What would it feel like on the surface of a planet while it collides with another planet?

Question

Let's say that something horrible has happened and a Mars-sized planet is knocked out of orbit and is hurtling towards an Earth-sized planet. How much time will they have? How will this affect the inhabited planet?

Obviously the impact itself means the end of life on that world, but this is happening on an astronomical time scale and they'll have years before the actual impact.

My base question is this: What will the target planet (Especially its inhabitants) experience as the other planet draws nearer?

Assumptions:

• The impacting planet orbited the star at roughly the same speed as the other planet
• Something (asteroid strike?) has caused the orbit of the impacting planet to degrade such that it is spiraling in towards the star
• The angle of impact will be acute, as one planet effectively "merges" into the other planet's spot
• Speed of impact will be slow (I don't know what qualifies as "slow" in astronomical terms, feel free to extrapolate as needed)

Answer 1

Virtually everything in Mindwin's answer is wrong.

The slowest possible approach of the rogue to the earth would occur with a Hohmann transfer orbit, and in this case orbital energies dictate a closing speed of about 3 km/sec. However, this ignores the gravitational attraction between the earth and the rogue, which will boost this closing speed to about 9.5 km/sec. Time to contact from 10 times the moon's orbit is about 14 days. At this distance, the area of the rogue's disk is about 1/50 that of the moon. Time to impact from crossing the moon's orbit is about 27 hours. A this point tides are about 8 times greater than normal. So, no massive tsunamis until a few hours before impact, and this will affect only small part of the earth.

Furthermore, since the orbit is essentially tangent to the earth's orbit, it will appear in the sky at 90 degrees from the sun, directly overhead at dusk, and will present a "half-moon" appearance.

Since the earth's atmosphere is about 30 km deep, the rogue will not appreciably affect the earth's atmosphere until less than 5 seconds before impact. No vortex. With a relative velocity near 10 km/sec, a tangent path from sea level to 30 km is about 2,000 km, so for a near-miss the atmosphere will be affected for a duration of (at most), about 3 minutes. No hoovering. Just an enormous shock wave.

Since Mars' surface gravity is about 40% that of earth, just at contact the apparent gravity at ground zero will be reduced to about 60% of normal. No floating. And on the other side of earth things get heavier by about 4%. No crushing gravity, I'm afraid.

Centrifugal force will be irrelevant, and there will be no swirling water. A head-on collision (well, head to tail) will simply liquefy the two bodies. The collision zone will be, especially at first, expanding hypersonically away from the point of impact. The folks on the far side of the planet will not have to wait a day to feel things, as the shock wave will propagate through the planet in less than 20 minutes.

Well, OK, everybody dies.

If, somehow, the rogue is thrown into an orbit which meets the earth head-on, the closing speed will be about twice the earth's orbital velocity (plus a bit for gravitational attraction), or about 60 km/sec. This is even quicker and more spectacular. But in the end, everybody dies.

EDIT - As trichoplax has pointed out, my calculations on the apparent gravity at impact were wrong. They assumed that the earth and rogue will be stationary, but obviously they will not. Instead, each is in freefall toward the other. The center of the earth is closer to the rogue than a point on the far side of the earth, so the acceleration of the earth will be greater than a person standing on the far side. Since the earth is considered (for the moment) to be a rigid body, that same greater acceleration will apply to the surface of the earth under the person's feet. If not for the attraction of the earth for the person, the two would drift apart. If Re and Rr are the radii of the earth and rogue, and Me and Mr their masses, the net force on a person on the far side will be $$F = \frac{GM_e}{R_e ^2} + \frac{GM_r}{({R_r +2R_e}) ^2} - \frac{GM_r}{({R_r +R_e}) ^2} $$ Letting the rogue be Mars-like, $$Rr = 0.53 R_e \text{ and } M_r = 0.107 M_e $$ $$F = (\frac{GM_e}{R_e ^2})(1 +\frac{.107}{2.53^2} - \frac{.107}{1.53^2}) = 0.97\times \frac{GM_e}{R_e ^2} $$ so not only will people not be crushed by the added gravity, they will be 3% lighter. Just before they die.

Likewise, for the point of impact, surface gravity will be 27% of normal. So, still no floating. Just before they die.

Answer 2

Since the actual impact will only last minutes, on the far side it will be kinda like this as the shockwave approaches:

T-10 minutes: 20C and sunshine
T-5 minutes: 20C and sunshine
T-4 minutes: 20C and sunshine
T-3 minutes: 20C and sunshine
T-2 minutes: 20C and sunshine
T-1 minute: 20C and sunshine (is that a shadow on the horizon?)
T-0 minutes: 4,000 C and death.

