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Questions tagged [orbital-mechanics]

Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft.

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Does the “17” really mean anything with respect to GNSS orbits being rational factions of a sidereal day?

This answer suggests that the orbital periods of the four large GNSSS contstellations are linked to the sidereal rotation period of the earth by rational numbers with 17 or 34 as the denominator in ...
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Understanding formula for geocentric position of Moon

I am trying to understand a basic formula in a Celestial Mechanics reference. The formula is for the position of the Moon in a geocentric frame. The reference claims in line (981) that this should be ...
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How are co-located satellites positioned relative to each other?

I'm trying to learn about co-located GEO satellites. When I first heard about them, coming from a planetary orbital mechanics background, the first thing I pictured was based on the epicyclic ...
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How can I calculate the $\Delta V$

I'm learning about Astrodynamics, and I would like to ask how can I calculate the $\Delta V$ for an interplanetary mission. I used a website for the porkchop plots (http://sdg.aero.upm.es/index.php/...
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For the mathematical relationship between J2 (km^5/s^2) and dimensionless J2 - which one is derived from the other?

The term $J_2$ seen in discussions of nodal precession of orbits (e.g. sun-synchronous) appears to come in two flavors. For example, in the Wikipedia article Geopotential model it has a value of $1....
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Did Rosetta improve on models of non-gravitational effects on comet 67P's orbit?

Question: Since the presence of the Rosetta spacecraft near comet 67P allowed for a detailed mass measurement, extremely precise position and velocity determinations, and physcal measurement and ...
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What are the effects of orbital reflectors and advertising on scientific observation?

There's a couple of startups and initiatives which are proposing "space billboards" which have been getting a lot of bad press recently. Most of the proposals involve a bunch of cubesats with large ...
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Software for orbital sketches

I am looking for recommendations on software that can create orbit sketches such as the image below. Adding an informative sketch to an article can help readers better understand the idea. I am ...
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Is the Parker Solar Probe's semimajor axis being so close to that of Mercury's just coincidence? Does it help somehow?

Parker Solar Probe will end up in it's Sun-kissing orbit though a series of Venus flyby's, which roughly pins Parker's aphelion at Venus's semimajor axis or about 108 million km. Its final perihelion ...
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Outlier detection algorithm for orbit determination, how do different ones compare?

I have been reading about outlier detection algorithm for improving determinations based on realistic data. Outlier exclusion is an important tool in any analysis of realistic noisy data. There are ...
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Small-bodies landing strategies

Looking at past landing missions on small bodies such as Eros, Itokawa and 67P I found different landing strategies Eros: NEAR Shoemaker probe performed 4 impulsive pre-planned open-loop maneuvers to ...
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Locally optimal control for low thrust transfers with J2 and other Earth perturbations

Background As detailed in this answer low thrust optimization is subject of ongoing research. There are a few closed-loop control laws which allow to achieve a locally optimal (but close to globally ...
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Solutions to a “Lambert's problem in velocity”

Lambert's problem (also here and here): $$\mathbf{r_1}, \mathbf{r_2}, t \rightarrow \mathbf{v_1}, \mathbf{v_2}$$ was solved by Johann Heinrich Lambert via Lambert's Theorem (see references below). ...
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Are ion thruster trajectories classified as brachistochrones?

Usually, when performing trajectory optimizations for chemical rockets, the limiting factor is the $\Delta v$ budget. That means velocity change is the optimization factor, with things like transfer ...
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extreme longevity of geostationary orbits

I was speculating that geostationary satellites could be around possibly for billions of years (i.e. much beyond the point where plate tectonics would have erased all traces of human civilization on ...
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Help converting from ECEF frame to ICRF frame for interplanetary mission design

I am writing a piece of software which propagates the orbit of a spacecraft from an initial Earth orbit to an interplanetary orbit. My simple two-body propagation works fine in either a geocentric or ...
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Laplace Method for Orbit Estimation

I have a question about Laplace angle-only method for orbit determination where the line of sight vectors are being interpolated. I read somewhere that the method fails (due to a matrix being singular)...
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Proper handling of terminal guidance of finite burn closed-loop execution for rendezvous conditions?

