Questions Show: Shooting Lasers at the Moon and Losing Contact with Rovers

Reflector left on the Moon by NASA astronauts.

Reflector left on the Moon by NASA astronauts.

This week we find out how hard it is to hit the Moon with a laser, and if scientists lose contact with the Mars rovers when they go behind the Sun.
If you’ve got a question for the Astronomy Cast team, please email it in to and we’ll try to tackle it for a future show. Please include your location and a way to pronounce your name.

  • Shooting Lasers at the Moon and Losing Contact with Rovers
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      When we shoot lasers at the Moon to hit the targets left by the Apollo astronauts, do we need to “lead” the laser beam to hit the target, or does the beam disperse enough that we don’t have to aim precisely?

      Do we lose communication with the Mars rovers and other spacecraft at solar conjuction?

      Could the Hubble Space Telescope and other satellites “capture” a small object and have it go in orbit around it?

      Could planets form in the accretion disk around black holes?

      What is in the space between atoms — could there ever be nothing?

      If you put all the asteroids together in the Asteroid Belt, would you have enough material to make a planet?

      Is the International Star Registry for real?

      Why don’t Hot Jupiters Ignite?

      Could there be moons around moons?

      If  we could drill a hole through the center of the Earth and a person went down there, would gravity tear them apart?

      Could Hot Jupiters be failed stars?

      Does our Moon orbit the equator?

      Where did all the water on Earth come from?

      If Universe was infinite and we steered an object near progressively more massive object and used a gravity assist from each object, could we approach the speed of light?

      Transcript: Shooting Lasers at the Moon and Losing Contact with Rovers

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      Fraser Cain: Hey Pamela, you’re in England again?

      Dr. Pamela Gay: I’m actually sitting at Chris Lintott’s desk recording while I watch construction workers outside, so we apologize if there are any accidental sounds like dentist drilling, in the background.

      Fraser: Right. So all the background noise is them, what, tearing the university apart?

      Pamela: It really looks like they are systematically replacing really, really, really old sidewalks.

      Fraser: Alright, then you can be excused for this.

      Pamela: [Laughter] Okay.

      Fraser: So, this week we want to know how hard it is to hit the moon with a laser and if scientists lose contact with the Mars Rovers when they go behind the sun. Now if you have a question for the Astronomy Cast team, please e-mail it to and we’ll try to tackle it for a future show. Please include your location, and a way to pronounce your name.

      Let’s get on with the first question. This is from Rob. Given that it takes about 1.28 seconds for light from the moon to get to us, and it takes about 1.28 seconds for light from the laser to hit the moon, would it if you were shooting a laser at the moon you would have a delay of total 2.56 seconds, so would we need to lead the target by 2.56 seconds? Or does the laser pulse shot from Earth disperse enough to encompass the entire moon?

      Let’s see if I understand this. We’ve talked about this a bit. When the Apollo astronauts were on the moon, they left a couple of mirrors, really nice mirrors. Scientists have been fine-tuning their calculations for the distance of the moon by shooting a laser pulse from here on Earth up to the moon, bounces off the mirrors back to Earth. Then they know a time of how long the laser pulse took and that tells them how far the moon is. But it does take time with the speed of light. So do scientists have to take into account the motions of the moon as they shoot their laser?

      Pamela: Yes, but not as much as you might worry. The laser beam, and they actually do this at McDonald Observatory, they have a lunar laser ranging station out there. They have a 30 inch telescope that they use to both first shoot the laser beam they reflected off the mirror and shoot it out to the moon, and then capture returning laser light.

      Now, a laser beam, by the time it gets to the moon, it’s about a kilometer and a half wide, so about a mile in diameter at the moon surface. The moon is moving at a pretty good clip through space. It’s moving at about a kilometer per second. We do at a low level have to anticipate where the moon is going to be when the laser gets there, but it’s not as though we are worrying about getting something in the size of a quarter to hit something the size of a refrigerator.

      It is more like, well, the target and the laser beam aren’t all that different in size and by the time the light ends up getting back to Earth, we’re only counting a few dozen photons if we’re lucky. So if the entire beam doesn’t entirely line up, it’s okay. We just need some part of that laser beam to end up hitting the reflector.

      Fraser: Okay. So the beam disperses so wide that it’s not that complex of a job. It still is bonking into the moon or into the mirrors and it’s still coming back. They don’t really need to take all of that motion into account. Okay. That’s cool. But what if it’s something further that they are trying to shoot at, say Mars? Then would they need to take that into account?

      Pamela: [Laughter] We have to really start worrying about the strength of a laser beam once we start getting at things that far away. But we have worked on things like this. And yes, we do worry about where things are going to be in time when the laser beam gets to them. It is a fairly straightforward calculation and it’s sort of like imagine trying to capture a little tiny Mini Cooper a football field away in your normal digital camera field of view.

      You have a fair amount of time to snap that picture during which the Mini Cooper is going to be somewhere in your field of view. If you’re trying to get it perfectly centered it’s a lot harder. But as long as you’re trying to get the Mini Cooper somewhere in your field of view as it drives through your field of view a football field away you’re pretty much okay.

