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I'm rather skeptical of this comic:

T-Rex claims an astronaut with a gun can shoot the Sun

I think a bullet's speed is insignificant to the orbital speed around the Sun, but is such possible without the astronaut falling into the Sun? Maybe with an existing rocket gun?

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    $\begingroup$ cf "Men of Good Will" by Ben Bova, where US and USSR forces get in a gun battle on the moon, then have to co-operate because the stray bullets go into low lunar orbit. isfdb.org/cgi-bin/title.cgi?540869 $\endgroup$ – Organic Marble Mar 28 '18 at 18:47
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    $\begingroup$ @RussellBorogove It's obviously possible to shoot into the sun if you're already falling into it, hence the safety requirement. $\endgroup$ – Cees Timmerman Mar 28 '18 at 19:30
  • $\begingroup$ I'm wondering if gravity assists with earth, moon, and possibly other planets could result in slowing the bullets speed with respect to the sun and altering it's orbit so that it would go into an orbit that intercepts the sun. It only take a bit more than 3 km/s delta v from a relatively low earth orbit to achieve trans-lunar injection. Shooting the bullet from the moon might be a better starting point. It could take years or decades to reach the sun, but it might be possible? $\endgroup$ – rcgldr Mar 29 '18 at 2:12
  • $\begingroup$ What counts as reaching the sun? Hitting the chromosphere? The photosphere? $\endgroup$ – JdeBP Mar 29 '18 at 21:04
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    $\begingroup$ @JdeBP That seems like an insignificant difference to me, given that the solar gravity involved if one of those is achieved would almost certainly draw in whatever remains of the bullet. $\endgroup$ – reirab Mar 29 '18 at 22:51
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The Earth's orbital speed around the sun is about 30km/s. Firing a bullet from the vicinity of Earth's frame of motion (e.g. from low Earth orbit) to hit the sun would require cancelling out most of that velocity -- about 25 times the speed of a normal rifle bullet. This might be achievable with a specialized weapon with a very long barrel and a very small bullet, but not by anything "off-the-shelf".

Since the orbital velocity to be canceled is perpendicular to the direction towards the sun, the bullet would need to be fired in the opposite direction -- "backwards" along Earth's orbit, counter-intuitively, not directly at the sun. Once out of Earth's gravitational sphere of influence, the bullet will be approximately motionless with respect to the sun, and the sun's gravity will then pull it straight downward.

Due to conservation of momentum, the astronaut firing such a gun would be accelerated in the opposite direction from the bullet; since we're postulating a very small bullet, the astronaut might mass 100,000 times as much as the bullet, so would pick up just a fraction of a meter per second velocity in the opposite direction -- a significant impulse but not a dangerous one if the astronaut is expecting it (and if the gun is designed in such a way that it can be fired aligned with her center of gravity rather than with her shoulder, which would impart spin).

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Apr 2 '18 at 18:25
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T-Rex with his tiny brain overlooks the Coriolis force. If the astronaut pointed the gun at the Sun and shot a bullet, it would miss spectacularly. The orbital motion of the Earth makes for a sideward motion of about 30km/s.

Depending on how you approach the problem of throwing stuff (radioactive waste and bullets are the same problem) in the Sun, you either need to get rid of angular momentum, momentum, or velocity (again, basically the same problem: $\Delta v$).

T-Rexen were so bad at orbital mechanics, they went extinct because of it: they didn't develop a space program and couldn't deflect that big rock coming to get them. Homo sapiens have a space program and are having a crack at the rock issue :-)

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Apr 2 '18 at 18:25
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Yes, you can shoot the sun with an ordinary rifle and, yes, it would be dangerous.

While a typical rifle's muzzle velocity is nowhere near the delta-v needed to hit the sun(Approximately 30 km/s, from low Earth orbit), that is what we have the slingshot maneuver for.

Gravity assist is how all of our space probes get anywhere already. So our enterprising dinosaur merely needs to calculate an aim for close a nearby moon or planet -- to start a slingshot trajectory that eventually hits the Sun.

Because the bullet cannot do any course corrections, and because the bullet would be much more prone to deflection by the solar wind, the aim calculations would have to be extra precise.