I'm writing some software which uses Optimal Control Theory / Calculus of Variations / Pontrayagin's Minimum Principle / Primer Vector Theory to do closed-loop execution of finite burns (in KSP). ...
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Differences between SGP8 and the standard SGP4? Is it ever used in practice?

The Wikipedia article Two Line Element Set says: The SGP4 model was later extended with corrections for deep space objects, creating SDP4, which used the same TLE input data. Over the years a ...
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Piggybacking to Enceladus

The latest NASA New Frontiers selection did not select any finalist proposals to Enceladus, which is one of the most promising targets for life in the solar system. It did however select Dragonfly, ...
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How will the Lunar Gateway go to L2 and L1?

In a talk to the press which can be heard in the YouTube video NASA Administrator Jim Bridenstine Explains the Lunar Gateway, NASA director Jim Bridenstine says after about ...
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Variation in Earth's aphelion and perihelion

I see from Astropixels.com that Earth's aphelion and perihelion each vary by a little over 29,000 km, while the earth-moon barycenter wobble is about 4,000 km. Is Astropixels.com correct? Are Jupiter ...
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(How well) Did Juno provide any insight into the Flyby Anomaly?

I ran across an old BBC article titled Juno: The spacecraft putting sling theory to the test — Why a probe passing our planet on its way to Jupiter might end a decades-old mystery, and reveal ...
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Atmospheric Entry Simulation: Numerical Inaccuracy

I am trying to make an atmospheric entry/aero-assist simulation. For the propagator, I have used conventional equations of motions in planet fixed reference frame: where the scalar states are- ...
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How to calculate if a co-location intercept is possible in a ballistic counter-battery scenario

Let us presume that we have a scenario with ballistic objects. As an example, one object $m_1$ is launched at time $t_0$ from Earth Location $a$ towards Earth Location $b$, both of which are in the ...
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Differential Equations for Rocket Launch in Polar Coordinates

I'm trying to understand the equations of motion for a rocket launched from earth, through the atmosphere, and into LEO. I found a set of ordinary differential equations which describe the motion of ...
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If a sun-synchronous orbit passes over the poles twice a year, how can it have a constant (e.g. 98°) inclination?

I am sooooo confused. The answer to Are sun synchronous orbits possible for any place on Earth? ends with In short, yes, sun-synchronous orbits are possible for any location on Earth. and mentions ...
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Formula in “Hidden Figures” Movie

In the scene where Katherine Johnson/Taraji Henson has to calculate "Go / No-Go," which I take to mean the operational go-no-go orbital-insertion criteria. She calculates a value for R, using this ...
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Resonances and the seven sisters recently discovered

In reference to the discovery of seven planets around Trappist-1, I'm just wondering if any of these planets are in an orbital resonance and if that is the case, what the implications for habitability ...
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Launch windows for on-orbit rendezvous

Suppose you've got a spacecraft in an orbit similar to that of ISS (let's say 51.6 degrees, but about 19 km lower). Suppose also you want to launch a second spacecraft to rendezvous with the first, ...
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Spice PCK planet orientation model

I'm looking at this SPICE kernel, which contains orientation data for planets and other bodies. Model description and examples in "Orientation models" section look pretty simple. But actual numbers ...
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Would you arrive sooner on Mars from Earth if you wait or if you leave immediately, when it's furthest from Earth?

Awkwardly phrased title aside, the question is as follows: If you wished to launch from Earth to head to Mars at the moment when the two planets are furthest apart, would you arrive sooner if you ...
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Algorithmic methods or techniques to find conjunctions in large ensembles of state vectors?

Suppose I wanted to answer the question Will Starman/Roadster pass particularly close to any asteroids in the next few years? or try to predict satellite conjunctions around Earth (e.g. Celestrak's ...
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How would propagation differ between a GEO and a Retrograde GEO?

