Talk:Specific orbital energy

Regarding the Sign of semi-major axis
I have never seen a different equation for hyperbolic SE. Hyperbolic trajectories are normaly associated with negative semi-major axes, which correctly gives a positive energy when using the general form of the energy equation. Why present it this way?

Here is what is currently in the article:

For a hyperbolic trajectory this specific orbital energy equation takes form:


 * $$\epsilon = {\mu \over{2a}}\,\!$$

--Keebler71 18:09, 2 June 2006 (UTC)

Characteristic energy
The article says "For a hyperbolic orbit, it is equal to the excess energy compared to that of a parabolic orbit. In this case the specific orbital energy is also referred to as characteristic energy." Our article on characteristic energy says characteristic energy is twice specific orbital energy, which also matches the formulas given.--agr (talk) 03:51, 22 April 2012 (UTC)

Earth Orbits example: LEO
The specific orbital energy for low Earth orbit appears to be incorrect. Given m1 >> m2 and GM for Earth is 398,600 km^3s^-2; given a 300km circular orbit (therefore a=r=6,678km) we get: e = -398,600 / (2 * 6,678) e = -398,600 / 13,356 e = -29.84 Mj/kg

Alternatively, potential energy -u / r is roughly -59.69 Mj/kg for r=6,678km and kinetic energy v^2 / 2 is roughly 29.85 Mj/kg for v = 7.726 km/s, yielding -29.84 Mj/kg. Freshgroundcoffee (talk) 23:56, 28 February 2013 (UTC)

Semi-major axis
There's a potential point of confusion that's worth pointing out. The equation "epsilon = - mu / 2a" is correct, but for an elliptical orbit of a small body around a large one (e.g., earth) it's easy to confuse the semi-major axis of the orbit with the radius at apoapsis from the center of the large body. Under that (wrong!) interpretation, specific orbital energy would be independent of the radius at periapsis.

But r_sub_a is not the semi-major axis. It's "(r_sub_p + r_sub_a) over 2". That's probably worth pointing out. Agnostic Engineer (talk) 00:50, 26 January 2015 (UTC)

Assessment comment
Substituted at 06:40, 30 April 2016 (UTC)

🌈 Ice.Queen (talk) 04:36, 6 February 2017 (UTC)