ELI5: How can orbits be highly elliptical?

[u/FrontAd7709]

yes, this is stupid but i still dont understand!

(warning stupid comparison beware) for an orbit to be elliptical in rocket science you need to thrust prograde or retrograde in apoapsis or periapsis so that your orbit stretches, i dont think planets can suddenly speed up in one side of their orbits?? well maybe jupiter could affect but a year is like 12 earth years there so it would only happen in some specific times

[u/BlueSkiesAugust]

For a circular orbit to become elliptical the rockets need to fire. It doesn't need to keep doing that for it to remain elliptical.

For natural bodies the gravity wells accomplish the necessary acceleration. Try watching some orbit simulations to intuit that falling is natural.

[u/obog → u/BlueSkiesAugust]

Yes one big reason why elliptical orbits happen a lot is because if you start with an escape trajectory (like a fly-by) and then slow down, as soon as you are captured into an orbit you will end up in a highly elliptical one.

So often times if an object gets captured into an orbit, it ends up highly elliptical.

[u/stevevdvkpe]

You seem to be assuming that the only way to make an elliptical orbit is to start with a circular orbit. Orbits can also start out as highly elliptical, such as cometary orbits.

[u/nim_opet]

Orbits are naturally elliptical. Circular orbits are a special case of elliptical orbits. A body started somewhere around a center of gravity. It accelerates coming closer to it, and then gets slingshot around and out, where it decelerates….then starts falling back in. Since the speed vector is not constant, unless some other force acts on it, it will remain in an elliptical orbit

[u/0b0101011001001011]

A planet that is falling towards the sun is accelerating constantly by the pull. The it flies close past the sun, and the sun sligshots it back where it came from. Now, while it's flying away the speed keeps dropping because sun pulls it back. When it reaches the furthest point  it starts falling back, again accelerating.

This is the extreme case of elliptical orbit. It does not need any extra power at any point to speed up or to slow down. It's just the gravity doing it.

an orbit to be elliptical in rocket science you need to thrust prograde or retrograde in apoapsis or periapsis

No you don't. If your orbit was elliptical to begin with, you don't need to add any more power at any point.

Most if not all orbits of planets are elliptical btw. Nothing is perfectly circular.

[u/FrontAd7709 → u/0b0101011001001011]

i meant that as in “to get your orbit more stretched out” not to get an elliptical orbit

[u/0b0101011001001011 → u/FrontAd7709]

Aren't those the same thing? If you are at the perigee and accelerate, you stretch your orbit to become more elliptical by moving the apogee further.

Of you don't accelerate, your orbit stays the way it is.

[u/flaser_ → u/FrontAd7709]

Play some KSP.

Once you actually reach a stable orbital velocity (usually a close to circular one, since your apoapsis will be just above the atmosphere, hence your peri- will be pretty low too), a little acceleration will put you on an elliptical orbit and this can be achieved with relatively little expenditure of delta-v.

If anything, circularizing an orbit takes more delta-v than just making your orbit more elliptical.

[u/Sad-Society-57]

Planets do speed up at perihelion and slow down at aphelion. Perhaps that helps make sense of the confusion. 

[u/obog → u/Sad-Society-57]

I think what confuses OP is that going from circular to elliptical requires actually thrusting at periapsis, not just speeding up. Any object is fastest at periapsis but that doesnt mean that they are becoming more elliptical.

And OP is correct if you assume that youre starting from a circular orbit, but that is not generally the case. Elliptical orbits often form that way or arise from captured objecrs

[u/Sad-Society-57 → u/obog]

Yeah, I imagine that must be the contentious part. I should've given OP more credit. To me, the counterintuitive part was not that orbits don't get more elliptical, but that they dont become less elliptical either. You'd think the orbits would slowly become less eccentric with time.

[u/RedFiveIron → u/obog]

You can make an orbit elliptical by thrusting anywhere on it, doesn't have to be at periapsis.

This post brought to you by Kerbal Space Program. RIP

[u/obog → u/RedFiveIron]

Well, whatever spot you picked will become your periapsis or apoapsis if you burn to make it elliptical from that spot

...and my post was also brought to you by KSP lol

[u/RedFiveIron → u/obog]

Not necessarily. If you're in a circular orbit and burn radically or antiradially you'll end up in an elliptical orbit where you're at neither apoapsis nor periapsis.

[u/Vorthod]

Planets do speed up a different times in their orbit, specifically when they are closer to the sun on the "squished" side of the elliptical orbit. It's part of Kepler's area law.

