Weightless or Heaviness Descent into Planet spaceship

[Albertgauss]

TL;DR Summary

Does an astronaut feel heavier or more weightless during descent on a planet when his spaceship slows down?

I was working on something where I was trying to imagine an astronaut flying into Jupiter. I convince myself that as the spaceship descends, as it slows down, whether or not the astronaut feels heavier or feels more weightless. I believe the answer is that the astronaut will feel heavier and I would just like someone to confirm this--- is this correct? I attach a slide of the Newton's laws I think are pertinent to the situation. I know the normal force acting on the astronaut is the perception of the weight that he feels.

Also, because this would not pertain just to Jupiter, is it true that astronauts in any spaceship that come to a landing on a planet will also necessarily feel heavier during their descent. Any idea what Earth astronauts feel for the situation when space shuttles land, just ballpark? as I cannot find this answer on the web?

 

[hmmm27]

Weight is a combination of mass and acceleration. Standing on Earth (or Moon, etc) we feel the gravitational pull (acceleration). An astronaut launching straight up, or landing feels the pull of the planet plus the acceleration of the rocket.
So, yes.

 

[Ibix]

If you feel an acceleration equal to the local gravity then you are traveling at constant speed with respect to the ground. In principle it's possible to descend at a constant 1m/s or something, feeling only your weight at that altitude (so slowly increasing to your normal weight if you are doing this on Earth). There'd be a small bump on landing. But that is fuel hungry.

A lot of capsules fall into the atmosphere, so will feel probably quite a high acceleration until they've slowed to terminal velocity, then whatever normal gravity is where they are, one hell of a bump when the chutes deploy, then back to normal gravity until another big bump on landing/splashdown.

The shuttle would be similar, I expect, with increased weight as it brakes into the atmosphere, then normal weight once it enters aerodynamic flight.

 

[DaveC426913]

It might help to look at it the other way around.

An astronaut always feels the gravity from a nearby planet - unless his orbital speed is fast that the curvature of his orbit creates significant acceleration toward the planet, cancelling out some or all of the gravity.

So, start with his weight as if he's just floating above the planet. Then subtract any apparent weigh loss due to his orbital curvature (if any).

And add any weight due to any deceleration.

 

[jbriggs444]

An astronaut always feels the gravity from a nearby planet - unless his orbital speed is fast that the curvature of his orbit creates significant acceleration toward the planet, cancelling out some or all of the gravity.

I would word it differently. An astronaut never feels the gravity from anything, near or far unless either (1) the thrusters are turned on or (2) His name is Beowulf Shaeffer and is approaching a Neutron Star.

What you see and "feel" as the "force of gravity", is your craft (or your room) moving on a non-inertial trajectory and the floors or your seat pushing you to follow that non-inertial trajectory. You feel like you should be moving along with the craft and you infer something moving you otherwise. You can feel the same thing in a car going around a corner.

Your "proper acceleration" due to the thrusters can be anything from near zero to more than three gees and can vary as you descend from orbit to end hovering in Jupiter's atmosphere.

For exotic trajectories that intersect with the planet, I believe that it is possible to experience nearly zero gees all the way to the point where the craft begins hovering. [You get into a very eccentric elliptical orbit that passes through the planet and then decelerate slowly until the orbit ends up with a tangent to the point where you want to hover. Then you hit the 3 gee button and hover]

 

[DaveC426913]

I would word it differently.

I was thinking of it from the POV of the OP writing a story and trying to figure out when the astronaut feels weight. Do you feel weight when you're decelerating into Jupiter's atmo?
I figure my perspective would show that the only time you wouldn't feel Jupiter's gravity is when you're moving at (Jupiter's) orbital velocity.

Beowulf Shaeffer and is approaching a Neutron Star.

 

[jbriggs444]

Do you feel weight when you're decelerating into Jupiter's atmo?

You will feel the effect of deceleration. This may be much greater than three gees.

Suppose that you start from a moderately high circular orbit. You will have a deorbit burn to drop you into an elliptical orbit that intersects with the top of the atmosphere where you want to end up. This can be as gentle a burn as you please.

Then you will be moving at very high speed relative to the atmosphere. You can use aerodynamic braking. This can start fairly gently. But as you lose speed, you'll either lose altitude and thereby encounter thicker atmosphere and greater deceleration. Or you'll hover on your thrusters to maintain altitude, but then experience increasing proper acceleration as the required support force increases.

 

[DaveC426913]

You will feel the effect of deceleration. This may be much greater than three gees.

Yes, but I believe the OP's question is related to when do you (also) feel the weight from gravity.

 

[jbriggs444]

Yes, but I believe the OP's question is related to when do you (also) feel the weight from gravity.

The answer is still never. Unless the thrusters are on. Or unless the craft experiences positive buoyancy. You do not feel gravity. You feel the force of the seat on your pants.

