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Method for spaceship to be absolutely still in space

by Ringmistress
Tags: absolutely, method, space, spaceship
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Ringmistress
#1
Jul2-12, 08:15 AM
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Hi, my knowledge on physics is very rudimentary but I have been googling.

So, I wanted to ask is there a way for a spaceship to be absolutely still in space?

I thought about having an eye-shaped spacecraft and thrusters on all sides (360 degree), but if they're all on at once the force would probably destroy the spaceship?

I'm still a bit confused about Newton's first law. I understand that the velocity is supposed to remain constant unless acted upon by an external force.

Does that mean once you accelerate, you can turn off the engines and you'll forever be moving, but will you be moving at the same rate unless you go near some sort of external force like a magnetic field? Or regardless of encountering an external force, will a spacecraft eventually slow down?

Also, why don't objects stay still in space? There is no external force but objects drift?

Sorry if I seem like a total idiot. I barely did any science in high school.

All this stuff comes up after reading a sci-fi book and I can't stop thinking about what-ifs.

Thanks everyone! Much appreciated.

P.S. I did try to google how an object can stay still. But all I could find was satellites and using lagrange's points. Not really what I was after.
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Astronuc
#2
Jul2-12, 09:09 AM
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Still with respect to what?

It would be difficult for a spacecraft to be 'absolutely' still since all other objects are moving, so the local gravitational field is constantly changing. Perhaps it is theoretically possible to have thrusters arranged to thrust against the gravity field.

From a practical side, the purpose of a spacecraft is either to get somewhere, or maintain a stationary orbit for observation.

Thrusting levels can vary from fractions of N to kN, so multiple thrusters wouldn't necessarily crush a spacecraft.
Nik_2213
#3
Jul2-12, 09:14 AM
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'Still' relative to what ? Even between planets, you have the Earth's orbital velocity as a departure boost. Without it, you fall into the Sun. Then the Sun is in galactic orbit, and the galaxy is moving relative to Local Group etc etc.

Different if you mean approaching a docking with some other craft in orbit. Then you have attitude thrusters to do the fine work. If the body is an asteroid, with small but significant gravity, you'll need to account for that with your thruster firings....

meldraft
#4
Jul2-12, 09:16 AM
P: 280
Method for spaceship to be absolutely still in space

In space, objects that already have an initial velocity continue to move because there is no friction or some other mechanism that would allow them to dissipate their kinetic energy. It's like pushing something on ice, it will continue to move until it hits an obstacle.

There are things that can affect moving objects in space, such as planetary objects, black holes and, in general, gravitational fields. However, space is so vast, that the likelihood of a moving object actually hitting something is in general very small for the timescales that we human generally perceive in our daily lives (i.e. an asteroid can be quite likely to hit something during a time range of a billion years, but highly unlikely during the next century).

Also, if Einstein taught us something, is that nothing can be considered "still". Everything is moving in respect to something else.
Drakkith
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Jul2-12, 04:05 PM
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Quote Quote by Ringmistress View Post
So, I wanted to ask is there a way for a spaceship to be absolutely still in space?
First, it may help to understand that there is NO such thing as an "absolute" frame of reference. By that I mean that you cannot say that you are moving and something else is not, or that something else is moving and you are not. It turns out that according to an observer (person or object we are viewing the problem from) the laws of physics don't care who is moving or who isn't. For example, a baseball moving at 90 mph towards a batter could say that everything else is moving 90 mph and it is simply staying still. If you are riding in a car, to you everything is moving by and you are staying still, while to someone at a bus stop you passed you are most definitely moving and they are still. Both of you would be correct to say that the other is moving. Does that make sense?

So we could modify your question to ask if it is possible for a ship to remain stationary relative to a specific object or frame in space (such as the frame of reference of the CMB, the cosmic microwave background), in which case the answer is yes. A couple of thrusters could maintain station with a good degree of accuracy as long as there were no large sources of gravity nearby, such as a planet or star, in which case you would need larger engines instead of small thrusters.

I'm still a bit confused about Newton's first law. I understand that the velocity is supposed to remain constant unless acted upon by an external force.

