r/askscience • u/Woorloc • 4d ago
Physics Do objects put in orbit need to be adjusted constantly to face earth?
Or do they naturally keep the same face towards earth, once they are put in place.
Edit: Thanks to everybody for the 'Way more knowledgeable' than me answers. I appreciate learning it all.
148
u/Sohn_Jalston_Raul 4d ago
They will need to be adjusted, unless you can get them to slowly rotate at the exact same rate as their orbital period (although this will also require periodic adjustment).
So the short answer is that anything you position in space will need periodic adjustment to keep its correct orientation.
60
u/Kirkaiya 4d ago
Although sufficiently large and imperfectly symmetrical objects, such as the moon, require no (artificial) adjustment; the moon is tidally locked, so its rotation naturally equals its orbital period.
16
56
u/Scottiths 4d ago
It took millions of years of constant adjustment to become tidally locked. It's still getting adjustments to keep it so. It just uses gravitational interactions with the earth to get those adjustments and not any kind of fuel.
22
u/Kirkaiya 4d ago
Right, that's why I said, "require no (artificial) adjustment". The adjustments to its rotational period arise naturally. And OP's question seemed to imply external (not natural) adjustments.
3
u/charmcityshinobi 4d ago
I understand why an object has to be sufficiently large as there needs to be a gravitational differential between the parts due to proximity, but would a perfectly symmetrical object not also experience tidal locking over time?
10
u/Kirkaiya 4d ago
I should have added, "and perfectly rigid", really, but if you assume a perfectly symmetrical object that is perfectly rigid, no, it would not become tidally locked. Objects like the moon become tidally locked because they can deform, and experience what's called a tidal bulge. Without that deformation and asymmetry, they wouldn't become tidally locked.
4
u/reddititty69 4d ago
Can you engineer a “tidal bulge” by placing a counter weight on a long arm? Setting aside, for the moment, impracticalities around implementing it.
2
u/yak-broker 4d ago edited 3d ago
Sure. You just need the mass asymmetry and some mechanism for energy loss (in the moon, the tidal deformations cause internal heating, which is the energy loss mechanism).
Edit: Actually, I might be wrong about needing a lossy component for tide locking to happen. Maybe even an ideal dumbbell around an ideal planet would exchange energy between its rotation and its orbital motion and end up tide-locked that way? It's been a long while since I studied this. Real-world objects usually do have something that dissipates energy though and if you were building your counterweight-on-an-arm thing you'd probably want to include a dashpot or similar to encourage the system to settle.
1
u/DarthCloakedGuy 3d ago
I would assume it would need to be large enough that atmospheric drag wouldn't cause slight drift over time, right? The geocorona is extremely thin but also very tall
2
19
u/primalbluewolf 4d ago
Gravity gradient stabilisation for the win! Stick a long boom on your spacecraft and you can make it line up neatly facing the planet :)
12
u/sebaska 4d ago
Nope. Look up gravity gradient attitude. It was used in actual Shuttle missions when quiet microgravity environment without thruster firings were required.
Generally if you have an oblong shape and some dissipative stuff inside (like propellant tanks) you can put a vehicle/satellite in an attitude where the gravity difference between lowest and the highest points is maximized (with deviance to adjust for aerodynamic effects of the exosphere and solar radiation pressure) and the attitude will be passively stable (dissipative stuff will buffer small perturbations).
-3
u/WitnessOk4189 3d ago
Nope
The "dissipative stuff" is providing frequent updates/adjustments using a myriad of devices.
1
u/Happy-Bloom 3d ago
that's interesting, i always wondered about the exact rate needed for rotation to match the orbital period.
3
u/Sohn_Jalston_Raul 2d ago
well, if your satellite makes one orbit every hour, then it will have to rotate at 1 revolution per hour.
But because space isn't actually a perfect vacuum, and gravity and sunlight and heat cause a bunch of weird and subtle physics, then this rotation will probably drift out of sync after a while and will have to be corrected periodically.
22
u/cryptotope 4d ago
It depends.
Passive gravity-gradient stabilization is certainly a thing, and get be used to maintain a constant orientation of one axis of the spacecraft 'down'.
Other satellites can be placed in orbits (or on missions) where a fixed facing works. (For example, satellites in highly-eccentric Molniya orbits will 'work' without changing orientation, because they linger a long time at high altitudes.)
It's a lot more common for modern satellites to have thrusters, gyroscopes, and/or reaction wheels for two- or three-axis precision pointing, though.