 

[russ_watters]

I was working on something where I was trying to imagine an astronaut flying into Jupiter. I convince myself that as the spaceship descends, as it slows down, whether or not the astronaut feels heavier or feels more weightless.

What does "more weightless" even mean?

 

[DaveC426913]

What does "more weightless" even mean?

Presumably OP meant antonym of heavier, i.e. lighter - than normal ... gravity .. of ... er ... the planet in question?

 

[hmmm27]

What does "more weightless" even mean?

I think he's trying to determine how gravity and coordinate deceleration add up.

 

[Albertgauss]

Some clarification. I wasn't able to get to this post until now.

"An astronaut always feels the gravity from a nearby planet"

I think you what mean here is that gravity always acts on the astronaut from a nearby planet, which I agree with. Yes, the 1/r^2 law never goes away.

What I am trying to understand is what would a person feel when there is deceleration towards the planet to slow them down. I know that on Jupiter with g =25, that means a person weighs several hundreds of pounds, which as I have understood is such a large force on the person that the heart cannot even pump blood. Astronauts taking off for launch have their net acceleration in the same direction as a decelerating astronaut slowing down on approach to the planet; in the former case the astronaut is speeding up (velocity increasing) away from the planet as they launch away, in the case I am considering the astronaut is also accelerating away from the planet (or decelerating towards the planet), but their velocity decreases as they slow down. This would be the thrusters of jbriggs444 comment. I read some books where the astronaut said that during launch, they feel a constriction or tightness on their body which they attributed to the increased weight of takeoff. I admit that this post has helped me to flush out that I may not have understood quite what I read in this regard. Anyway, I was trying to understand if an astronaut decelerating would feel something similar since they would have the same net upwards acceleration.

"An astronaut never feels the gravity from anything, near or far unless either (1) the thrusters" from JBriggs444.

But what about when you stand on the ground or sit in a chair and the normal force acts on you? The Normal force just balances your weight in both cases, and I can feel the chair/floor touching me. I went skydiving once and was in freefall for a little while but I didn't feel anything on my body. It was these two experiences I have been thinking of where I try to understand if I can perceive my weight or not.

"I figure my perspective would show that the only time you wouldn't feel Jupiter's gravity is when you're moving at (Jupiter's) orbital velocity." This is DaveC's comment. I would agree here, but wouldn't this also be a freefall, like when I was skydiving?

"You will feel the effect of deceleration."

Maybe this helps to clarify what I am really after. What is felt in the body? again, I am thinking of the astronaut on the launch pad with the constriction in their chest. I understand that if my ship accelerates in the non-inertial frame I will see things in my ship move past me (there is a NASA video showing what I think pertains to this where the astronauts aren't touching the space station but then it accelerates--they stay still and the ship moves past them)

"This may be much greater than three gees."

Yes, certainly, I agree there.

"Presumably OP meant antonym of heavier, i.e. lighter."

Yes, correct, I could not think of the correct antonym. What is the correct antonym, if there is even one?

 

[jbriggs444]

But what about when you stand on the ground or sit in a chair and the normal force acts on you? The Normal force just balances your weight in both cases, and I can feel the chair/floor touching me.

You cannot feel [uniform] gravity. It acts uniformly on all parts of your body, so it results in no stress anywhere. Free fall in deep space far from gravity is the same as free fall in an elevator shaft on Earth. Both are the same as free fall in an orbit around Jupiter. Free fall in a circular orbit around Jupiter feels the same as free fall in an elliptical orbit that will end up impacting into Jupiter.

What you can feel is stresses on your body. If you are standing on the planet's surface, you feel the support compressing your feet. You feel the muscle tone it takes to keep yourself upright. You feel the effort it takes to raise your arms away from your sides. [Your muscles contain what amount to strain gauges/position sensors -- they can tell how your body parts are positioned and how they are deflected by the stresses required to maintain your posture]

The acceleration you can feel and measure with an accelerometer (essentially a known mass mounted on a set of spring scales) is called "proper acceleration". This is the acceleration that is produced by the real forces other than gravity^(*) on your space craft. If the craft is not in the atmosphere, the only such real force is from the rocket motors and attitude thrusters.

For a spacecraft that is floating within the Jovian atmosphere or which is suffering extreme wind resistance as it comes to a stop, one also has to consider the real forces of buoyancy and wind resistance. Both are real forces and will produce proper acceleration.

(*) Proper acceleration ignores so-called "inertial" forces which act equally on all parts of a body in proportion to the mass of each part. In Newtonian mechanics, gravity is the only inertial force which is considered to be a real force. Fictitious forces such as the Centrifugal or Coriolis force are all inertial and do not count toward proper acceleration. [In General Relativity, gravity loses its place as a real force and is just another fictitious force, so then it for-sure does not cause proper acceleration]

 

[Albertgauss]

Okay, I get it now. That makes a lot more sense. I certainly appreciate everyone's help in this. I feel this is all cleared up. I see how very confused I was, but I learned a lot from this discussion.