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My imposter syndrome is acting up again after I posted several comments under:

Now I feel like I don't know anything. So I'd like to ask:

Question: What is the difference between SPICE kernels and JPL Development Ephemerides? Is a DE just a bunch of kernels on a string?

Maybe it has a table of contents as well?

Answering my question requires knowing the internal anatomy of both. I am not asking for differences in how they are used - no duck typing answers please! Let's go under the hood.

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    $\begingroup$ It is essentially the same numbers in a different file format. The SPICE file format is much more complex and flexible than the DE file formats. The binary DE format is actually a bit ambiguous, so there's no reliable method write code for future versions. The SPICE format makes up for that. But the ASCII format is the easiest. The link at the end is an article describing the DE formats, I am currently (slowly) working on one for the SPICE formats, but the SPICE format is also covered in the SPICE documentation. astrogreg.com/jpl-ephemeris-format/jpl-ephemeris-format.html $\endgroup$ Jun 27, 2023 at 12:22

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(Difference between Development Ephemerides and 'Spice' kernels?)

'Development Ephemeris' is the name given since about the early 1960s to successive versions of standard solar system data prepared at the Jet Propulsion Laboratory and used by NASA in its operations. In effect, each version has been an almanac, giving data to represent current best estimates of the trajectory of selected object(s) over time, in whatever digital/computable form has been most convenient at the time of issue.

Part of the history of the DE series is given at https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris . The JPL/NASA data was at first developed out of older standard data used for the astronomical ephemerides, but it had to be refined in several ways before it could be applied for NASA's purposes. Most especially, tracking requirements meant that the reference data had to provide a good approximation to distance and velocity measures in order to guide the instrumentation in the tracking or radar-ranging of space objects, e.g. moon, planets, or spacecraft in the close vicinity of the moon. The older astronomical data arose out of optical astronomical data which placed heavier emphasis on visible position coordinates, and only gave distance and velocity data with much less accuracy, often not good enough to keep tracking instrumentation on target. So the 1960s and early 1970s saw a great deal of bridging development to get from the old optical-based data towards provisionally-improved reference data that would also support tracking operations, with further generations of the improved data following on from that.

From this it can be seen how the designation 'DE' or Development Ephemeris has referred to underlying solar-system data of some generation or other, without reference to a specific form in which it is presented. The specifics of DE versions have been described over the years in a number of JPL documents and reference texts. Early on these were basically internal 'inter-office memoranda' (IOMs) many of which are now publicly available. Some of the history and links to versions both recent and older are at ftp://ssd.jpl.nasa.gov/pub/eph/planets/ .

By contrast, the 'spice' kernels represent definite digital formats containing recent generations of the reference data. Their coverage has also been extended beyond the original DE coverage of sun, moon and planets, so that kernels now cover also a range of minor planets, satellites and other trackable astronomical entities. Some of them, in binary format, designated 'spk' files, had their formats described a few years ago e.g. at https://arxiv.org/abs/1507.04291 , but there have also been more recent developments, available e.g. via https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/ .

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  • $\begingroup$ Thanks for your thorough and insightful answer! I'm getting there, but this kind of thing is hard for me. Can we say that 1) both DEs and kernels contain ephemerides of objects in the solar system expressed as lists of interpolation coefficients for a series of timepoints, and 2) A single kernel is for a relatively limited chunk of time for a single object, but a DE is (usually) for a collection of objects? Have I still got it all wrong - is any of that right? $\endgroup$
    – uhoh
    Jun 28, 2023 at 22:27
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    $\begingroup$ @uhoh : thanks for your post. I agree broadly with your statement (1). Then, under (2), if we are talking about spk and pck files, I believe they cover a broader range than statement (2) : for example some spk files contain a complete DE version embedded within, while others cover other bodies not included in the DE data series. Also, it seems (see arxiv 1507.04291) that 'Spice' referred to a software toolkit, not to data, the spk and pck files were designed to be used by that toolkit. (Sorry to be hazy about the last point, personally I still use the JPL data-formats for DE and INPOP data.) $\endgroup$
    – terry-s
    Jun 29, 2023 at 22:11
  • $\begingroup$ Okay very nice, concise summary of a complicated (but not complex) situation. Thank you! $\endgroup$
    – uhoh
    Jun 30, 2023 at 0:26

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