All eyes on #Rosetta part 1: #ALICE

Note: In this – hopefully – daily series of postings I’ll highlight one of the many instruments on board of the Rosetta spacecraft and the Philae lander.

Hello Space Geeks, in this first posting of this series we’ll talk about the ALICE-experiment on board of Rosetta. First, what does ESA say?


is an imaging ultraviolet spectrometer, which will determine the composition and temperature of the surface and the characteristics of the gas molecules in the coma.

Principal Investigator (PI): Alan Stern, Southwest Research Institute, Boulder, Colorado, USA.


So, what is a spectrometer? It is basically a camera with a prism in it’s optics. As you know, due to the difference of refractive indices of the vacuum in space and another medium like glass – that of the prism – the light get’s bend differently depending on it’s wavelength, or, color. This is the principle a spectrometer uses: it takes the incoming light from a target, spreads out the spectrum, and projects the resulting light-pattern onto a regular CCD-sensor included in every digital camera:

Figure 1: Diagram of a dispersion prism; by Wikipedia user Suidroot; licensed under the terms of GFDL and CC-BY-SA-3.0,2.5,2.0,1.0; Source:

After the light is decomposed into it’s individual frequencies, the intensity of these colors is recorded and translated into a diagram, like so:

Figure 2: Light spectrum of a fluorescent lamp; by Wikipedia user Anton; licensed under the terms of GNU FDL >= 1.2; Source:

But why the hassle? Because the reflected light from the target tells you something about it’s composition! The ambient light from the sun has a certain spectrum; when this light is reflected by the target – for instance 67P’s surface – and from what colors are absorbed or reflected, you can tell from which elements the surface is made of without touching the object of interest!
In Figure 2 you see the emitted spectrum of a fluorescent lamp; Mercury, this infamous metal being liquid at room temperature, has just about this atomic spectrum when heated – it glows in distinct colors which is unique to this element (Compare “mercury atomic spectrum” at Wolfram|Alpha).

And this doesn’t only apply to the reflected or absorbed colors of the surface, but also about the light which passed through the coma, the “tail” of the comet.

3D mockup of ALICE; “The ALICE ultraviolet imaging spectrometer”; ESA; Copyright ESA/AOES Medialab; Source:

That means, as soon as ALICE is operational, we already get to know from what the comet and the surface is made of even when Philae wasn’t even deployed yet. I don’t know about you, but I find this pretty darn cool. :-)

Update: I forgot to mention: ALICE was sucessfully powered on at the 18th of March. Yay!

2 Responses to All eyes on #Rosetta part 1: #ALICE

  1. […] OSIRIS system for now. OSIRIS was already switched on a few days ago (along with ALICE, more in the first posting of this series) so I’m a bit in a hurry to catch up with all the already working instruments […]

  2. […] is a mass spectrometer? If you’ve read my posting about ALICE, the ultraviolet spectrometer, you already learned that lights of different colors get “bend” differently in a prism […]

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: