How Fast Do Galaxies Circle Each Other?

How fast are both galaxies orbiting each other? Is there a NASA reference?
NGC 5256 is a pair of galaxies in its final stage of merging. It was previously observed by Hubble as part of a collection of 59 images of merging galaxies, released on Hubble’s 18th anniversary on 24 April 2008. The new data make the gas and dust being whirled around inside and outside the galaxy more visible than ever before. This image is composed of data gathered with the Advanced Camera for Surveys and the Wide-Field Camera 3. Image credit: ESA/Hubble, NASA

NGC 5256 is a pair of galaxies in its final stage of merging. It was previously observed by Hubble as part of a collection of 59 images of merging galaxies, released on Hubble’s 18th anniversary on 24 April 2008. The new data make the gas and dust being whirled around inside and outside the galaxy more visible than ever before. This image is composed of data gathered with the Advanced Camera for Surveys and the Wide-Field Camera 3. Image credit: ESA/Hubble, NASA

Originally posted on Forbes!

The Milky Way and Andromeda are plunging towards each other, aimed nearly directly at each other, and proceeding at a pace of about 110 kilometers every second. Given the enormous distance between our two galaxies, these two will still take some three billion years to bridge the space between them, even though 110 kilometers every second would get you from New York to Tokyo in about a minute and a half.  At that speed, you could travel to the Moon in about an hour, and jet between the Earth and Pluto in about two years. By comparison, New Horizons’  journey took nearly 10 years to cross that same distance.

This is not a particularly rapid pace for a collision between galaxies - most studies of interactions between two galaxies (at least in the relatively nearby universe) choose galaxies which are moving at less than 300 kilometers per second, relative to their companion. Even then, the typical encounters happen at a relatively slow pace, slightly less than 100 km/s. If you’re interested in collisions, the slower the speed the better.

Why is that? Similar to why ‘Oumuamua didn’t hit the Sun after travelling for so long, if galaxies pass by each other at very high speeds, they don’t spend very long influencing each other. If you go to clasp hands with a friend, when you’re both walking at reasonable, low speeds, you’ll find it easy to grab onto each other and keep your hands clasped for a little while. If you imagine trying to grab onto a hand extended from a car (don’t do this), the length of time that your hands could possibly be in contact with each other is so short that at best you’re looking for a high-energy high five instead of a handshake.

Similarly, the longer two galaxies spend near each other, which they do when they’re moving slowly, there’s much more time for the two galaxies to distort each other into fantastical shapes, and the slower they go, the less energy they have in order to escape the gravitational clutches of the other galaxy. If the two galaxies are moving slowly enough, then they will sink together and scramble themselves into a single, messier, larger galaxy, keeping all the stars that had made them up before their crash together. This is the future for the Milky Way and Andromeda - while the Sun will remain in orbit around the new center of our newly enlarged galaxy, the skies will be dramatically changed.

Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the center of the other, until ultimately, the two merge into one massive galaxy. NGC 2623 is in the late stages of the merging process, with the centers of the original galaxy pair now merged into one nucleus, but stretching out from the center are two tidal tails of young stars, a strong indicator that a merger has taken place. Image credit: NASA, ESA and A. Evans (Stony Brook University, New York, University of Virginia & National Radio Astronomy Observatory, Charlottesville, USA)

Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the center of the other, until ultimately, the two merge into one massive galaxy. NGC 2623 is in the late stages of the merging process, with the centers of the original galaxy pair now merged into one nucleus, but stretching out from the center are two tidal tails of young stars, a strong indicator that a merger has taken place. Image credit: NASA, ESA and A. Evans (Stony Brook University, New York, University of Virginia & National Radio Astronomy Observatory, Charlottesville, USA)

There are lots of places in the universe where galaxies can orbit at much much faster speeds. We don’t expect them to collide in the same spectacular fashion as the Milky Way and the Andromeda galaxy do, because they’re moving so much faster. In galaxy clusters, which are home to hundreds or thousands of galaxies, the relative speeds between any two galaxies can be much, much faster - up to thousands of kilometers per second. At a thousand kilometers per second you’d reach Tokyo from New York in ten seconds flat, take six and a half minutes to get to the Moon, and make it to Pluto in a little under three months. At that speed, even if the galaxies come near each other, they're the equivalent of trying to grab your friend's hand from the window of a high speed train - over very quickly. Only a direct hit between the disks of two galaxies would cause these same kinds of streamers to appear we see from the slower collisions. Given the amount of space between galaxies, even in the relatively dense regions of richly populated clusters, that almost never happens.