A Binary Equatorial Synchronous Retrograde Orbit is an orbit that would travel in the opposite direction of GEO and would pass over the same point on Earth twice per day at the same altitude of GEO. ...
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How do I calculate the Characteristic Energy for a rocket to Near Rectilinear Halo Orbit where the Deep Space Gateway is gonna orbit moon?

How do I calculate the Characteristic Energy for a rocket to Near Rectilinear Halo Orbit (NRHO) where the LOP-G or the Deep Space Gateway is going to be orbiting the moon? Or if someone already has a ...
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Geometry of full global coverage with 4 supersynchronous satellites, 3 or 16 control stations

in order to provide a truly global coverage of the Earth from space, including the polar regions, a minimum of just 4 satellites is required. Placed in precise 63.4 degree supersynchronous orbits, one ...
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Why is DSCOVR in a Lissajous orbit instead of a halo orbit (to stay out of Sun exclusion zone)?

Halo orbits are a sub-class of Lissajous orbits. See this answer for (much) more on that. DSCOVR's orbit will put it in it's Sun Exclusion Zone in about 2020 where the communications line of sight ...
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How was the radius of Venus measured so accurately (± 3 km) in the mid 1960's, before Venera 4 and Mariner 5?

In several answers to the question When did planetary scientists realize Venus' surface pressure was almost 100x that on Earth? How did they find out? the known radius of Venus is mentioned. It was ...
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variables affect the trajectory in a stochastic way

What are the different variables affecting the trajectory of a rocket in a stochastic way for getting potential three-sigma trajectory?
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Were the Apollo lunar ALSEP transmitter signals ever analyzed or used after the experiments were shut down?

This answer to the question Why were the “perfectly functioning” seismometers placed by Apollo 12, 14, 15 and 16 astronauts all shut off in 1977? links to a NASA News Release No: 77-47 dated September ...
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Compute heliocentric ecliptic coordinates from Kepler's laws

I'm writing a program to compute the heliocentric ecliptic position of planets and asteroids. I want to output rectangular coordinates oriented according to the ecliptic: positive X in the direction ...
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Use of thrusters for Collision Avoidance Maneuver

I'm building an orbit propagator that gives the position of two satellites at time T. The position are expressed in ECI and after that are being transformed in LVLH frame (as mentioned ECI to LVLH ...
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Computing the local orbital frame vector directions and angles of gyroscope direction in the satellite

In the context of the study of a cubesat (little satellite), I am asked to define the local orbital frame and compute the local orbital frame vector directions as a function of time (...
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Understanding shape of DSN's 'kinky uplink protocol' keeping frequency in coherent transponder's optimum bandpass?

This tweet by deep space SDR enthusiast and amateur radio operator Edgar Kaiser @df2mz shows a squiggly tone from MRO. Mars Reconnaiscance Orbiter (MRO) X-band downlink. The signal shows this ...
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Quantitatively, how deep are the stable equilibrium points in GEO? How much delta-v to move from one to the other?

This answer discusses a spacecraft in GEO using a two-impulse (circular → elliptical → circular) raise & lower maneuver to move (phase) itself by 180 degrees in longitude then return to synchrony. ...
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Determining the spacecraft position vector with respect to ICRF frame for Venus flyby

For an interplanetary mission design to Saturn, I have considered doing multiple gravity assists starting with Venus. I am using the JPL Horizons ephemeris and ESA Lambert Solver. I have the ...
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Are 2 satellites joined better than 2 apart?

Could new satellites be designed to intercept other older functioning artificial satellites or space junk already in orbit to add weight to the new satellite in order to last longer in orbit?
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Why might NASA need to “practice moving a spaceship in different orbits in deep space”?

The NASA.gov feature Q&A: NASA’s New Spaceship says Q: NASA’s really building a spaceship?! A: That’s right! NASA is working with its partners to design and develop a small spaceship ...
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Spacecraft-fixed coordinate system

I am working with data from a polar-orbiting satellite (inclination 97 degrees) with limited attitude information. I am trying to define a coordinate system which is fixed with respect to the ...