+++++++

Q: Is Pluto a Jovian or a terrestrial planet?

A: As we discussed at some length in the lecture on Pluto, it's neither. It's size is that of a terrestrial planet, while its density places it somewhere near the Jovians. Whether you consider a planet at all is a matter of semantics: its composition is that of a large comet, and it orbits roughly in a region called the Kuiper Belt, where many comets orbit.

++++++++++++++++

Q: How many satellites does the earth have?
A: One natural and several thousand artificial satellites.

+++++++++++++++++++++

Q: Are there mutual eclipses for other planets like those of Pluto and Charon? Do the moon and the planet need to be very similar in size for this to happen? Close to each other?

A: No, and you just stated the reason for it---the two objects need to be similar in angular size for a noticeable blockage of light to take place; it's just simple geometry. But actually the moons of many planets often pass in front of the planet, though we don't call these mutual eclipses (technically, I suppose they are). Take a look at the picture of Jupiter's moons passing in front of the planet on page 278. We don't call these eclipses because if the moon is only a small fraction of the angular size of the planet, you don't see a significant blockage of light---it winds up looking like a dot passing across the face of the planet, and we call it a "transit".

+++++++++++++++

Q: How are "captured" moons captured?

A: They are presumably asteroid-like or comet-like bodies orbiting the sun (in other words smaller bodies formed as part of the accretion process that also formed the planets) that were moving in orbits that eventually brought them close enough to a much larger planet so that the planet's gravity bent their motion into an orbit around the planet.

+++++++++++++++

Q: Why do the outer planets have so many moons?

A: We don't know, but you can probably make an intelligent guess. Since the planets themselves are larger, there was probably a lot of debris around them to form into moons. Also because these planets have strong gravitational fields, they're more effective than the smaller terrestrial planets in capturing moons.

++++++++++++++++

Q: Can spacecraft like the Galileo get out of orbit now that they're in orbit? After all, Jupiter has a large escape velocity.

A: I suspect you may be wondering whether we could travel to a planet, and then bring back material. Not easily. You have to carry enough fuel with you to escape from the planet, as you imply. And that's not easy. I don't think that Galileo has anywhere near enough to do it, and future missions to planets (except Mars) are not designed to go and come back, for the simple reason that it's so expensive to carry the fuel. We need the space for scientific instruments. Check NASA websites for information on future space missions.

+++++++++++++++++++++

Q: How do we know the surfaces of all these frozen planets are composed of water ice? Couldn't it be some other gas?

A: We identify the composition of surfaces by looking at the spectrum of light they reflect. Of course the light is sunlight, which has its own absorption lines, but if we compare the spectrum of direct sunlight (what you get by looking directly at the sun) with the spectrum of light reflected off the planet, the difference is that the reflected light contains additional absorption lines from the planet itself. And yes, there are other gases present, most notably frozen ammonia, methane, and some carbon dioxide. But water is the most abundant.

++++++++++++++

Q: When the rotation of the earth and the revolution of the moon are finally synchronized, will the moon always be facing one side of the earth? And will there be tides?

A: Yes, the moon will always be facing one side of the earth. There will be essentially no tides then due to the moon, since the earth will rotate at the same rate the tidal bulge is rotating (the period of the moon). There will, however, be smaller tides due to the effects of the sun, which will rise and fall with the period of the solar day, since the earth will not be tidally synchronized with the sun. Because the day will be much longer then, the tides will go up and down more slowly, too. By the time all this happens, however, the sun will probably have become a red giant and engulfed the earth (we'll talk about this in next semester's course).

++++++++++++++++++

Q: Can a small planet with a very high density have moon go around it---the planet is small but has more mass than the moon?

A: Yes. The mass, not the size or the density is what determines the strength of an object's gravitational field.

+++++++++++++++++

Q: Could a moon escape from the gravitational pull of a planet?

A: I suspect that the only way to do this, other than putting rockets on it, would be if it collided with some other object--a passing asteroid similar in size to the moon. But the collision would be more likely to obliterate the moon than to eject it.

+++++++++++++++++

Q: We see all those large impact craters on the moons of planets. What's the chance of a large impact occurring today?

A: Impacts do occur even today; the larger the impactor, the rarer it is. Small grains of sand and gravel hit the earth at the rate of 100 tons a day; you can see them burning up as shooting stars every five minutes or so on a any clear, moonless night. Basketball size rocks probably fall every few months or so somewhere on earth (usually in the ocean). Objects the size of a car hit the earth every few decades, and objects the size of a building (a hundred feet or so) every millennium. One such "building-sized" object, the Tunguska event, flattened hundreds of miles of trees in Siberia in 1909; the impact produced as much energy as a 20 megaton nuclear bomb. And asteroid-sized objects, 10 km or so in size, like the one that destroyed the dinosaurs, probably hit every 100 million years or so.