Any astronaut (human or dinosaur) that can both do those calculations, and devote not insignificant resources to firing the bullet(s) would be a dangerous creature indeed.

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    $\begingroup$ why would shooting the sun be dangerous? $\endgroup$ – Pixelomo Mar 29 '18 at 8:58
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    $\begingroup$ @Pixelomo, the shooting itself is not particularly dangerous. The person who could and would pull it off, is. $\endgroup$ – Brock Adams Mar 29 '18 at 9:05
  • $\begingroup$ Isn't a typical bullet much denser than a typical spacecraft, making it therefore much less affected by solar wind? $\endgroup$ – Sean Mar 29 '18 at 21:14
  • $\begingroup$ @Sean, it's not just the density. It's the mass, the time of exposure, and the drastically increased intensity as the bullet approaches the sun (much, much closer than course-correcting spacecraft that have also been designed for such effects). Think of the bullet as an extra stiff bit of comet's tail. It won't move as much or glow as brightly (until it's too late), but the effect can cause a miss if not accounted for. $\endgroup$ – Brock Adams Mar 29 '18 at 21:18
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Staying safe (i.e. not falling into the sun) requires an orbit. According to orbital speed of planets,

Uranus is the second slowest planet with an orbital speed of 6.81 km/s. This equates to 15,233 miles per hour.

Neptune travels around the sun at a speed of 5.43 km/s or 12,146 miles per hour. Although this is a very high rate of speed, Neptune still has the slowest orbital velocity of any of the planets.

As pointed out by other answers, the bullet would have to negate that relative speed. According to NASA:

Most modern rifles are limited to velocities below 2 km/s (4,500 mi/h). WSTF HVI two-stage light gas gun launchers use highly compressed hydrogen to accelerate projectiles at velocities in excess of 7.5 km/s (16,800 mi/h). These velocities simulate impacts of particles on spacecraft and satellite materials and components.

So shooting the sun with a handheld gun would work, but only far beyond Neptune, and the astronaut would have to brace against his ship to not tumble away.

For example, a .223 AR-15 with a muzzle velocity of 975 m/s would have to be fired from at most (lack of air pressure should increase muzzle velocity) (gravitational contant (m3⋅kg−1⋅s−2) 6.674e-11 times solar mass (kg) 1.98855e30 divided by 9752) 139,609,022,485.207 km from the Sun, that's 933.23 AU, 31 times as far as Neptune but within the aphelion of Sedna.

As the followup comic suggests, it would be easier to shoot other stars:

Shoot other stars!

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Yes it's possible, although difficult, and there is no risk.

When firing, the astronaut has to allow for the changes in angular momentum as the bullet nears the Sun, known as the Coriolis effect. The astronaut, gun and bullet are currently on an orbital path round the Sun, and he needs to make sure the bullet actually gets there. If he misses the Sun, he's created a small metal comet instead. This needs some serious number-crunching to get the right trajectory. It isn't a risk though - there are countless (literally; we don't know how many there are!) meteors on various trajectories around the sun, of various sizes. One bullet isn't a big deal here.

The astronaut also has to make sure that nothing else will intercept the bullet on the way there. This will require some significant calculations. If the astronaut is near the Earth, she will need to make sure it has escape velocity to leave Earth's orbit. She will also need to check that during its trip, the bullet does not go near enough to Mercury or Venus's gravity which could capture it; and she will need to check the effects of all the planets' gravity on the bullet's path. This makes the number-crunching even harder. Even if the bullet stays away from Mercury and Venus, the difficulty of setting up a trajectory to allow for the gravity of the planets will increase the chance of a miss. Still no risk though.

Then we have risk to the astronaut from the gun. The reason a gun has recoil is that the breech is sealed. This forces all the gases produced in the detonation to go out the front, pushing the projectile. This gets maximum energy to the projectile, so it goes faster. It would be perfectly possible to design a recoil-less space gun without a sealed breech - only half of the energy from the detonation would go into propelling the projectile, but that's not such a problem if the priority is stopping the astronaut being shot backwards. Essentially you'd have a small bazooka. Cartridges would need to be redesigned, of course - most likely you'd end up with miniature rockets rather than the current design of a solid projectile and propellant with a disposable base.