Answer 3

There is a game on Steam called "Universe Sandbox²", which focuses on simulation of planets and stars.

I opened up our solar system and added another Earth with almost the same orbit and speed, so actually they would never hit each other except due to their own gravity. This is what happens:

Original setup (2017-07-18 03:31pm):

Begin of collision (2017-07-18 04:47 pm). Propably some people still live.

11 minutes later (2017-07-18 04:58 pm).

29 minutes later (2017-07-18 05:16 pm).

So after half an hour, Earth almost looks like in the early days. Due to the collision energy, the heat dramatically increased, melting everything.

Answer 4

Based on the first assumption:

• The impacting planet orbited the star at roughly the same speed as the other planet

This means that the impacting planet is in the same orbit as our inhabited planet, since speed and orbit are much the same thing.

Obviously this isn't a stable arrangement, but it's more stable than it seems on the face of it, because as an object picks up speed, its orbit gets longer, and as its orbit gets longer, the time it takes to orbit increases.

• Something (asteroid strike?) has caused the orbit of the impacting planet to degrade such that it is spiraling in towards the star

Orbits don't spiral, unless you have some constant source of energy slowing the object down. Satellites spiral down because the atmosphere produces drag. This couldn't be the case with another planet. In fact, our moon is being accelerated into a higher orbit away from Earth. Unless the other planet were well inside of the Moon's orbit, close enough to already be doing widespread damage just by interacting with our atmosphere and heating it up, there is no way for the other planet to spiral down to our inhabited planet.

• The angle of impact will be acute, as one planet effectively "merges" into the other planet's spot

The angle will actually be face-on, if our first assumption is true. I'll get to that in a bit.

• Speed of impact will be slow

Again, not if the first assumption is true.

Really it comes down to whether the uninhabited planet's orbit suddenly became very elliptical (that is, if Mars's orbit suddenly changed so that its closest point to the Sun is within Earth's normal orbit, but its furthest point is still at its original orbit), or whether we have two bodies that have been sharing the same orbit around their star and are about to meet their inevitable ends.

WhatRoughBeast has already provided an excellent answer in case it's a Hoffman Transfer orbit (including the fact that Mindwin's answer is spectacularly wrong), so for that scenario, I'll only repeat the most important detail, that the speed of impact will be 9.5 km/sec, and I only repeat it in case WhatRoughBeast's answer somehow goes missing.

I'll also add, since it's asked in the question but I haven't seen a good explanation in any other answers yet:

From the point of view of someone on the surface, but far enough over the horizon to not see the impact, your first and only warning will be an earthquake that gets stronger very quickly, over the course of a few seconds, until the ground is shaking too violently for you to stay on its surface... You will be thrown about like a ragdoll, each impact more violent than the last, until you sustain sufficient brain injury or spinal cord injury that you lose consciousness... Once the rumbling starts, you'll probably have 20 seconds of consciousness. You'll be too busy being thrown around to notice that the ground is heating up from all of the friction, and within 5 minutes, the ambient heat will be enough to cause any carbon based life forms to spontaneously combust.

From the point of view of someone who can see the impact: Things get hot very quickly. You might have a second of consciousness if you're behind a mountain.

Now, on to the meat of my answer:

If the two planets are traveling at the same speed (first assumption from the question), they'll be in the same orbit.

Orbits are tricky, though. As you gain more speed, the size of your orbit increases... The bigger your orbit, the longer it takes you to complete that orbit. Thus, if you want to slow down compared to another body in the same orbit, you pick up speed. (That is, you go further away from your star, and just as Venus orbits the Sun more often per Earth year than Mars does, you'll be orbiting your star less often.)

Now, with two rocky planets, there's a lot of gravity, which means that as they get closer, there's a lot of acceleration. They will both attract each other, and if they're both the same size, they'll attract each other equally.

The planet in front will slow down, and the one behind will speed up.

But since we're orbiting, any slowing down and speeding up will affect the size of our orbits. The planet in front will get closer to its star, and the planet chasing will get further away.

The first time this happens, someone on the surface will see an incredibly bright planet. Brighter than Venus or even the International Space Station, and you might even be able to make out its illuminated side (it'll look like a half circle). Assuming our inhabited planet is the ahead planet, the chasing planet will be visible from dusk to midnight.

The orbits of both planets then get more elongated, but only astronomers and people who keep track of time would notice at first.