      Fraser: Okay, let’s move on. This comes from Eric Splinter. Will we lose contact with the Mars Rovers, or any other piece of equipment that we sent out there, when it’s on the far side of the sun, and if so, for how long?

      I guess this is the question, right? Earth and Mars are both orbiting the sun and there is times when the Earth and Mars are opposite sides of the sun. So, can we communicate with the Rovers when this happens?

      Pamela: Unfortunately, we lose every year basically about 12 days when we just can’t communicate with the Rovers anymore. This has happened several times. We’ve always gotten recovery signal when Mars comes out the other side of the Sun, so it’s not a huge deal.

      However it is something that we have to deal with with everything. With Mars it’s about 12 days. Other objects are going to have other lengths of time that we have to worry about things being out of reach.

      Fraser: You can imagine the Messenger is going to be at Mercury, right? So it’s going to be going around and around and around with Mercury, losing contact. It takes 88 days so you could imagine losing contact with it pretty often.

      Pamela: But the nice thing is that since Mercury is so much closer to the sun, it’s on a much faster orbit. While we lose contact with it much more frequently, the amount of time that we lose contact for is going to be much shorter. It all balances out in the end. We more frequently lose it but then it comes back quicker, versus we lose contact less frequently but for longer periods of time with Mars.

      Fraser: Right. Same deal with Cassini and Saturn I guess?

      Pamela: Right. And we know how to deal with this and the nice thing about these Rovers and these space explorers they are pretty much self-contained units. They know how to take care of themselves. They know how to call home when they come back from the other side of the sun.

      Fraser: Alright, good. So, this is something that NASA scientists are well aware of and they take into account. I don’t know what they do during the 12ish days that the mission goes dark. Maybe they all schedule their vacations for those 12 days. [Laughter]

      Pamela: It’s probably a good time to go away, or at least sit down and crunch data instead of worrying about what your Rover is up to.

      Fraser: Right, the next question comes from Ken Smith from Ft. Erie, Canada. Ken wants to know: Could something the size of the Hubble Space telescope capture a pebble or speck of dust that was flying in space and keep it in orbit around itself?

      I guess the question then is, as we know anything with mass has gravity. Would something even with a relatively small amount of mass, like the school bus-sized Hubble Space telescope, be able to capture something and have it go into orbit around it?

      Pamela: This is really a problem with differences in energies. The escape velocity for something as small as the Hubble Space telescope is such that basically while a good exhaled breath would cause you to fall out of orbit in order to orbit the Earth; you don’t need to have a huge matching of velocities for something small coming along. If it’s going at similar to the Earth’s velocity, there’s a good chance that it can get captured.

      With the Hubble Space telescope, you would have to have something that is going at the exact right angle, at the exact right velocity. Even with that, just the difference from going from the sun side to the dark side, that change in energy would be enough to cause something to no longer be orbiting the Hubble. If there is just too much going on and it’s too easy to escape that orbit you can’t really get anything orbiting the Hubble Space telescope.

      Fraser: When we talk about things in our solar system being captured by…into orbit…doesn’t it usually require a three body interaction? That’s almost impossible to just capture an object into orbit you need to have that three body interaction to kick something into the orbit, right?

      Pamela: It doesn’t require a three body problem if you have objects that are all very similar in mass. Having three different bodies working together to change which two are orbiting one another, orbiting around a central gravitational center of mass for the system, then you need the three bodies.

      If you have something like Jupiter, something like Titan, if you have a large enough difference in mass, it’s not that difficult for the larger body to capture the smaller body. You’ll often end up with things that are in highly elliptical orbits. Orbits that are very elongated and flattened, and to get those orbits to circularize, that’s where the third body starts coming in. Or you need some sort of frictional slowing or some other extra force that will slowly remove energy from the system to get the orbit to not be so elongated.

      With something where you have a small body like the Hubble Space telescope, school bus sized or maybe something huge like the planet Earth the gravitational pull of the Hubble is essentially negligible compared to the planet Earth. It’s only when you have these giant systems like Jupiter that they are able to very easily capture things that happen to be flying by.

      This is where we end up, for instance, with Neptune’s moon Triton. We are fairly certain that it’s just a captured quiper belt object. Many of Jupiter and Saturn’s moons are probably just captured asteroids. These various objects just fell into orbit because their orbit around the sun happened to line them up with the orbit of one of these planets going around the sun in such a way that they were captured, settled into orbit. Also it is other factors that eventually circularize these orbits.

      Fraser: So if the astronauts go up to service Hubble again, they’re not going to notice a constellation of specks of dust moving in orbit around Hubble. But…

      Pamela: No. That’s just not going to happen.

      Fraser: But maybe we’re being kind of, we’re taking this question too literally, right? We’re taking into account the gravity of the Earth and how it’s impossible for it to actually capture. Maybe that’s the word we’re focusing on.

      If we took an object with the mass of Hubble, moved it out further into space where it’s not interacting gravitationally with anything else really, and then an astronaut very carefully dropped a speck of dust and gave it a little nudge so that it was, so it had perfect escape velocity or not quite escape velocity or orbital velocity going around Hubble, it would, right? You would end up with a speck of dust going in orbit around a mass?

