Earth and Moon From an Alien's Perspective

krygny writes "NASA's Deep Impact spacecraft (whose extended mission is called EPOXI) has created a video of the moon transiting Earth as seen from 31 million miles away. Scientists are using the video to develop techniques to study alien worlds. 'Our video shows some specific features that are important for observations of Earth-like planets orbiting other stars,' said Drake Deming of NASA's Goddard Space Flight Center... 'A "sun glint'" can be seen in the movie, caused by light reflected from Earth's oceans, and similar glints to be observed from extrasolar planets could indicate alien oceans. Also, we used infrared light instead of the normal red light to make the color composite images, and that makes the land masses much more visible.'" Here are links to the two videos, one red-green-blue and the other infrared-green-blue.

Comments

Missing something

[Space cowboy]

Perhaps I am, but 31,000,000 miles doesn't seem that far away from an astronomical perspective - in fact it seems pretty darn close. A single light-year is about 5,878,625,373,183.61 miles (from Wiki), so 31M miles is roughly 1/190,000 of a light year.

The nearest star is ~4.2 light years away, so our potential alien visitor would have to travel a very long way towards us (and in that case why not come the last 0.0001% of the journey!) before this was a useful property.

Now I realise you can only take a video from as far away as your spacecraft really is, but I'd expect to see extrapolations to realistic distances before you start to claim things like "Making a video of Earth from so far away helps the search for other life-bearing planets in the Universe". - that's a bold claim, after all. I'm sure there's a standard line somewhere about extraordinary claims requiring extraordinary evidence to back them up...

I dunno, perhaps I'm just a grumpy old physicist, but there's all sorts of effects that only come into play at astonomical-scale distances (and the relativistic-scale speeds that commonly occurs between bodies that far apart), I guess I'd like to have seen more data and less hand-waving.

Simon.

Re:Missing something

[iamlucky13]

You're correct that the distances are wildly different, but for some observation techniques that really doesn't matter much. The distance between the earth and the moon compared to the distance to the spacecraft is small enough to be just as negligible. There's no reason why something that works 30 million miles away shouldn't work 30 trillion miles away. The only real differences are the brightness and resolution (well...perhaps some of the spectrum may be reduced by the interstellar medium, but that's a pretty specific factor).

You can't resolve objects at the separation of the moon and earth from 30 trillion miles away, not even with the Hubble or Keck telescopes, and especially not with spectroscopes that can give you clues to what chemicals are present on those bodies. By studying star wobbles an astronomer might infer the presence and mass of a "planet," but that won't tell him if it's really a single planet or a planet-moon system. Look at the video and notice that as the moon crosses the earth, the total reflected light from the earth and moon would be decreased by the ratio of the area covered (about 7%) because the moon is blocking part of it. From that, the astronomer can infer not just the presence of the moon, but the relative sizes of the planet and moon.

Assuming the Space Interferometry Mission goes forward as planned, the astronomer might eventually be able to get a spectrum from the planet without being washed out by the parent star. By watching how the spectrum changes during such transits, they can figure out what elements and compounds (like water) are likely present on the planet, and what ones are present on the moon.

It may sound far out, but it's already being done with exoplanets and their stars, and transits of Pluto and it's moon are how we got a lot of our information so far about those two bodies.

Wow

[FlyingSquidStudios]

I was going to post the usual attempt at witty snarkiness, but then I actually watched the video... seeing the Moon actually moving around the Earth like that, it actually made my heart skip a beat. Seeing us that way with my own eyes someday, as unlikely as it may be, is something I really long for.

Cool, but then again....

[Anonymous Coward]

That's cool but then again, I'm a sucker for any movie I'm actually in.

Hey..

[elemnt14]

..I can see my house from there!

That's no moon...

[oodaloop]

Whoops, reached my limit for "That's no moon" comments in a single day. No no, don't get up. I'll show myself out.

A word from the EPOXI team

[ScienceTim]

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.