The solar system is indeed pretty much a flat sheet, with the major planets all orbiting in a very thin plane surrounding the Sun. Part of the reason we don’t tend to send spacecraft in the 'up' direction, out of this thin plane, is simply that there’s not very much...
The Earth rotates around its own axis once every twenty-four hours. The Moon, on the other hand, rotates once around its own axis every 28 days, and once around the Earth in that same 28 days. The end result of this combination is that the same side of the Moon is always facing the Earth. As the Moon moves to be directly above a different portion of the earth, its face also turns at exactly the same rate, so that only one hemisphere of the Moon is ever visible from our home here.
If the Moon turned at any other rate (either faster or slower), we would eventually see all sides of the Moon, and what is currently the lunar far side would be a much more familiar sight to us. If we spun up the Moon to one rotation every 24 hours, how dramatic would this be?
I’m assuming that we’re not changing the Moon’s orbit here - so that the Moon would still orbit the Earth once every 28 days. This means that the rising and setting of the moon would happen in the same way as they do now - slightly later every day, and the phases of the moon would remain the same, because the phases are simply the combination of the Moon’s location in its orbit around the Earth, and what fraction of the near side of the Moon is illuminated by the Sun. So we would still have a new moon and a full moon about once per month. What would certainly change is which portion of the moon is illuminated.
Speeding up the Moon’s rotation so that it spins once every 24 hours is a pretty dramatic change. That means the Moon has to rotate the full 360 degrees of a circle in 24 hours, which puts us at 15 degrees of rotation every hour. That may not sound like a lot, but over the course of an evening, which we’ll say is an average of 12 hours (half of our Earth’s 24), that means that the Moon has rotated by 180 degrees. A full moon could rise with the familiar near side facing us, and by the time it sets, 12 hours later, we’d be looking at the unfamiliar jagged territory of the lunar highlands - what is currently the lunar far side. In a six hour period, you’d expect the Moon to rotate by 90 degrees. If you were in a half-moon phase, where only half of the Moon’s face is illuminated, you would expect that illuminated portion to change completely, twice over, every time the Moon rose above the horizon.
However, if the Moon truly did rotate once every 24 hours, the two sides would probably look much more similar to each other than they do now. Part of the intense cratering of the far side of the Moon is because it is constantly facing “outwards” towards space, and it’s an easier target for interplanetary fragments of rock to hit, than the somewhat protected, Earth-facing side. If the Moon rotated faster, these meteoroids would have a pretty even chance of hitting any face of the moon, and the cratering would probably be more evenly distributed.
It’s fun to think about how this kind of situation might have influenced our calendars - since our months are roughly based on the lunar cycle, perhaps we would have used the appearance or disappearance of certain features of the moon as a smaller unit of time. But we certainly wouldn’t have grown attached to one side of the Moon - what we see now as the near side would be just as normal to us as the far side.
Hello all! I have a very exciting announcement!
In conjunction with the lovely folks at Icon Books, I am pleased to unveil the cover for Astroquizzical: A curious journey through our cosmic family tree. This book is based on the content of Astroquizzical over the past few years, and will be coming out in the UK on March 8th, and in the US on June 12th!
It’s easier to get your head around this scenario if we start with a much simpler version of this, moving at much slower speeds. So let’s say we have a convertible, driving at 40 miles an hour, and a passenger in that car can throw a ball at 10 miles an hour. If the passenger throws the ball straight up while the car is moving, the passenger can catch that ball when it comes back down. Someone observing this scene from the side of the road would say that the ball is moving to the side along with the car, while the passenger inside the car would tell you that the ball didn’t move horizontally relative to the car. (This means that when the ball came back down, it landed in the hands of the person throwing it, instead of hitting the front or back of the car.) Both statements are perfectly correct, and both the side of the road watcher and the passenger in the car would tell you that the ball flew upwards at 10 miles an hour, and then back down.
This kind of thought experiment illustrates an important point in physics - motion in the horizontal direction and motion in the vertical direction can be treated completely separately from each other. If I’m just interested in describing the motion of the ball up and down, that can be done regardless of what the horizontal motion of that ball is doing - any horizontal motion simply takes a vertical path up and down and stretches it out sideways.
Once we speed things up to fractions of the speed of light, the concepts of special relativity begin to apply, but this basic division of motion remains constant. Two things do change, though, and the first is that we need to be much more careful with where our watchers are when we describe what it is that they see. The other change is that the conversion from what the passenger in the convertible sees to what the person on the side of the road sees is more complex than simply remembering to add in the motion of the car.
If our shuttle exits the larger spaceship, and has two rockets on it, one on each end, so that it can move at a speed of one-tenth the speed of light at a perfect 90 degrees, and then stop, and go in reverse back to the spaceship, we have effectively replicated our slower situation. A person on the spaceship would say that the shuttle isn’t moving at all horizontally (in the direction the spaceship is traveling) but is bouncing outwards and back without shifting along the spaceship from its berth. Because it’s moving at 90 degrees to the direction the spaceship is traveling, there isn’t anything that would prevent the shuttle from coming straight back to its dock on the main spaceship.
An observer on a nearby planet would see the spaceship passing by at four tenths the speed of light, and they would see a shuttle leave the main craft, appear to drift outward at an angle, and then drift back inwards, meeting back up with the main craft. That planet based observer would measure the speed of the shuttle to be higher than 0.1c, because it’s got a horizontal speed that the observer on the spaceship doesn’t see, along with its motion at a right angle to the spaceship.
But what if the shuttle doesn’t go out at a right angle to the spaceship’s direction of travel? In that case, what the watching people see on the spaceship and on the planet they’re zipping past might differ a little more. If our shuttle can reverse directions instantaneously, going from 0.1c “forwards” with the spaceship to 0.1c “backward” right away, then someone on the spaceship would simply see the shuttle moving at 0.1c relative to the ship, which is stationary under their feet. As long as the shuttle is always moving at a fixed speed, it should be able to reverse course and catch back up to its dock.
The planetary observer, meanwhile, would observe a whole host of things changing. (And if we wanted to make things super complicated, we could look at how much time each set of observers is dealing with.) Velocities don’t add in the same way when you’re moving at significant fractions of the speed of light, and so the shuttle would appear to be moving at different speeds relative to the planet, depending on whether the shuttle was moving against the flow of the spaceship (0.29c) or along with it (0.48c).
The shuttle could wind up in a situation where it couldn’t reach the spaceship again if it could slow itself down enough. In the above scenario, I’ve assumed that that shuttle is always moving at a fixed speed, but if the shuttle could change its speed so that instead of being stationary relative to the spaceship, it were stationary relative to the planet, it would not be able to accelerate back up to the speed of the spaceship. Then it would be stuck, lagging ever further behind the spaceship it came from.
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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.
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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