++++++++++++++++++++++++++++++

Q: If Jupiter, or some other gas giant planet, were to have a collision, why wouldn't the collision change the planet?

A: It would, but not permanently --check out page 344 in our book to read about the collision of comet Shoemaker-Levy with Jupiter, and to see pictures of the "hole" in its atmosphere, the size of the earth. But the damage disappears with time---most of the material of the impactor (in this case, a comet) wound up scattered in the outer atmosphere of Jupiter. The rocky core, if there is one, was not affected.

++++++++++++++++++++++++++++++

Q: Could we land on the rocky core of a giant planet?

A: There's no way to get there. Tens of thousands of miles of hot liquid like between the clouds and the core of Jupiter and Saturn. Not a friendly place. An atmospheric probe dropped into Jupiter by Galileo traveled only a few thousand kilometers into the atmosphere before we lost contact with it.

++++++++++++++++++++++++++++++

Q: If the collision that broke apart Miranda did it while the planet was forming, why didn't it heal itself more completely?

A: We can imagine cases where the blow is not enough to completely pulverize the material, and several chunks are cracked off to reunite later; or where the moon was almost solid, but not quite, and the collision just deformed one side which then cooled quickly?..We'd only expect total healing if the moon were liquid at the time of the collision or if the collision pulverized the moon enough to bring the accretion process essentially back to square one.

++++++++++++++++++++++++++++

Q: Will the eventual collision of Mars and its moon have any effect on the earth?

A: The moon's only about 15 km across, hundreds of times smaller than the planet. It will dig a pretty big hole on Mars. It's possible some debris might escape from the planet and, after orbiting the sun for a few million years, hit the earth---like the Martian meteorites we've found in Antarctica recently.

++++++++++++++++++++

Q: Why doesn't the public hear about these space missions?

A: Well, it's probably true that unmanned space missions don't get the press of shuttle flights by senators; but when a spacecraft arrives at a planet, there's usually a lot about it on the evening news and in the papers. There's usually about one TIME or NEWSWEEK cover each year devoted to some of these results. And the Web has lots of information on space mission that's very actively accessed by the public. Two years ago, when the Mars rover, Sojourner, was taking pictures on the surface, there were millions of hits a day on the rover's website. Check out all the links on our web pages for these missions, and you'll see just how rich the resources are.

+++++++++++++

Q: Are there any plans to send anything beyond the solar system?

A: No, because it would take far too long. Two spacecraft, the Pioneer, and one of the Voyagers, are several billion miles out, beyond the distance of Pluto, and they will eventually leave the solar system. But there was no mission planned---they will eventually just be lost in space. Even at the high speeds they are traveling at, it will take about 50,000 years before one of them even comes moderately close to another star beyond the sun.

++++++++++++++

Q: When will the next meteor shower be? How about the next comet?

A: There are about a dozen regular meteor showers each year; the Perseids, on August 11, are among the brightest, but you can check the sky calendar linked to our website to keep track of others. Or look in your book, page 346 for a list of meteor showers. As to comets, the only bright one that returns frequently enough for us to anticipate it is Halley's Comet, next due in 2061. The other bright ones, like Hale-Bopp two years ago, surprise us--appearing as if from nowhere, because their periods are thousands of years or more, and the last time they were visible, there was no one to record the date and time of their passage.

+++++++++++++++

Q: In recent years there have been discoveries of other planets circling other stars. Are those system similar to ours?

A: About a dozen nearby stars have been found to have planets. We can't actually see the planets, only the effects they have on their stars---the stars move back and forth slightly in response to the gravitational pull of the orbiting planet. We know enough about them so far to say that they don't seem to be quite like our planetary system. About half of the new planets are Jupiter-sized objects closer to their stars than Mercury is to our sun. We can't figure out how such large objects can accrete so close to the star. According to our current understanding of how planets accrete, the hot star should sweep the region near it so clear of material that a large planet couldn't form close by. Keep tuned; we'll learn more about these objects in the next few years as people make more observations of them.

+++++++++++++

Q: Is there life elsewhere in the solar system? Elsewhere in the universe?
A: We don't know. But my educated guess is not in the solar system; surely elsewhere in the universe. In fact there are so many planets in our own Milky Way Galaxy that I think there may be life on many worlds in the Milky Way. The odds, I think, favor it. But they're just odds, right now; there's no evidence that there is life anywhere but earth.