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28 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 ...
Mark's user avatar
  • 15.4k
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 ...
David Hammen's user avatar
  • 75.3k
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-...
uhoh's user avatar
  • 148k
21 votes

How do satellite trackers work when showing a "satellite"?

How do satellite trackers work when showing a "satellite"? Satellite "tracking" websites do not "track" anything. You could blow up the satellite, and that little dot ...
uhoh's user avatar
  • 148k
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, ...
Jack's user avatar
  • 9,966
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 ...
Steve Linton's user avatar
  • 19.6k
17 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 ...
amkas90's user avatar
  • 456
17 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 ...
gosnold's user avatar
  • 1,308
17 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)...
David Hammen's user avatar
  • 75.3k
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 ...
notovny's user avatar
  • 5,544
14 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 ...
Uwe's user avatar
  • 49.2k
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, ...
Ryan C's user avatar
  • 8,047
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 ...
notovny's user avatar
  • 5,544
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 =...
David Hammen's user avatar
  • 75.3k
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 $e$, $a$, $i$, $\omega$, $\...
David Hammen's user avatar
  • 75.3k
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 ...
Organic Marble's user avatar
12 votes

How do satellite trackers work when showing a "satellite"?

None of the current services show actual 'live' data. This is due to physics which mean achieving global coverage of LEO would require a site looking DOWN from high orbit. Rather we have a multitude ...
GremlinWranger's user avatar
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 ...
uhoh's user avatar
  • 148k
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 ...
Anton Hengst's user avatar
  • 10.9k
10 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 ...
David Hammen's user avatar
  • 75.3k
10 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 ...
Orbital Fun's user avatar
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.
sequoia's user avatar
  • 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 ...
Russell Borogove's user avatar
10 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}}}\...
Ryan C's user avatar
  • 8,047
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 ...
Tristan's user avatar
  • 17.3k
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 ...
Russell Borogove's user avatar
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 ...
Tristan's user avatar
  • 17.3k
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. ...
Christopher James Huff's user avatar
9 votes
Accepted

Differences between numerical propagators

You should check to see what propagator GMAT is using. A constant step size RK4 method is going to drift from the true solution of the ODE over time, so if GMAT is using an adaptive step size method ...
Alfonso Gonzalez's user avatar

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