      Pamela: You could do that. Again, just the slightest energy difference would cause the speck of dust to probably fall out of orbit. Just a slight…

      Fraser: I said careful…

      Pamela: I know…I know…

      Fraser: Very careful. And away from other gravity. And maybe, shielded from the radiation of the sun. Maybe surrounded by a ball of shielding that is perfectly smooth and round. [Laughter]

      Pamela: Anything is possible when it comes to orbits. It’s whether it’s stable. And in this case, yeah, you could get it to happen, but it wouldn’t stay there. So, it’s a very tenuous system.

      Fraser: Right. With your pesky reality jumping into cause problems, I’m going to go with my answer.

      Pamela: Okay.

      Fraser: Yeah. This is in a perfect world. Absolutely. It’s just mass, right? Mass orbiting mass. That’s all.

      Pamela: Mass orbiting mass. And it’s not going to stay there the second anything hits it. Any slight bit of energy, but sure.

      Fraser: See, you bring reality back into this. I don’t understand…

      Pamela: I’m sorry. I’m having a realistic day.

      Fraser: Clearly. I needed to have a fantasyland. Okay. Let’s move on. This comes from Sam McAdoo from Connecticut. I know that an accretion disk can form around a black hole when it’s feeding on matter, can planets form from this? And if so, have any been discovered?

      So, black hole is actively feeding as we said. It is choking on material and so this great big fat accretion disk forms around it which, I guess, smells very similar to the accretion disk that might form around a newly forming star. So, could you get a planet in there?

      Pamela: Not so much. Here it’s again a problem with energies. In order for planets to form, you have to be able to have the atoms stick together. You have to be able to have molecules forming. You have to be able to basically have the thermodynamics of the system allow one blop of material to quite happily, chemically bond another blop of material.

      But as you heat things up, the motions of the individual atoms get higher and higher and higher, such that these atoms don’t want to stick together. In the case of accretion disks around black holes, you have disks’ matter that is so hot that they are actually radiating away light.

      In some cases the reactions going on in the centers of these disks can be exactly the same as the reactions going on in the centers of stars. With these high energy reactions taking place, you just can’t end up with any planets forming, any chemical bonds forming, and instead, the material just acts like swirling plasma happily radiating away light, but not forming any planets.

      Fraser: We talked a bit about this with the tidal forces. You’ve just got such different gravity pulling at different distances as you move through this accretion disk. As you said, you just are not going to get things to clump together. It comes back to that old spaghettification problem.

      If you fall into a black hole, your feet are going to be pulled way harder than your head, and you’ll be yanked into spaghetti. That is sort of the same process that is going on in the accretion disk.

      Pamela: And just differential velocities. One edge of the planet-sized sphere of space would be orbiting at such a different velocity than the outer edge of that same spheroid planet-sized flub of space. You could just imagine a planet getting sheared apart if you were to use Star Trek’s transporter beams to suddenly plop Jupiter in with these accretion disks.

      Fraser: I think you did hit on something that is kind of interesting which is that you can get the conditions that are the same as stars especially around the super massive black holes right? You can get star-like blobs in the accretion disks, just places in the accretion disks that are the temperature and pressure of a star, and it’s giving off radiation like a star. And then it dies, right?

      Pamela: And this is an entire disk of material that is behaving like this. When we look at QSOs, when we look at quasars out on the edges of the universe, these are galaxies that giant accretion disks around their central super massive black holes and this entire accretion disk is giving off so much light that these most distant galaxies shine like nearby stars.

      Fraser: So, no planets, but stars shaped like rings. Which is pretty cool! Alright, so Joseph Ferguson asks: What is in the space between atoms? Is there ever nothing? And how could there ever really be nothing?

      Whooo. [Laughter] Alright. So space is a vacuum and it’s mostly nothing but there are a couple of atoms every square, I don’t know what, meter of space or something like that. What about the space in between those atoms? Is there anything there?

      Pamela: This is where you get down to the I sound almost like Clinton “What is the definition of nothing?”

      Fraser: Well, I guess you’ve got electromagnetic radiation, right? I mean, you’ve got light and stuff passing through the region.

      Pamela: Right. So you have all those spaces permeated basically by the cosmic microwave background. But then you also have so that’s light energy, but you also have what we call vacuum energy. We are fairly certain that pretty much every cubic meter of space has three protons of energy contained within it across the entire universe.

      We’re not quite sure where all this energy comes from. We’re not quite sure how to describe it. We think it might be the cause of the acceleration of the universe’s expansion, the dark energy that we’re trying to sort out so desperately. Beyond energy, there are places where we don’t have nice, solid particles that you can study within a laboratory. There are places in space that are nothing but energy. And that’s kind of cool to think about.

      Fraser: Right, this is where you’ve got these virtual particles that pop into existence and self annihilate and disappear again. What about, I don’t know, like space itself? I mean, space doesn’t let you move faster than light, right, so there’s something in there. It’s kind of hard…I see what you mean. It’s like “what do you mean by nothing?”

      Pamela: Yeah. And it’s not that the space doesn’t let you not move faster than the speed of light. That has more to do with Einstein’s theory of relativity. Space itself just is. It’s sitting there going “I’ve got energy. I’m expanding.” That’s pretty much all it does. It sits there with its bubbling of virtual particles happily expanding away being an igmatic.

