The Nature of the Universe, Time Travel and More...

[Asbo Zaprudder]

Didn't Star Trek do parachute jumps from orbit? B'elana tried it in the holodeck in an episode of Voyager and Kirk did it as a sport but that scene was cut from one of the movies.

There was a proposal by General Electric for an emergency "bail-out" system capable of bringing a single astronaut safely down to the surface from Earth orbit called the Manned Orbital Operations Safety Equipment system or MOOSE:
MOOSE - Wikipedia
Some sort of heat shield is necessary, of course. It's not like jumping from a static platform as Alan Eustace, Felix Baumgartner and Joseph Kittinger did.

 

[Gingerbread Demon]

There was a proposal by General Electric for an emergency "bail-out" system capable of bringing a single astronaut safely down to the surface from Earth orbit called the Manned Orbital Operations Safety Equipment system or MOOSE:
MOOSE - Wikipedia
Some sort of heat shield is necessary, of course. It's not like jumping from a static platform as Alan Eustace, Felix Baumgartner and Joseph Kittinger did.

Neat idea but yeah abandoned.

 

[Will The Serious]

Some sort of heat shield is necessary, of course

That would be because the entry speed was high? Would it work from outside the Van Allen Belt or is that too far away to reach the ground in a reasonable amount of time?

-Will

 

[publiusr]

Most of the water evaporates as per the state diagram for water at zero pressure. However, this process removes energy from the water molecules that remain so that they transition to ice.

Triple point - Wikipedia

Water ice is such a tricky thing…expanding when everything else contracts while cold…kicking up features on Pluto….

 

[Asbo Zaprudder]

That would be because the entry speed was high? Would it work from outside the Van Allen Belt or is that too far away to reach the ground in a reasonable amount of time?

-Will

Orbital velocity is high; escape velocity is higher. I don't know what you mean by reasonable nor what the exact scenario is that you propose. What is the purpose of doing the calculation?

 

[Will The Serious]

Orbital velocity is high; escape velocity is higher. I don't know what you mean by reasonable nor what the exact scenario is that you propose. What is the purpose of doing the calculation?

I'm writing a little fanfic, and I'm curious. I was wondering if, by reasonable, it might take too long for a ship-free astronaut to fall, from a near zero velocity synchronous orbit starting point, to accelerate towards the ground. Maybe days? Longer; such that they might need more than environmental protection? Maybe they would also need a water supply or some food. The distances and the slower speed could mean a long time to the ground.

-Will

 

[Asbo Zaprudder]

A geosynchronous equatorial orbit (GEO) aka a geostationary orbit only has zero velocity with respect to a point on the equator at the surface of the Earth, which is not an inertial frame of reference. The orbital speed with respect to an inertial frame of reference such as the background stars is about 7,000 mph or 11,300 km/hour, so roughly 1.94 miles per second or 3.1 km/s. You would have to exert a suitable delta-v to deorbit in such a way that the path intersects the Earth's surface. The delta-v from the surface of the Earth to low Earth orbit (LEO) is between 9.3 and 10 km/s, and from LEO to GEO is 3.8 km/s (although GTO is the usual intermediary and requires slightly higher delta-v). So, a suitable delta-v would be greater than 3.8 km/s. Say you could adjust your velocity so that you'd fall straight down from an altitude of 35,786 km (22,236 miles). A numerical integration of the equation of motion* dr/dt = √(2GM(1/r - 1/R)) from geosynchronous orbital radius to the Earth's radius suggests it would take about four hours. That's the minimum time, assuming you're not also propelling yourself to increase the acceleration. GEO satellites carry some fuel for station keeping and to retire themselves into a graveyard orbit, but usually not enough to deorbit.

*Derived from conservation of energy.

 

[Will The Serious]

That is very helpful, Asbo Zaprudder. Thank you.
When you typed delta-V, you meant a static change such as, V2 - V1, not an acceleration?

This change would allow gravity to take over so the astronaut could fall out of orbit? So, V is in reference to the orbital velocity and a falling body, to fall out of a GEO would consequently enter the outer atmosphere at a minimum sheer speed of about 3500 mph (3.8 k/s).

In the low density of atmospheric gasses in the outer atmosphere, does that seem like a significant frictional heat generator. Once in the atmosphere, the gas medium would return the body to a geosynchronous orbit +/- the wind speed.

-Will

 

[Asbo Zaprudder]

Delta-v (more known as "change in velocity"), symbolized as Δv and pronounced delta-vee, as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a manoeuvre such as launching from or landing on a planet or moon, or an in-space orbital manoeuvre. It is a scalar that has the units of speed. As used in this context, it is not the same as the physical change in velocity of said spacecraft.
Delta-v - Wikipedia

Delta-v can be applied over a short or long period, but the time taken will affect the trajectory. Applying acceleration or deceleration too rapidly will turn a human into jam, and they'd be dead, Jim.

