Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Earth actually has two moons.

news.discovery.com

Post reply on HN
Print view View original

41–50 of 71 posts

Re: Earth actually has two moons.

#41

Post by joshAg »

harshpotatoes wrote: post #39
keenerd wrote: post #35

Earlier quoted context omitted.

No. If the Earth's mass were to vanish, the Moon would continue to orbit the Sun. It would be a bit more wobbly, but the Earth is not an essential part of the system. If the same happened to Jupiter, all of those moons would fly out of of the solar system. (Or at least go cometary.) None of the orbits would survive.

I don't see how this and Kepler's laws fit together. The moon's mass is significantly different from the earth. How could the moon have the same orbit as the earth if the earth wasn't there?

the orbit of a satellite does not depend on the mass of the satellite.

the period and orbital velocity are dependent on the mass of the orbited object and the radius of the orbit.

Re: Earth actually has two moons.

#42

Post by waqf »

joshAg wrote: post #40
white_raven wrote: post #29

Earlier quoted context omitted.

yea but saturn's rings are made of ice, moons and planets are made of rock just like asteroids not ice like comets.

what's the difference between rock and ice? is there a boundary where if the melting point is below a certain value then the substance is ice, and if it's above that value it's rock? it's not like rock can't melt in the right circumstances.

Ice is made of small molecules like water, methane or carbon dioxide.

Re: Earth actually has two moons.

#43

Post by keenerd »

Retric wrote: post #38
keenerd wrote: post #35

Earlier quoted context omitted.

No. If the Earth's mass were to vanish, the Moon would continue to orbit the Sun. It would be a bit more wobbly, but the Earth is not an essential part of the system. If the same happened to Jupiter, all of those moons would fly out of of the solar system. (Or at least go cometary.) None of the orbits would survive.

That has to do with the distance between the sun and Jupiter not just Jupiter's mass. http://en.wikipedia.org/wiki/Deimos_%28moon%29 Deimos would continue to orbit the sun in much the same way that the moon would if it's planet disappeared. (Moon: 1.022 km/s, Deimos: 1.3km/s, Callisto's 8.204 km/s. Earth 29.78 km/s, Mars 24.077 km/s, and Jupiter 13.07 km/s) Edit: http://en.wikipedia.org/wiki/Escape_velocity To escape…

Your Deimos (and Moon) velocities are off, as those are velocities relative to Mars and Earth comparatively. The real question is, does 24.1km/s +- 1.3km/s keep Deimos in orbit without Mars. 5% change in velocity will probably knock it a bit out of whack, but I am not doing the perigee/apogee calculations right now.

PS, The Earth was the original example of 'flat' and nothing is ever allowed to change as we gain better understanding.

Re: Earth actually has two moons.

#44

Post by Retric »

keenerd wrote: post #43
Retric wrote: post #38

Earlier quoted context omitted.

That has to do with the distance between the sun and Jupiter not just Jupiter's mass. http://en.wikipedia.org/wiki/Deimos_%28moon%29 Deimos would continue to orbit the sun in much the same way that the moon would if it's planet disappeared. (Moon: 1.022 km/s, Deimos: 1.3km/s, Callisto's 8.204 km/s. Earth 29.78 km/s, Mars 24.077 km/s, and Jupiter 13.07 km/s) Edit: http://en.wikipedia.org/wiki/Escape_velocity To escape…

Your Deimos (and Moon) velocities are off, as those are velocities relative to Mars and Earth comparatively. The real question is, does 24.1km/s +- 1.3km/s keep Deimos in orbit without Mars. 5% change in velocity will probably knock it a bit out of whack, but I am not doing the perigee/apogee calculations right now. PS, The Earth was the original example of 'flat' and nothing is ever allowed to change as we gain bett…

Everything's orbital velocity was taken relative to what they orbit. My point is if you take Jupiter and +/- the moon's orbital velocity you get a more extreme orbits than you do with Mars +/- Deimos. So the new orbits say just as much about how far they are from the sun vs what they orbit.

PS: The Earth is not the original definition of flat.

Re: Earth actually has two moons.

