# Tag Info

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### Why does this plot show Starman's speed relative to the Sun fluctuating so often?

As was astutely noted by Hans, the period of the movement was about 25 days. It turns out that is the time it takes for the sun to rotate once. When I was grabbing the data from JPL Horizons, I listed ...
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### 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 ...
• 75k
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### Is this what station keeping maneuvers look like, or just glitches in data? (SOHO via Horizons)

You are most likely seeing an artifact of how JPL represents its ephemerides for fast numerical computation. JPL integrates the equations of motion over time. This inevitably results in mismatches ...
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### How to calculate the planets and moons beyond Newtons's gravitational force?

"Question: How to calculate the planets and moons beyond Newtons's gravitational force?" Uhoh, your comment invited further sources on this. (Kudos, by the way, for all the work and the interesting ...
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### How to calculate the planets and moons beyond Newtons's gravitational force?

Let's add an approximations to take into account some of the General Relativity (GR) effects — at least for bodies orbiting close to the massive Sun — and start to look at $J_2$ the lowest order ...
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### Getting state vectors from JPL Horizons ephemerides

Keywords: Horizons tutorial I'll work one example all the way through, and include the exact output you should get so you can check your results. Then you can change one item at a time to get the rest ...
• 149k
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### 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
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### Where to find the best values for standard gravitational parameters of solar system bodies?

If you consider the ephemeris files put out by JPL as the final say in terms of simulations, then the data they put in the PKC files alongside the ...
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### Why hasn't DSCOVR's trajectory determination been updated in JPL's Horizons after 2015-Aug-4?

No new DSCOVR spacecraft trajectory data has been made available by this non-NASA (NOAA) mission since May 2017. (Flight projects are not required to provide trajectory data to Horizons and, in this ...
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### I've almost learned to spell Chebyshev, why has JPL switched to Hermite interpolation for DE438?

Not seeing that. I downloaded de438.bsp, and it in fact uses only Чебышёв position polynomials. (Or Chebyshev, Chebychev, Chebysheff, Chebychov, Chebyshov, Chebycheff, Chebyschev, Chebyschef, ...
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### Are there any sources with high-confidence state vectors for GRAIL-A?

Partial answer too long for several comments: tl;dr: The drift you are seeing is just as likely (if not more) to be a difference in your acceleration models as it is an "inaccuracy" (...
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### How do we track the exact location of the spacecraft which is millions or billions of miles away from us?

How do we track the exact location of the spacecraft which is millions or billions of miles away from us? We don't track the exact location of spacecraft. There are always errors in measurements, ...
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### Why do the LRO's orbital elements appear to constantly oscillate - except recently?

JPL HORIZONS features orbits interpolated from actual data rather than pure simulations. I suspect that what happened here is that there is no orbital data for the three sections of data that don't ...
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### How is the Chebyshev method used by JPL?

The $a_i$ are the coefficients for the Chebyshev approximation. As you say, NASA gives us those. That is what you find in the DE files, e.g. de430.bsp. (Don't click on that unless you want to download ...
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### What happened to time in 1962? (JPL Horizons changes displayed time from UT1 to UTC for dates starting in 1962)

I recommend searching for the string 1962 in https://www.ucolick.org/~sla/leapsecs/timescales.html to see the list of changes that happened to the way time was reckoned as of 1962-01-01. To save you ...
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### How can the data rate be increased in noisy outer space?

Yes we have solutions: Higher-power transmitter More directional emitting antenna (by increasing the frequency for instance) Larger receiving antenna Less noisy electronics Electronics with less ...
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### Does JPL's Horizons have better MU69 state vectors for the New Horizons flyby somewhere?

Thanks to Jon. D. Giorgini at NASA JPL for pointing out that there is indeed a much newer solution in Horizons. The trick is to use the newer, though unofficial designation Ultima Thule when ...
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Your question is slightly ambiguous, a file containing the Chebyshev coefficients wouldn't be an ephemeris, but it's something you could use to generate an ephemeris. Regardless, the article Format ...
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### Why does Earth's North Pole RA flip by ~180 deg around AD2000 relative to SSB? (JPL Horizons data)

Is this an artifact to do with J2000 and ICRF? It's an artifact of choosing to use right ascension and declination to describe the Earth's orientation. Right ascension is undefined at a declination ...
• 75k
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### Python API for JPL Horizons?

callhorizons is depricated now and refers to the python library astroquery which now seems to be the way to go. astroquery (GitHub, readthedocs) is "an astropy ...
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### Juno's original orbit around Jupiter - is this apsidal precession? If so, need expression

My question is: Is this motion actually apsidal precession due to Jupiters non-spherically-symmetric gravitational potential? Yes, that apsidal precession results from Jupiter's oblateness. ...
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### Calculating spacecraft thrust manoeuvres with JPL data

You're going to have to go digging, but the information you're looking for can be found in the Planetary Data System, which has files with spacecraft events including maneuvers.
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### How can I know if NASA Horizons output is measured, calculated, estimated or what-else?

No, these are from measured orbit determinations. However the data does go into the future somewhat, as you can see from the table below. The "od" you see in the rows means "orbit determination", and ...
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### How to obtain Chebyshev coefficients directly from the JPL Horizon's interface?

The best and most concise answer is here The "gold standard" is the SPICE toolkit. You can read more about using spice in this excellent answer. The JPL Development Ephemerides are lists of the ...
• 149k
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### Are there any sources with high-confidence state vectors for GRAIL-A?

What you want is the SPICE Kernels (SPK) from https://pds-geosciences.wustl.edu/grail/grail-l-lgrs-5-rdr-v1/grail_1001/spk/ . They are named like sssttaaYYYY_DDD_yyyy_ddd.spk, where ...
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### Is Cassini's 183rd burn; some kind of engine record?

Hard to say. Cassini's "main engine" is a modernized version of the attitude thrusters used on the Apollo spacecraft: MMH/NTO bipropellant thrusters delivering about 440N thrust. Every maneuver made ...
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### JPL Horizons original reference orbit data for Juno - how to retrieve now that it's been updated?

Comments from @oefe pointed me to the solution! I'll post it here. Since I'm not an expert I'll keep the explanations minimal to avoid saying anything misleading. A good reference is the SPK required ...
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### How do we track the exact location of the spacecraft which is millions or billions of miles away from us?

While this site does not give a very detailed answer, it gives a relatively good idea on how this is achieved: The JPL has five groups, that handle the navigation together. Ephemerides Group ...
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