Say you attempt to decelerate from 3,200 m/s to 0 m/s at xg (where 1g = 9.81 m/s^2; acceleration due to gravity at the surface of the earth) takes 326/x seconds, so 326 seconds at a deceleration of 1g. The fully fuelled Apollo CSM had a thrust of ~90,000 N and a mass of ~30,000 kg, unfuelled mass was ~12,000 kg, so the acceleration was 3 to 7.5 m/s^2 or about 0.3g to 0.75g. The maximum burn time was 750 seconds. If thrusting in the direction of travel, deceleration lowers the perigee until it intersects the earth. The trajectory remains an ellipse, however - it doesn't become a straight line. While thrusting lowers the altitude, the orbital velocity goes as roughly 1/√r, where r is the distance from the centre of the earth, so it increases. Counterintuitively, you actually speed up because you lose potential energy and gain kinetic energy.

The radial velocity on entering the Earth's atmosphere when merely falling from GEO can be calculated from the formula I gave previously. The geocentric gravitational constant GM = 3.986x10^14 m^3/s^2; GEO is 42,164 km from centre of mass; top of atmosphere is 6,378 + 100 km (Kármán line) = 6,478 km from centre of mass, so the inward radial speed is 3.25 km/s or 11,710 km/hour at the Kármán line. However, there is also tangential velocity and as you're not actively braking the whole way down, you're really on an orbit that now intercepts the atmosphere of a rotating Earth. Re-entry velocity from LEO is typically 7.8 km/s, from the Moon, 11 km/s. You'd need more thermal protection than returning from LEO.

 

[publiusr]

Lots of counterintuitive things in LEO.

Microgravity environment means a tether will always point coreward in free fall.

Two craft docking. One fires thrusters to move straight forward...you wind up "over" your docking target.

That craft...if flying backwards aft pointing towards it's forward orbit...if it fires? It slows and drops...a de facto retro fire.

What was that phrase Larry Niven used..."East is...." ah....my memory...

 

[Asbo Zaprudder]

Someone been playing KSP?

 

[Disposable_Ensign]

I don't think a rotating universe is the best solution to the whole time-travel thing. It works much better (and fits with Trek) if you look at the String Theory approach. Given large extra dimensions (ADD), there are tons of alternate universes, if you will, on different 4D branes. Time travel, in such a conception, is achieved by traveling to different points on another brane (if this were possible).

 

[Asbo Zaprudder]

Yes, David Deutsch first suggested in 1991 that time travel's paradoxes could be resolvable if you travelled to alternate realities. String theory was not invoked.

Temporal Paradox: Parallel Universes
Time Travel Simulation Resolves “Grandfather Paradox” - Scientific American

I'll let you resolve the rotating universe scenario with Kurt Gödel when you can time travel.

 

[Disposable_Ensign]

Yes, David Deutsch first suggested in 1991 that time travel's paradoxes could be resolvable if you travelled to alternate realities. String theory was not invoked.

Temporal Paradox: Parallel Universes
Time Travel Simulation Resolves “Grandfather Paradox” - Scientific American

I'll let you resolve the rotating universe scenario with Kurt Gödel when you can time travel.

I only brought it up, because it is our current understanding of what alternate universes are.

 

[Asbo Zaprudder]

I only brought it up, because it is our current understanding of what alternate universes are.

Sorry, but brane theory is not the only game in town - there are lots of alternatives. See, for example, Max Tegmark's book Our Mathematical Universe: My Quest for the Ultimate Nature of Reality. A lot of this stuff is essentially metaphysics and might well be unfalsifiable - at least in our lifetimes. Like string theory, brane theory is not even wrong if it makes no predictions that would differentiate its viability from alternate theories if empirically tested. Perceived theoretical elegance is completely inadequate and unscientific.

 

[publiusr]

I'll let you resolve the rotating universe scenario with Kurt Gödel when you can time travel.

I tried that and all I got was Zwicky telling me how spherical I was....before I reminded him that everyone else calls me a square...sharp edges....but I do have my points.

 

[Asbo Zaprudder]

Putting on the Fritz!

 

[Disposable_Ensign]

Sorry, but brane theory is not the only game in town - there are lots of alternatives. See, for example, Max Tegmark's book Our Mathematical Universe: My Quest for the Ultimate Nature of Reality. A lot of this stuff is essentially metaphysics and might well be unfalsifiable - at least in our lifetimes. Like string theory, brane theory is not even wrong if it makes no predictions that would differentiate its viability from alternate theories if empirically tested. Perceived theoretical elegance is completely inadequate and unscientific.

It has more scientific data behind it than philosophical thought experiments or other theories. If we use the math that brane theory provides, we can accurately predict things in the real world.

 

[Will The Serious]

It has more scientific data behind it than philosophical thought experiments or other theories. If we use the math that brane theory provides, we can accurately predict things in the real world.

Math theory isn't scientific data, it's logic. What empirical data is there in Brane Theory?

-Will

 

[Asbo Zaprudder]

I'm also waiting for the empirical evidence...

...I think it'll be some time coming.