# Kepler satellite - question about its current troubles **URL:** \ **Category:** Factual Questions\ **Created:** [May 15, 2013, 9:58pm UTC](https://boards.straightdope.com/t/kepler-satellite-question-about-its-current-troubles/658455 "2013-05-15T21:58:32Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [May 15, 2013, 9:58pm UTC](https://boards.straightdope.com/t/kepler-satellite-question-about-its-current-troubles/658455/1 "2013-05-15T21:58:32Z")
[The Kepler satellite and mission](http://kepler.nasa.gov/) are dedicated to identifying so-called exoplanets (planets orbiting stars other than our Sun). In particular, the hope is to locate and then study those exoplanets which share enough characteristics with Earth to make them good candidates to support life as we know it (e.g. not too close or far from its sun, similar mass, similar temperature, etc.). As described today in [this New York Times article](http://www.nytimes.com/2013/05/16/science/space/equipment-failure-may-cut-kepler-mission-short.html?hp), all is not well with Kepler. In particular, its reactions wheels, which are needed to keep the satellite oriented appropriately and pointed in the right direction, are malfunctioning. The mission may therefore end two years earlier than planned thus limiting any conclusions that might be drawn from its observations. So, finally, my question. In the same New York Times article linked to above, Dennis Overbye (who is a pretty competent astronomy/physics journalist) says, “Since Earth transits only once a year, two more years would have given astronomers a chance to see more transits of the planets they are looking for. Without the extra time, the data will be noisy, . . .” What does the Earth’s transit have to do with identifying exoplanets? Did Overbye misspeak and really mean to say that ‘since the _exoplanets_ of interest have an orbital period of around one Earth year, two more years would have given astronomers a chance to see more transits of the planets they are looking for’? And, if it’s not an error, can you please teach me how the “earth’s transit” relates to Kepler’s data gathering. Thanks! ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [May 15, 2013, 10:09pm UTC](https://boards.straightdope.com/t/kepler-satellite-question-about-its-current-troubles/658455/2 "2013-05-15T22:09:22Z")
That’s pretty much it. If you were at another star watching our system (and the alignment is right which is one out of a couple hundred), you’d only see Earth pass in front of the sun for a pretty short period of time once a year. So, yes, a couple of years of missed data either means a bunch of other stars that would have been watched won’t be OR for a given star where you’ve seen one or two blips you won’t get to see the third or fourth (which is a much more solid measurement). ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [May 15, 2013, 10:19pm UTC](https://boards.straightdope.com/t/kepler-satellite-question-about-its-current-troubles/658455/3 "2013-05-15T22:19:39Z")
Thanks. But, just to be clear, you agree that Overbye’s reference to _Earth’s_ transit was irrelevant, no? ---
**Author:** ![MEBuckner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mebuckner/32/2896_2.png) [@MEBuckner](https://boards.straightdope.com/u/MEBuckner)\ **Post date:** [May 16, 2013, 12:35am UTC](https://boards.straightdope.com/t/kepler-satellite-question-about-its-current-troubles/658455/4 "2013-05-16T00:35:29Z")
> [@KarlGauss](#): > > Thanks. > > But, just to be clear, you agree that Overbye’s reference to _Earth’s_ transit was irrelevant, no? I’d say it’s not completely irrelevant, but that he left out a couple of steps in the chain of reasoning. _Given that_ we’re interested in looking for other “Earth-like” planets, which will presumably be orbiting more-or-less “Sun-like” stars at distances (at least roughly) comparable to the distance from our Sun to Earth, the laws of physics say a body in an orbit that’s around one [A.U.](http://en.wikipedia.org/wiki/Astronomical_unit) from its parent star has to take something like one Earth year to complete its orbit. A planet that takes something more like one Earth day, or a hundred Earth years, to orbit its parent star _has_ to be either very, very close to its star or very, very far away from its star. Even if you posit a planet orbiting a very, very dim star or a very, very bright star, there are going to be a lot of other issues with having that planet be truly Earth-like, and it seems like a better bet to look for more-or-less sunlike stars and planets in orbits around those stars such that they complete one orbit, maybe not every 31,557,600 seconds _exactly_, but something on that order. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 16, 2013, 12:42pm UTC](https://boards.straightdope.com/t/kepler-satellite-question-about-its-current-troubles/658455/5 "2013-05-16T12:42:30Z")
> [@KarlGauss](#): > > But, just to be clear, you agree that Overbye’s reference to _Earth’s_ transit was irrelevant, no? It isn’t irrelevent, but (at least from the cited comment) the reason isn’t made clear. Kepler is looking for transits by a target exoplanet about its star. However, in order to make better than order of magnitude estimates of the orbital arguments of the planet, the observations need to be made from different aspects, e.g. different positions of the Earth as it orbits around our star (Sol). Although the change in aspect is tiny–at most, 2 AU, or 0.0000316 light-years–it may be enough to establish speed, inclination, orientation of the semi-major axis, et cetera, i.e. all the parameters to evaluate whether the planet may be in a habitable zone (as we would consider it, a zone where liquid water could potentially exist throughout the orbit) about its star. Without multiple measurements of a transit from different aspects, the orbital elements are very uncertain, whereas combinations of transits and aspects can allow scientists to reduce the error bands using Bayesian methods of error estimation. Remember that Kepler doesn’t directly observe the planet; only the very, very slight dimming asit transits its star. As an analogy, consider trying to estimate the speed and length of a freight train blindfolded from half a mile away from the track. This is actually a trivial exercise in comparison to what the Kepler program is doing to determine the elements of exoplanets. Stranger ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [May 16, 2013, 2:58pm UTC](https://boards.straightdope.com/t/kepler-satellite-question-about-its-current-troubles/658455/6 "2013-05-16T14:58:24Z")
VERY helpful responses. Much obliged.