The Earth's axis is tilted 23 degrees from the plane of its orbit. The Moon's orbit is much closer to the same plane as the Earth-Moon orbit of the Sun.
Hi, ScienceTim here, from the EPOXI team. Let me correct some misconceptions.
The purpose of this experiment is to make a measurement of the Earth's spectrum at low spectroscopic resolution that allows us to simulate what an observer would detect from outside the solar system. Although we have spatial resolution in this movie, our scientific results will be obtained by adding up all the light in each of our filters in order to explore the ability to deduce properties of the Earth in unresolved data (we actually have 7 filters, not just the 4 that we show, plus a near-IR spectrometer). This information can be used to evaluate the engineering requirements for future space missions that will have the actual purpose of detecting and characterizing extrasolar terrestrial planets. Such a mission will be able to collect very few photons, so it will be required to do its job with very limited information.
Why not just simulate the Earth computationally, since we know a great deal about it? We do this, of course. Converting our detailed knowledge into an accurate simulation is not straightforward, however. Radiative-transfer techniques employ a variety of approximations, depending on the situation, and those approximations may require us to know something that would not be available for an actual extrasolar planet -- as an easy example, the pressure scale height is important for some methods. The EPOXI observation, and others like it that we acquired on earlier and later dates, provide an empirical test for those models. Once we have an empirically-tested model verified, we can apply the techniques from that model to the problem of modeling the apparent spectrum of nearly-Earthlike and not-at-all Earthlike terrestrial planets.
Keep in mind that this measurement is an interesting and useful exercise in the value of empirical test, but it is not the primary mission element. Currently, the primary mission element is observations of stars with known planets, to investigate these systems more deeply. We will finish in another month or so. Then we cruise for about a year, then we have a close flyby of another comet, after which the mission will be over. We have lots of good stuff coming.
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The Earth's axis is tilted 23 degrees from the plane of its orbit. The Moon's orbit is much closer to the same plane as the Earth-Moon orbit of the Sun.
Hi, ScienceTim here, from the EPOXI team. Let me correct some misconceptions. The purpose of this experiment is to make a measurement of the Earth's spectrum at low spectroscopic resolution that allows us to simulate what an observer would detect from outside the solar system. Although we have spatial resolution in this movie, our scientific results will be obtained by adding up all the light in each of our filters in order to explore the ability to deduce properties of the Earth in unresolved data (we actually have 7 filters, not just the 4 that we show, plus a near-IR spectrometer). This information can be used to evaluate the engineering requirements for future space missions that will have the actual purpose of detecting and characterizing extrasolar terrestrial planets. Such a mission will be able to collect very few photons, so it will be required to do its job with very limited information. Why not just simulate the Earth computationally, since we know a great deal about it? We do this, of course. Converting our detailed knowledge into an accurate simulation is not straightforward, however. Radiative-transfer techniques employ a variety of approximations, depending on the situation, and those approximations may require us to know something that would not be available for an actual extrasolar planet -- as an easy example, the pressure scale height is important for some methods. The EPOXI observation, and others like it that we acquired on earlier and later dates, provide an empirical test for those models. Once we have an empirically-tested model verified, we can apply the techniques from that model to the problem of modeling the apparent spectrum of nearly-Earthlike and not-at-all Earthlike terrestrial planets. Keep in mind that this measurement is an interesting and useful exercise in the value of empirical test, but it is not the primary mission element. Currently, the primary mission element is observations of stars with known planets, to investigate these systems more deeply. We will finish in another month or so. Then we cruise for about a year, then we have a close flyby of another comet, after which the mission will be over. We have lots of good stuff coming.