# 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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### Roughly express how much of our Moon's apparent libration amplitude seen from the Geocenter is due to different effects?

My understanding is that when tidal locking of a body first happens it transitions from a state where the average rotation period is faster than the orbital period to a state where they are the same ...
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### Getting back from the Moon

This link describes the way back from the Moon surface to the orbiting spacecraft. But how that spacecraft got back to the Earth? It is a long journey. How much rocket fuel was needed, in terms of ...
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### Are the proposed Launch UK facilities in the wrong places?

I live in the UK. I was recently told that the UK Space Agency (UKSA) was doing a project to launch rockets in Newquay. I did some more research and found that, according to the UKSA, Newquay will not ...
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### How is the space probe powered to travel a huge distance in a deep space mission?

As of March 2019, the New Horizons space probe was about 4.1 billion miles (6.6 billion kilometers) from our planet (Earth) and traveling deeper into the Kuiper Belt at nearly 33,000 miles (53,000 ...
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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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### Why can't I just use dot product angles to get satellite vision of a point on Earth?

Right now I'm using MATLAB and can easily get the ECEF coordinates of the satellite and the point on the Earth. Since ECEF is cartesian, why can't I just rearrange:  r_{es}\cdot r_{ps} = \|r_{es}\| ...
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### Time dilation between Mars and Earth due to different mass

I am not sure I understand the Time dilation correctly but if mass has dirrect impact on time...than lets consider cosmological event - one orbit of planet Earth around Sun. This event will person on ...
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### Accuracy of converting from TLE/Orbital Elements to Cartesian if used for other propagator?

Say that I wanted to propagate a real-life satellite based on an initial position in space. However, the only source of data I can get is from a tracking website like CelesTrak or Space-Track, where ...
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### Simply explain for the 2nd “cosmic speed”

Many sources can explain about first, second and third cosmic velocities, but the explanations contain difficult formulas and are not easy to understand. For me, I can check it with powerful magnet ...
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### Will the Emirate Mars Mission, Tianwen-1 and Mars 2020 Mission get closer to each other or spread out on their way to Mars?

The brand new Where Is Roadster page Spacecraft launched to Mars in 2020 (announced here) currently says that it has been: 13 days, 3 hours, 47 minutes and 13 seconds since the Emirates Mars Mission ...
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### Why are lunar orbits unstable?

I've seen a few times now that objects in orbit around the moon tend to crash into it in relatively short time frames. The orbits tend to become more elliptical until they intersect the surface, is ...
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### Orbit Determination of ISS just with a telescope and a watch?

The question Is it possible to represent an orbit with just a periapsis position and 2 angular velocities? got me thinking, suppose I take a telescope and took 3 measurements of ISS angular position ...
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### Is it possible to represent an orbit with just a periapsis position and 2 angular velocities?

I was thinking of how complex the orbital elements are and got to wondering whether you can represent orbits in a more simple way. If you start with an ECI coordinate frame and you know the spherical ...
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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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### Could a solar sail composed of smart glass stay near the L1 point of Venus?

From answers to this question i've learned that the Lagrangian L$_1$ point of Venus is not stable, despite the almost circular orbit of the planet and the fact that it has no moon. Nevertheless would ...
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### An orbit in the corner of isosceles right-angled triangle

Is it possible to have a non-escape orbit that is equidistant to two celestial bodies (for example, Earth and Mars) at all times? If possible, how much does it cost to keep an object (which is much ...
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### What is the sign of Earth's J2?

From Wikipedia's Geopotential model; The deviations of Earth's gravitational field from that of a homogeneous sphere and the question For the mathematical relationship between J2 (km^5/s^2) and ...
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### Lagrange Coefficients for Hyperbolic Orbits

Here is the expressions for f,g and f dot for elliptic orbits. I would like to find similar expressions for hyperbolic orbit case.
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### Roughly how many kinds of closed or periodic orbits are there in the circular restricted three-body problem?

Question: Roughly how many kinds of closed or periodic orbits are there in the circular restricted three-body problem? Has anyone made an attempt to enumerate and classify/categorize them? Notes: If ...
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### What makes Earth-Mars transfers more non-Hohmann; inclination or eccentricity?

Answers to Why do many Mars missions launch now, if the Hohmann transfer orbit is the most propellant-saving one? and discussions below them made me wonder what makes Earth-Mars transfers more non-...
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### Delta-v penalty for stopping in a distant Earth orbit versus using atmospheric reentry for braking and returning to Earth from Mars?

Comments below this answer to What precautions are planned to prevent an accident or anomaly from releasing Martians into Earth's environment? say: Seems to me like a high earth orbit lab might be a ...
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### Can we go anywhere in space with only THREE reaction wheels and ONE chemical thruster?

I had a discussion the other day with some of my peers and they were very keen on the fact the it is possible to change your orbit as desired using a single thruster and a three reaction wheels. The ...
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### How could one determine the acceleration of a satellite given its ephemeris data, taking into account the earth's oblateness?

I am looking to determine the acceleration vector of a satellite orbiting the earth at a certain point in time given its ephemeris data for that same point in time. I am taking into account the ...
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### Is there a basic 2 or 3 body space simulator

I would like an educational tool to allow a child to 'drag and drop' or 'add' a planet to a workspace, adjust the x, y and z velocities as well as the size and mass. OF the numerous simulators I have ...
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### Compute eccentricity and angular momentum vectors of a binary

I having some trouble finding the direction vectors (in 3D) of the eccentricity and angular momentum of a binary. The issue is that the binary has only been described in Keplerian orbital elements and ...
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### Optimum delta-v burn to change periapsis or apoapsis at an arbitrary point on an elliptical orbit?

This question is explicitly not about doing burns at the apoapsis or periapsis in order to raise/lower the other apsis. Of course those are going to be the most efficient maneuvers, and the burns ...
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### Will the new “TLE format” be orbitally-mechanically better than traditional TLEs?

Celetrak's A New Way to Obtain GP Data (aka TLEs) by Dr. T.S. Kelso 2020 May 27 explains the arrival a new infoblob format that in several ways is "better" than the ancient TLE, and contains ...
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### Where will the DSCOVR mission Falcon 9 2nd stage rocket body end up?

This answer to Why would a mission to Sun-Earth L1 have an instantaneous launch window? suggests that this was quite a delta-v-intensive launch. It left the F9 2nd stage in a very high 17 day orbit ...
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### Why can't custom-made TLEs for the DSCOVR launch booster in orbit around Earth work with SDP4?

This fascinating answer to Why has no TLE been published for the DSCOVR satellite and the Falcon 9 R/B? presents links to custom-made TLEs for the Falcon 9 rocket body that put DSCOVR into orbit, and ...
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### Navigating in space with only cameras

I'd like to figure out how to determine where something is in space (solar system) based only solely on measurements that can be preformed on location visually/with a camera. Let's say I'm floating in ...
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### Delta-v difference between Mars and Moon

Considering a simple Hohmann transfer, does it take less Delta-v and propellant to go to Mars than to the Moon
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### What is the cheapest possible way to land a crew of 2 on the moon?

During the Apollo moon missions, a single very large rocket sent both the Apollo CSM and the LM to the moon in a single shot and it took only around 3 days to get there. However, after the Apollo ...
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### How to get semi-major axis from TLE?

This should be fairly straightforward, but, how do I get the semi-major axis (a) from at TLE. For example if I have the TLE: ...
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### Keplerian to cartesian elements issue

I am trying to convert between keplerian orbital elements to cartesian position and velocity but something is going wrong. The issue is that I (as a check) compute some keplerian orbital elements with ...

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