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, I’m currently preparing my trip to the DLR in Cologne this Friday (my accreditation was confirmed), so I missed a posting yesterday. Also there’s a family and a day job to take care of! :-)
Today I’d like to talk about the two mass spectrometers on board of the Rosetta spacecraft, ROSINA and COSIMA. First I hand over to ESA what they have to say about these instruments:
ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) contains two sensors which will determine the composition of the comet’s atmosphere and ionosphere, the velocities of electrified gas particles, and reactions in which they take part. It will also investigate possible asteroid outgassing.
Principal Investigator: Kathrin Altwegg, Universität Bern, Switzerland.
COSIMA (Cometary Secondary Ion Mass Analyser) will analyse the characteristics of dust grains emitted by the comet, including their composition and whether they are organic or inorganic.
So ROSINA will investigate gases, where COSIMA will investigate dust; but going to to the technology, they pretty much work the same.
What 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 – the prism “sorts” the different colors by their wavelengths and projects it to a digital camera. There we record which colors occur and how bright each color is. We also learned that from the color-composition we can learn from what stuff is made of.
A mass spectrometer uses a similar principle; it takes a sample of gas, sorts the different substances by their weight (instead of color), and counts how much particles (instead of photons) of each mass hit a detector.
So how do you separate atoms by their mass? I’ll give you a a simplified explanation. You use a magnet! First we have to ionize the sample so that it’s electrically charged. What you do is you fire negatively charged electrons at the (neutral) atoms. The electrons hit the atoms at high speed, kicking out the electrons orbiting the atom: Having lost their electrons, only the atoms’s nucleus remains. Since the nucleus only consists of neutrons (having no charge) and protons (being positively charged), the atomic nucleus is now positively charged. Now you accelerate the positively charged atoms (now called ions) in an electric field up to a velocity of few kilometers per second. Now the magic happens: This beam of ions is forced into a curve by another electric field and through a magnet. Since heavier objects in a curve are “lazier”, they don’t take the curve as sharp as lighter objects. A mass spectrometer does exactly this; if the beam of ions is put into, let’s say, a right curve, the heavier ions will arrive more to left after the curve, the lighter ions more to the right. All these ions, now sorted by their mass, hit the detector in different places. The detector counts how much of each at which position arrived and then shows you of what the gas is made of!
This video shows us a lab-grade mass spectrometer as used on earth and explains the principle of operation.
Obviously ROSINA and COSINA are much smaller and less heavier than lab-equipment on earth; after all, lifting a kilogram of material into space onboard of an Ariane 5 costs around 15,000 Dollars, so smaller is better :-) (120 Mio, USD / 6,700 kg payload; Rosetta had a launch mass of 3,000 kg but the Ariane 5G had a unique configuration, so these number may be way too small; I couldn’t find proper numbers, but let’s forget about the money for now:)
Rosetta will send the results back to earth and scientist will analyze graphs like this:
And this is exactly what ROSINA does.
But… how does COSINA then analyze dust? Dust is much bigger than singular atoms or molecules from a gas. It’s easy: Heat it up and vaporize it! COSINA includes a little oven, where dust samples are heated up and brought into the vapor-phase. And then it’s pretty much the same as with ROSINA; ionize, accelerate, bend around a curve, shove the ion-beam through a magnet to amplify the seperation and let them fellow bang into a detector. Count hits, create spectrum, send to earth. Voila!
And that’s how you figure out what gases and dust is made of; on earth and in outer space.