With a gun like this, an astronaut could safely shoot anything without the problem of recoil shooting him in a random direction. Of course he does have the problem of keeping the exhaust away from himself, but that could be solved by placing the gun on a shoulder-mount (like a bazooka) instead of being hand-held. Of course he also has a potential problem of stray projectiles entering orbit and hitting him on a future lap round the asteroid/moon/planet, but that's a separate problem.

Without a specially-designed gun, the astronaut will need to make sure she opposes the recoil. Mounting the gun at their centre of mass will ensure she gets driven backwards cleanly, without spinning; an MMU pack would then allow her to decelerate and return to where she was. It's tricky but not impossible. Even without mounting the gun at her centre of mass, the MMU would allow recovery from the spin, although it would be harder to stabilise herself again for the next shot.

And finally we have the risk to the Sun of it being hit by a bullet. The New Scientist thinks the effects of a direct meteor strike would be spectacular but they do not mention an existential risk to the Sun. Based on that, the impact of a small metal object on something the size of the Sun (mass of 1.989×10^30kg, thank you Google) is negligible.

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  • $\begingroup$ Space is huge, so it's unlikely to hit something with a small gravity well. To avoid planetary interference, orbiting at a 90 degree angle to their orbits should work. $\endgroup$ – Cees Timmerman Mar 29 '18 at 15:26
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It's worth noting that they published a follow-up comic that basically says what the other answers have covered, that it's not really possible (although it does raise the hope that you can shoot other stars, but that's another question)

enter image description here

The image has this text on it

If you're saying, hold on, I'll just build a gun that fires a smaller gun that fires a bullet to get around this, then congratulations, you have just invented multi-stage rockets.

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Yes, an astronaut could in theory shoot the Sun. I wish to answer this question in exactly the same way as several other answerers, but I believe there is a much more concise explanation that is more intuitive for readers of this site. One can invoke the "Coriolis Effect", but this is an abstract and complicated way of going about things. The same nett explation, i.e. an alternative account of what Newton's laws imply for this situation, can be made in terms of conservation of angular momentum and the idea of simply "de-orbiting a bullet that is initially in Sun orbit". In the simplest terms, we simply need, with our weapon, to impart the delta-V on the bullet that is needed to de-orbit the latter from its initially stable, almost circular Sun orbit.

After launch, the orbital angular momentum of the bullet about the Sun stays constant. One calculates the orbital angular momentum as the product of the radius to the Sun and the component of the bullet's velocity at right angles to this. The angular momentum vector points normal to the orbital plane. Whatever happens after this point, this angular velocity vector - direction and magnitude - stays constant, since the bullet undergoes no interaction that changes this vector. So the bullet cannot leave the plane of orbit, and, moreover, as it nears the Sun, its tangential speed increases so as to keep the angular momentum magnitude constant.

So it will miss the Sun unless its orbital angular momentum is very small. It doesn't have to be precisely nought, because the Sun is a nonzero size target, but for all practical purposes, the Sun is small and we must fully de-orbit the bullet so that it can fall into the Sun. And that means imparting a delta-V to pretty much fully counter the initial 30 kilometer per second tangential velocity of the Earth relative to the Sun.

So, our delta-V, roughly, is the stupendous figure of 30 kilometers per second.

Our bullet may weigh 10 or 15 grams. That makes for quite a recoil. Indeed, a 150kg astronaut (fully kitted) will undergo a delta-V of about 3 meters per second in the opposite direction from the bullet. The shooter's weapon will need to be well supported in their suit to avoid serious tissue injury from the launch.

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Yes, it is possible, but the astronaut must choose carefully its shooting point.

Drawing from Rusell Boorgrove's good answer, the orbital speed of the Earth is about 25 times faster than a bullet. Therefore, the astronaut needs to be in an slower orbit - 25 times slower. For a given eccentricity, orbital speed is inversely proportional to the square root of orbit's major axis. Therefore, assuming the same eccentricity of Earth's orbit (near circular), the astronaut can place himself in a circular orbit around the Sun with radius 625 AU, far beyond the Kuiper belt.