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How Much Closer To The Sun Does The Moon Travel?

Does the Moon get closer to the Sun than the Earth?
Photographed by an Expedition 28 crew member onboard the International Space Station, this image shows the moon at center, with the limb of Earth near the bottom transitioning into the orange-colored troposphere, the lowest and most dense portion of the Earth's atmosphere. Image credit: NASA

Photographed by an Expedition 28 crew member onboard the International Space Station, this image shows the moon at center, with the limb of Earth near the bottom transitioning into the orange-colored troposphere, the lowest and most dense portion of the Earth's atmosphere. Image credit: NASA

Originally posted on Forbes!

 

In order to tackle this question, we have to understand a bit about the geometry of the solar system, and how both the planets and the moons which orbit those planets behave.

Most of the planets in the solar system circle the Sun in a very thin plane, meaning that if you drew the orbits out on a sheet of paper, you wouldn’t be missing out on any hidden geometry of our solar system. With the exception of Pluto, there’s very little vertical motion in the solar system that would be obscured by drawing it out on flat paper.

The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.Image: NASA. Orbital lines: wikimedia user ZYjacklin. Public domain.

The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.Image: NASA. Orbital lines: wikimedia user ZYjacklin. Public domain.

Moons are under no particular obligation to follow this pattern, and it’s often thought that if a moon of a particular planet is doing something particularly odd with its orbit, we can use that information to guess that it might have arrived at that planet in an unusual way, rather than forming around that planet. Neptune’s moon Triton is a good example; not only is it angled quite sharply with regards to the plane of the solar system, but it also goes “backwards” - it orbits in the opposite direction of Neptune’s rotation. These have been taken as hints that Triton didn’t form around Neptune, but formed elsewhere, and got trapped around Neptune after being jostled too near to Neptune's gravitational well.

If the Moon happened to orbit in a circle the way a hula hoop rolled on its edge moves, forever tumbling along the direction of the Earth’s travel, then the Moon would never get any closer to the Sun at any point in its orbit. These sorts of orbits aren’t impossible, though in our solar system, they're non-standard.

Our Moon's orbit is, in fact, quite close to perfectly flat with respect to the direction that the Earth travels. It’s tilted by only five degrees relative to Earth’s orbit around the sun. If your arms, like mine, are about six feet from fingertip to fingertip, five degrees is about 3 inches away from horizontal. If you hold both arms out sideways, point one index finger up, and one index finger down, the tips of your fingers are about five degrees offset from the line drawn by your arms.

Earth–Moon system (schematic). Image credit: NASA, arrangement by wikimedia user brews_ohare. Public domain.

Earth–Moon system (schematic). Image credit: NASA, arrangement by wikimedia user brews_ohare. Public domain.

Five degrees of an offset means that the distance between the Moon and the Sun will vary almost exactly by the distance between the Earth and the Moon. Everything is moving in the same plane, so drawing it out on a sheet of paper won't be ignoring much geometry. The Moon’s orbit is also quite close to circular, which again helps with this - there’s no long, comet-like orbit for our Moon, which is why we see it as very nearly the same size in our skies. So with all that behind us, how close could the Moon get?

The Moon orbits the Earth at a distance of about 238,900 miles from our home planet. The Earth, in its turn, orbits the Sun once every year (by definition), at a distance of about 93 million miles from the Sun. Because we know the distance between the Sun and the Earth, and the distance to the Moon from the Earth, if we line everything up just right, then we can place the Moon directly in between the Earth and the Sun. We know this situation happens - this is how we get solar eclipses, when the Moon lines up exactly between the Earth and the Sun.

Annular eclipse. Taken from a 8" Reflector with a solar filter. Image credit: wikimedia user Smrgeog, CC BY SA 3.0

Annular eclipse. Taken from a 8" Reflector with a solar filter. Image credit: wikimedia user Smrgeog, CC BY SA 3.0

This configuration subtracts 238,800 miles off of the 93 million miles which separate the Earth from the Sun. So in the end, even though we have a pretty ideal setup, the Moon can’t ever get that much closer to the Sun. At best, the Moon manages to get a grand total of 0.25% closer to the Sun than the Earth.

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