# 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. For the movements of celestial bodies, see [celestial-mechanics].

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### Gravitational keyhole for spacecraft flyby?

Is there a term for the area near a planet a spacecraft must pass through for a gravitational-assist flyby, or does 'gravitational keyhole' apply to gravitational assists as well as for asteroids?
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### How to interpret and use characteristic energy C3 in this case?

In this answer I've interpreted the x axis of the plot in the question (also shown below) labeled as $C_3$ (characteristic energy) to be the "excess $v^2$ above escape velocity from Earth", and so I'd ...
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### How to find instantaneous mass of a central body that results in a given osculating orbit?

This question was underspecified, so I'm going to try again. Osculating orbital elements are Keplerian elements of a hypothetical orbit around a specified center which would be tangent to a specified ...
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### How to find instantaneous positions of a central body of specified mass that results in an osculating orbit?

If I understand correctly, osculating orbital elements such as those posted in JPL's Horizons represent the mathematical osculating (tangential or "kissing") Keplerian orbit about a specified location ...
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### Game for teaching basics of orbital mechanics

I am looking for a game that would be suitable for teaching basic orbital mechanics concepts to primary school students (grades 6-8). Contrary to most of the questions here asking about spaceflight ...
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### Why is it easier to escape the solar system than get to Mercury or the Sun?

It has been mentioned several times on this site that it is "easier" (less delta-v, and hence less fuel) to reach the escape velocity of the solar system, than to reach the planet Mercury or ...
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### Is there any reason why a spacecraft would not be able to turn around without a gravity assist?

Let's say you have a spaceship on its way to Mars; halfway there you decide you need to come back to Earth instead. Is there any reason a spacecraft would be unable to do this? Let's assume it began ...
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### How to calculate delta-v required for a planet-to-planet Hohmann transfer?

How do you calculate the delta-v required to do a Hohmann transfer from a circular orbit around one body to a circular orbit around another? I'm assuming you'd need to know the masses of the two ...
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### Why is the ribbon of decommissioned geosynchronous satellites skewed?

I was looking at this 3D map of satellites orbiting earth and noticed something interesting. Around the green ring of active geosynchronous satellites, there is a ribbon of what I assume are obsolete ...
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### Space travel using constant acceleration drive: Earth to Europa

Long story short: I'm writing sci-fi and taking my protagonist to Europa. He's got 2 weeks to one month to get there from Earth, give or take a few days. That sounds, of course, preposterous in this ...
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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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### Orbiting Earth before heading to Moon

Each of the lunar Apollo missions (Apollo 8-17) entered Earth orbit immediately after launch. Each mission then left Earth after a few orbits by burning the S-IVB engine, and headed off to the Moon. (...
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### The Martian: Does it really take a supercomputer to calculate spaceflight maneuvers?

My preemptive apologies for asking a question about a movie, and the spoilers within said question, but considering the widespread support for its scientific plausibility, I'm hoping you'll let it ...
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### What is a “Sailboat Island of Stability”?

The New Horizon's team, in response to an announcement of a press conference from the Hubble team, posted some speculations as to what the press conference might be about. On the list includes "Small ...
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### Where to find the best values for standard gravitational parameters of solar system bodies?

Where can I find the best values to use for standard gravitational parameters of solar system bodies for numerical simulation experiments? Here are two sets I've put together. The first set is from ...
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### Lowest possible lunar orbit and has any spacecraft achieved it?

Because the Moon has no atmosphere a spacecraft can theoretically cruise at a very low orbit. What is the lowest possible lunar orbit and which spacecraft achieved it? Was it the Lunar Module of ...
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### Calculations Apollo astronauts were trained to do by hand if loss of guidance computer and/or data & coms link to Earth?

This answer to the question is it possible to navigate space travel with no computer? currently contains the line Yes, as long as you don't care where you end up but I am not so sure this is true. ...
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### Why has no TLE been published for the DSCOVR satellite and the Falcon 9 R/B?

So, usually a few days after every launch, I go out and check out the published TLE data for each satellite, made public courtesy of CelesTrak. It generally includes all the orbital parameters for ...
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### Why does this plot show Starman's speed relative to the Sun fluctuating so often?

After the launch of Starman, during the inaugural flight of SpaceX's heavy lift vehicle, the Falcon Heavy, PearsonArtPhoto set up a website to track various stats about Starman and his Roadster. A ...
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### Do you need 0 km/s velocity to crash into the sun?

I was reading a popular thread about the delta-v required to escape the solar system compared to the delta-v required to crash into the sun. I get it: the earth itself already has a high speed (29.7km/...
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### How does orbital eccentricity affect positions of Lagrange points $L_4$ and $L_5$?

It is often said that the $L_4$ and $L_5$ points are "60 degrees ahead and behind" a planet like Jupiter. Clearly this is true only in the case of circular orbits. In more elliptical orbits, I assume ...
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### Does trajectory of an object orbiting a planet depend on the object's mass? (With hypothetical Apollo example)

For example (hypothetically), if an Apollo crew of one of the lunar missions went on EVA just after Trans Earth Injection burn and carefully released a small bolt, or a plastic package from their food,...
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### What Earth gravity model is used for different types of calculations?

When performing orbital calculations, under what circumstances should I assume: a perfectly spherical Earth a reference ellipsoid a geoid no particular format at all
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### The plane of the orbit of Juno around Jupiter is not the ecliptic plane. How did it get into this plane?

