# What will happen to Earth when hit by a Neutron Star at the Speed of Light?

by Zdenka
Tags: earth, happen, light, neutron, speed, star
 P: 35 Here's one for everyone to ponder.. Firstly, this question has always bugged me since I was born. It's not impossible to imagine an Alien civilization 1.5 billion years more advanced than us, using the method of 'Hurling Neutron Stars at 99% the speed of light, to destroy planets and their inhabitants. My question is: If a Neutron Star 20 miles wide, traveling at .99c smashes into the Earth squarely, would the Earth 1. Shatter into a million pieces? or.. 2. Nothing would happen?. Like putting a pin through a large clay ball, the Neutron Star is too small to damage Earth. Since it's traveling so fast, it would just cause a very small tunnel through the Earth, but the soil (ie Earth) and water would seal it up quickly. Please share me your thoughts on what'll happen.. or you might have some other scenarios.
 Emeritus Sci Advisor PF Gold P: 2,351 At a diameter of 20 miles, a neutron star would mass about 10 times that of the Sun. At this mass, more damage would be done by its intense gravity than by the collision of the star itself. Earth would be in its Roche limit when it was still some 11,000,000 miles away. At 6378 km (the radius of the Earth) its force of gravity would be 1.7 million g's Large amounts of the Earth would be absorbed by the neutron star, adding to its mass, and the rest would be strewn by tidal forces.
P: 15,325
 Quote by Janus Earth would be in its Roche limit when it was still some 11,000,000 miles away.
True, although at .99c, the neutron star would cross that distance and be through Earth in less than a minute.

P: 2,194

## What will happen to Earth when hit by a Neutron Star at the Speed of Light?

 Quote by Zdenka Firstly, this question has always bugged me since I was born.
Really? You came out of the womb wondering what would happen if a neutron star collided with the earth at close to the speed of light?
P: 15,325
 Quote by Nabeshin Really? You came out of the womb wondering what would happen if a neutron star collided with the earth at close to the speed of light?
No, he was born at 6 years old.
 P: 35 hahaha! I love the responses, humans! Especially from Janus... I've never thought of Earth getting torn apart even before the Neutron Star has reached it. Well then, I guess we're all doomed if an Alien race decides to take up on my offer, lol :) Btw, I am not 6 yrs old! :)
P: 35
 Quote by DaveC426913 True, although at .99c, the neutron star would cross that distance and be through Earth in less than a minute.
Upon second thoughts, I think you're right. Janus, you talk about Roche limit, but I remember reading about black holes (which produces as much gravity as Neutron stars), and that you'd need to be at the Event horizon to be affected by it's gravity.
[/quote]This is incorrect. A black hole's gravity reaches far beyond its event horizon. The event horizon just marks where the escape velocity equqls the speed of light. [quote]

The event horizon is only 20miles away from the core. What I'm trying to say is that I doubt that a Neutron Stars gravity would be able to cause damage to earth at 11,000,000 miles away!

At the Roche limit, anything not nailed down will start to float away (including the atmosphere and oceans)

When the neutron star reaches the distance of the Moon's orbit, the tidal force across the Earth will equal 30 g. Even things nailed down will start to rip loose.

At the distance of geo-sync orbit the tidal forces across the Earth reach over 27,000 g.

At 2000 km, the tidal forces over a 1 meter distance is 34 g. (for a 5'8" person, this would be the equivalent of hanging from your heels with 58 times your weight hanging from your head.)

etc.
Mentor
P: 15,403
 Quote by Zdenka but I remember reading about black holes (which produces as much gravity as Neutron stars), and that you'd need to be at the Event horizon to be affected by it's gravity.
That's not correct. At large distances, a black hole has the same gravity as a star of equal mass.
 P: 20 The kenetic energy of the impact would be on the large side. Edited to add, actualy thinking on it it would probibly be sufficient to ignite oxygen in nuclear combustion. The energy of the impact into the crust would travel round the world as a mega earthquake and perhaps have sufficient energy to melt much of it. As it entered the core there would be a tussle between the bow shock wave and gravity, what would win is beyond me but either the earth would be sucked into the star or the pressure wave of the impact would tear it appart.
P: 2,194
 Quote by dorlomin The kenetic energy of the impact would be on the large side.
Hrm. For m=10 solar masses and v=.99c, I get the kinetic energy to be ~10^48J.

Wow.
P: 15,325
 Quote by Nabeshin Hrm. For m=10 solar masses and v=.99c, I get the kinetic energy to be ~10^48J. Wow.
The kinetic energy is the least of your problems. The material of the Earth will rend itself into its component atoms, which would then plate themselves onto the surface of the neutron star in a layer a few microns thick.
P: 15,325
 Quote by Zdenka Btw, I am not 6 yrs old! :)
No. You were born at 6 years old, which is why you were capable of being bugged about neutron stars "since you were born". Come to the meetings!
P: 2,194
 Quote by DaveC426913 The kinetic energy is the least of your problems. The material of the Earth will rend itself into its component atoms, which would then plate themselves onto the surface of the neutron star in a layer a few microns thick.
Indeed. Someone just mentioned kinetic energy so I figured I'd give a ballpark.

The plating of atoms is a pretty cool concept, too. And the neutron star would continue to mysteriously hurl through space at .99c unabated... only a few microns larger.
 P: 35 Such interesting responses, guys! thanks! I never thought of all the gravity involved would affect the earth since the Neutron Star is traveling at the speed of light, and would arrive at earth before the 'gravity effect' kicks in. At most I thought the N.Star would just be like a heavier version of the Asteroid that killed off the dinosaur 64 million years ago.. So lets assume the gravity doesn't destroy the earth, so would then the Earth be split apart by the Neutron star or would it absorb the impact like an Asteroid.
P: 35
 Quote by DaveC426913 No. You were born at 6 years old, which is why you were capable of being bugged about neutron stars "since you were born". Come to the meetings!
hahaha! I get ya... I was actually born 12 years overdue in the womb, which gave me plenty of time to wonder about Alien mysteries. ;-p
 P: 30 Maybe the star would travel straight through the earth taking it with it due to its large gravity.
 P: 35 My calculations show that the Neutron Star will simply leave a 20 mile diameter hole in the Earth, but our planet will remain intact, even though it's constantly spewing out volcanic lava due to the damage. The 'super' gravity WON'T gobble up Earth because the Neutron Star is traveling way too fast (99% speed of light) to have enough time for the gravity effects.
P: 69
 Quote by Zdenka My calculations show that the Neutron Star will simply leave a 20 mile diameter hole in the Earth, but our planet will remain intact, even though it's constantly spewing out volcanic lava due to the damage. The 'super' gravity WON'T gobble up Earth because the Neutron Star is traveling way too fast (99% speed of light) to have enough time for the gravity effects.

I tend to agree. Gravity reacts at 'C'. If the neutron star has a velocity of .99C then it is almost 'on top of us' before we run into gravitational problems.
If the Neutron Star entered at the equator the transit time would be something in the area of 1.21 X 10ˉ5 s through the earth.
At that speed, even the rotational velocity of the earth I don't think would have much effect on the dimensions of the tunnel.

 Related Discussions Introductory Physics Homework 3 General Physics 15