter.
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