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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 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 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. $\endgroup$ – Draco18s Sep 22 at 18:45
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    $\begingroup$ I recommend to play Kerbal Space Program. $\endgroup$ – Firzen Sep 23 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 at 15:36
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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. $\endgroup$ – Henning Makholm Sep 22 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.) $\endgroup$ – Ilmari Karonen Sep 23 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 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.) $\endgroup$ – Martin Bonner Sep 23 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 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. $\endgroup$ – Cecil Rodriguez Sep 23 at 6:11
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    $\begingroup$ I think Randall Munroe can make a nice poster from this $\endgroup$ – theist Sep 23 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 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 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 at 9:02
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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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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 at 13:42
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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 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 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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It depends on the last thrust it had & direction it is moving to. Cause in space velocity of moving object remains same until it come across any gravitational forces.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    $\begingroup$ This does not address the problem fully. For instance, there are always gravitational forces acting on a spacecraft within the solar system. How does that affect the velocity? $\endgroup$ – Hohmannfan Sep 22 at 17:36
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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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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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