Does that mean once you accelerate, you can turn off the engines and you'll forever be moving, but will you be moving at the same rate unless you go near some sort of external force like a magnetic field? Or regardless of encountering an external force, will a spacecraft eventually slow down?
Once you turn off your engines after accelerating you will go on forever. Or close to it. The interstellar medium is filled with a very very low density gas, perhaps a few atoms of hydrogen per cubic meter, so you would encounter a very small amount of drag. For our purposes we can simply ignore it though. Until you hit something you will keep going unless you purposely decelerate. Gravitational sources such as stars and planets will alter your path, but generally won't change your net velocity. (You speed up on approaching a star, but slow back down after passing it and moving away, so no net gain or loss)

Also, why don't objects stay still in space? There is no external force but objects drift?
Objects in motion will remain in motion. They are moving because they were moving in the past. Interactions between objects can make things move too, such as a comet impacting an asteroid and sending pieces of it flying out.
Chronos
#6
Jul2-12, 09:48 PM
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How would you ever know you were 'still'? Motion, or lack thereof, is purely relative to other entities in the universe.
D H
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Jul2-12, 11:45 PM
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Quote Quote by Ringmistress View Post
So, I wanted to ask is there a way for a spaceship to be absolutely still in space?
You've been beaten up enough on the "absolutely still in space" issue. Hopefully you get the picture.

So let's make your question more realistic: Is there a way for a spaceship to be still with respect to something else in space? Here's a real example: A spaceship approaching the Space Station so as to dock or berth with the Space Station. A bit of NASA nomenclature: It's called docking when the approaching vehicle flies all the way to the docking port. It's called berthing when the last bits are done with the Station's robotic arm. Think of the difference between a ship coming into port completely under its own power (docking) versus one pushed/pulled the last bit via tugboats (berthing).


I thought about having an eye-shaped spacecraft and thrusters on all sides (360 degree), but if they're all on at once the force would probably destroy the spaceship?
It probably wouldn't destroy the spaceship, but it also wouldn't accomplish the goal. Thrusters are never perfectly aligned and do not perfectly balance. Nothing is perfect in the real world. Why not just turn them all off -- after coming to rest with respect to the target, that is.

I'm still a bit confused about Newton's first law. I understand that the velocity is supposed to remain constant unless acted upon by an external force.

Does that mean once you accelerate, you can turn off the engines and you'll forever be moving, but will you be moving at the same rate unless you go near some sort of external force like a magnetic field? Or regardless of encountering an external force, will a spacecraft eventually slow down?
You'll forever be moving at the same rate until encountering some other external force.

Also, why don't objects stay still in space? There is no external force but objects drift?
Because there's nothing to change their velocity.


So back to our spaceship coming into dock or berth. I'll look at berthing because that's what SpaceX just did with its Dragon vehicle. (This is also the technique used by the Japanese HTV.) The final action by the Dragon was to get to a "berthing box" (a place where it can be grappled by the robotic arm), zero its velocity with respect to the very slowly rotating Space Station, and cut its engines. At this point it was up to the astronauts to grab the Dragon with the Station's robotic arm and maneuver the Dragon to it's docking port. Now let's step back a few hours. The Dragon did not just fly up to this berthing box and come to a rest. It was a very complex and very slow process. NASA and Russia's space agency are very leery of something damaging their Space Station. Building the Space Station took a long time and a whole lot of money.

The Dragon instead had to follow a curved path to a point below the berthing box. It then started moving up toward the berthing box, but it first had to come to rest with respect to the Station at a couple of hold points. Once SpaceX and NASA were convinced the Dragon was behaving correctly the vehicle was allowed to slowly continue to the next point.

So how did the Dragon (and the HTV) come to rest with respect to the Station, at least for a short time? It's not simple. Thrusters are never perfectly aligned, and thrust is not infinitely controllable. The process is done slowly to make sure everything is within control. Sensors and software constantly determine the relative position, velocity, orientation, and rotation between the vehicles. The guidance system has a predetermined plan for how things should proceed. The navigation system determines how much these observed values disagree with the planned values. Finally, the control system fires thrusters as commanded by the guidance and navigation systems to correct the discrepancies.