Did Juno start off its course on the ecliptic plane from the earth orbit and then left it at insertion to Jupiter's orbit over its north pole or it was always in that plane?
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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 precisely do we know mass of the ISS?

Everyone knows that in space, every gram counts. Knowing how much mass an orbital object has is necessary for precise motion calculations. So how precisely do we know the mass of ISS? I'm sure we can ...
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### If something “falls off” the L2 or L1 point, where will it go?

The L1 and L2 points are thought to be unstable "saddle" points, meaning that there is stability in two directions of movement, but not in the other. That raises an obvious question - when a ...
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### Why did the rendezvous attempt fail on Gemini 4?

It is said in Wikipedia, that On the first orbit, McDivitt attempted to rendezvous with the spent Titan second stage. This was unsuccessful for a number of reasons: NASA engineers had not ...
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### No need for gravity assist with Falcon Heavy?

Elon Musk is quoted to have said, after the launch of Falcon Heavy, that «it can launch things direct to Pluto or beyond, no stop needed, don’t even need gravity assist or anything». Now, I am not an ...
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### Why do the LRO's orbital elements appear to constantly oscillate - except recently?

I plotted some of the orbital elements of the Lunar Reconnaissance Orbiter from JPL's Horizons database, and I see that there are constant oscillations. The period of the semi-major axis and ...
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### How was Juno's arrival set up to be on the evening of July 4th?

Presumable it is largely a coincidence that Juno arrives at the perfect moment for American pride in NASA's achievements, but not entirely. The video below of the trajectory, released in July 2011 ...
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### What is “propulsive passivation” and why will the SpaceX STP-2 mission do it?

The Teslarati article SpaceX’s Falcon Heavy flies a complex mission for the Air Force in launch video discusses the upcoming third Falcon Heavy mission STP-2 and quotes the caption on the linked ...
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### Why doesn't the ISS use ion engines to maintain altitude?

So the ISS uses small thrusters on the Soyuz (usually) to perform occasional reboost burns to raise its orbit. This goes through, from what I can tell, around 3,500 kilos of fuel per year. Would it ...
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### How did the Apollo guidance computer handle the Earth-Moon system's rotation around the Sun?

In this comment I just wrote: "...but at all other times it is just doing math to calculate its trajectory." The math needed a stable clock Timing accuracy of the Apollo Guidance Computer? (also 2, ...
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### Has anything ever executed an orbit change such as between ISS and Hubble?

ISS and Hubble have vastly different orbits - different altitudes, different inclinations, requiring a massive delta-V to transfer from one to the other. My question is: has anything ever had to ...
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### How to determine an orbit of a satellite for a collision detection?

I'm trying to validate an collision detector, and for that I'm starting from a pair of initial conditions for a satellite. After that I will use an RK method to integrate. How can I determine a pair ...
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### How exactly are satellites drifted to slots in different orbital planes?

The Iridium constellation is being replaced by new satellites. There are six planes with 11 satellites + one spare each. Since the Falcon 9 launcher can only carry ten satellites per launch, filling ...
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### Apollo 11 mission report shows velocity well below escape velocity thousands of km on the way to the Moon

This is from the Apollo 11 mission report. It says the vehicle was going at 7.6 km/s when the CSM separated from the third stage. It was not in orbit at this point, it was thousands of KM from earth. ...
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### 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 : $$X = \frac{\sqrt{1-e^2}*\sin(v)}{1+e*\cos(v)}.$$...
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### Did Tiangong-1 turn the engines on?

The following graph shows the Tiangong-1 radius vector at the perigee, semi-major axis and apogee (scaled to 6371 km just to show an approximate altitude) and its orbit eccentricity: I used the SGP4 ...
314 views

### The sorting of perturbational effects by the power

I have developed an orbital propagation tool, which considers the effects of Sun, Moon, Earth harmonics, SRP, drag, relativistic correction. First question Considering the LEO orbit, I would sort ...
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### Why is DSCOVR in a Lissajous orbit? Wouldn't a halo orbit completely avoid the 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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### Circular to elliptical orbit delta V requirements

this might be a dumb question, but i've been recently trying to calculate the delta V to deorbit a satellite, and I've run into a problem. Assuming a 400km circular starting orbit(and disregarding ...
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### Gravitational assists from bodies other than planets

Gravity assists are pretty important for interplanetary travel. NASA uses them mainly because they can save time, energy, propellant, and expenses. But so far, all of the gravity slingshots have used ...
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### What is/was legacy TLE ephemeris type 2? (TLE, line 1, column 63)

Wikipedia's Two-Line Element Set entry for line 1, column 63 (1.12) says: Ephemeris type (internal use only - always zero in distributed TLE data)13 13Celestrak Frequently Asked Questions: Two-Line ...
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### How can I “debounce” these TDRS satellite inclinations? (reconstruct the zero-crossings)

The plot below is from the question Why do the geosynchronous TDRS satellites have this distribution of inclinations?, and the explanation says Inclination and mean motion (rev/day) for TDRS ...
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### How many active or inactive artificial satellites are in Mars orbit?

I just read in the BBC news that four nations have placed satellites in Mars orbit. How many active or inactive artificial satellites are there in orbit around Mars (2014)?
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### Docking in orbit

What systems are used for range, speed and alignment when docking in orbit? Whether automatic, human controlled arm, or Apollo style human controlled ship to ship? Playing Kerbal Space Program, I ...
Studying again orbital maneuvers, I found the following plotting the cost of a Hohmman transfer between coplanar circular orbits (black line), in terms of $\Delta V$ against the orbit ratio $r_f/r_i$. ...