      Fraser: Right. I guess when we talk about it like the grid of space, right?

      Pamela: Yeah.

      Fraser: And that’s something. It’s not nothing.

      Pamela: But it’s not stuff.

      Fraser: But it’s not something. It’s not nothing, but it’s also not something.

      Pamela: It’s not anything that fits in tweezers.

      Fraser: Right. But yet it is kind of there and it does have its effect on the universe. So, we’d have to go back to the original question: “What do you mean by nothing?”

      Pamela: Right.

      Fraser: It’s a killer question. I don’t know if we did it justice, but it sounds like a whole show, isn’t it.

      Pamela: It’s a question for the philosophers.

      Fraser: We need to get a bunch of astronomers drunk and then ask the question.

      Pamela: [Laughter]

      Fraser: Alright. This question comes from Cole, from San Diego, California. My question is about the asteroid belt. I wonder if this is a failed planet? And could over a great amount of time, maybe form another planet?

      So, the asteroid belt I guess he is talking about is the one between Mars and Jupiter. It is a collection of asteroids with the largest being Ceres and Vesta. If you took all those asteroids, bundled them all together, would you get a planet?

      Pamela: No.

      Fraser: No. How much planet would you get?

      Pamela: I wouldn’t even really measure it in how much planet you would get. If you take all the mass, it’s about 4 percent of the mass of Earth’s moon. And that’s kind of sad.

      Fraser: 4 percent of the moon?

      Pamela: Of the moon.

      Fraser: Whoa.

      Pamela: Right. And its four largest objects: Ceres, Vesta, Pallas and Hygiea. These four objects contain half the belt’s total mass. We’re looking at remove those four giant asteroids and you have in everything else two percent mass of the moon.

      A lot of people, thanks to science fiction, have this image that the asteroid belt is this rocky, dense, hard to fly through, dangerous for spacecraft, place. But the reality is in general, if you’re standing on an asteroid, you can’t see another asteroid anywhere near you.

      Fraser: Right, right. And so, not a failed planet.

      Pamela: Not a failed planet.

      Fraser: Right. So, this question comes from K. G. Graubux from Hanover, Virginia. While listening to the radio I constantly hear commercials about the International Star Registry where you can name a star and it is recorded and copyrighted. Is this process recognized by professionals, or just a money grab from the less informed?

      Pamela: This is actually one of the saddest things I have to deal with in this as a professional astronomer. This is not a recognized anything.

      Fraser: Let’s give a little bit of background. If you come from Mars, and you’ve never heard of this, there are companies that will I guess allow you to name a star. You give them money and they will go to their vast star catalog and name the star whatever you want to name it. They’ll give you a certificate, and congratulate you and give you coordinates if you want to go and find the star that you have named. So, that’s the offer, and what is from an astronomer’s point of view, what’s the deal?

      Pamela: Basically they have taken a generic catalog of there’s a star here, star here, and a star here, put an empty spot next to the position of each of the stars, and they started putting their own table of names for those stars within their databases.

      This is the equivalent of me going out to the Colorado Rockies, using The U.S. Geological Survey’s, map of where all the mountain peaks are, and then going to my friends and say “hey, I’m writing a book of mountain names, give me your name and I’ll assign it to a mountain peak. Then I’m going to copyright my particular naming of the mountains.” Now my naming of the Colorado Mountains, those aren’t the mountain’s official names. The official names of mountains that comes from the U.S. Geological Service.

      With stars and planets, the official names of these objects come from the International Astronomical Union. There are a lot of very set rules of how you can and can’t name things. These lists are just somebody’s fictional lists. No more valid than me going out and renaming Pikes Peak to Pamela’s Peak. I might like my name better, but it’s not the official name.

      The thing that always saddens me about this is you can go to children’s stores and you can get a stuffed toy that names a star after your newly born child. You can commemorate a star in the name of a loved one who has just died. All you’ve done is basically given money to some company that is going to sell a book to other people that, well, it’s just, as many words as fit on the paper and none of it is acknowledged by anyone outside of that company.

      Fraser: I guess it’s not recognized by the astronomy community in any way, shape or form.

      Pamela: No.

      Fraser: Renaming mountains or renaming buildings, the Fraser State Building in downtown Manhattan. It’s the same process. You can just kind of imagine what that means. I think is a slight educational benefit to this. In that I’ve talked to a bunch of amateur astronomers, people who work at observatories, who work at planetariums, and someone will come out and say “can you find my star for me?” Some they won’t even get real stars they pick numbers they just make them up. These aren’t real stars. Some of the more legitimate ones, if you can call them that, will find real catalogs of stars and really try and keep it organized.

      So you can take your document to a planetarium and the person running the telescope can find your star for you, but it’s not really your star. It’s not really named after you. All you are really doing is the same thing as hopping in a plane and someone showing you a mountain and saying “ha, ha…there’s Pamela’s Peak.” And then say, “That’s Pike’s Peak.”

      Pamela: Yeah.

      Fraser: Yeah. It’s too bad. It’s crazy popular. Someone is making tons and tons of money. It’s just not recognized by anybody. It is for entertainment purposes only.

