# Tag Info

46

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 the target (center) as "coord@10". I should have omitted the "coord", as that means coordinates, or in other words, a point on the surface. Without that, it ...

23

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 using the Earth-Moon barycenter and assuming that is an object (it isn't, either). You are asking Horizons to compute the osculating Keplerian elements of these ...

18

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 between the integrated state and observations. These errors are used to adjust initial states and the integration is then re-performed. The cycle stops when the ...

12

The prior answer cleared up the issue I was having. It is true that with only the absolute magnitude (H) and albedo (p) you can estimate the size. The formula is the following: $$D(km) = \frac{1329}{\sqrt{p}}10^{-0.2H}$$ For objects in my search range, here are some examples: (object name) (JPL diameter estimate) (formula result) Aten 1.1 km 1....

12

"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 results that you gave in your own reply.) Have you seen what was done by Steve Moshier (S L Moshier) in the early 1990s? He gave a complete replication of the ...

11

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 you would have thought that this might be the right period. It is true that the Earth-Moon barycenter might move along a more representative Keplerian orbit than ...

10

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 multipole term for a body's gravitational potential beyond the monopole term $-GM/r$. Newton: The acceleration of a body in the gravitation field of another body of ...

8

As Wikipedia mentions, Absolute Magnitude is a standard measurement of how bright an object would be, if measured from $1\ ㍳$ (Earth Mean distance from sun), away. Furthermore, it uses the same logarithmic scale to measure relative brightness, such that a $1.0$ magnitude change relates to a $10^{0.4}=2.512$ scale. Put all of the pieces together, and you ...

8

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 of the things you need. Go to https://ssd.jpl.nasa.gov/horizons.cgi Ephemeris Type Select the Vector Table: Target Body Type the word "Earth" into ...

6

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, errors in the behaviors of the spacecraft when it makes maneuvers, and errors in our models of the solar system. Exactness (perfection) is unattainable. It is far ...

6

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 SPK files are the authoritative value, because that's the data used in the simulations. These values are available at https://naif.jpl.nasa.gov/pub/naif/generic_kernels/pck/gm_de431.tpc For completeness, JPL ...

6

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 affiliated package that contains a collection of tools to access online Astronomical data. Each web service has its own sub-package.", where making Horizons queries is just one of many options. It's ...

6

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, Chebyscheff, Tchebyshev, Tchebychev, Tchebysheff, Tchebychov, Tchebyshov, Tchebycheff, Tchebyschev, Tchebyschef, Tchebyscheff, Tschebyshev, Tschebychev, Tschebysheff, ...

6

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 a >100MB file.) NASA/JPL needed a way to provide high resolution and high accuracy functions of time for the positions of the planets, and the most compact way ...

5

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 exhibit the oscillation. We know for a fact that there is no raw data for one of those sections because it is in the future. I spoke to a controls expert and he ...

5

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 some time, that culminates in: coordination of UT on 1962-01-01 Following the direction of the IAU, all observatories in the world adopted new longitudes ...

5

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 refers to an update to the navigation state based on a series of radiometric tracking activities over time. No, if you mean bouncing radio signals passively off ...

5

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 searching for it: Note that the new solution for Ultima Thule has an "MB" or major body designation! This solution is based on all observational data available to ...

5

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 case, the original 2015 trajectory data was provided to the DSN for tracking support). If there is a source of current DSCOVR trajectory information (no doubt it ...

4

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 reading: http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html I was pointed to the following folder, which has solutions in kernel form as .bsp ...

4

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 by the Apollo CSM or LM would have used several of these thrusters in short bursts. It would not surprise me if that amounted to several hundred fires per ...

4

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 signal loss More efficient modulation (which may require playing on the bandwidth) I suggest you look into what is called the link budget equation to find more ...

4

Based on this answer in spice-discussion, one must input the time explicitly as TDB, like this: tdb = '2017-09-01 12:05:50 TDB' et = spice.str2et(tdb) Then the results match exactly: CSPICE_N0066 [ 19219587.76713952 97944303.7160171 42842235.3192082 ] [-34.6088113 4.64408183 4.27890913]

4

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.

4

From the tutorial: To select a point on the surface of a target body (limited to those with an IAU rotational model), use this form (units are degrees and km): {g: E.Long, latitude, h@}BODY [geodetic/planetographic coords] {c: E.Long, DXY, DZ@}BODY [cylindrical coordinates] For example: "g: 348.8, -43.3, 0 @ 301" specifies the crater Tycho ...

4

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 Chebyshev coefficients you seek, along with some additional information. They can be downloaded as text files or binary. If you are familliar with Python you can ...

4

To understand the meaning of the formula, one needs to face its origins in general relativity. GR is not just about corrections to Newtonian expressions for gravitational forces and accelerations. It is more profound: The key point is that there is no longer a simple relationship between space and time coordinates and physical measurements of space and time. ...

3

If you haven't looked there before, the starting place for all NASA data is the NASA Space Science Data Coordinated Archive (NSSDC). Back in the day when I used to work with ISEE-3/ICE data, they were the place were you went to get your data or you sent it to archive your data. I'm not familiar with the data format you are asking about, but the orbital ...

3

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 Calculates the positions of astronomical objects at predicted times. Orbit Determination Analysts Study radio transmissions and images from cameras to determine the ...

3

I just want to add that the relativistic correction term mentioned in the answer by uhoh, which is the "post-Newtonian expansion" at the "1PN" level, approximate relativistic effects by introducing a repulsive $1/r^3$ term. The expression is used by the JPL so you have to use it if you want to get as close to there ephemeris as possible. Even though you get ...

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