27
votes
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 ...
- 12.4k
23
votes
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 ...
- 71.4k
22
votes
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
21
votes
Accepted
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
19
votes
Accepted
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
18
votes
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
16
votes
Accepted
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 ...
- 1,268
16
votes
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
15
votes
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
15
votes
Accepted
How to get true anomaly from time?
An exercise that was left unsolved from last year's class gives me this equation :
$$
t-t_{p} = \sqrt{\frac{a^3}{\mu}}*(\arcsin(X) - e*X)
$$
where :
$$
X = \frac{\sqrt{1-e^2}*\sin(v)}{1+e*\cos(v)...
- 71.4k
14
votes
Accepted
I'm building a CubeSat for a short 8-day mission in LEO followed by a fairly quick reentry, what range of orbits to consider?
You want a very low altitude, to maximize drag, to make the object deorbit quickly even if you never manage to establish control from the ground. The constraint is that you don't want to go too low, ...
- 6,753
13
votes
Accepted
Required Orbital Components to Fully Define Orbit
You need them all, but can omit Mean Anomaly at Epoch and Epoch if you don't care where the object is at any given time, and just care about the orbit itself.
Inclination and Right Ascension of the ...
- 5,060
13
votes
Why does a launch due south (180° azimuth) not translate into a polar orbit from Vandenberg?
If the Earth did not rotate, a 180° launch azimuth would result in a 90° inclination orbit.
To compensate for the rotation, the azimuth should be bigger than 180°.
If the rocket were launched from the ...
- 48.2k
12
votes
Accepted
Calculate True Anomaly at future point in time with hyperbolic orbits
Not surprisingly, one needs to use hyperbolic functions as opposed to trigonometric functions with regard to hyperbolic trajectories. The motivation is simple. Let's start with Kepler's equation, $M =...
- 71.4k
12
votes
How much delta-v does it take to go from one arbitrary orbit to another?
I've got a set of Keplerian orbital elements $e_0$, $a_0$, $i_0$, $\omega_0$, $\Omega_0$, and $\theta_0$, and I'd like to get to a different orbit with orbital elements $e$, $a$, $i$, $\omega$, $\...
- 71.4k
12
votes
Accepted
Major axis larger than total of apogee and perigee?
On the linked page, the apogee and perigee numbers don't include the radius of the Earth (they are measured from some theoretical Earth surface). They are altitudes. The semi-major axis does include ...
- 171k
11
votes
How to obtain UTC of the epoch time in a satellite TLE (two line element)?
The best guess available about the time scale used in TLE files is from the epic Revisiting Spacetrack Report #3 report from Celestrak, which they have put online here:
https://celestrak.org/...
- 931
11
votes
Accepted
Why does NASA Horizons say that Earth inclination is not 0 at J2000 epoch?
Here's the heart of your problem:
Center : Solar System Barycenter (SSB) [500@0]
The ecliptic is defined in terms of the Earth's mean (or average) orbit ...
- 71.4k
11
votes
Accepted
Why is the sidereal period of the Earth 362.392667 days?
What's going on?
You are learning:
what osculating orbital elements are and are not,
that real orbits are not Keplerian!
@DavidHammen's answer is of course spot-on correct, but I understand why ...
- 149k
11
votes
Accepted
Is there a low Earth orbit with a 24-hour day night cycle?
There are not! To obtain a 24 hr day/night period, you have to pass through the Earth's shadow every 24 hours. This means you're orbiting once every 24 hours (as no orbits precess rapidly enough to ...
- 10.3k
10
votes
Deviation of semi-major axis
Ok, that's embarrassing:
You just have to add earth's radius (traditionally the equatorial radius) of about 6378 or 6378.137 km to apogee or perigee heights to get distances to the center.
- 311
10
votes
Accepted
Perigee location?
I'm assuming atmosphere isn't a consideration.
The azimuth angle (north-east-south-west orientation) doesn't matter at all for this question -- it determines the orientation of the ellipse, but has ...
- 164k
9
votes
Accepted
Deriving the changes in Keplerian Elements induced by small impulses
If we assume a perfect two-body problem, absent perturbations from external bodies or non-spherical gravity sources (i.e., perfect conic orbits with no precession or variation), your constraints ...
- 17.1k
9
votes
What's a Brouwer-Lyddane mean semi major axis, or any other, for an orbit in a lumpy gravity field?
The Brouwer-Lyddane Transformation is based on two articles:
"Solution of the Problem of Artificial Satellite Theory Without Drag," D. Brouwer, The Astronomical Journal, Nov. 1959, pp.378-396
"Small ...
- 418
9
votes
Accepted
Determine orbit type from TLE
According to Wikipedia, field 8 of TLE line 2 is the "mean motion in revolutions per day"; you can determine the orbital period from this.
For geosynchronous orbit, you should expect 1.0 ...
- 164k
9
votes
Is it possible to plot the ground track of a satellite with just azimuth & elevation angles without range?
Yes. This is a classical astrodynamics problem of orbit determination.
The technique you would use is called Gauss' method. It allows you to determine an approximate orbit from three timed ...
- 17.1k
9
votes
How much delta-v does it take to go from one arbitrary orbit to another?
What you're looking for is Lambert's problem, which is used both for trajectory design and orbit determination, and to produce porkchop plots. Your hunch that this is not a simple problem is correct. ...
- 9,244
9
votes
Accepted
Why are there six orbital elements?
$$\newcommand{\F}{\mathbf{\vec{F}}}\newcommand{\p}{\mathbf{\vec{p}}}\newcommand{\q}{\mathbf{\vec{q}}}\newcommand{\v}{\mathbf{\vec{v}}}\newcommand{\r}{\mathbf{\vec{r}}}\newcommand{\a}{\mathbf{\vec{a}}}\...
- 6,753
8
votes
How (the heck) are military satellites with (apparently) classified TLEs still showing up on sat map websites?
There are a few ways.
Visual tracking
Tracking of it's RF emissions
Radar tracking.
I suspect it's the first two. If you know about where something is, you can see it visually, and track it's RF ...
- 120k
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