Smaller Planet = Less Gravity?

How small would the planet have to be so that gravity was not strong enough to kill you from a high drop? 1/2 the size? smaller? (eg falling out of a 7 story window and safely landing)

You eventually stabilize your descent when falling due to air resistance. However I guess with a much smaller planet you would have about as much air as the moon does. So you might not die falling but you would die of asphyxiation.

Will also depend on what the planet is made of (it's density).

Oh yes, a foam rubber planet would probably be OK. It would have other issues, of course. :slight_smile:

It’s rather more complex than that.

Less gravity means less atmosphere, less atmosphere means less air resistance, less air resistance means little or no “terminal velocity” effect, so “falling” from a sufficient height may still result in a lethal velocity. To have negligible risk of gravitational impact, the planet may have to be quite small indeed - of the order of a few miles.

And the atmosphere can be lost even if a planet has substantial gravity - more so than the moon which has essentially no atmosphere.

On the other hand if you limit the distance to a “seventh story” drop, then it may be possible on something not too much smaller than the moon.

Space suits are rather heavy.

As pointed out earlier it is the mass of the planet that is important not it’s size as such.

What kills you when you fall is that you hit something solid and the sudden deceleration subjects your body to forces in excess of what it can tolerate.

Newton’s second Law is F=ma where F is force, m is mass and a is acceleration.

On Earth parachutists probably want to hit solid ground at a lot less than 5m/s if they can possibly help it - which is not really very fast. If you take 5m/s as the speed at which you have a fair chance of surviving a fall relatively uninjured that would let you calculate how far you could fall on any given planet or what mass the planet could be to let you survive a fall from a given height.

Of course if you land on something that decelerates you slowly that makes a huge difference. There are plenty of videos on the internet of people leaping from several stories into snow and walking away unhurt.

On a related note, gravity may be increasing. If the doc tells me I've gained a few pounds, at least I will have an excuse :wink:

If the doc tells me I've gained a few pounds, at least I will have an excuse

I read somewhere recently that a gravitational map of the world has been made and that your body weight can vary by 1% depending on where you are on the planet. That sounds a lot but assuming its not a typo you may wish to choose where your doctor is located.

I read somewhere recently that a gravitational map of the world has been made

And the cartographer is about to return home.

And the cartographer is about to return home.

That is some satellite. I used to think the oceans were flat, but apparently not. It also seems melting ice sheets will actually cause sea levels to fall in some areas because of the gravitational shift caused by the redistribution of mass. Currently ocean levels have been falling (or at least not rising as fast as expected) because the damned Australians have been stealing a lot of water :slight_smile:

That is some satellite

Amen to that.
The accelerometers are phenomenally sensitive

"Imagine a snowflake, which has a fraction of a gram, slowly falling down on to the deck of a super tanker. The acceleration that the super tanker experiences from that snowflake is comparable to the sensitivity of the instruments.” (GOCE Mission Manager Rune Floberghagen)

AWOL:

That is some satellite

Amen to that.
The accelerometers are phenomenally sensitive

"Imagine a snowflake, which has a fraction of a gram, slowly falling down on to the deck of a super tanker. The acceleration that the super tanker experiences from that snowflake is comparable to the sensitivity of the instruments.” (GOCE Mission Manager Rune Floberghagen)

And then they took that sensor, strapped it to several hundred kg of rocket fuel, and blasted it into "space".

Your bones would be a lot weaker in low gravity so it could self-cancel.

radman:

If the doc tells me I've gained a few pounds, at least I will have an excuse

I read somewhere recently that a gravitational map of the world has been made and that your body weight can vary by 1% depending on where you are on the planet. That sounds a lot but assuming its not a typo you may wish to choose where your doctor is located.

yes, from a satellite called "GRACE" Page Redirection

the distance and starting velocity also make big difference: if you are traveling near the speed of light i think how to stop is your real problem..

Some interesting things happening with gravity at the moment;

Apparently the current official value for the force of gravity is an average, the idea being that as measurements become more precise the different experiments will converge towards a fixed value. Strangely though some experiments are suggesting that the value is actually changing. It is probably an experimental error, but scientists seem to be scratching their heads and some are getting excited.

Plans are also afoot for a tremendously sensitive experiment involving several (3?) satellites millions of kilometers apart trying to detect small changes in the relative movement of masses caused by gravity waves.

