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Pretty much what the title is saying. I feel like I am missing something fundamental here and this is driving me crazy.

Does a spaceship which is out of Earth's gravity drift to the Sun eventually?

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    $\begingroup$ The ship is in an orbit, with a lot of speed: the speed it had from being on Earth, combined with the speed it got from burning fuel. It's not like an out of fuel boat drifting on the ocean. $\endgroup$
    – PM 2Ring
    Sep 21 '19 at 20:52
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    $\begingroup$ Think about what stops the Earth falling into the Sun. It doesn't even have engines. $\endgroup$
    – badjohn
    Sep 22 '19 at 11:20
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    $\begingroup$ Experiment to try at home: get some string, a nail, and a baseball. Hang the baseball on the string over the table, wait for it to come to rest. Place the nail directly under the baseball such that the nail will have to push the baseball aside. Do so, moving the baseball away from the nail. The nail represents the sun and the baseball your ship (we put the mass here to avoid air resistance). Now give the baseball a shove in pretty much any direction not directly towards the nail. This is also the carnival game Swinger, which is rigged by placing the bowling pin/nail off-center. $\endgroup$ Sep 22 '19 at 18:45
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    $\begingroup$ I recommend to play Kerbal Space Program. $\endgroup$
    – Firzen
    Sep 23 '19 at 13:58
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    $\begingroup$ To the OP...the comment about playing Kerbal Space Program may sound like a flippant joke, but it is probably not intended that way. Honestly, it is a fantastic tool for gaining a high level understanding of orbital mechanics, answering questions like this. If you're interested enough to ask questions like this, you may find it fascinating to play. (No, I do not have any financial interest in the company that makes it, or any other unstated reason to recommend it.) $\endgroup$
    – Beska
    Sep 24 '19 at 15:36

10 Answers 10

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What you're missing is some combination of the following:

  • objects launched from Earth orbit are still in orbit around the Sun,
  • objects in orbit don't need fuel to stay in orbit.

All the planets stay in orbit around the sun because orbits are generally long term stable.

A spacecraft flying from Earth to Mars follows a loop, like in the picture below.

enter image description here

Source

Most of that time, the engines aren't firing, it's just an orbit with higher eccentricity that takes some fuel to enter into, but once in that orbit, it travels that arc naturally.

Escaping Earth takes a lot of fuel, and additional fuel is used to enter the longer orbit after which, it's flying towards Mars, playing catch-up in a sense.

When close to Mars, adjustments and rockets to reduce velocity.

What would happen to the craft depends on where it runs out of fuel, but you said between Earth and Mars. It would either just enter a slightly longer than Earth orbit or, perhaps maybe crash into Mars, though I think a near miss is more likely.

What you're describing, almost happened once, voyager 1 to Jupiter, though the article's final sentence, the author writes rather poorly saying

"would have gotten almost to Jupiter, and then come back toward the sun, which would not have been good"

That's only partly accurate. It would have almost made it to Jupiter, then stayed in a more elliptical orbit, moving closer to the sun for a while, only to fall further away again as the orbit continued.

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    $\begingroup$ One minor correction: It is Mars that catches up with the (slower, relative to the sun) transfer orbit. $\endgroup$ Sep 22 '19 at 22:26
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    $\begingroup$ Both you and @HenningMakholm are sort of right. The transfer orbit has a shorter period than Mars' orbit, so Mars starts out ahead and the spacecraft will catch up to (and even get ahead of) it. But the spacecraft slows down as it gets further away from the Sun, and its orbital velocity at aphelion (where it will encounter Mars) is slower than that of Mars, so the encounter will indeed occur with Mars catching up to the spacecraft from behind. Orbital mechanics is weird like that. (Also, playing Kerbal Space Program is a great way to hone your intuition for this stuff.) $\endgroup$ Sep 23 '19 at 9:08
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    $\begingroup$ "objects in orbit don't need fuel to stay in orbit" I tend to disagree. Even ISS needs to fire its engines from time to time, otherwise its orbit will decay $\endgroup$
    – DeepSpace
    Sep 23 '19 at 14:17
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    $\begingroup$ @DeepSpace The ISS in particular needs to fire its engines because its orbit is very close to earth. Satellites in geostationary orbit don't need to do that (but they may have small engines to keep themselves above the same bit of the earth). The moon is also in orbit around the earth, and doesn't need to fire its engines at all. (In fact, the moon is receding - very slowly - from the earth.) $\endgroup$ Sep 23 '19 at 14:35
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    $\begingroup$ @DeepSpace You're technically correct, however the ISS is slightly different to this case in that it is constantly skimming the outer edge of the atmosphere. Gravitational interactions with other planets on an elliptical orbit described by this answer would add and remove velocity from the spacecraft and change its orbit over time eventually into some form of unstable orbit but that could be in a literally astronomical timescale $\endgroup$
    – Cam Waite
    Sep 24 '19 at 11:41
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It is a mistake to talk about being "out of Earth's gravity" as if there a fairly sharp boundary where Earth's gravity dropped to zero. If you double your distance to an object, the force of gravity drops to 1/4 of the original force. So it does eventually become negligible, but there is no sharp dividing line.