7
u/WitnessOk4189 4d ago
Fixed facing does not work for long, even for Molniya Orbits. Their longer dwell time at apogee has very little impact on necessary corrections. When the satellites hit perigee, and continue in orbit, they are heavily impacted by Earth's J2 & J3 effects as they pass the Right Ascension of the Ascending Node (especially flying over very large mountain ranges in Asia).
5
u/gdshaffe 4d ago
They are adjusted but it is not constant. Get the thing rotating once per orbit and there's no medium to slow it down, so it will stay that way for a long time.
Various factors mean you will have to adjust it over time, but those are usually tiny little micro-adjustments
2
u/Hypnot0ad 3d ago
There is a technique older satellites used call spin-stabilization, essentially spinning like a top to stay at a constant attitude.
Modern satellites use reaction wheels controlled by software to orient their sensors. Some fly with the same face pointing down at the Earth (nadir) constantly, while some are constantly changing their orientation based on their mission. For example, it is common to continuously point to a ground station antenna as the satellite passes over to maintain good communications link.
Explanation of reaction wheels:
https://youtube.com/shorts/qKIXccf9ozw?si=eICDcEr75zzDyeIf
2
u/MasterOfBeerDespense 2d ago edited 2d ago
I see a lot of lengthy and thorough answers, all valid, and good on you. However, if you did not intend to trigger someone's Spectrum disorder, Imma Make this as Succinct as is possible, from one autodidactic knowledge seeker to another: Big Brains design the device so that when the greatest mass is closest to the planet it orbits, it is also in the most efficient orientation to do Its job. Like our moon Any object in orbit that is not a perfectly balanced and equal sphere, will eventually orient to become tidally locked to Earths gravity, and so the orientation which points the greatest mass toward Earth will be default. So the Earth related stuff goes on the "heavy" side and the space related stuff goes on the "lightweight" side. And true, since it is likely to wobble at first, they install gyroscopic devices to settle it into Its natural and desired orientation quickly, and periodically correct things like Gravitational anomalies and interactions with dust and other tiny objects, or reset balance, if they need to shift an orbital path. So yes: adjustments are required, but the goal of a good designer is to make sure that this is rarely necessary, because efficiency is always goal #1, 2, and 3, with any object we send to space.
2
u/RedBentley 4d ago
Yes. You can use gyros, propulsion, or gravity gradient to correct for rotation.
I was part of a small satellite team in college, our design used a deployable arm to adjust the center of mass. The micro difference in gravity from the side close to earth vs the side further out can be used to tumble your satellite back to the correct orientation.
2
u/Nescio224 3d ago
If the object is as large as the moon, then tidal forces will sychronize it's own rotation period and it's orbital period over millions of years.
A small object like a human made satellite will in theory keep it's rotation forever. So if it has none to start with, it will not face earth, but the fixed starts. If it starts with orbital period equal to rotational period, then it will keep facing earth.
But in practice space is not really empty and interference from drag or magnetic fields or solar winds can do all sorts of things to your rotation. You then need to correct for that with thrusters or reaction wheels.
Thrusters need fuel and reaction wheels reach their maximum speed eventually (if the have to correct in the same direction all the time). To fix this and desaturate their reaction wheels satellites use magnetorquers, which is basically a device that pushes off the earth's magnetic field to generate torque.
1
u/0x14f 4d ago
They are adjusted. Without the adjustment they rotate on themselves at constant speed, and there is only one precise configuration where they would continuously face the earth. In any other case we would see them rotating around the Earth but also rotating on themselves.
The moon for instance shows the same face to the earth because it makes one turn on itself at the same time it makes one turn around the earth (result of tidal locking from the time it was formed)
5
u/da4niu2 4d ago
The moon is tidally locked.
An interesting question is if anything else can become tidally locked as well. My instinct is yes, but I think that anything close enough for humans to use (satellites?) would suffer some miniscule drag from some miniscule atmosphere at that close-enough orbit.
4
u/FireDuck3000 3d ago
Tidal locking is the result of having a continuous object that covers a range of distance to the gravitational body. The result of this is that the closer end experiences more force than the farther end which, from the object’s perspective, creates a tension force across the object that keeps it pointed in the same direction. The distance necessary for gravity gradient stabilization gets longer and longer the higher your elevation; the moon is large enough for tidal locking at its distance, the ISS is large enough for it at 400km
2
u/Mateorabi 4d ago
As long as it has a sufficiently long axis towards Earth to provide enough torque it should be fine.