After a couple of years, things are reversed... Our inhabited planet has sped away from the chasing planet, and is now doing the chasing. As it approaches the rogue planet, it becomes visible in the morning sky, from midnight to dawn (and even visible during the day, if you know where to look, until noon). Our inhabited planet gains speed, the rogue planet loses speed, they get a few million miles closer than they did before, and miss each other by a large margin once again... this time, with our inhabited planet in a larger orbit and our rogue planet in a smaller orbit zipping away.

This cycle repeats for a couple centuries (a mere blink of an eye in astronomical terms... the Earth is 4.5 billion years old; this is 1/20,000,000th the time frame... There are comets that visit the sun once every million years). Now things get interesting.

Towards the end of the cycle, there are no moons around these planets, if there ever were any. The gravity battle has pulled all satellites away, natural or artificial. The fourth to last orbit, the rogue planet gets to within 1/5th the distance of the Earth to the Moon. (For scale, imagine a typical classroom. If the Earth is the size of a basketball, the Moon is a grapefruit... and both would be in opposite corners of the room.) There would be earthquakes and volcanoes during the weeks that the two planets are closest to each other... During the month leading up to the encounter, the rogue planet is hidden by the glare of the star, but after the encounter, it dominates the night sky well past midnight. Careful observations would be able to see the rogue planet moving across the background stars during the evening of closest approach.

9 months later, things are considerably closer, earthquakes are larger, and the timing is reversed... The month leading up to the encounter, the rogue hangs heavy in the night sky, and after the encounter disappears into the glare of the star.

9 months later, greater earthquakes damage every standing building, destroying most... the planet passes within 20,000 miles of ours, appearing out of the sun's glare, and absolutely dominating the night sky, their relative speeds are so great and both bodies are so close that you can see the other planet spinning above you.

9 months again, the rogue planet comes... The night before it passes, it starts to enter your planet's penumbra (out of focus shadow... an area experiencing a partial eclipse), then slowly creeps into the umbra (full shadow, are experiencing a total eclipse).

The two planets will collide with all of the force of a head-on collision, about 60km/s.

Anyone who can see the impact will die immediately. The atoms of their bodies will be stripped apart faster than the neurons carrying the information about what's going on could process that data. If they're close enough to "see" the flash, their brains will never experience the sensation.

There won't be a low rumble that will turn into an earthquake that will throw people against other objects to their deaths, like in a "slow" 9km/s impact... There will be one shockwave that moves at supersonic speed through the mantle of the planet, and as soon as the ground beneath your feet experiences that shockwave, the ground will be moving so fast that you'll just go splat.

Planetary collisions are nature's way of asking how that space program is coming along...

Answer 5

The Discovery Channel did a special on a large asteroid collision with the Earth. This video is a simulation of what would happen. They include some "first person" perspectives of what it would look like on the surface of the Earth.

Detailed simulation of large asteroid impact with Earth.
https://www.youtube.com/watch?v=bU1QPtOZQZU

A planet sized impact would be similar, just amplified by 3-4 orders of magnitude. Here's a lengthy discussion including simulations of what would happen:

Less detailed simulation of Theia impact with Earth.
https://www.youtube.com/watch?v=ibV4MdN5wo0

SPOILERS: (Hover to read)

Everyone dies. People die as soon as the supersonic shock wave gets to them. That might give some people hours, I'm not sure since I didn't run the numbers. To add insult to injury, enough debris would be flying around from such an impact that any humans in orbit or on the surface of the moon would also die.

Answer 6

the compression of the atmosphere would be similar to many millions of atomic bombs going off, so actually the entire planet would be enveloped in something similar to an atomic shockwave, where the cars move 100ds of meters sideways in the wind, without even thinking about the ground movement happening at the same time. The ground movement would occur prior to the wind effects, moving very fast and very far compared to an ordinary earthquake.

The air itself would become alot less clear because of all the thermal and compression effects happening within it, water would vaporise be compressed while the wind would be going at 1000kph, so you wouldnt see very much.

There is a documentary on nova with scientists discussing the effect of a neutron star invading the solar system, it's totally false because the iron of the earth would compress against the crust like a huge hypervolcano, and pop out of the breach in the crust and fly into space, long before stones would start to fly around.

It depends on the velocity of impact, typically at meteoric velocities.

The effect would be similar to an large earthquake of which the main shockwave would be transported directly through the mantle rather than around the crust. The effect would be different depending on the angle from the percussion.