      Pamela: Yes.

      Fraser: So the next question comes from Jordan James from Bowser City, Louisiana. Since some of the hot Jupiters get very close to the star and therefore very hot, why did it not ignite and go up into flames? I’m not sure what the ignition temperature is in space, but the auto-ignition temperature in air is about 550 Celsius. Is there some reason why they don’t ignite, or do they simply not get hot enough? What Jordan is talking about are these hot Jupiters, it’s these…

      Pamela: Hindenburg.

      Fraser: The hot Jupiters, these newly discovered planets orbiting stars incredibly close. They are incredibly hot temperatures. What are they made out of? What would a planet like this be made out of?

      Pamela: These are objects that are majority hydrogen and a fair amount of helium. They also have ammonia and methane. Other different trace amounts of molecules and atoms within their atmospheres. But what they aren’t is a large part oxygen. That is kind of the key.

      Fraser: Right, right. So, like Jupiter, they are 85 percent hydrogen. And if I was to take Jupiter, bring it to Earth somehow…

      Pamela: [Laughter] without destroying the planet Earth.

      Fraser: Without destroying the planet Earth somehow. Don’t worry about the scales, we’re in Fraser’s Fantasyland right now, and put a match to it, what would happen to Jupiter?

      Pamela: Well if you brought it to the planet Earth and you were able to stick some small corner of the Jupiter atmosphere within the Earth’s atmosphere, you could probably ignite that small corner. Now the problem is first of all, Earth weight is small, the Earth atmosphere is small and we’d be pretty much get distributed all over Jupiter gravitationally. The key is Earth’s atmosphere has oxygen.

      If you were able to bring just some small corner of Jupiter into some small corner of our atmosphere without our atmosphere flying all over the place gravitationally, then you could ignite some of Jupiter’s atmosphere.

      What you need is an oxidizer and this was actually something when I looked up this question I was like “hmmm, I wonder…” And it’s because I’m used to things burning in terms of atomic things like Hindenburg. It caught itself actually on fire, flames, giant badness, people burning, wasn’t nuclear reactions.

      To ignite hydrogen or helium, or just about anything else, you need some sort of an oxidizer that will allow flames to allow the chemical reactions to take place. Out in space you don’t have oxidizers. So out in space, you don’t have burning planets, and this is probably a good thing.

      Fraser: So then, what could you do to ignite it? I guess, have it form nuclear fusion, right?

      Pamela: Well, nuclear fusion, but to get that though, you need to have a much denser, much hotter, much more massive system. If you took Jupiter and added more Jupiters and added more Jupiters until you have something that is at least a couple tens the size of Jupiter in size, something 10 to 20 sized Jupiter, you might be able to get a little bit of what the burning in the core. Once you start getting something 30 or 40 times the size of Jupiter, you start getting into brown dwarf categories. You have to start getting something significantly bigger to allow those nuclear burning processes to take place.

      Fraser: So, even though they are very close to the star, and the star can be crazy hot, and the surface of the planet could be thousands of degrees, no big deal.

      Pamela: No big deal. No oxidizer that works.

      Fraser: Just a hot planet. Hot Jupiter. No place you want to live.

      Pamela: No.

      Fraser: If you’re worried that one of these things are going to ignite because you want to live there, don’t worry about it. You’ll be fine. So, let’s move on. This one comes from Chris. And Chris, you did provide me with a way to help pronounce your name, but I didn’t get the reference, so I’m sorry.

      Do we have any evidence for naturally orbiting debris around the moons? Moons around moons? Moons around moons around moons? It’s like a Dr. Seuss question. If we can’t observe such phenomena in our own solar system, is there reason to believe that such a behavior could occur elsewhere? Moons around moons? Could you get that?

      Pamela: Yes.

      Fraser: You could?

      Pamela: Yes.

      Fraser: So the moon, say Earth’s moon could have a moon going around it? Well, that’s a spacecraft, right? We’ve done it.

      Pamela: Right. So right now, if you go out, go to the moon. You have the Japanese spacecraft, you have the Indian spacecraft, and you have the Chinese spacecraft all happily, for the most part, orbiting around the moon.

      Now truth be told moon not the most spherical mass. Once you start getting close down to the surface, it does have just enough atmosphere that it causes frictional losses of energy of the things trying to orbit it. It is not the most stable place. You’re not going to stay in orbit forever around the moon. But as you start looking at some of the really big planets in our solar system; it’s not impossible to get things that are for a period of time orbiting these giant moons.

      You can end up with debris around Saturn’s moons quite happily orbiting. You could end up with smaller bodies orbiting larger bodies orbiting Jupiter. Now again, this isn’t the most of stable of places to be because it is a very complex gravitational field and you end up with a lot of weird torques going on, but you can for a period of time have naturally occurring satellites of these natural occurring moons.

      Fraser: Right. And we’re already far into this. You’ve got the sun which is going around the center of the Milky Way, so that’s one moon, right? And then you’ve got the Jupiter going around the sun, so you’ve got a moon around a moon. And then you’ve got one of Jupiter’s moons going around Jupiter, and that’s a moon going around a moon going around a moon.