#45

Post by keenerd »

joshAg wrote: post #37
keenerd wrote: post #35

Earlier quoted context omitted.

No. If the Earth's mass were to vanish, the Moon would continue to orbit the Sun. It would be a bit more wobbly, but the Earth is not an essential part of the system. If the same happened to Jupiter, all of those moons would fly out of of the solar system. (Or at least go cometary.) None of the orbits would survive.

If Jupiter were to orbit the sun at the same distance of the earth, what would happen to the orbits of the moons if its mass suddenly disappeared? Would they still be cometary (at the least) or would they settle into a planetary orbit?

At 1AU, orbital velocity is 42.3 km/s. Io does 17.3 km/s relative to Jupiter. Io would be gone. Calliso does 8.2 km/s and I highly suspect that would be too fast to maintain a planetary orbit at 1AU.

The orbital velocity of Earth is the escape velocity of the solar system, from Earth (42.1 km/s). Fun fact, this means that if we ever wanted to dispose of nuclear waste by dumping it in a star, we'd need much less rocket fuel to hit Alpha Centauri than the Sun.

Re: Earth actually has two moons.

#47

Post by joshAg »

keenerd wrote: post #45
joshAg wrote: post #37

Earlier quoted context omitted.

If Jupiter were to orbit the sun at the same distance of the earth, what would happen to the orbits of the moons if its mass suddenly disappeared? Would they still be cometary (at the least) or would they settle into a planetary orbit?

At 1AU, orbital velocity is 42.3 km/s. Io does 17.3 km/s relative to Jupiter. Io would be gone. Calliso does 8.2 km/s and I highly suspect that would be too fast to maintain a planetary orbit at 1AU. The orbital velocity of Earth is the escape velocity of the solar system, from Earth (42.1 km/s). Fun fact, this means that if we ever wanted to dispose of nuclear waste by dumping it in a star, we'd need much less rocke…

Jupiter has an orbital velocity of only 13.7 km/s, so even if Io is going the right direction and all the elliptical planes are parallel, isn't that still just a max orbital velocity of ~40 km/s, which is slow enough for orbit at 1AU?

It'd be interesting to see what happens at Jupiter's orbit, thouh. Does Io always leave? What about Callisto? It seems like there would be a large part of their orbit that would send them off into cometary orbits (or worse) depending on what direction they are going when scotty beams jupiter aboard the enterprise.

Re: Earth actually has two moons.

#49

Post by philh »

Retric wrote: post #38
keenerd wrote: post #35

Earlier quoted context omitted.

No. If the Earth's mass were to vanish, the Moon would continue to orbit the Sun. It would be a bit more wobbly, but the Earth is not an essential part of the system. If the same happened to Jupiter, all of those moons would fly out of of the solar system. (Or at least go cometary.) None of the orbits would survive.

That has to do with the distance between the sun and Jupiter not just Jupiter's mass. http://en.wikipedia.org/wiki/Deimos_%28moon%29 Deimos would continue to orbit the sun in much the same way that the moon would if it's planet disappeared. (Moon: 1.022 km/s, Deimos: 1.3km/s, Callisto's 8.204 km/s. Earth 29.78 km/s, Mars 24.077 km/s, and Jupiter 13.07 km/s) Edit: http://en.wikipedia.org/wiki/Escape_velocity To escape…

The original example of 'computer' was someone paid to do arithmetic.

Re: Earth actually has two moons.

#50

Post by maaku »

white_raven wrote: post #29
maaku wrote: post #27

Earlier quoted context omitted.

I think he's referring to Saturn's rings, which is largly composed of objects like the “moon” in this article.

yea but saturn's rings are made of ice, moons and planets are made of rock just like asteroids not ice like comets.

Saturn's rings are made of the exact same stuff as its moons, because that's what they are. One or more of Saturn's moons broke up millions of years ago under gravitational/tidal forces, and over time the debree spread out into a ring.

And BTW, there are plenty of examples of moons made of ice, and at the temperatures of the outer solar system ice can be harder than rock (and BTW, Neptune and Uranus are called "ice giants"--care to guess why?).

Post reply on HN