Before going for the big experiment the European Space Agency is sending up a single satellite to measure movement between a couple of 2kg cubes of gold and platinum (?) as a proof of some of the technology involved. Here is where things get interesting though. As a variation on the original plan for this satellite it is being suggested that it could be driven to a "saddle point" where gravity should be zero. There is a theory, Modified Newtonian Dynamics (MOND), that the force of gravity does not fall uniformly with distance in a traditional Newtonian way. Aparently if the ESA satellite is driven to a saddle point it would make MOND and several other gravitiational theories "stand out like a sore thumb" if they are correct.

Lets hope they go for it and do find something interesting. This could be very big and very exciting.

radman:
Some interesting things happening with gravity at the moment;

Apparently the current official value for the force of gravity is an average, the idea being that as measurements become more precise the different experiments will converge towards a fixed value. Strangely though some experiments are suggesting that the value is actually changing. It is probably an experimental error, but scientists seem to be scratching their heads and some are getting excited.

Plans are also afoot for a tremendously sensitive experiment involving several (3?) satellites millions of kilometers apart trying to detect small changes in the relative movement of masses caused by gravity waves.

Before going for the big experiment the European Space Agency is sending up a single satellite to measure movement between a couple of 2kg cubes of gold and platinum (?) as a proof of some of the technology involved. Here is where things get interesting though. As a variation on the original plan for this satellite it is being suggested that it could be driven to a "saddle point" where gravity should be zero. There is a theory, Modified Newtonian Dynamics (MOND), that the force of gravity does not fall uniformly with distance in a traditional Newtonian way. Aparently if the ESA satellite is driven to a saddle point it would make MOND and several other gravitiational theories "stand out like a sore thumb" if they are correct.

Lets hope they go for it and do find something interesting. This could be very big and very exciting.

do you mean earth gravity (9.81 m/s^2) or the gravitational constant (6.67e-11 someunit )?

There was an interesting article in last Scientific American about LIGO project e.a. to detect gravity waves.

do you mean earth gravity (9.81 m/s^2) or the gravitational constant (6.67e-11 someunit )?

Gravitational Constant.
LIGO is earth bound and uses laser at right angles with a path length (after bouncing from mirrors) of hundreds of kilometers.

Laser Interferometer Space Antenna (LISA) will be doing a similar thing but with a path of millions of kilometers!

LISA will detect gravitational waves in a differnet frequency range to LIGO so they compliment each other.

For me though what is exciting is that just the LISA proof of concept satellite (due to launch next year?) might completely change our understanding of gravity - if somebody has the nerve to drive it to a "saddle point".

I do most of my science reading in the New Scientist. There have been a couple of Apollo 8 astronauts writing to the letters page recently so I seem to be in good company.

Considering how many tons of space debris fall on Earth each year, the value should go up at some very small rate.

Surface gravity of Saturn is supposed to be close to that of Earth. It is much more massive but far less dense.

IIRC a moon of either Jupiter or Saturn has a dense poisonous atmosphere that is constantly being replenished. Our Moon could have one, it just can't keep it but no I don't know how long, probably years.

Mars might keep an atmosphere if it had a bigger moon to power core activity. There would be an engineering feat! Combine Phobos with Deimos and a bunch of asteroids, drop comets and asteroids with volatiles on the planet to get started, or try the route from Kim Stanley Robinson's Mars series. At 1/4 G on the surface you could fall from a good bit higher than you'd like on Earth.

I had a science teacher who talked about jumping from a 30 foot practice tower for practice back in the round parachute days. Maybe he was just trying to scare us. I've jumped from a 1 story roof and done the martial arts roll on the sloping lawn with no big problem (shoulder hit the heater fill pipe cap on the third roll, it was sore for days) when the ladder down was full and the police were coming (we were throwing M-80's off the roof, could hear the sirens, it was jump or get caught).

There's people who've survived 5 story falls when they hit all spread out. Operating word is survived.

My recent favorite was the wing suit guy that landed in the big prepared boxes (there may have been balloons or bubble wrap involved) thing. He hit at 100+ mph and the sound of collapsing cardboard was kinda neat!

Has anybody had lifeguard training where you jump off a 12-ft board and not have your head go under? It works off a 16-ft board too!

GoForSmoke:
Mars might keep an atmosphere if it had a bigger moon to power core activity

death-star in reverse!!!
but we don't have the technology yet