      Then we could put a spacecraft around one of Jupiter’s moons and it would be pretty stable. That would be a moon around a moon around a moon around a moon around a moon. Could you go one more? I guess it’s just how stable you can keep the whole thing going.

      Pamela: It’s all about what are the different energies necessary to knock things out of orbit. The smaller the nesting doll on this case, I guess is the best analogy, the easier it is to get knocked out of orbit such just the difference of being hot to being cold, from going in the sunlight out of the sunlight, be enough to destabilize the orbit.

      Fraser: I mean there are moons around asteroids, which is cool.

      Pamela: Yes.

      Fraser: In fact, it seems like it is a lot more common than people ever thought and in some cases multiple moons around fairly small asteroids. You’d be surprised where you can get these things working out.

      Pamela: And if you haven’t seen it before, there’s this really great image of the asteroid Ida and its little satellite, the asteroid Dactyl.

      Fraser: The next question comes from Levi Blackman from Denton, Texas. If I could somehow drill to the center of the Earth protected from heat and pressure, how would gravity pull on me? Would I be suspended? Would the gravity from the Earth tear me apart? What about the center of the sun?

      So, we’re going to take Levi, we’re going to dig a shaft right down the center of the Earth and lower him down and let go in the middle of the Earth. What will happen?

      Pamela: This is actually one of those really fun, although students always hate us when we do this problems give to students who are studying mechanics. If you were to take and put, not just a hole all the way to the center of the Earth, but actually bore a hole all the way in one side out the other. Make it completely lined, completely safe, pump it so that it has consistent vacuum inside of it and then stick a person with breathing gear inside this tube and let go at the surface of the Earth, it would actually be like sending them on a bungee cord mission.

      You take this person and you let go and they shoot because they are gravitationally being sucked down toward the center of the planet. When they get to the center of the Earth they have so much kinetic energy that they just keep going. If it is a completely frictionless environment they will keep going until they get to the same distance from the center of the Earth, but on the other side.

      They will keep bouncing back and forth, back and forth. It’s all about conservation of energy. They have nothing but gravitational potential energy at the surface of the planet and let go in the center of the planet they have nothing but kinetic energy. That kinetic energy causes them to keep going to the other side.

      If you add just a little bit of air to this tube, have them reach out to the sides of the tube in terror and just these small amounts of friction can cause them to lose some of this energy and cause them to eventually end up settling to the very center of the planet, where they just sort of hang out going “oh, it’s hot in here.”

      Other than that, they have all of this mass around them, but most of the gravitational poles are balanced such that the left side is pulling on them equal to what the right side is pulling on them equal to how their head is being pulled equal to how their feet are being pulled.

      Fraser: So they are weightless.

      Pamela: They are weightless. They’re not getting yanked apart. They are just sort of hanging out in a very hot, unpleasant place to be.

      Fraser: Wow.

      Pamela: Same thing will happen in the center of stars, except it is much more difficult to build you a nice, safe little chamber.

      Fraser: Right like 15 million degrees Kelvin.

      Pamela: Exactly.

      Fraser: Well, that’s really cool. Yeah, there was a sort of interesting thought of an experiment that someone had suggested at some point. Build a transportation system where you bore a hole, say from San Francisco to Los Angeles. You do a straight line while the Earth curves around this hole, and then same deal, you know.

      You make it as frictionless as possible; put a train in it, the train will roll downhill halfway to the journey between San Francisco and Los Angeles. Then it will roll uphill for the remainder of the journey. You could actually travel from between any two points on Earth with no energy because you would be rolling downhill one way and uphill the other way.

      Pamela: So, the big killer in all of these different situations is friction. If it wasn’t for friction, yeah, we’d have these perfect transportation systems. We’d have everything so much cheaper and easier, but reality is we have friction all over different machines and it’s friction that prevents these wonderful imagined realities from happening.

      Fraser: But, tell me if I’m wrong, if I recall correctly, the amount of time for the journey would always be the same?

      Pamela: What do you mean? The amount of time to get to the center and the amount of the time to get from the center to the final point?

      Fraser: Right. If you go straight through the Earth it would take the same amount of time if you would take a more slanted route, because you’re not going straight down, you’re going at a different angle…a less deep angle.

      Pamela: No, that’s not entirely true. You can change the amount of time that it takes but what will always be true is your velocity at the end. So if I go down a slope that I started at an altitude of 3,000 feet and ended at 0 feet. If I do that at an extremely steep angle, or I do that spread out across several miles, it’ll take me a different amount of time as I accelerate down the slope. But my end velocity, the conversion of gravitational potential energy to kinetic energy, the end will always have me going the exact same velocity by the time I hit my low point.

      Fraser: Right. Cool. Alright, so if you get to the center of the Earth, you’re weightless and you won’t be torn apart. So let us know how that works out for you when you do this. Remember it’s hot.

      Okay, Brandon Fahrencamp asks: Is it possible that these hot Jupiters are just failed binary star systems where the companion star was not able to accumulate enough mass to ignite?

      So, we’re talked about the hot Jupiters before. Are they just failed stars? Like is Jupiter a failed star?