The power of the shockwaves would be enough to create waves of force lifting the crust in heights in between 10 meters and 1000 kilometer, depending on what rebound of the wave you first catch, cars would literally be flung very far into the air a few seconds after the initial schockwave occured, and the intensity of the earthquake would increase until the entire surface of the earth was wabbling and cracking by in waves at least 100 kilometers tall.

The precise model of the surface and the earth's crust's interaction with the viscous and compressed inner materials of the earth would be a function of the earth crust's thickness and the size of the shock waves traversing the mantle, disassembling the crust in the first moments while the observer was alive.

Answer 7

Assuming the following:

1. You are standing at the equator
2. You weight 200 pounds
3. Mars hits the Earth and the precise opposite location of where you are standing
4. The Earth is spinning at 1,040 mph at the equator

Your knees would buckle and your own body would hit the floor of the Earth with about 200,000 pounds of pressure. You would disintegrate your own self.

If it hit in such a way that sent you flying down the street then you would collide with something at about 1,000 mph. It would probably snap your neck before you even knew what happened.

Answer 8

There's more than one possibility that could take place in this kind of scenario, the most obvious thing that would likely to happen is you getting crushed like an ant in a second, when the natural disaster occurs. And unfortunately, I don't think you'll be able live long enough to feel how it is.

Answer 9

https://youtu.be/lEIGjXbtQwY?t=76

According to this video, a planet the size of Mars did collide with Earth. 4.5 billion years ago.

They collided at a speed of 25,000 miles an hour (~70 miles/second). The entire surface would have been liquefied with molten rock. Debris flies out into space as a result and then recombines due to gravity. And the result was our Moon.

Answer 10

I will call "Mars" the rogue planet (the one knocked out of orbit) and "Earth" the planet that is about to be hit. But it is not really Mars, because OP stated a planet with the same mass as Earth.

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Disclaimer: this is not a physics essay. The time frames are relative, since OP did not state the speed of approach. If anyone wants to delve into this aspect and do some calculations, by all means do it. I tried to do some storytelling here, like a script for a catastrophe flick.

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As the Mars approaches, its first influence will be on the tides. The side of Earth nearer to mars will begin to experience huge tides. As it approaches, it will become mega-tsunami in size. This will begin to be felt when it is around 4 lunar distances from Earth (since Mars is the same size as Earth in the question's scenario, it should have gravity six times that of the moon).

It will also have an effect on the day cycle. Every day the clocks will be off by some seconds at first, and then a minute. Most people won't notice it, but weather scientists and government will be aware very soon.

As Mars passes behind Earth and in front of Sun, it will be visible in the sky at night. Amateur astronomers will find it really soon, and after a couple weeks everyone will see it. A huge red "second moon". As it gets even closer, night will be brighter and brighter (and even more red).

also, the crashing planet may be not red. But read the top of the answer again, please.

As it approaches more, it will begin to pull people. Not the "omg i am being lifted up" (not yet) type of pull, but our 9.8m/s gravity pull won't hold anymore. It will be way less on the side facing Mars, and a bit less on the other side (because there are two Earth radius from one side to the other).

When a crash is imminent (one week away, lets say), the huge tsunami waves will destroy everything near the shores (some miles inland. Kiss Holland and Japan goodbye, except for the mountains).

Days before the crash the atmosphere of both planets will merge. If the atmosphere of Mars is poisonous, most people will die. Soon the oceans will also be sucked, and begin to cross from Earth to Mars. Like Dxun from star wars:

Also, most water and gases will escape into space in a ring-shaped areaaround the mid-point of both planets, because there is not enough pull to keep fluids from reaching escape velocity.

Hours before the crash, earth, dirt and anything not affixed to the ground will be pulled too. Any surviving humans will now have the "i'm flying" feeling.

The crash will not be like a ball hitting another. At the geological size and scale we are talking about, you can treat dirt and earth rocks like fluids. The upper crust of both planets will be ripped apart by the centrifugal force and gravity, and begin to swirl together with the water from the oceans, trapped between the gravity field.

Anything living inside this swirling vortex will be crushed.

The humans on the other side will feel a heavy gravity pull, and also tectonic activity throughout the entire planet, since the magma beneath the crust's tectonic plaques will be pulled too by the tidal forces.

But it is just a matter of a day, as the earth spins around and the vortex between Earth and Mars vacuums away the Earth surface like a knife in an orange skin.

The released heat will boil the entire planet. The ice caps on both planets will melt and boil. Humanity will die entirely.