Two Concepts for the Terrestrial Planet Finder

EccentricAnomaly writes: "This NASA press release and this space.com story discuss two concepts for the terrestrial planet finder, a mission that will look for Earth sized planets around other stars. One concept is an infrared interferometer the will nullify starlight while amplifying the infrared light from any planets. The other concept is a visible light coronagraph - basically a larger version of Hubble that will block out the light from stars so that it can see any pale blue dots."

Infra-red?

[AndrewRUK]

I might be being stoopid here (but hey, the only other psts here are a troll and two ACs posting the article) so bear with me...

Do stars not give out any infra-red? Because, if they do, surely that would swamp any I-R detector, and the planet wouldn't be seen.

Of course, IANAA (...astronomer) so if anyone wants to explain why I'm wrong, I'd like to know.

Re:Infra-red?

[LordSah]

The warmth you feel from the sun is infrared radiation. So yes, they give out lots of infrared. I don't know much about I-R detectors however...

Re:Infra-red?

[CheshireCatCO]

The warmth you feel from the sun is infrared radiation.

Not true. About half of the energy you feel as "warmth" from the Sun is from visible light alone. If you added in the near IR and near UV to get what is often called "shortwave radiation" (as opposed to longwave infrared, like most of what Earth and you and I emit), you get the overwelming bulk of the energy that heats you up when you lie in the sun. For some reason, it's come to be a common misconception that IR warms you, and visible doesn't. The association is probably because we think of hot bodies (like people) with IR. But the Sun is just a very hot body, so the physics is all the same.

Re:Infra-red?

[LordSah]

Thanks for clearing that up. Isn't the heat from fires and stoves mostly IR? A campfire doesn't emit much visible light, but it emits a lot of energy that's very warming.

I would reckon the common misconception comes from elementary school teachers...that's where I got it :)

Re:Infra-red?

[CheshireCatCO]

It depends on where the peak emission is at. That, in turn, is temperature dependent. For a stove at a several hundred Kelvin, the peak is in the IR, so you're perfectly correct about the main radiative energy coming from IR.

Re:Infra-red?

[mmarlett]

Well, IANAA either, but stars (suns) put out just about everything we can "see" or detect -- infrared, ultra-violet, microwaves, x-rays, blah blah blah. I would presume that one could calabrate an infrared detector to be sensitive to a narrow range of temperatures that would allow one to block out the ultra-hot star but still see a warm planet against a cold blanket of space.

NASA has a bit more on that here [nasa.gov].

The most important graph for this question:
"Infrared TPF concepts would use multiple telescopes configured into an interferometer and spread out over a large (30 meter) boom. The telescopes must operate at extremely low temperatures, and the spacecraft would necessarily be much larger. However, the image contrast requirement is much easier at infrared wavelengths -- only a million to one -- and thus the system optical quality is easier to achieve. "

Re:Infra-red?

[RevRigel]

Actually, since stars are nearly ideal black body radiators (with the exception of some absorption lines), they have a peak frequency at which they emit radiation. For stars like the sun, this is in the ultraviolet region. It never gets as high as X-rays or gamma rays; not even in small amounts. It takes much hotter gas to emit X-rays (the sun is around 6000 K, I believe X-rays start showing up around 30000 K or 60000 K).
The sun does emit radiation throughout the spectrum below its peak wavelength, but as the frequency approaches zero so does the energy emitted -- so you would see a fair amount of infrared, but very little ULF radio.

Re:Infra-red?

[Avatar1000]

From one of the AC posters:

- Infrared Interferometer: Multiple small telescopes on a fixed structure or on separated spacecraft flying in precision formation would simulate a much larger, very powerful telescope. The interferometer would utilize a technique called nulling to reduce the starlight by a factor of one million, thus enabling the detection of the very dim infrared emission from the planets.

So, yes - stars give off considerable IR radiation; yes, the planned mission has a way (a method called Nulling) of coping with this potentially blinding glare.

Related article in Discover Magazine...

[BMazurek]

There is/was a related article in the March 2002 issue of Discover Magazine.

There is an online version of that story [discover.com].

To seek out new life and new civilizations...

[faldore]

I think it would be easier to go out there and see for ourselves. Somebody invent warp drive!!!

New game plan.

[El Jynx]

I'm beginning to wonder why don't take a more drastic approach. What they could do is get, say, a dozen heavy-duty telescopes up into orbit, only this time put them in orbit around the sun, not the earth, and put them at a fair distance - say, between Mars and Jupiter. Then they let all 12 scopes takes pictures of the target in as many different spectra as they are able. This done, all you have to do is beam the images back to earth and let some big fat mainframe calculate the differences in image; what you get then is an image of your target that is lightyears clearer than anything they can produce now because you have effectively created a telescope with a diameter of the orbit of the 12 sattelites. Make a dish that big and you won't have imaging problems for quite a while (although delegating the rights to use it will be difficult since everyone will be jumping at the chance :P)

But I would guess this problem is similar to that of the space elevator: the costs of building something on such a scale is prohibitive at this time for any one nation. Maybe if we all got together it would be doable, though. I personally think they should do the space elevator first, then we can launch the lenses from space; way cheaper.

Re:New game plan.

[Antity-H]

It is true that building the space elecator would allow us to do lots of things, including making lenses in 0 grav, which i think should allow us to make far better and far wider lenses than we have already. However the problem remains with the project that not only it is economically impossible to realise, but it is not even technically possible, and might not be for a very long time. I don't have the reference nor the time to look it up, but i remember reading that only carbon nano tubes could be used to realise the cable for the space elevator. Now, a few days ago there was that funny little post on slashdot about nanotubes exploding after a flash of light, so that's it for the space elevator. in addition i am not even sure your 12 satellites could perform so well, there are too many variables in the orbits calculus, which would make it virtually impossible to have them move in a precise formation, not to say that between mars and jupiter we have a nice little asteroid belt ( therefore high particle density) .. guess what a dust particle could do to your lenses not to mention a rock the size of your fist.

Re:New game plan.

[RevRigel]

What you're proposing is an interferometer. Radio telescopes like the Very Large Array are interferometers as well. You are correct that interferometers increase resolution to the resolution of a large telescope of the same baseline (but with the light/radio gathering power of the actual diameter of telescopes used, of course).
In order to make an interferometer work, you have to combine the signals completely in phase (to within 1/10 wavelength, as a general rule). So if you were doing work in the 20cm band, your cable lengths from each antenna would need to be the same to within 2cm. Optionally, signals from radio telescopes can be recorded (such as those on opposite sides of the globe, for really long baseline), and lined up later, on computers. This only works because we have data acquisition systems that can work at MHz and GHz frequencies. Optical frequencies are more like 10^15 Hz.
In order to do optical interferometry, you have to combine the light paths to within a tenth wavelength (40-50nm, or several hundred atoms). This interferometry can only be done in real time, with hard optics. At least until we get attohertz electronics and data acquisition working.

The space interferometry mission will put an interferometer in space in the next 10 years or so, but its baseline is only 30 feet or so. Still very cool.

I like the interferometer idea

[Rhinobird]

one of the cool things that can be done with a set up like that, is that the planets stick out like sore thumbs in the data. Also, you get to do spectral analisys on the data to find out the compnents of the atmospheres (or surfaces?) Unfortunately, you don't get to see any groovy little blue dots...just some funky looking false color images with bright spots...