      Pamela: This is where we have to start looking at the system in which these different objects form. When we look at binary stars what we generally say is you have a large blob of gas and dust that as it was collapsing down it splits sort of like when you look at a twins forming, you end up with this split. Then both stars end up forming out of their own separate pockets of gas and dust from this original one large pocket of gas and dust.

      When you have planets forming, what you have instead is giant pocket of gas and dust that continues to collapse and never splinters, but instead of ends up with eddies within in and those eddies within it end up forming the planets. It’s the dynamics of how the system forms that we use to try and say this one thing is a failed star, this is a brown dwarf. This other object is an actual planet, a Jupiter.

      This is the only way we have to really start differentiating between the largest of the hot Jupiters and the smallest of the stars, what was the situation in which they formed where in some instances you can get giant star with small brown dwarf sized objects forming like a planet around it.

      But in another system, you might end up with a red dwarf and a brown dwarf that is the exact same mass. But because the system evolved differently because the gas split in different ways and these two objects really did form from this splitting of one blob of gas and dust, they are technically different objects.

      Fraser: Right. So at the end of the day, planet, star, brown dwarf it just comes down to mass. A hot Jupiter may have three, five, seven times as much mass as Jupiter, but that is still a tenth of what you need to really be a star. You could say that it is a failed star if you could take 10 more of them and mash them all together, and then you’d get a star. But I think that is your definition. To say that it is a failed star is sort of, you’d have to make it so much more bigger, much more massive. That is why we talk about Jupiter. For Jupiter to be a star you’d have to find 80 more Jupiters, put them together and then you’ve got a star. There’s your failed star.

      I’m trying to think of some sports analogy like me running a marathon in the Olympics. Am I a failed marathon runner, you know. I barely run, I never have run, you know. If I went out and I trained a lot then maybe I can be a marathon runner, but it’s almost genetically impossible for me to do so I think that is the analogy.

      I guess with a hot Jupiter, it’s the same deal. Add seven or 10 more of them, then you’ve got yourself a star. I think it’s neat, I think that you can end up with a hot Jupiter as forming in place in the accretion disk around a star. While a binary system is where the clouds fragment into two separate creatures, right?

      Pamela: Right. And this is where you look at both the mass and the how it formed when you’re dealing with these border cases “well it looks like a brown dwarf; no it looks like a super Jupiter.” So we differentiate the most confused cases where the masses are right on the boundary and looking at the formation history.

      Fraser: But at the end of the day, mass is mass.

      Pamela: Mass is mass.

      Fraser: Right.

      Pamela: It either has nuclear fusion or it doesn’t.

      Fraser: Right, right. Okay. Les Cam from Arizona wants to know: Does our moon orbit the equator? Well?

      Pamela: No. This is actually one of those…

      Fraser: It doesn’t?

      Pamela: No it doesn’t. It orbits the center of the planet Earth. But the moon doesn’t care if it’s going over the North and South Pole. It doesn’t care if it’s going over the equator. All that it cares about is the center of its orbit and the center of mass of the Earth moon system which is within the planet Earth near the Earth’s centered mass.

      All it cares about is where the center of mass of our system is and it’s going to orbit that center of mass of the system. To figure out where it is, it’s going to go where is the center of the planet Earth’s mass? Beyond that it doesn’t care. It could be orbiting like I said over the North and South Pole. It could be orbiting exactly along the path the sun takes through the sky.

      It could be doing whatever it wanted. As it turns out the orbit of the moon is actually slightly tilted relative to the path the sun takes through the sky and nowhere near the point over the planet Earth that happens to equal the equator.

      Fraser: And that’s why we don’t get eclipses once a month.

      Pamela: No. In fact we only get an alignment of the sun, Earth and moon, about every six months.

      Fraser: Right, right. So if it did orbit right over the equator you’d get solar eclipse, and then two weeks later lunar eclipse. Then solar eclipse, lunar eclipse. But in fact, as you said, they are actually broken up about every six months.

      That is because you have the moon inclined a little bit off of the Earth’s plane and so they have to line up. There you go. Cool.

      Moving on, David Schaffer from Medford, Massachusetts, wants to know, and wants to explain to his seven year old, where did all the water on Earth come from?

      Pamela: This is actually highly controversial.

      Fraser: Great. Explain that to a seven year old. [Laughter]

      Pamela: The leading idea right now is planet Earth formed, planet Earth formed with water, sun got hot, sun dried out planet Earth, and no more water. But, our solar system conveniently stored water out in its outer edges in the form of comets.

      These comets came along and bombarded the surface of the Earth. When they melted they left behind the oceans. So the way to think of this you can imagine a completely dry driveway that has been baked by the sun but then it snows. When the snow melts you are left with puddles on your driveway.

      Fraser: Where is the controversy coming from?

      Pamela: We don’t know how much water got baked. We don’t know what fractions of the oceans come from comets versus original water. It is in trying to figure out the details, that hopefully a seven year old isn’t too worried about, it’s in trying to figure out the details that we sort of getting ourselves into a “hmmm” moment.

      Fraser: Right and there are some of the theories that it wasn’t comets that somehow early on the Earth was able to grab all that water and not be dried out. I think each one of these theories has some stuff going for it, but also has some stuff some big problems with it that scientists haven’t worked out.

