## News from #Rosetta and #MIDAS

April 3, 2014

Dear space geeks, I had a little chat with the gals & guys from Rosetta’s MIDAS-Team and I’d like to sum up the relevant information as a little consolidated Twitter-timeline.

OK, cool, now that is something! Then they posted a schematics of the MIDAS atomic force microscope and I just couldn’t resist asking more:

I surely will do that! But also be sure to read their bit about the latest software upgrade.

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## All eyes on #Rosetta: About micro-gravity, solar radiation pressure and solar winds

March 28, 2014

Good Morning space geeks, I’m almost ready for my trip to the German Aerospace Center in Cologne to watch Philae’s commissioning in their Microgravity User Support Center (MUSC). But since I have a few hours to kill I thought I share a few thoughts with you about the microgravity in 67P’ vicinity and how the solar wind and solar radiation pressure affects Rosetta’s orbit around 67P. Warning: May contain mathematics :-)

So, what are the forces acting on Rosetta while lurking around 67P? The primary forces are:

• Gravity of 67P
• Solar radiation pressure
• Solar wind pressure

There’s also the gravity of the sun, but since 67P and Rosetta are very close to each other and share the same orbit around the sun, we neglect this for now.

Before starting, I ask you to remember a few basic pinciples.

Newton’s Law of Universal Gravitation. This law say that the force (in Newton) between two masses is the product of their mass multiplied with the gravitational constant, divided by the square of their distance: $F=\frac{G*m_1*m_2}{d^2}$

So, what’s the force acting between Rosetta and 67P if Rosetta is on a circular, 1000 meter orbit? From Wikipedia I learned that 67P’s mass is about $3.14*10^{12}\,kg$; Rosetta’s mass is about two metric tons, so $2000\,kg$. The gravitational constant is, as (as we think) everywhere in the observable universe, $G\,\approx\,6.67\,*10^{-11}\,m^3kg^{-1}s^{-2}$. $F=\frac{6.67*10^{-11}\,m^3\,kg^{-1}\,s^{-2}*3.14*10^{12}\,kg*2000\,kg}{(1000\,m)^2}\approx 0.4191\,N$

So, only about 0.4 Newton; that is not a lot. As the ever helpful Wolfran|Alpha search engine suggests, this is the peak force your fingers exert on your keyboard while typing.  When I first did this calculation I really wondered how you could possibly keep Rosetta in a stable orbit around 67P with such little forces at work when Rosetta is constantly being bombarded with solar radiation and when the solar wind keeps blowing.

But as always with physics, assumptions are nice, but if you can do the math, just do it. So: Is the combined forces of the solar wind and solar radiation pressure big enough to affect Rosetta and would Rosetta need to do a lot of correction burns to keep it’s orbit stable?

Let’s head over to the radiation first. To figure out the force we need the solar radiation pressure and multiply this with the effective cross-section of Rosetta. I assume a cross-section of 1.5 m^2 – that might be wrong, but the principle of calculation stands.

First, a definition which was deliberately marked with “DO NOT CITE”, but I found this definition most easy to understand:

“Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light. If the radiation is totally reflected, the radiation pressure is doubled.”
Source: http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Radiation_pressure.html

So, what is the power flux (measured in Watt per meters squared) at Rosettas current position? Rosetta is, as of today, about 4.25 AU away from the sun. The power flux on earth is around $1370 Wm^{-2}$ so we need to use the inverse-square law to calculate the power flux at 4.25 AU.

The Inverse power law states: $S=S_0*(r_0/r)^2$

Hence: $1370\,Wm^{-2}*(1\,AU/4.25\,AU)^2=75.8\,Wm^{-2}$

However, Rosetta is not absorbing all the power 100%; I have no figures, so I assume it absorbs about 50% ( $d=0.5$) of all the radiation. We will now calculate the effective radiation pressure from the power flux at Rosetta’s current position. $P=S/(c/(1+d))$ $P=3.793*10^{-7}\,kg/m^2$

Now multiply by Rosetta’s effective cross-section to convert the pressure to force: $F=P * 1.5\,m^2 = 5.69*10^{-7}\,N$

So… compared to those 0.4191 Newton, that’s not a lot! But what about the solar wind? The formular for calculatng the solar wind pressure at a distance of 1 AU is: $P=1.6727*10^{-6}\,N/m^2*n*V2$

where pressure P is in nPa (nano Pascals), n is the density in particles/cm3 and V is the speed in km/s of the solar wind. Wolfran|Alpha told me today the parameter for n and V2: $V2 = 438.7\,km/s$ $n = 1.4\,Protons/cm^3$

We also need to apply the inverse power law again to draw conclusions for the distance at 4.25 AU: $P=P_0*(r_0/r)^2$ $P=1.6726*10^{-6}\,Nm^{-2}*(1\,AU/4.25\,AU)^2$ $P=9.26*10^{-8}\,Nm^{-2}$ $P*1.5\,m^2=1.39*10^{-7}\,N$

Not a lot either! Now add these forces and compare to the gravitational forces between 67P and Rosetta: $F_{wind} + F_{radiation} =$ $1.39*10^{-7}\,N + 5.69*10^{-7}\, N = 7.079*10^{-7}\, N$ $7.079*10^{-7}\,N << 0.4191\,N$

So we showed that solar wind and radiation is so little that it won’t affect Rosetta’s orbit much.

