The Kepler telescope was launched from Earth on March 7 2009 and placed in an independent orbit around the Sun, with a semimajor axis of 1.0132 AU, period of 372.53 days, and an eccentricity of 0.036 according to Wikipedia.

Kepler needs to be far enough from Earth so that Earthshine (sunlight reflected by Earth) can not contaminate it's measurements. The camera is considered a photometer - Kepler's main mission is to detect extrasolar planets by measuring tiny shifts in the brightness of stars caused by distant planets transiting their stars.

I believe that there are other reasons to keep Kepler away from Earth as well. But I am wondering why it was decided to put Kepler in an orbit that continues to move farther and farther from Earth.

Kepler's design lifetime 4 years but it is continuing to produce scientific data even now, over seven years since it was originally commissioned.

My question is about its orbit. Currently it is about 137 million kilometers from Earth, almost 1 AU, and it continues to move farther from Earth all the time (see plots below). I understand that Kepler performs a substantial amount of preprocessing of the roughly 93 million pixel images before sending them to Earth, and downloads are done periodically, perhaps once per month. So the long distance does not seem to be causing too much of a communications problem.

But I'm wondering why it was decided to put Kepler in an orbit which constantly drifts farther from Earth. Is there some benefit to this, or is it the only way to keep it from returning to the vicinity of Earth too quickly?

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above: plot of the motion of the inner solar system bodies projected on to the plane of the Ecliptic in a rotating (synodic) frame based on Earth's orbit. Data from the JPL Horizons ephemeris database. The Earth is the blue "blob" to the right, and the Kepler telescope moves slowly behind the Earth every year. The wiggles are due to the eccentricity of both the Earth and Kepler.

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above: plot of the distance between Kepler and Earth as a function of time. Data from the JPL Horizons ephemeris database.

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above: plot of the solar angle between Kepler and Earth as a function of time. Data from the JPL Horizons ephemeris database.

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above: CCD array of Kepler's Photometer, credit: NASA.

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    $\begingroup$ Stopping it from drifting would cost fuel. Drifting causes no harm. Why do the burn?? $\endgroup$ Oct 10, 2016 at 19:20
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    $\begingroup$ Out of curiosity, did you use Matlab for those plots? The style resembles those output by Matlab (though iirc matplotlib produces similar plots). $\endgroup$
    – JAB
    Oct 10, 2016 at 19:45
  • $\begingroup$ @LorenPechtel Yep that makes sense! I'm wondering if there is another orbit that would separate from Earth even more slowly and require even less fuel, or if this is some how near the minimum already. $\endgroup$
    – uhoh
    Oct 10, 2016 at 23:00
  • $\begingroup$ @JAB I used Python and Matplotlib. There is some visual similarity to Matlab, but everything in the plot are just python objects. The orbits are handled within JPL's Horizons system, and I've just saved their output in Ecliptic coordinates to my computer. The only math that I've done myself here is a simple rotation transform to keep the earth on the +x axis. $\endgroup$
    – uhoh
    Oct 10, 2016 at 23:09

1 Answer 1


Initially, Kepler was slated to go to L2. However, when preparing Spitzer, NASA found that a heliocentric, Earth-trailing orbit takes less propellant to reach than L2. For Spitzer, this enabled the switch to a smaller launcher. This finding also applies to Kepler.

  • $\begingroup$ OK that's helpful. I suppose if Kepler was designed to operate normally at about ~1AU, then it could also operate while it is less than 1AU. I'll take a look at those links. Was there some kind of conceptual orbital mechanics breakthrough here, or was it more like obvious in hindsight? $\endgroup$
    – uhoh
    Oct 10, 2016 at 14:54
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    $\begingroup$ Figure 7 in this pdf is interesting, as is the paragraph immediately above the figure! L2 requires regular station keeping which means weight and loss of observing time. Once in a heliocentric orbit, propulsive maneuvers for orbit adjustments are not necessary. $\endgroup$
    – uhoh
    Oct 10, 2016 at 15:12
  • $\begingroup$ SEL2 looks like a much better place for Kepler than a heliocentric orbit. If Kepler had worked for 30 years as HubbleST, its science yield would've grown exponentially with time. Transiting needs time. $\endgroup$
    – LocalFluff
    Apr 17, 2018 at 15:07

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