I haven't done the math, but the astronaut could shot from a slightly less distant point if he were in the aphelion of a very elliptical orbit.

Interestingly, the easiest way to shot things in Earth (from the surface) is to get close and shot straight to the target, but the only way to shot the Sun seems to get very far away, shot at right angle and wait some centuries for the bullet to fall into it.


Edit after comments challenging the fragment "wait some centuries for the bullet to fall into it (the Sun)."

According to the third law of Kepler, the time needed by a bullet at rest to fall into the Sun is proportional to $R^{3/2}$. Then, assuming that it's true that a bullet at rest needs 65 days to fall from 1 AU (Earth's orbit) to the Sun, falling from 625 AU would need:

$$65·625^{3/2}=1015625 days = 2780.63 years$$

Therefore, the sentence "wait some centuries" could be replaced by "wait 27.8 centuries".

Note: the 65 days data I took from comments could also be calculated from the third law of Kepler just comparing free fall with Earth's orbit: .5^(3/2)*365.25/2=64.56.

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    $\begingroup$ I doubt you could shoot buildings from Earth orbit without adjusting for your orbital speed. $\endgroup$ – Cees Timmerman Mar 29 '18 at 15:29
  • $\begingroup$ Of course you can't shot the Sun from Earth orbit. For this reason the answer proposes a large orbit around the Sun. $\endgroup$ – Pere Mar 29 '18 at 17:44
  • $\begingroup$ @CeesTimmerman And you can't shoot buildings with a gun from Earth orbit, but that is unrelated to the answer, isn't it? $\endgroup$ – Pere Mar 29 '18 at 17:50
  • $\begingroup$ 65 days should be enough for a bullet to drop from 1 AU into the sun. $\endgroup$ – Cees Timmerman Jun 26 '18 at 14:01
  • $\begingroup$ But first you need to stop the bullet. You can't stop it with a gun because the Earth is a lot faster than a gun. $\endgroup$ – Pere Jun 26 '18 at 14:52
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If by "shoot", you just mean "propel a bullet into the sun", then yes. The Voyager probes have reached escape velocity, and the amount of energy needed to counteract orbital velocity is comparable to the energy of escape velocity, so it follows that it would be possible to create a craft capable of flying into the sun, putting a bullet on it, and calling that "shooting at the sun".

If you mean "fire from a normal gun", and you only require it be done without violating the laws of physics, then that's quite clearly possible. There's nothing in physics preventing a spaceship from canceling out its orbital motion, an astronaut shooting at the sun, and the spaceship then accelerating back up to orbital velocity.

Now, if you want it fired from a normal gun, at the current level of technology, that would be much harder. There are three things we need: get rid of orbital velocity, get close enough to the sun to aim, and get back to orbital velocity. From what I can tell, sharpshooters can hit person-sized objects from a few kilometers away. It's probably easier to aim in space, so let's say that a sharpshoter could hit a 1m target from 10 km away. That's a ratio of 10k, so an astronaut should be able to hit the sun from 10k times its diameter, which is about 1 million km. So that's a distance of 10 billion km, which is about 100 times the distance from Earth to the sun. So that part looks good.

Voyager has a mass of about one ton, or about 10 times the mass of a human. So shedding orbital velocity for a human should be roughly comparable to doing so for Voyager (there would be complications such as that Voyager used gravity assists). However, we would also have to include enough fuel to then get back to Earth. According to this article, 72% of Voyager was fuel, so there was about four times as much fuel as payload. According to this article, Voyager cost 865 million dollars, so if we need four times as much fuel to get back, and the cost is proportional to fuel, that's about $3.5 billion. Quadrupling the fuel would probably increase the cost by a factor significantly less than four, and there are a lot of costs that Voyager had that this would not, but there would be a lot of costs that this would have that Voyager didn't (Voyager wasn't trying to keep someone alive). So as an order of magnitude estimate, it appears that it would cost around a billion dollars to do this. So: possible, but ridiculously expensive.

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