Also, it is not Earth's gravity that keeps us from falling into the Sun. Remember, we are in orbit around the Sun too. We are moving at the same speed as Earth, and that's the speed you need to be moving to have an orbit of this radius. So if the Earth disappeared but all the people remained, we would all suffocate but we would keep orbiting the Sun.

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    $\begingroup$ This made me imagine 7 billion corpses orbiting the sun. Not bad imagery for a little over a month until Halloween. $\endgroup$ Sep 23 '19 at 6:11
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    $\begingroup$ I think Randall Munroe can make a nice poster from this $\endgroup$
    – theist
    Sep 23 '19 at 10:54
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    $\begingroup$ @CecilRodriguez All of the corpses together would have a relatively centralized gravity well, so eventually (I'm unsure how long) they would all merge together into one tiny corpse asteroid(?). Pretty metal. $\endgroup$
    – Onyz
    Sep 23 '19 at 18:12
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    $\begingroup$ For those who really needed to know, a sphere with the volume of all the humans on earth would be about 500m radius, its mass about 385,000,000t would give us a gravitational acceleration of 0.0001m/s^2. $\endgroup$
    – Avi Cherry
    Sep 23 '19 at 19:35
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    $\begingroup$ @Onyz My intuition would say that we would rather scatter because of the speed we have on the earth surface due to its spin that gets overwhelmend by earths gravity. I do not think humankind has enough mass to counter this speed. But this is just a guess, and people near the poles would have different speed than these on the equator, so maybe there is a spot where this changes.... $\endgroup$
    – Kami Kaze
    Sep 24 '19 at 9:02
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Building on the already great answers, it takes a significant amount of effort to reach the Sun, for example it's 55 times more energy than is required to get to Mars if starting from Earth orbit: It's surprisingly hard to go to the Sun.

The Earth is traveling in its orbit at approximately 30km/s and an object leaving Earth would need to actively decelerate to lose that speed before it would fall into the Sun. A craft running out of fuel would just continue in its existing orbit.

For further reading, see in answers to:

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    $\begingroup$ I'm surprised this hasn't been mentioned in other answers; if you're starting from an Earth orbit, and then fire your rocket straight towards the sun, you will not hit the sun. Orbital mechanics are extremely unintuitive, and it doesn't help that most science fiction depicts constant acceleration = constant velocity and "banking" turns. Scott Manley has an excellent series on orbital mechanics and a video dedicated to The Most Confusing Things About Spacecraft Orbits. $\endgroup$
    – amcgregor
    Sep 25 '19 at 13:42
  • $\begingroup$ @amcgregor, if you're in Earth orbit and fire your rocket straight towards the Sun, you can hit the Sun. It just takes far, far more energy than firing backwards against Earth's orbit. $\endgroup$
    – Mark
    Feb 28 '20 at 0:17
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Current spacecraft designs do not consume propellant while en-route to Mars because there is no need to apply any thrust. A launch to Mars would begin with an acceleration to at least low-Earth-orbit velocity. The spacecraft might then briefly orbit Earth, or simply continue accelerating to escape Earth's gravity and attain a trajectory which is a transfer orbit. Either way, at some point still very near Earth (relative to Mars), the spacecraft will shut down its engines and coast along the transfer orbit (an orbit around the Sun which more-or-less crosses the orbits of Earth and Mars). This orbit intersects Earth's orbit at the time and place Earth was during launch, and will intersect Mars' orbit at the time and place where Mars will be upon arrival. Upon arrival, the spacecraft will have to decelerate somehow to enter an orbit around Mars and/or enter Mars' atmosphere to land there. If the spacecraft fails to decelerate into orbit, it will remain on its transfer orbit, periodically crossing the orbits of Earth and Mars.