## And @Philae2014 is awake! #Rosetta

March 28, 2014

Good morning, commissioning starts! :-)

What a beauty :-)

## Rosetta: OSIRIS’ first light!

March 27, 2014

Already taken on the, 20th and 21st of March, I wonder what took them so long to release it although I kept asking them every other day :-)

And here’s the press release: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_sets_sights_on_destination_comet

## All eyes on #Rosetta part 6: #MIDAS

March 27, 2014

Hey, this is an easy one: ESA actually wrote a detailed description of MIDAS so today I can concentrate on another system :-)

Read more on ESA’s Rosetta Blog:

http://blogs.esa.int/rosetta/2014/03/26/introducing-midas-rosettas-micro-imaging-dust-analysis-system/

## All eyes on #Rosetta part 5: #ROSINA and #COSIMA

March 26, 2014

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.

http://www.esa.int/Our_Activities/Space_Science/Rosetta/ROSINA

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.

http://www.esa.int/Our_Activities/Space_Science/Rosetta/COSIMA

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.

## All eyes on #Rosetta part 4: #CONSERT

March 24, 2014

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, welcome to the fourth posting of this series about the scientific instruments on board of Rosetta and Philae! It’s Monday and only four more days to go until the DLR will host it’s big recommissioning-event in Cologne, and boy, am I excited.

Today I’d like to present you CONSERT, the “Comet Nucleus Sounding Experiment by Radio wave Transmission”-experiment. This experiment consists of two parts, first a transmitter on board of Rosetta, and a transponder installed on the lander Philae. CONSERT was built in cooperation of the Max-Planck-Institute for Solar System Research and the l’Institut de Planétologie et d’Astrophysique de Grenoble.

CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) probes the internal structure of the nucleus. Radio waves from the CONSERT experiment on the orbiter travel through the nucleus and are returned by a transponder on the lander.

Principal Investigator: Wlodek Kofman, Institut de Planétologie et d’Astrophysique de Grenoble, Grenoble, France.

Basically CONSERT works as follows: After Philae landed on 67P, Rosetta will fly around the comet until it’s facing the opposing site. Now, CONSERT’s trasmitter will send a 90 MHz Radar-signal right at the comet and Philae’s transponder – sitting on the other side of the comet – will try to pick up the signal, modify the signal by some data (that’s what a transponder does) – and send it right back, through the comet again, to Rosetta. Transponder Operation Principle; Waveform 11 shows what Rosetta initially sends out, Waveform 12 shows the data Philae want’s to encode (time of receive, signal strength, phase-, frequencies-shifts if any), Waveform 13 shows the modulated transponder signal Philae sends to Rosetta; From US Patent “Radar transponder operation with compensation for distortion due to amplitude modulation”; Source http://www.google.co.in/patents/US7864097

Rosetta then will pick up the modified (“modulated”) signal and record all the data:

• How long did it take from Rosetta to Philae? (Philae encoded this in the signal)
• How long did it take from Philae back to Rosetta? (Rosetta compares the time of receiving the transponder signal with the encoded timestamp)
• How faint was the signal, when Philae received it? (means: How large is the attenuation of the comet)
• Did the polarization change?
• Was the signal somehow otherwise modified? (e.g. phase-shift)

Rosetta and Philae will conduct multiple, many many thousand send-receive experiments and at the same time record their position. From all the data then ESA will be able to calculate the inner structure of the comet:

• How many layers are there and how thick are these?
• From which material is the comet made of?
• What’s the density?
• Are there regions in the comet more or less dense?
• If yes, how do these look like? Seismic Tomography; this is an example of using earthquakes to determine the inner structure of earth; CONSERT will apply the same principle, the earthquakes are here the Radar-sender, the receiver an antenna instead of an seismometer. The Department of Earth & Atmospheric Sciences , Cornell University; Source: http://www.geo.cornell.edu/geology/classes/Geo101/101images_spring.html

CONSERT won’t be useful before Philae landed, but it’s a rather interesting instrument. Until now we can only guess about the internal composition of a comet. We made educated guesses, but you never now until you had a real look at it.