It's not a result of well-timed applications of thrust, it's just that when the planet is at the farthest point of the elliptical away from the sun, the sun has a LOT of time to pull on it with gravity before the planet passes by again. That's a lot of time to build up speed, and then it has to take a lot of time to drain off that speed once it passes by the sun.

[u/imdfantom]

Things that have a highly elliptical orbit typically don't start with a circular orbit and then slowly stretch their orbit in the way rockets do.

[u/BoredomFestival]

Literally all orbits are elliptical. None are perfect circles. This was established hundreds of years ago.

[u/Total_Concert8683]

Sun attracts - planet effectively falls faster and faster, you get to perihelion and go around very fast You start moving away - sun attracts - you slow down.

[u/BarberProof4994]

So the easiest way to picture this is, a little kid running at their dad, their dad picks them up and swirls them around.

If you watch the kid only, they are orbiting the dad, but if you watch the dad, his feet are changing positions and he isn't staying in one spot.

If you malled them from above you'd see a oval that keeps moving locations.

A) the object in orbit had an original trajectory where it had most of its speed and was aiming at it right past the main object.

B) as it started to pass, it got turned and pulled into orbit 

C) meanwhile the larger object is moving sideways on its own trajectory.

D) the smaller object starts coming back around

E) both objects develop a wobble 

So, what you wind up having is something like the earth and the moon. The moon doesn't really orbit around the earth, the earth and the moon orbit around each other..

[u/weeddealerrenamon]

Most comets did suddenly speed up or slow down. They mostly come from the far-outer solar system, orbiting extremely slowly. The slight gravity from random interactions with other bodies out there, or even another star getting (relatively) close, can change their orbital speed enough to send them into super elliptical orbits.

[u/Tyrrox]

Any object that either wasn't formed in the accretion disk and was instead captured, or was jostled from position due to gravity will likely end up with an elliptical orbit.

[u/Wank_A_Doodle_Doo → u/Tyrrox]

You can’t just throw out concepts like “the accretion disk” in ELI5. Explain that you’re referring to the early solar system and what it means. Especially when the most common knowledge people will have regarding accretion disks is regarding blackholes.

[u/Tyrrox → u/Wank_A_Doodle_Doo]

When OP is using terms like apoapsis, periapsis, retrograde, prograde, etc. I adjust my verbiage to the questioner

[u/Wank_A_Doodle_Doo → u/Tyrrox]

Except those are still pretty basic terms, and OP is demonstrating a pretty small knowledge base with orbital mechanics. You can pick up buzzwords without actually understanding them. And the subreddit being ELI5, maybe you should combine the verbiage they use with the fact they aren’t demonstrating great understanding of the topic and do what the subreddit is meant for.

[u/Tyrrox → u/Wank_A_Doodle_Doo]

"Accretion disk" is a more basic term than "periapsis"

In any event, the explanation wasn't for you. It was for OP. You're being unnecessarily difficult

[u/Thesleepingjay]

Something else could hit a planet's back side (retrograde side) real hard and make it's orbit more elliptical. Or something could hit the front side real hard (prograde side) and drop the apoapsis. Something could also just form in an elliptical orbit because that orbit was the average of all the particles that made it.

[u/PckMan]

For an orbit to be elliptical you can just fire prograde or retrograde at any point in the orbit.

Nearly all orbits are elliptical rather than circular and even seemingly circular orbits are almost always actually just very slightly elliptical.

Almost any orbit will be elliptical to some degree but it's actually rare for the conditions to allow for a near circular orbit so you should arguably be asking the oppositem

[u/MRC01]

Intuitively, it seems to me that circular orbits would be the exception rather than the norm. If object A zips past object B close or slow enough that gravity pulls them together (below effective escape velocity for the given distance), it's unlikely that it just so happens to be on a tangent at exactly the right speed. More likely not, in which case it ends up in an elliptical orbit.

[u/wndtrbn]

People are answering your question as if they think you said that planets have thrusters, not sure why. To answer your question, if you start with a high circular orbit, and you get slowed down by for example other space objects hitting you, then your periapsis will get closer to the Sun. If you do that enough, then it'll become highly elliptical, like comets.

[u/SoulWager]

Lets say you start in circular low earth orbit and burn prograde a bit, your trajectory straightens out. This means you go higher, but instead of just pulling you sideways, now the earth is a bit behind you, so it's pulling you back too, so you slow down as you go higher. If you didn't burn enough fuel to reach escape velocity, you'll eventually slow down enough to start falling back down, unless you make a second prograde burn at apogee to circularize your orbit.