      As you said, we don’t know. We think it’s one or the other, but we don’t know. There’s evidence for each possibility. So, where the Earth came from, we don’t know.

      Pamela: But for now, I’d go with the snow storm on a dry driveway.

      Fraser: There you go. Water from comets. So, last question. This comes from Jeff Carlin. Imagine the universe to be infinite. If we launch an object into space we’re able to steer it towards progressively more massive objects, say black holes, and use gravitational assists from each object, could we reach or approach the speed of light?

      So, okay, background. So gravitational assists, that’s where we fire a spacecraft and have it go towards a planet which is orbiting the sun like Jupiter. As it is sucked in by Jupiter, it gets a gravitational assist and it gets to go faster in its orbit in its velocity towards its final target right? So, could we scale that up and take a spacecraft and launch it at this star and then that star and this black hole, how fast could we get going?

      Pamela: The catch with gravitational assists is you have to be in an orbit that is carrying you in the same direction that that object is already going in. If you simply fall toward an object that is not moving, you’re going to accelerate toward it, and then decelerate away from it. You’re not going to actually increase your velocity at all.

      Fraser: I think we did cover this question before. This is where you had the analogy where if you had a car going down a hill and then back up the other side, and you do get to go faster because you’re falling down the hill. You’re being pulled down by gravity, but then you go back up the hill on the other side and you end up at the same speed you were going before. The orbit, as you were saying, the orbit is the thing that matters, right?

      Pamela: Right.

      Fraser: It’s the orbit, not the gravity.

      Pamela: It is the fact that because of the orbits you end up spending more time getting accelerated toward Jupiter if you’re going in the same direction Jupiter is orbiting in, and less time as you fall away from it.

      Imagine you were able to find some magical combination of objects scattered throughout our infinite universe, such that you did always get a gravitational boost. In this case, you could slowly but given a long enough period of time, get closer and closer to the speed of light.

      The thing is, as you got closer and closer to the speed of light your actual mass, you’re not going to change the number of atoms you have at all. The gravitational effects of your mass, the energy effects of your mass, are going to get larger and larger and larger as well.

      It is eventually going to be you giving the gravitational assists to the objects that you’re approaching instead of them accelerating you. So, you could get going pretty fast, but you’re never actually going to get to the speed of light. The ability to get accelerated is going to decrease more and more the faster you are going.

      Fraser: Right and I hate to be reality guy in this question. But to sort of scale up the gravitational assist, I would launch my spacecraft and I would have to launch it towards an object, a star or something, I guess that is further out from the center of the Milky Way than the sun is right? So the spacecraft would approach the star would be drawn in by the gravity, and experience more gravity on the way in and less gravity on the way out, so it would get a gravitational kick.

      Then maybe I would aim towards another star that was a little further away from the center of the Milky Way and I could hop from star to star to star and pick up my velocity. But I think, I guess what, and tell me if I’m wrong here, the mass of the object doesn’t really matter, right? So if it’s a black hole with a tremendous amount of gravity, that cancels out. It’s the orbital velocity of the object around the center of the Milky Way.

      Pamela: You need both.

      Fraser: No, I understand. Doesn’t the gravity of Jupiter cancel out and isn’t it that what you’re really picking up when you do a gravitational assist is you’re getting a boost from the speed that Jupiter is going? The difference of speed that Jupiter is going around the center of the Milky Way, or I’m sorry, around the sun.

      Pamela: No. Unfortunately, you need to have you actually have to have the really large mass.

      Fraser: Because it’s the time. Okay, I got it.

      Pamela: Right. So, you need to have both the time that you’re getting accelerated by the object and you have to have that gravitational acceleration. You also have to have the orbital velocity. You have to have the combination of the both to get the good assist.

      Fraser: So the best assist would be a very strong gravitational attractor going very quickly around, away from you.

      Pamela: Yes.

      Fraser: So, you would get sucked in very fast. Then you would get slingshot out the other side. But the key here is it’s all about the velocities. If you have a black hole that is moving in the same speed as you and the same kind of velocity, then it’s pointless.

      Pamela: Right.

      Fraser: Right, right and once again if you’re a reality guy, to find this you must reach some I guess, would you reach an upper limit, or would you always be able to extract a little more velocity?

      Pamela: You’d eventually reach an upper limit where you’re actually accelerating the object toward you and losing your own energy to it.

      Fraser: So, you could star hop in the Milky Way and I think that is a great example, because there’s lots of things to star hop to. Each time you’re going to go a little faster, little faster, little faster, so you could theoretically get yourself up to relativistic speed to where then the whole system falls apart, because now you’re the one that is pulling these objects back out of their orbit. But you could do it.

      Pamela: As you go to your limit of relativistic speeds, I’m not quite sure what you’d find that is orbiting faster than you. But in our infinite universe where infinite things are possible, they might exist. You can certainly get yourself going at a pretty good clip.

      Fraser: Right, right, okay. Otherwise, you’re going to be going “cool.” That was a great question. That was awesome. Well I think we made up for a lack of question shows the last couple of weeks, but keep the questions rolling. We’re getting back into our game, so thanks for everyone who asked these questions. We’ll talk to you on our next show, Pamela.

      Pamela: Sounds great, Fraser. I’ll be talking to you later.