Interview to Dr. E. Myles Standish from JPL/NASA ( Jet Propulsion Laboratory )
    By Rui Miguel Fernandes

    Summary: The JPL ephemeris files, are the most accurate in the world. The man behind its calculation is Dr E. Myles Standish, that worked for years in the JPL's headquarters in research - still he hasn't stopped.
    He's the reason why Cosmos hasn't stopped trying to improve itself after 8 years...
    This interview was conducted before his retirement.
    Let's find out a little more about him.

    Cosmos - This is not exactly a question, but talk a little bit about yourself - where did you graduate, etc.

    Dr. E. Myles Standish - BA and MA: Wesleyan University in Connecticut; PhD: 1967 Yale University, also in Connecticut. After I received my degree at Yale, I stayed there and taught astronomy for four years. Then I came to JPL where I have worked ever since. At JPL, I have always concentrated on the planetary ephemerides (knowledge of where the planets are and how they are moving). So, I'm like a map-maker; but, the map is in motion. The navigation team at JPL needs to know this information when they send a spacecraft to one of the planets. As the years have gone by, they have wanted the information to be more and more accurate.
    Most of my papers are related to the ephemerides and are published in the major astronomical journals. Once in a while, I have published the results of some historical research, but even then, these have been subjects somehow related to planetary positions. The JPL planetary and lunar ephemerides are used throughout the world; the planetary positions in all of the major national almanacs are based upon the JPL ephemerides: previously, "DE200";now "DE405".

    Cosmos - Are you a personal observer of the stars and planets, that is, are you a pratical astronomer, or do you prefer the math related to it - the pencil and the computer?
    Dr. E. Myles Standish - I never use a telescope, but I certainly do use measurements made by others using optical telescopes (now with CCD's attached), radio antennas (planetary radar and VLBI), lunar laser ranging, and spacecraft tracking data (ranging and doppler). On the other hand, I do spend most of my time on a computer, processing the measurements and the motions and positions of the planets and moon.

    Cosmos - Is celestial mechanics your favourite issue in the astronomy field?
    Dr. E. Myles Standish - My job requires celestial mechanics, astronomical measurements, physics, numerical analysis, and computing. I think that my favorite is the computing. I have seen the amazing improvements made in computers over the past 4 decades, so I am fully aware of how powerful (and inexpensive) today's computers are. It's a joy to have them and use them.

    Cosmos - When and how was created the idea of generating the JPL ephemeris files? When we're they computed?
    Dr. E. Myles Standish - In the 1960's, JPL realized that the existing ephemerides were not good enough for spacecraft navigation, so they had no choice: JPL had to improve the ephemerides themselves: there is a big saving of fuel if the ephemeris is good: then one does not need to make big mid-course corrections to compensate for a bad target position. We have been improving the ephemerides ever since. When a significant improvement becomes possible (more observations, more accurate observations, better handling of the observations, etc.), then we create a new ephemeris.

    Cosmos - What principles did you use in Physics to elaborate them and what computational techniques we're required ?

    Dr. E. Myles Standish - Three things are needed in order to create an ephemeris:

    a) the equations of motion which are the forces acting upon the planets and moon - these come from our knowledge of gravitational physics,

    b) a computer program to numerically integrate the equations of motion (analytical formulae are not accurate enough for modern-day ephemerides), and

    c) the initial conditions (positions and velocities at some given date) and related constants (masses, etc.) - these are determined by a least squares adjustment of them in order to make the ephemerides fit the observational measurements as well as possible.

    Cosmos - How accurate are them?
    Dr. E. Myles Standish - The measurements of Sun, the four innermost planets (Mercury, Venus, Earth, Mars), and the Moon are accurate to a few tens of meters, due to the highly accurate measurements of them. However, since the asteroids affect the motions, and since the masses of the asteroids are not well known, it is not possible to compute the forces from the asteroids with high accuracy. Therefore, the positions and motions of the inner planets are known to an accuracy of about 1 kilometer.
    The accuracy of the outer planets is much worse: 100 km at Jupiter on out to a few thousand kilometers at Pluto. This is because the only measurements of the outer planets are optical - much less accurate than the radar and spacecraft measurements of the inner planets.

    Cosmos - What we're the primary and secondary difficulties in their calculation in the physics and computational areas? How did you manage to program the initial conditions and physical constants?
    Dr. E. Myles Standish - The key for the planets is the variety, accuracy, and time-coverage of the observations. These allow the accurate adjustment of the initial conditions and constants: in effect, one uses the existing ephemeris, produces what that ephemeris predicts for the value of each measurement, and then compares these "computed" values with the "observed" values. A mathematical computation ("least squares", invented by Gauss) then gives what corrections should be applied to the initial conditions which were used for the existing ephemeris. Finally, one adds these corrections to the existing initial conditions and uses the new initial conditions to produce a new ephemeris.

    Cosmos - Someone - a portuguese mathematician - asked me: why Chebychev polynomials in the interpolation ?
    Dr. E. Myles Standish - Chebychev polynomials seem to be accurate, economical, and efficient. Of course, they are not the only choice, but certainly they are a good one.

    Cosmos - Can we make a brief comparison chart in terms of accuracy of the JPL ephemeris and, let's say the VSOP87 or the ELP2000/82?
    Dr. E. Myles Standish - VSOP87 and ELP2000/82 were fit to JPL's DE200 (an ephemeris that we made 24 years ago). Therefore, it doesn't make much sense to compare them with DE200. If the comparison were to be made and if it showed any significant differences, it would come from the weakness of the fit of those ephemerides to DE200. On the other hand, EPM2000 from my colleagues in St. Petersburg, Russia, was made in a way very similar to the ephemerides made at JPL. Comparisons with that ephemeris are quite useful, though one must keep in mind that both EPM2000 and the JPL ephemerides have used mostly the same observational measurements.

    Cosmos - Are there any plans for improve the ephemeris or, more important, to expand the time span?
    Dr. E. Myles Standish - The ephemerides are improved every few years or so - when more and better measurements become available. Also, we have extended the coverage of DE405 from 10,000 BC to 10,000 AD. It's fairly easy now, with the fast modern computers that are available.

    Cosmos - What are you currently doing in the JPL headquarters - what kind of research?
    Dr. E. Myles Standish - Currently, we are processing spacecraft measurements from the MGS and Odyssey spacecraft, both of which are orbiting Mars. The measurements are both ranges (distances) and VLBI (angular directions). The combination is giving us excellent positions of Mars at the present time - useful for the upcoming arrivals of spacecraft at Mars this month and next month.

    Cosmos - What are your future plans of research?
    Dr. E. Myles Standish - I am in the process of writing descriptions of the whole ephemeris process so that others here can assume the job of refining the ephemerides.

    Cosmos - Would you like to leave a message to all that follow your work?
    Dr. E. Myles Standish - To anyone who follows this work, I would like to say that I hope you enjoy this field as much as I have; it has been interesting for me and has given me great satisfaction.

    Cosmos - Thank you very much for this time.
    Dr. E. Myles Standish - You're very welcome.


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