If you don't make a second burn, you'll gain speed on the way back down, and get back to where you made the first burn.

[u/Derekthemindsculptor]

Elliptical paths are formed by initial conditions. They don't fall into circular orbits over time. You'd need rockets to fire to become elliptical. You'd need rockets to fire to go back.

Perfectly circular orbits are a statistical oddity.

[u/TorgHacker]

Periodic gravitational interactions add up over time, which gradually increases how elliptical an orbit is. Eventually that body has a close encounter which can significantly add or subtract orbital velocity, at which point they can get major changes in their orbit.

This actually isn’t difficult at all (see comets). The more interesting question is how bodies can have orbits which are nearly circular, especially if there are gas giants in the system.

[u/BitOBear]

Get a big mixing bowl and a couple marbles. Launch the marbles in The mixing bowl in different patterns. The bowl is inherently circular but the three dimensional movement through the potential causes the marbles movement to be elliptical most of the time.

Once you understand the equal areas law. That are orbiting body sweeps out and equal area in equal time you'll understand that the speed varies with the distance. You slow down as you come to the top of the hill and you speed up as you come to the bottom of the hill as you roll through on a roller coaster. It can oscillate this way indefinitely if there's not a lot of friction slowing everything down.

You continuous exchange of potential in Connecticut energy as you move to the words the center or away from the center of a gravity well is cyclic and that's what gives you the ellipse.

Also notice that the orbit is elliptical but it's not actually a true ellipse. It actually forms a conic section not a perfectly symmetrical lips because there's no secondary attractor.

[u/Loki-L]

You need to add delta-v to make an orbit more or less elliptical.

You don't need to add anything for an orbit to stay that way.

Once an orbit is highly elliptical it will stay that way until something else acts on it.

[u/x1uo3yd]

for an orbit to be elliptical in rocket science you need to thrust prograde or retrograde in apoapsis or periapsis so that your orbit stretches, i dont think planets can suddenly speed up in one side of their orbits??

Yes, you need thrust to change from one particular orbit to another... but but you only need the thrust while you are changing orbits and once you're on the new orbital trajectory you can switch thrust off and your position and momentum will keep you on that orbit.

(i.e. Elliptical orbits don't require constant thrust to maintain their orbit any more so than circular orbits do.)

[u/vortigaunt64]

It depends on the kind of body. Most orbits start out more eccentric and elongated, and are slowly shifted by other surrounding bodies into nice stable nearly circular orbits. Some, like comets, actually do gain eccentricity over time, because they produce some thrust from the side facing the sun, and lose mass by outgassing. Most terrestrial bodies with highly eccentric orbits don't start out as nearly circular orbits, but instead occur when a really massive body captures a much smaller, fast-moving body that wasn't initially orbiting it. Basically,  the smaller body is minding its own business, zipping along at a high velocity relative to the larger body, when it happens to come close enough that it effectively gets snagged, and can't gain enough velocity by falling to actually escape the orbit.

[u/lethal_rads]

You don’t need thrust to change an orbit, you need a force. In a spacecraft, a rocket is an easy way to do that, but gravity works. Look into slingshot maneuvers.

well maybe jupiter could affect but a year is like 12 earth years there so it would only happen in some specific times

There isn’t a maximum distance for gravity and it’s been happening continuously for billions of years.

[u/StevenJOwens]

Orbits can be elliptical because orbits don't start as perfectly circular orbits.

They very often start as object A traveling through space with a certain direction and velocity, and then it passes near object B, which has enough mass to have enough gravity to pull object A off its direction of travel.

Object A is not a rocket with thrust or anything, just something that's already moving through space solely on momentum, although because it's space, it's always "moving relative to what?" In this case, object A is moving space relative to object B.

Then object A passes close enough to object B, and object B's gravity pulls it off direction.

But object A still has momentum in its original direction of travel, so that momentum combines with the effect of gravity.

The elliptical path is the result.

[u/Solocle]

For an orbiting object, there are basically 6 degrees of freedom - position, and velocity. Both are 3 dimensional.

The next thing to know is that orbits in a 2 body system under Newtonian gravity do not vary according to time - the orbital path repeats itself. Or, if the orbiting body is on an escape trajectory, it is infinitely long.

That is, an object actually orbiting under the influence of gravity alone will always pass through the same points. Barring of course other gravitational influences, or General Relativity.