There are ideas for very high ISP propulsion systems which could theoretically shorten the time required to get to Mars. These systems would produce a small, constant acceleration - initially in a more-or-less prograde direction, but then turn around at more-or-less the halfway point to produce a retrograde thrust to slow down enough to be captured by Mars' gravity upon arrival. If such a system failed en-route, the vehicle would almost certainly miss Mars completely and find itself in an orbit around the Sun, probably with an apoapsis beyond Mars and periapsis somewhere between Earth and Mars.

In either case, a spacecraft becoming derelict enroute to Mars would not fall into the Sun; it would remain in a solar orbit unless/until it collides with a planet, perhaps after one or more perturbing planetary close approaches.

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A spaceship in space is very different from a car on a flat road. If your car runs out of fuel, the friction between the tyres and the road cause the car to slow down until it eventually stops. In space there is almost no friction (because space is close to a perfect vacuum), so little that for the rest of this answer I will pretend there is none at all.

To get out of orbit of Earth, you need to move really fast. If you run out of fuel once out of the orbit of Earth, you will continue to move really fast because there is no friction in space. Your path will be bent by the gravity of everything, but only nearby (think inside the solar system) and massive (think the Sun, Earth, Jupiter, etc.) objects will have a noticeable effect. The closer and more massive the object, the bigger the effect.

While it is possible to speed up or slow down while passing these objects, you have to be close to them, and normally it takes a precise orbit to achieve this. So basically, you'll just keep drifting really fast unless you're really unlucky and crash into something. But because space is a vacuum, there's almost nothing to crash into.

Because your path is continuously being bent, you will most likely end up in orbit around something, and because the Sun is the most massive object in the solar system (about 99.86% of the total solar system mass), you'll most likely end up in orbit around the Sun.

It's possible if your initial aim was good enough that you could crash into Mars, assuming that was your destination and you weren't just planning on sailing past it.

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    $\begingroup$ the closer and more massive the object - the bigger its sphere of influence. -if your initial [velocity (not aim); speed and direction] was good enough you wouldn't need to do a e.g., 'tans-lunar injection' to achieve Orbit insertion $\endgroup$
    – Mazura
    Sep 22 '19 at 22:12
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    $\begingroup$ I think this answer gets to the crux of the matter, which seems to be that the OP is perhaps misunderstanding space travel. It' not like you take off from Earth and burn fuel at a constant rate until you get to Mars (as if you were driving a car). It's more like throwing a dart - there's a huge (but short) initial impulse, after which you're pretty much just coasting. $\endgroup$
    – dwizum
    Sep 23 '19 at 16:26
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Normally a spacecraft would use most of its fuel just leaving the earth. Retaining some secondary fuel for maneuvers. If it runs out of fuel early during liftoff it depends on how early it ran out to know what happens. it could either fall back to earth or end up in earth's orbit.

You'd expect if it runs out just a little before planned burnout time the spacecraft would be on a course to Mars but would have an altered trajectory.

During a planned launch to Mars, the spacecraft will be in free motion moving at a high velocity to get it to Mars.

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As mentioned by everyone else, a spaceship doesn't need fuel to keep going in space.

But besides that, not only the spaceship might keep moving in orbit around the Sun, depending on the angle and speed that your spaceship was going between Earth and Mars, it might even escape the Sun's gravitational pull and drift forever in empty space away from the Sun.

As an example we have Voyager 1, which left our solar system and is moving towards other stars to never come back.

In this case, it didn't gain the speed necessary for that just between Earth and Mars. But that doesn't mean it can't be done. ;-) If your theoretical spaceship has enough propulsion moving away from Earth, it might gain the speed it needs.

