# Talk:681: Gravity Wells

Why is Earth's well's depth listed as 5478km but as 6379km in the inset? Compare with Mars which has 1286 in both places. 87.174.225.131 07:21, 12 April 2013 (UTC)

Best guess is either a goof, or that the lower number is just for Earth itself, while the greater number is for the Earth/Moon system as a whole. Proportionally speaking, we have the largest moon in the solar system, so maybe it wouldn't nicely fit in the Earth well as easily as Mars's and Jupiter's moons do.--Druid816 (talk) 08:28, 12 April 2013 (UTC)
It may be the height needed to go from one gravity well to another. You don't have to get all the way up to escape speed for that.
Randall wasn't kidding about the Sun being "very very far down"; its well is 100 times deeper than Jupiter's!
Wwoods (talk) 19:47, 12 April 2013 (UTC)
OTOH, from the table above i'm thinking that the 5.4 might be the Venus figure, and it was wrongly placed besides Earth...
Secondly, what i found interesting was that the Earth's 6.4 looks so much like its radius! I wonder if it's merely a coincidence, or there's a connection between the two... -- 141.101.99.233 21:25, 30 October 2013 (UTC)
The table is great, it must be included in the article; layout and time is just my problem right now. PRO TIP: Do not care about the x-axis.--Dgbrt (talk) 22:18, 30 October 2013 (UTC)
The fact that the density of the Earth is 5478 kilograms per cubic kilometer makes me pretty sure it is a typo. Fewmet (talk) 03:04, 4 July 2014 (UTC)
Hehe, you might be right. That's the best explanation. It would be a strange coincidence otherwise. But your units are wrong: a cubic kilometer of water, ice-cream or Natalie Portmans would be already something like a billion kilograms. Or a trillion, if you are American. Oh, you might be American. In this case: happy 4th of July! -188.114.102.35 12:39, 4 July 2014 (UTC)
Thanks for catching that (and for the July 4 wishes). It should be kilograms per cubic meter. Looking into that, though, leaves me less sure that is the origin of the problem. I thought I had multiple sources for Earth having a density of 5478 kg/m3, but can find only one (and not a very compelling one at that). I have sounder sources for 5513 kg/m3. 5514 kg/m3, 5515 kg/m3, 5520 kg/m3 and 5540 kg/m3. It may be trivial in that all round to 5500 kg/m3.
It was corrected on the poster version. Earth's well in the main graphic is marked as 6379km, just like the inset.108.162.216.86 00:19, 21 August 2014 (UTC)
I solved for the wells on Earth, Moon and Mars using the equation Randall gave and masses and equatorial radii from NASA, getting 6371 km, 287 km and 1286 km, respectively. Fewmet (talk) 23:07, 5 July 2014 (UTC)

The Oberth Effect mentioned in the title text is well-explained here (assuming you are not intimidated by the algebra in squaring a binomial). The gist of it is that using a bit of fuel in a rocket thrust will increase the rocket’s kinetic energy . The higher the kinetic energy at the time of the thrust, the greater the increase in kinetic energy. It works because the energy of the fuel goes into increasing the kinetic energy of the ship and the kinetic energy of the spent fuel. The faster you go, the greater the portion of the energy the ship gets.

The “gravity assist” is also known as the slingshot effect. The Wikipedia explanation is good, especially with its diagram. In it a spaceship (or other body) accelerates toward a planet (or moon, star, etc.) in the same direction that body was going. The ship picks up a little of the body’s momentum and so goes faster, although only according to an external reference frame. An observer at rest with respect to that other body would actually see the ship approach and depart with the same speed.

The title text reference to orbital speed is unclear to me. I suppose it just means that the given gravity wells assume you are at rest on the surface of the planet. Then being in orbit (and necessarily having an orbital speed) would mean you are part way out of the well already. Fewmet (talk) 02:57, 4 July 2014 (UTC)

If the first stage of a rocket is still supplying lift for a while after its fuel is used up and the stage is cut adrift, would there be any saving in waiting for the next phase to cut in when forward motion is almost ended rather than continuing the burn immediately from the second stage?

The higher the vehicle gets the more productive the fuel becomes.Or is it preferable to continue the journey as fast as possible? -- Weatherlawyer (talk) (please sign your comments with ~~~~)

My first instinct would be to say burn as continuously as possible. If you wait until your speed is almost zero, you have to use a whole load of energy (fuel) to get back to the speed you were going in the first place. --Pudder (talk) 17:12, 27 January 2015 (UTC)

Hence the need to use ultra light containers in the first stage? -- Weatherlawyer (talk) (please sign your comments with ~~~~)

Maybe the typo is based on Randall's days at NASA? It might already incorporate gravity assists and the Oberth effect. That number might even be what NASA was using as the minimum potential with known cost-effective techniques. flewk (talk) 21:22, 8 January 2016 (UTC)

## Earth's Geosynchronous Orbital Altitude

In the XKCD strip, the artist states above Earth in the lower right popout that the geosynchronous altitude is well below top of Earth's gravity well. While the rest of his strip is a wonderful representation of the science behind gravity wells, this one bit is not accurate. A geosynchronous altitude for Earth is nearly 36,000 km, not under 6000 km. Kudos for the rest of the strip, though.