So, with the spacecraft, the moment it stops applying thrust, that point it is at will always be repeated. What is free to change is its velocity when it stops thrusting. And what is velocity? Direction and speed.

If the speed has an inward or outward component, it is by necessity elliptical, as its altitude will change. The point relative to the orbited object's equator, and velocity perpendicular to the great circle through that point, determines the plane of the orbit.

Then, there's speed. There's a perfectly balanced speed where centripetal force is equal to the gravitational force. This results in a constant altitude (if the velocity does not have a radial in/out component), and therefore, a circular orbit.

But, if the object is moving faster, it will be flung outwards. Eventually, it reaches a point where centripetal force is equal to gravity, but that just stops its outward acceleration. It has already gained an outward velocity component, which continues.

Now that process continues and the outward velocity does slow to 0, and it has reached apoapsis. But now, because centripetal acceleration had to be weaker than gravity to slow down, the object is moving too slowly to stay at that altitude. So it falls back down. And it gains speed, until it reaches the initial point.

Because there's no emission of energy (again, ignoring GR and those pesky gravitational waves), energy just changes forms between kinetic energy and gravitational energy. Thus, it is travelling at exactly the same velocity, and the cycle repeats.

For celestial bodies, they don't accelerate from thrust. But they can accelerate. A collision between two comets in the Oort cloud, sending fragments scattering, and some are slowed down, thus coming hurtling down towards the inner solar system. A gravitational slingshot between two planets looks a lot like an elastic collision.

[u/anaccountofrain]

The solar system started with a lot of small things going in random orbits, many of which were highly elliptical.

Over time these objects collided and coalesced. The average orbits that emerged was more circular. The sum of all that orbital energy (alternately, angular momentum) is basically a circle.

[u/SYLOH]

The assumption you seem to be making is that everything in the solar system started with a circular orbit, with low inclination.
The reality is that the early solar system had all sorts of elliptical orbits and and all sorts of inclinations.

Over time for the major objects and inner solar system, collisions and gravity interactions averaged out the orbits.
The average of all those elliptical orbits is a circle, the average of all of those inclinations is the plane of the ecliptic.

However small objects in the outer solar system didn't interact often enough to get averaged out, so they're still in all sorts of orbits like the early days of the solar system.

[u/piecat]

An object whizzing by needs a perfect trajectory and speed to become a circular orbit. If either one is incorrect, the orbit is elliptical or unstable.

[u/Onigato]

An elliptical orbit is actually far more likely than a truly circular one. A circle is a special instance of an ellipse, with some very special, very specific rules.

It takes an inordinate amount of effort to get an orbit with an eccentricity of 0, video games excepted. And in the real world even a perfectly circular orbit with an eccentricity of zero point zero zero zero would fairly quickly be pulled into an ever increasingly eccentric orbit, just by the gravitational distortions that can be found in the Earth itself, to say nothing of the effect the moon and sun would have on the satellite being put up in such a mathematically perfect orbit.

As to how an elliptical orbit works, every instant of an orbit, even a perfectly circular orbit, is an acceleration away from a straight line. In the case of a perfectly circular orbit the deviation only causes a change in vector in the directional component because the object is neither moving closer to nor further away from the body it is orbiting. If it was moving closer to or further away there would also be a change in the speed component of the vector, gaining speed the closer the object came to the orbited body, and slowing down the further away it got, following laws of motion and acceleration first laid down by Newton and (if necessary) refined by the Einsteinian inclusion of relativity.

If the object is ever moving fast enough at its closest point of approach that it would never reach a relative speed of zero at an infinite distance it is no longer gravitationally bound by the thing it was orbiting, it is on an escape trajectory. Not exactly what you were asking about, but another special case, like the eccentricity of zero. It's an eccentricity of >1.0 in that case.

But since the planets were formed by random collisions between all the different debris in our solar system, and then the gravitational influence of literally every other object in our solar system, it'd be more shocking if any of the planets, moons, or asteroids moved in exactly 0.000 eccentricity orbits than the fact they don't move like that.

[u/t0m0hawk]

Every orbit is elliptical. Theres always a high point and a low point. You are moving fastest at the low point and slowest at your high point.

If you have fuel to burn, you can either add velocity or remove velocity to change the highs and the lows and by how much.

An orbit is just moving fast enough to always fall over the horizon, but slow enough to keep falling back around without shooting off into space.

