Lowest orbital altitude

[a_lost_packet_]

It is not clear to me if you really have to jump to be "in orbit". :?
But it is a fact that we all literally trace an 'orbit' around the center of the Earth. It may not be a 'gravitational orbit', but an orbit it is. Maybe that is what a_lost_packet meant :?:

Sorry, I sort of put my foot in my mouth there.. which is certainly not conducive to achieving orbit by jumping. I was only trying to emphasize the idea that a certain velocity is necessary. I should have included a disclaimer there. I'll edit to include one.

One can't attain "orbit" by jumping on Earth because, for one thing, one can't leave the atmosphere.

As aremisasling stated:

...An orbit is any path that has enough velocity tangential to the surface of the body it is gravitationally bound to in order for it's path to not intersect the atmosphere or surface of that body during one revolution. ..

So, "No" you can't achieve orbit by jumping on Earth. A fact that I'm sure plenty of jumping people are thankful for because we all seem to enjoy breathing quite a bit..

 

[orionrider]

But it would not take much jumping to hop from a large asteroid or even a small moon. :shock:
With a top speed of about 12m/sec, Usain Bolt could easily escape Phobos
http://www.iaaf.org/LRR09/news/newsid=50606.html

 

[aremisasling]

Here's a nice visual representation of gravity wells with some nice comments about escape velocities for the Martian moons.

http://xkcd.com/681/

 

[Woggles]

Here's a nice visual representation of gravity wells with some nice comments about escape velocities for the Martian moons.

http://xkcd.com/681/

Thanks. What an awesome chart!!!

 

[MeteorWayne]

I don't think "mountains and changes in density" would cause an orbit to decay. They do not substract or add energy to the spacecraft. I think once you achieve a stabel orbit over an airless planet, at any altitude, your spacecraft would continue indefinitekly.

You are wrong. That is why low lunar orbits have limited lifetimes, despite the lack of appreciable atmosphere.

 

[a_lost_packet_]

Earth's gravity map.

Heightmap is exaggerated, obviously, to show the differences in influence in regards to Earth's local gravity variations.

 

[orionrider]

Excellent, thank you Lost_Packet!

Here the legend for this map: http://nasadaacs.eos.nasa.gov/articles/ ... avity.html
This map, created using data from the Gravity Recovery and Climate Experiment (GRACE) mission, reveals variations in the Earth's gravity field. Dark blue areas show areas with lower than normal gravity, such as the Indian Ocean (far right of image) and the Congo river basin in Africa. Dark red areas indicate areas with higher than normal gravity. The long red bump protruding from the lower left side of the image indicates the Andes Mountains in South America, while the red bump on the upper right side of the image indicates the Himalayan mountains in Asia.

 

[nimbus]

A possible extended mission of MESSENGER (assuming the one year primary mission is successful & MESSENGER is still operational, at least the main instruments), is that the Periherm (lowest point in an orbit around Mercury, apoherm the highest) of MESSENGER may be reduced to 30 KM / 18 miles, to image portions of the surface at better than one metre resolution, but also to measure smaller MASCONS.

Andrew Brown.

Wow!! hehe.

 

[CalliArcale]

A possible extended mission of MESSENGER (assuming the one year primary mission is successful & MESSENGER is still operational, at least the main instruments), is that the Periherm (lowest point in an orbit around Mercury, apoherm the highest) of MESSENGER may be reduced to 30 KM / 18 miles, to image portions of the surface at better than one metre resolution, but also to measure smaller MASCONS.

Andrew Brown.

Wow!! hehe.

*drools*

That will be awesome. Just for the pictures alone, it will be awesome, but mapping out the gravity field will reveal a heck of a lot about Mercury's internal structure. Totally awesome.

 

[Captain_Salty]

What would happpen to a vehicle like a car, that reached orbital velocity on the ground, of a perfectly spherical atmosphereless planet?
Can it be weightless and still be in contact with the ground?

 

[orionrider]

Aha, it reminds me of trying to reach the speed of light, sort of
The less weight your car has, the higher 'slippage' becomes and the less traction you get. At 99.9% slippage becomes infinite and traction tends to zero. So your car never becomes 100% 'weightless'.

 

[Captain_Salty]

wouldn't a spinning tyre still get some grip (push) against the ground, even if it was weightless?

 

[MeteorWayne]

Why would it be in contact with the ground if it was "weightless"?

 

[Captain_Salty]

Because it starts off against the ground and there's no lifting force (at least until orbital velocity is surpassed) ? I guess it'd have to have no suspension, solid tyres and maybe a wheelie bar though :lol:

 

[nimbus]

It's really a philosophical question, not one with practical implications.. You'd have rolling resistance slowing it down and so slowing below orbital velocity. The only way you can have such an extraordinary contraption working as you describe is with perfectly frictionless wheels, from ground contact patch to roll mechanisms.

