Measuring an Alien World

How do you measure and analyze a planet that's one-and-a-half times the size of Earth and 300 light years away? How do we know the elements from which it is composed? The answer: With the incredible technology of a cryogenic telescope like the Spitzer and Kepler Space Telescopes.

Let's take a look at the Spitzer telescope and how it works. It's simple basics.

Cryogenic telescope assembly (CTA)
This amazing device has a liquid helium tank known as a cryostat. It cools the telescope down to 5K (-459°F). The telescope detects faint heat radiating from objects in space. Instruments must be kept extremely cold, so their own heat will not interfere with the measurements.

Figure 1

The four parts of the cryogenic telescope assembly.(Source: Caltech NASA/JPL)

The four parts of the cryogenic telescope assembly.
(Source: Caltech NASA/JPL)

In Figure 1, the telescope is the tubular-looking device vertically mounted atop the multiple instrument chamber, which sits on top of the cryostat assembly (which looks like your barbecue propane tank), surrounded by the outer shell.

Instrument chamber electronics
Here is where the electronics comes in. The instrument chamber contains three key electronic systems: an infrared array camera (IRAC), an infrared spectrograph (IRS), and a multiband imaging photometer (MIPS).

The IRAC detects light at near- and mid-infrared wavelengths. This is a four-channel camera (four different detectors measuring light of a particular wavelength). Simultaneous images are taken at the wavelengths sensed by each of the four arrays, which are 256 x 256 pixels in size.

Figure 2

The plot shows a strong signature of water vapor in the gas and dust surroundinga young star captured by Spitzer's IRS.(Source: Caltech NASA/JPL)

The plot shows a strong signature of water vapor in the gas and dust surrounding
a young star captured by Spitzer's IRS.
(Source: Caltech NASA/JPL)

The IRS functions like a prism and breaks up light in a rainbow of colors. Since each chemical element has a unique fingerprint in its light signature, we can tell what elements and molecules make up the object we are viewing. (Water was found on another distant planet with this method.) The detectors are 128 x 128 pixels in size, and the long and short wavelength modules each have their own entry slit to allow infrared light into the spectrograph.

Figure 3 shows a spectrometer using an array detector.

Figure 3

Spectrometer design using an array detector.(Source: Texas Instruments)

Spectrometer design using an array detector.
(Source: Texas Instruments)

Processing the CCD signal
Analog Devices' Erik Barnes wrote in an article on his company's website:

Accurately recovering and digitizing the CCD signal requires several operations, including correlated double sampling and dc restoration (clamping), gain, offset, and A/D conversion.

Correlated double sampling (CDS) serves two important purposes: it calculates the difference between the reference and data levels of the CCD signal, and it reduces some of the noise components in the CCD signal. Conceptually, the CDS is a differential-in-time amplifier: it takes separate samples of the input signal and outputs the difference between them.

Figure 4

CCD output stage.(Source: Analog Devices)

CCD output stage.
(Source: Analog Devices)

The MIPS is also an imaging camera. It detects light in the far-infrared wavelengths and can perform some simple spectroscopy. The detectors are different sizes for the different wavelengths: 128 x 128 pixels for 24 microns, 32 x 32 pixels for 70 microns, and 2 x 20 pixels for 160 microns.

Measuring a planet light years away
The Spitzer and Kepler Space telescopes have made the most precise measurements of a planet's radius outside our solar system. An uncertainty of just 74 miles on either side of the planet was unheard of until now. This is like measuring the height of a six-foot-tall person to within three-fourths of an inch on Jupiter.

To make this accurate measurement, both the Kepler and Spitzer telescopes watched the planet Kepler-93b move across the face of its star. This minute eclipse changed the infrared (Spitzer) and visible (Kepler) light received. This measurement was corroborated by both telescope methods and eliminated a false-positive result.

NASA discussed Spitzer’s accuracy last week on the agency's website:

Spitzer racked up a total of seven transits of Kepler-93b between 2010 and 2011. Three of the transits were snapped using a “peak-up” observational technique. In 2011, Spitzer engineers repurposed the spacecraft's peak-up camera, originally used to point the telescope precisely, to control where light lands on individual pixels within Spitzer's infrared camera.

The upshot of this rejiggering: [NASA scientists] were able to cut in half the range of uncertainty of the Spitzer measurements of the exoplanet radius, improving the agreement between the Spitzer and Kepler measurements.

In 2013, NASA discussed the repurposing of the Spitzer camera:

This camera was used during the original cryo mission to put gathered infrared light precisely into a spectrometer and to perform routine calibrations of the telescope's star-trackers, which help point the observatory. The telescope naturally wobbles back and forth a bit as it stares at a particular target star or object. Given this unavoidable jitter, being able to control where light goes within the infrared camera is critical for obtaining precise measurements. The engineers applied the Peak-Up to the infrared camera observations, thus allowing astronomers to place stars precisely on the center of a camera pixel.

Since repurposing the Peak-Up Camera, astronomers have taken this process even further, by carefully “mapping” the quirks of a single pixel within the camera. They have essentially found a “sweet spot” that returns the most stable observations. About 90 percent of Spitzer's exoplanet observations are finely targeted to a sub-pixel level, down to a particular quarter of a pixel.

So it was as simple as that.

17 comments on “Measuring an Alien World

  1. Sachin
    July 31, 2014

    Considering the fact that this instrument is supposed to be used in measuring and analyzing objects that are located more than 300 light years away, it is highly likely that many elements found there will be very different from anything we have ever seen. The IRS will give us the unique footprints of the elements it locates there but I cant see how this will be of much use considering the fact that the footprint will most probably be unlike anything ever seen before and cannot be identified by comparing it to any data we already have. Insightful piece though.

