# Tag Info

### Lowest possible lunar orbit and has any spacecraft achieved it?

The lowest orbit achieved would probably be PFS-2, a small satellite deployed from Apollo 16's service module. It was intended to go into a 55x76-mile orbit (88.5x122 km), but due to variations in ...
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### Why is the sidereal period of the Earth 362.392667 days?

Why is the sidereal period of the Earth 362.392667 days? It's not. You are doing three things wrong: You are using the solar system barycenter and assuming that is an object (it isn't). You are ...
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### Lowest possible lunar orbit and has any spacecraft achieved it?

"Lowest possible lunar orbit..." As pointed out in comments and in answers to the linked questions Are low, polar lunar orbits in general relatively stable? Moon orbit station-keeping delta-...
• 149k
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### Nuances of the terms (mean / osculating / Keplerian / orbital) elements

Your assumption is a good starting place and its good to be cautious about it. Many of us are guilty of abbreviation or outright misuse of the terms for convenience. Here's my rough guide, not meant ...
• 9,129
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### Can the Right Ascension and Argument of Perigee of a spacecraft's orbit keep varying by themselves with time?

You are correct to a point that the RA of the ascending node and argument of perigee won't change over time without some external force acting upon the satellite. In a simplified gravitational field, ...
• 9,884
Accepted

### Will crewed vehicles ever follow multi-flyby trajectories?

Given the mass costs in terms of consumables and the risk and support costs of keeping humans in space for longer, it seems unlikely that the multiple Earth-Venus flybys used by a lot of robot probes ...
• 19.2k
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### How (the heck) are military satellites with (apparently) classified TLEs still showing up on sat map websites?

There is an international network of observers of classified satellites, organized around the Seesat-L mailing list: http://www.satobs.org/seesat/seesatindex.html They typically look for satellite ...
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### When calculating the six Keplerian Orbital Parameters, why do we need both Eccentricity AND the Semi-Major Axis? Doesn't one tell you the other?

The two parameters are independent; any combination of non-negative semi-major axis and eccentricity between 0 and 1 results in a valid ellipse. Semi-major axis for an ellipse is half the longest ...
• 5,060
Accepted

### How to get semi-major axis from TLE?

The TLE gives mean motion ($n$) in $\frac{rev}{day}$. This needs to be converted to $\frac{rad}{s}$ which can be accomplished by multiplying the $n$ TLE value by $\frac{2\pi}{86400}$. Therefore, to go ...
• 426
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