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Any object with mass including a spaceship, without means of acceleration, having a given velocity or not, will eventually end up at a gravitational relation with another object of much greater mass. This eventually can be from hours to millions of years, depending on various parameters like, the most important ones, space and time the event begins and probable velocity of the object. For an object going from Earth to Mars the most important factor is when, since the positions of those 2 celestial bodies may vary significantly, from been as close as seen in relative 'display star system' pictures conveniently show all celestial objects next to each other or simply much far away as at opposite sun orbital position, means the sun is somewhere between Earth and Mars. So the answer can be probably not, the object will keep going until affected by some other greater mass or yes, in the rare case where its travel routing stops when inevitably crosses the sun or close enough to orbit and been destroyed out of heat.

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A spacecraft travelling from Earth is still in on the orbit around the Sun, travelling at a large speed. It will keep going around the Sun, indefinitely and passively.

In order to crash into the Sun, the spacecraft must have used its engines to drop its speed in such a way that the ellipsis of its orbit becomes narrow enough to intersect the Sun's surface.

This requires a lot of fuel. In fact, from the Earth's orbit and beyond, it's easier to reach Jupiter than it is to fall onto the Sun.

But if the spacecraft had managed to reduce its speed enough before running out of fuel — then yes, it will fall onto the Sun, gradually increasing the speed of its fall as it is dragged closer to the Sun.


But there is another possibility: the spacecraft might escape the Solar system and set off to the outer space, entirely passively!

It depends on how fast it was going when it ran out of fuel.

In order to escape the Sun's gravitational pull with engines off, the spacecraft must be travelling at the Sun's escape velocity, regardless of the mass of the spacecraft.

If it shuts down its engines near the surface of the Sun, it must have reached the speed of at least 617 500 m/s to escape the Solar system.

If it shuts down its engines further from the Sun (but not close to any planet), the speed needs to be smaller than 617 500 m/s. The exact speed depends on the exact distance from the Sun and the trajectory.

For example, Voyager 1 is travelling at around 17 260 000 m/s.

When the Juno spacecraft was travelling to Jupiter, it reached max speed of 618 000 m/s. If it did neither use it's engines nor pass behind any planets in order to slow down, then it would eventually escape the Solar system.


BTW, If you start from the surface of the Earth and want to escape both Earth's and Sun's gravity, you'll need whopping 16 650 000 m/s. But that number is valid only for a passive object not using it's engines, similar to a cannon shot, or an unreasonably powerful and quick-burning first rocket stage without other stages.

Luckily, you don't need to do that. Instead, you can keep using your engines along the travel, e. g. get into a distant orbit first with some fuel left and proceed from there.

Also, Earth rotation can help a bit.

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ontrary to the suggestion of the question, a space ship would not stop when it runs out of fuel. Even though there are objects in space like gas, dust and particles, they are in such low concentrations and so spread out that it is highly unlikely that a space ship would come into contact with anything.

This means that the space ship in question would not be slowed down by anything in space, or that it is highly unlikely that it would. This is because there would be no friction that would enable anything in space to slow down a space ship.

A space ship does not need constant thrust in order for it to move through space, unlike a ship on the ocean. Even when the thrust that comes from the fuel runs out, the space ship will still move forward thanks to the momentum that came from the fuel. What could a space ship do if it stopped because it ran out of fuel

This momentum will then carry the space ship forward on the same trajectory that it was already travelling, even when the fuel runs out and the engine shuts off.

This momentum will continue to carry the space ship at a constant speed. And since it would cost an extortionate amount of money to send fuel into space for a space ship, space ships will tend to be designed to use as little fuel as possible. Not only would it be incredibly expensive to get fuel to a space ship, it would also be heavy and impractical.

Since space ships need to conserve fuel, the engines are normally turned off for most of the journey. This is because the space ship will still be moving even without the use of fuel as has already been explained. The engines will then only be turned on to decelerate.

So, when you watch Star Wars or any other movie that is set in space, you may realise that if a space ship constantly has its engines on, this is an unnecessary use of fuel.

In answer to the question, a space ship would not simply stop if it ran out of fuel; its momentum would keep it moving through space.

And since it is impractical to send up fuel to replenish the fuel supplies of a space ship, this hypothetical space craft would likely keep moving until it collided with something, which is quite unlikely to happen in space.

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