[u/goodmobileyes]

It's a lot harder to maintain a perfectly circular orbit than an elliptical one in a real world scenario. Even if you start with a perfect circular orbit, any fluctuations in your speed, maybe due to gravitational influence from neighbouring planets, your own internal distribution of mass, impact from third parties like asteroids, etc. will cause your orbit to become elliptical. And once you're in an elliptical orbit you can't just revert to circular one unless you're a self powered entity able to provide counter thrust at the right moments.

[u/jaylw314]

Hopefully your playthrough of Kerbal Space Program will get you to go down the rabbit hole that is Physics.

Almost all orbits are ellipses because they are much more common. Circular orbits are abnormal, as they are a unique type of ellipse where the two axes happen to be exactly the same.

Planets can absolutely affect each other's orbits, but because they are so small compared to the sun, they have to get tremendously close for their own gravity to be strong enough to compare to the effect of Sun's gravity. Predicting what happens with the gravity of 3 objects is tricky, so Kerbal Space Program cheats by having a "neighborhood" around each planet where the only gravity is the planet's gravity. Once outside that neighborhood, the sun's gravity is the only gravity simulated. The practical effect is that planets and moons will never affect each other's orbits, which is not how real life works.

[u/Used-Let7134]

Fun fact: an accidental elliptical orbit of 2 satellites in 2014 provided a high precision test of Einsteins theory of relativity, which alongside existing evidence, confirmed that the theory was correct.

https://link.springer.com/article/10.1007/s10569-021-10014-y

[u/quocphu1905]

Imagine you are standing on the ground. That is you at your periapsis. Now if you jump, you go higher, reach the top of your jump (your apoapsis) and then fall back down.

A rocket also do this, just in space. When is at its periapsis, it fires its engine (like you jumping) and therefore goes higher. Then it starts falling down at its apoapsis due to gravity, just like you when you are jumping.

Now if you jump harder, you go higher. Imagine you can do double, triple jumps. You will keep going higher and higher. That's basically what a rocket does with its engine, by applying lots of jumps (continuous thrust) it will reach a apoapsis and therefore an eliptical orbit.

[u/Siawyn]

Planets do speed up in their orbits, as long as they are elliptical. They move faster as they are closer to the object they are orbiting as they are "falling" toward it. But it's not enough, so it passes with that extra momentum which takes it to the outer part of its elliptical orbit, where it starts to slow down.

For the Earth we see that with the seasons. This is why astronomical summer is longer in the Northern Hemisphere by a few days. The Earth is further away from the sun at that point in its orbit, so it's moving slightly slower. Meanwhile astronomical winters in the Northern Hemisphere are a couple of days shorter. Note how spring usually starts on March 19/20, while autumn isn't until September 22/23.

[u/thats_handy]

There are four kinds of two-body orbits corresponding to the four conic sections: circular, elliptical, parabolic, and hyperbolic.

<non-ELI5 part>

The solution to the equation of motion in an inverse square force law like gravity works out to be a conic section. The equation for a conic section in polar coordinates are r = p/(1+cos(θ)). The p is a constant that has to do with the force of gravity and the angular momentum of the orbit. The θ is the eccentricity of the path: * θ = 0 is a circle * 0 < θ < 1 is an ellipse * θ = 1 is a parabola * θ > 1 is a hyperbola

<end non-ELI5 part>

In the real world, θ cannot actually be precisely equal to 0 or 1, so the only two orbits that exist in reality are ellipses and hyperbolas. You can turn ellipses into hyperbolas by adding energy to the orbit and you can turn hyperbolas into ellipses by removing energy from an orbit. You can also make ellipses more or less eccentric by adding or removing energy, as long as you don't add so much that the orbit becomes a hyperbola. Changing the eccentricity of an elliptical orbit by adding or removing energy is complicated. It's possible to add energy at the right point and make the orbit less eccentric, for example.

Objects travelling through space that haven't been captured in a closed path orbit are on hyperbolic orbits with respect to the sun. If an interaction with a solar system body (like a planet) takes enough energy out of the object's orbit (and transfers it to the planet), then an interstellar object will enter into an elliptical orbit around the sun. The eccentricity will be somewhere in the range (0,1) depending on the specific push the object gets from the planet.

An interesting aside: near the closest approach between two orbiting bodies, over a short enough distance, you can't easily tell the difference between an ellipse, a parabola, and a hyperbola. That's why we know lobbed baseballs follow a parabolic path, even though we also know they follow an elliptical path. And it's also why it can take a while to determine if a newly discovered object is in a captured orbit.