This never works exactly right, so the process continues, but eventually the errors become within some predefined tolerance. The vehicle is at rest, to within tolerance. That tolerance is very, very small by the time the vehicle gets to the berthing box. The robotic arm can grab a slowly moving vehicle without damage to the arm or the vehicle if the relative motion is slow enough.
Seiten Maoh
#8
Nov15-12, 11:40 AM
P: 1
I was just thinking the same thing. There is no way to be still with respect to nothing or perhaps the center of the universe (hypothetically)? Hypothetically a spaceship is equipped with a warp drive that is able to warp through spaces. And the spaceship set its destination to a dead space far away from anything, there is no gravitational pulls, it's far away even from any galaxy. Will the ship be absolutely still when it comes out of warp? Or Even more wild, a jump drive instead, and just jump through spaces, will the ship still maintain its momentum in a deadspace when it exits the wormhole?

Like swimming in a river, by swimming against the current, you will find yourself not going anywhere(if you are really good swimmer that is), so is there a way to calculate the gravitational pulls and use thrusters to push against the pull so that you can neutralize all the momentums from planets, stars, galaxies? And in that sense what you would observe is that the moment the thruster is activated, you will suddenly see earth and everything around you moving rapidly away from you as they are being pulled by the galaxy's gravitational pulls?
BobG
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Nov15-12, 06:40 PM
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The heck with all the technically correct stuff. You should concentrate on the fun stuff.

Read Isaac Asimov's "The Billiard Ball".

"If I had been a quicker thinker, I would have known Ed was speaking the purest nonsense when he said that the billiard ball would slowly rise in the zero-gravity field. It _couldn't_ be so! If Bloom hadn't despised theory so, if he hadn't been so intent on being proud of his own ignorance of theory, he'd have known it himself.

"The Earth's motion, after all, isn't the only motion involved, young man. The Sun itself moves in a vast orbit about the center of the Milky Way Galaxy. And the Galaxy moves too, in some not very clearly defined way. If the billiard ball were subjected to zero gravity, you might think of it as being unaffected by any of those motions and therefore of suddenly falling into a state of absolute rest--when there is no such thing as absolute rest."
phinds
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Nov15-12, 07:11 PM
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Quote Quote by Seiten Maoh View Post
... There is no way to be still with respect to nothing or perhaps the center of the universe (hypothetically)?
There is no center to the universe, hypothetical or otherwise. You should read the FAQ in the cosmology section.
ImaLooser
#11
Nov15-12, 09:38 PM
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How about being still with respect to the average cosmic microwave background?
BobG
#12
Nov16-12, 06:18 AM
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Quote Quote by ImaLooser View Post
How about being still with respect to the average cosmic microwave background?
Quote Quote by Drakkith View Post
So we could modify your question to ask if it is possible for a ship to remain stationary relative to a specific object or frame in space (such as the frame of reference of the CMB, the cosmic microwave background), in which case the answer is yes. A couple of thrusters could maintain station with a good degree of accuracy as long as there were no large sources of gravity nearby, such as a planet or star, in which case you would need larger engines instead of small thrusters.
Yes, but note that you'd need to fire thrusters to do it, since there is no where far enough from everything else that you'd feel no forces at all. Your spaceship would drift unless all of the external forces acting on it were perfectly balanced. Since all of the other objects in the universe would be moving with respect to the CMB, the external forces would be constantly changing, making it impossible for them to remain perfectly balanced.

While Newton's First Law is true, and the external forces on an object may be so small as to be negligible for your purposes, there will never be a situation where an object actually has no external forces on it.

Considering you left one of those other objects in the universe, you wouldn't be stopping your spaceship. You'd be speeding up your spaceship and heading it in a direction that would be stationary relative to the CMB (like swimming upstream). Keeping in mind that it would never truly stay stationary for more than an instant.

The easier method would be to choose a reference frame that had you at the origin of it. Then you'd be stationary and all of the other objects would be moving relative to you.
Drakkith
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Nov16-12, 12:03 PM
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So you are saying you'd need thrusters Bob? That's what I'm getting from your thread.


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