  2. Sachin
    July 31, 2014

    The CTA tank, is an ingenious, albeit unconventional, thermal sensor. Its basic operation sounds pretty simple enough but I wish you had gone a little deeper into the details. From what I can tell, it uses heat signatures (to very minute levels) to detect the presence of an object. How does it deal with situations where it encounters multiple objects in space each emanating its own heat. Even more complicated since it seems to be designed to detect low temperature objects, what happens when it detects a high temperature one….maybe one above 50 degrees, wouldn't that heat its parts and possible cause them to interfere with its overall accuracy?

  3. Steve Taranovich
    July 31, 2014

    @SachinEE—there are so many challenges in an amazing effort like this. The talented scientists at NASA, JPL and others will have to approach each one step-by-step. Right now they have discovered this Earth-like planet 1-1/2 times our size, but probably too hot to sustain any life

  4. geek
    July 31, 2014

    @Steve: Any idea about the cost factor while using CTA? Does it cost far more than it would with conventional systems?

  5. Steve Taranovich
    July 31, 2014

    @tzubair–liquid helium frozen vapor from the helium tank is used to keep the instruments cold. The cryostat holds about 360 liters of liquid helium, and can cool the instruments to temperatures as low as 1.4 degrees Kelvin (roughly -457 degrees Fahrenheit, or -272 degrees Celsius) for more than 5 years. I'm not aware of anything else that would work as well and be lightweight for the spacecraft escape velocity from the Earth. More weight means more fuel needed.

  6. geek
    July 31, 2014

    “. I'm not aware of anything else that would work as well and be lightweight for the spacecraft escape velocity from the Earth. More weight means more fuel needed.”

    @Steve: No one can doubt its effectiveness in terms of accuracy. What I meant to ask was more linked with the use. In terms of developing the CTA technology and also making it mature, how challengingn is it going to be? Particularly when you know there's a huge cost that has to be incurred.

  7. Steve Taranovich
    July 31, 2014

    @tzubair—Nasa/JPL is always looking for a next-generation system. Here is one:


    NASA will use a purely mechanical cooling system with no cryogen and get down to 1.7 Kelvin! They will be based upon Stirling and Joule-Thompson coolers.

    Making it more mature is always a goal for all NASA systems, but there is not word as to the progess in that area yet.

  8. chirshadblog
    July 31, 2014

    @steve: Well that's bad but Im pretty sure that NASA will not give up on the idea. 

  9. Steve Taranovich
    July 31, 2014

    @chrishadblog—I hope the US does not give up on NASA—they need funding

  10. etnapowers
    August 4, 2014

    “The telescope detects faint heat radiating from objects in space. Instruments must be kept extremely cold, so their own heat will not interfere with the measurements.”

    The reliability on the long term of the instruments, working for a long time at very low temperatures, is a critical point to ensure the effectiveness of this amazing CTA.



  11. etnapowers
    August 4, 2014

    The need of NASA for funding is deeply related to the results that will be obtained. The companies like NASA, that work on challenging aerospace projects must be funded by the Government bodies to achieve missions that can improve the life of people , let's think for example to the earth atmosphere studies by the meteorological satellites and space stations.

  12. Netcrawl
    August 6, 2014

    @etnapowers its a tough fight for NASA, the agency is currently facing internal problem and huge budget cuts. I don't believe that these private companies could handle NASA's technical needs such as rocket programs, I admit I'm not a fan of SpaceX and their revolutionary space vehicles, I dont think that it can pushes boundaries like NASA should be doing. I don't even believe that partnering with these private companies would save the government a lot of many and resources like for example buying launches from commercial companies like SpaceX.

  13. Netcrawl
    August 6, 2014

    @Steve the massive cuts are going to hurt NASA, its forcing the agency to think something different, my greatest fear is that NASA would have to cut some important space missions (high-profile space explorations) just to fund another one. We're losing our edge in space exploration, we're giving up our position to our adversaries.

  14. etnapowers
    August 6, 2014

    @Netcrawl: I appreciate your post but I have to say that I am a fan of companies like SpaceX, because I think that the funds for scientific challenges, like the researches of the NASA company, might be raised also from commercial activities like the commercial space flights.

  15. eafpres
    August 11, 2014

    @Netcrawl–“I don't believe that these private companies could handle NASA's technical needs such as rocket programs”

    I also tend to disagree with you on this point.  Originally NASA had the best engineers and worked on all the leading edge technologies related to both space flight and high performance aircraft.  Over time the mission has spread out so much, it is unfortunate but no wonder they face budget cuts.  Given this, they become conservative, which leads to blocking really new technologies.

  16. eafpres
    August 11, 2014

    @sachinEE–“highly likely that many elements found there will be very different from anything we have ever seen”

    That is not an obvious conclusion for me.  We know that life can develop on carbon-based molecules.  We have a couple hundred years of looking at what you can make with carbon.

    But for another reason I think it is possible to do this work is that most of the molecules for which they are looking are simple ones.  CO2, H20, CH4, etc.  These have strong vibrational spectra that can be observed.  More complex organic molecules have very complex spectra due to so many energy levels; I doubt they could figure those out.

    But from looking at only a few simple ones we can draw a lot of conclusions.

  17. eafpres
    August 11, 2014

    @tubair–liquid cryogen cooling is mechanically very simple.  There is a big issue with any vibrations from any kind of active cooling.  

    One thing I don't know about is what they do with the small amount of He constantely boiled off.  Some how they have to get rid of the vapor, or perhaps the pressure gets high and higher, until 5 years from now it is all vapor and the temperature begis to rise.

    I recall from the film Apollo 13 they talked about cryo-stir.  In zero gravity the liquid and vapor phases co-mingle, which causes problem as well.

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