In any other circumstances, "in contact with the surface" and "orbital velocity" are mutually exclusive.

 

[aphh]

Perhaps the easiest way to conceptualize orbital velocity is to think about the force/acceleration that pulls you down and what is needed to negate that.

Earth pulls you down with an average constant acceleration of 9.8 m/s^2. To stay on circular orbit you would need constant acceleration of 9.8 m/s^2 of opposite direction.

For object in uniform circular motion (object in orbit) the acceleration, called centripetal acceleration, comes from constantly changing direction. Velocity is a vector, hence it has both magnitude and direction. Change of both, or just either one, means acceleration. On orbit your speed remains constant, but the direction changes constantly, hence you get constant acceleration. This will negate the pull by gravity.

Formula for centripetal acceleration for uniform circular motion is as simple as a = v^2 / r.

a = Centripetal acceleration
v = Orbital speed
r = Radius from the center of gravity

In the equation, if you replace a with 9.8 m/s^2 and r with the desired orbital radius, for example 6371 000 m + 355 000 m (average radius for Earth + average altitude for ISS), you can easily calculate the required orbital speed, that gives constant centripetal acceleration of 9.8 m/s^s, which negates the acceleration of gravity that constantly tries to pull you down.

Solving the equation with those numbers gives orbital velocity of 8123 m/s. With this speed the ISS would orbit the Earth in about 1.45 hrs, which is equal to 87 minutes.

In reality the speed is a little less and one orbit takes ~90 minutes, because the pull of earth's gravity is a little less than 9.8 m/s^2 at that altitude. The gravity becomes progressively less the higher up you go. It is still a rather good approximation especially for very low orbits.

Also note that the vector for centripetal acceleration points down to the center of gravity. The force created by centripetal acceleration will push you away from the center of gravity. Hence the acceleration of gravity and acceleration of uniform circular motion cancel each other, and you stay on orbit.

 

[Captain_Salty]

So the earth really has a natural ground speed limit, beyond which you will lift into an orbital projectory. Maybe that's when you fire off the down thrusters :mrgreen:

 

[MeteorWayne]

Again, it should be pointed out, that at the earth's surface, the orbital velocity is 17,686 mph, or Mach 23, so you would instantly be incinerated....

 

[aphh]

Again, it should be pointed out, that at the earth's surface, the orbital velocity is 17,686 mph, or Mach 23, so you would instantly be incinerated....

That's laughable. Belongs to Sci-fi or Unexplained.

 

[MeteorWayne]

What's laughable?

 

[ZiraldoAerospace]

What's laughable?

Yeah I know, what part of Wayne's comment was laughable? Do you know of a way to not be incinerated from going Mach 23 at ground level? Please do share.

 

[yevaud]

Any velocity over (ballpark) Mach 4 is considered "Hypersonic." As you get to those velocities in atmosphere, the air becomes highly compressed in front of the moving object and heats up greatly. In point of fact, at Mach 4 or more, our own airframes that are capable of achieving those velocities have heating from compression as a major consideration.

This is also the mechanism from which meteors "burn up" in the atmosphere - not from friction with the atmosphere, as is erroneously believed.

 

[BurgerB75]

If I recall correctly the SR-71 Blackbird's titanium skin was actually "loose" until it hit speed. This allowed for the skin to expand and not buckle. And this was at speeds less than Mach 4.

Fake Edit:

Found it: http://en.wikipedia.org/wiki/Lockheed_S ... d_airframe

The skin wasn't loose but corrugated.

 

[MeteorWayne]

by aphh » Fri Jul 23, 2010 8:51 pm

MeteorWayne wrote:Again, it should be pointed out, that at the earth's surface, the orbital velocity is 17,686 mph, or Mach 23, so you would instantly be incinerated....

aphh>>That's laughable. Belongs to Sci-fi or Unexplained.

by MeteorWayne » Sat Jul 24, 2010 1:36 am

What's laughable?

-so aphh, how come you've never replied to this? Couldn't get your foot out of the keyboard?

 

[Woggles]

If I recall correctly the SR-71 Blackbird's titanium skin was actually "loose" until it hit speed. This allowed for the skin to expand and not buckle. And this was at speeds less than Mach 4.

Fake Edit:

Found it: http://en.wikipedia.org/wiki/Lockheed_S ... d_airframe

The skin wasn't loose but corrugated.

Hi Burger, I just watch Great Planes last night. They said the wings, which hold the fuel, actually leaked out fuel till the skin got hot enough to seal.