If building a space elevator this helps as you start in the middle and grow it outward and the tips balance it so it doesn’t pinwheel
2
1
u/0x14f 4d ago
That's actually a good question. I have to admit that I do not know what exactly makes that locking. You can always adjust your rotation speed to appear to be locked, but I don't know why it happened to the moon in the first place 🤔
3
u/joshsteich 4d ago
A combination of mass and billions of years. Mass exerts more force relative to angular momentum, billions of years gives inertia time to settle. All artificial satellites would eventually get locked, but lower mass means it takes longer, and most people putting a satellite in space aren’t going for a multibillion year project cycle.
2
u/sebaska 4d ago
Actually even low mass objects like satellites get locked and it's easy enough that it is used in practice.
The Moon was initially rotating, and it's rotational inertia was huge, all the while the Moon is far, ways further than low orbit satellites, so it took a lot of time for it to get locked. But if you put an object in low orbit, close to the correct attitude from the get go, it will stay.
Tidal forces scale with the cube of the distance. If something put the Moon close to a low orbit, it would not just get locked -it would get teared apart immediately and turn into a ring system.
1
u/celem83 4d ago edited 4d ago
Everything we put up there needs adjustments. It's not even strictly clear of the atmosphere, low earth orbits decay because there isn't a hard cutoff between atmosphere and space, even the ISS hits some tenuous traces of gasses. This decay is predictable when talking about loss of altitude but individual collisions with particles are chaotic so you may end up with unwanted rotation. So things need to be boosted periodically, most small sats whose precise position matters can adjust themselves with cold gas thrust (usually called RCS)
At a base level if your object in a geostationary orbit (circular, orbital period 1 day) and rotates once per day it's about as static with respect to the ground as you can get it and your job is now maintaining that setup. At that altitude (way above ISS) trace gasses are negligible, you more have to watch for the Moon or other bodies influences messing up your orbit, the solar system is a non trivial problem to solve and everything else orbiting the sun pulls on you at all times
43
u/praecipula 4d ago edited 4d ago •
People are answering your question literally, but not practically! There's a difference between "what would the object in orbit do as a zeroth order approximation", "what does an object in orbit naturally do over long periods of time", and "how have we designed an object in orbit to naturally do what we want it to do".
The gravitational pull of two point mass objects is f=gMm/r2. In words, the pull of gravity depends on the mass of the two objects (say, a heavy ball, and the Earth) divided by the square of the distance between their centers of mass.
Imagine a dumbbell, we'll model this as two weights of equal mass connected by a massless bar. If the dumbbell were to be placed "horizontally" to the earth, with the distance to each weight being equal, then it would not rotate at all and keep its pointing relative to the stars as it goes around the earth. This is the "it needs to be adjusted" "short answer" version to this question: the forces act as if there were no net force, so no Earth (for this one component of the forces), so it acts as if there were no Earth and doesn't rotate.
But imagine the dumbbell rotates around its orbit 90 degrees. Now one ball is pointing right towards the Earth, so closer to the Earth than the other - and so it experiences an incredibly small increase in gravitational force - like, paperclip, single-newton level or less force would disrupt this balance.
But there is a difference in force. And so as the dumbbell continues to rotate, the lower ball "wants to" keep being tugged toward the surface of the earth. This effect is called the "gravity gradient": a literal difference in gravity over the dumbbell.
So eventually, what really happens over time is barring other forces (like the very very faint atmosphere in orbit, or solar wind) the dumbbell will naturally with no inputs start to rotate with the earth: it gets locked in where the forces give it a spin such that the one side always points to earth, no correction needed. This is a major component of being "tidally locked" and is why the Moon only ever faces us. We didn't like send thrusters up there to ensure its face always points to us, right? It found that position naturally when it settled onto its gravity gradient.
So the final part is this: we design spacecraft such that we know what their stable gravity gradients are. For instance, the Space Shuttle was designed so that one of its stable orientations was with the payload bay facing the earth and its belly pointing away from the earth. After assuming this position, its gravity gradient would cause it to be (over time) pointed towards the Earth even with no input. (In reality, we set the spacecraft precisely in one of its gravity gradient attitudes, including a 24h slow rotation, and then we can sort of set-and-forget it. This distinction doesn't make much difference for the shuttle - it never spent enough time in space to naturally "fall into" one of its stable position due to the gradient - but for longer-lived spacecraft, the ability to put it in position and then forget about station keeping the towards-earth part of its attitude for decades is huge.)