In a previous article I investigated what would happen if the Earth stopped turning entirely, either locking to the Sun or the background stars.
If it happened quickly, then results would be catastrophic, turning the whole planet into a blended slurry of mountains, oceans and trees, hurting past a hundreds of kilometers per hour. And if it happened slowly, it would still be unpleasant, as we stopped having a proper day/night cycle. But it wouldn’t be immediately lethal.
But would happen if the Earth somehow just stopped in its tracks as it was orbiting the Sun, as if it ran into an invisible wall? As with the Earth turning question, it’s completely and totally impossible; it’s not going to happen. And with the unspun Earth, it would be totally devastating and super interesting to imagine.
Before we begin to imagine the horrifying consequences of a total loss of orbital velocity, let’s examine the physics involved.
The Earth is traveling around the Sun with an orbital velocity of 30 kilometers per second. This is exactly the speed it needs to be going to counteract the force of gravity from the Sun pulling it inward. If the Sun were to suddenly disappear, Earth would travel in a perfectly straight line at 30 km/s. This is how orbits work.
If the Earth’s orbital velocity sped up, then it would go into a higher orbit to compensate. And if the Earth’s orbital velocity slowed down, then it fall into a lower orbit to compensate. And if the Earth’s orbital velocity was slowed all the way down to zero? Now we’re cooking, literally.
First, let’s imagine what would happen if the Earth just suddenly stopped.
As I mentioned above, the Earth’s orbital velocity is 30 km/s, which means that if it suddenly stopped, everything on it would still have 30 km/s worth of inertia. The escape velocity of the Earth is about 11 km/s.
In other words, anything on the Earth’s leading side would fly off into space, continuing along the Earth’s orbital path around the Sun. Anything on the trailing side would be pulverized against the Earth. It would be a horrible, gooey mess.
But even if the Earth slowed gently to a stop, it would still be a horrible mess. Without the outward centripetal force to counteract the inward pull of gravity, the Earth would begin falling towards the Sun.
How long would it take? My integral calculus is a little rusty, so I’ll draw upon the calculations of Dave Rothstein from Cornell’s Ask an Astronomer. According to Dr. Rothstein, the whole journey would take about 65 days. It would take 41 days to cross the orbit of Venus, and on day 57, we’d cross the orbit of Mercury.
As they days went by, the Earth would get hotter and hotter as it got closer to the Sun. Aatish Bhatia over at WIRED did some further calculations to figure out the temperature. A month into the freefall, and the average temperature on Earth would have risen to 50 degrees C. 50 days in and we’d be about 125 C. On the final day, we’d get up to 3,000 C… and then, that would be that.
Of course, this is completely and totally impossible. There’s no force that could just stop the Earth in its tracks like that. There is, however, a plausible scenario that might drag the Earth into the Sun.
In the far future, the Sun will turn into a red giant and expand outward, engulfing the orbits of Mercury and Venus. There’s still an argument among astronomers on whether it’s going to gobble up Earth as well.
Let’s say it does. In that case, the Earth will be inside the atmosphere of the Sun, and experience a friction from the solar material as it orbits around, and spiral inward. Of course, at this point you’re orbiting inside the Sun, so falling into the Sun already happened.
There you go. If the Earth happened to stop dead in its orbit, it would take about 65 days to plunge down into the Sun, disappearing in a puff of plasma.
“If the Earth’s orbital velocity sped up, then it would go into a higher orbit to compensate. And if the Earth’s orbital velocity slowed down, then it fall into a lower orbit to compensate.”
Is this right? The inner planets have a faster orbital velocity than earth and the outer planets slower. So it seems like it should be the opposite.
On the other hand, unless the change in speed were carefully coordinated, the orbit would probably become elliptical. I think, speeding up the earth in the same direction that it is currently moving would put it in an elliptical orbit which has a perihelion equal to the current distance from the sun to the earth and a higher aphelion. Can anybody say if this is right or wrong?
I detect a terminology conflict! 😳
Fraser has the concept right, but when he says “slowing down its orbital velocity,” what he really means is “lowering its specific orbital energy.” To put it another way, he’s just saying that “hitting the brakes weirdly makes objects go faster, but in lower orbits.”
Star Wars, et al., have given people the idea that maneuvering spacecraft should move similarly in space to (e.g.) WWII-era aircraft. Our intuition thus suggests that a docking Soyuz should point its nose at the ISS & hit the gas, but the resulting balance between forward speed & distance fallen (gravity) would be a higher, slower orbit pushing it further from the ISS with time — not good, unless you happen to have time to wait for the ISS to “lap” the Earth & come back around to you. Instead the Soyuz will “hit the brakes,” reducing its specific orbital energy, lowering its altitude (intuitive) while gaining orbital velocity (NOT intuitive) which allows the Soyuz to make up the distance with the ISS. Once they’ve made up enough ground they’ll “hit the gas,” pushing themselves back up to the ISS’s orbit so they can perform the docking.
RULE OF THUMB: more energy = higher, slower orbit; less energy = lower, faster orbit.
Just to address the last bit of your question, YES, adjustments made at particular times & in particular ways can have the effect of either “circularizing” or “de-circularizing” one’s orbit (briefly shudders at the assault done by rocket scientists upon the English language), but I understood Frazer to be speaking in generalities. Generally speaking then, a loss in energy means the same type of orbit as before, just lower & faster, and vice versa. Not always, to be sure, but mostly.
Keyword: Imagination. In this case another catastrophic end for mankind. Frazier seems to be kind of hung up on this idea? That the universe is out to get us! Worst case scenarios like this cater to fear. Fear is what polititians use to motivate. Are you trying to get elected?
Please explain why the Earth would stop orbiting the sun. I don’t get it…
Here’s a simulation. On October 10, 2017, Earth will suddenly lose all its orbital speed. The Moon keeps going, but Earth does not.
http://orbitsimulator.com/gravitySimulatorCloud/simulations/1468113656063_noV.html
85 days. No integral calculus necessary. Earth is suddenly at aphelion of its new orbit. Perihelion IS the Sun. So semi-major axis = 0.5 AU. Period of orbit = 2pi*sqrt(sma^3/(GM)). Earth’s fall is half of that orbit. t=pi*sqrt((0.5AU)^3/(GM)) = 85 days
ouch, does this means we won’t see any shadows on groundhog day?