## 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.

## 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 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.”

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.

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis; ESA; Source: http://www.esa.int/spaceinimages/Images/2002/11/The_Rosetta_Orbiter_Spectrometer_for_Ion_and_Neutral_Analysis

The Cometary Secondary Ion Mass [spectrometer]; ESA; Source: http://www.esa.int/spaceinimages/Images/2002/11/The_Cometary_Secondary_Ion_Mass

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:

Massenspektrum Tetrachlordibenzofuran; by Wikipedia user chris; License GFDL >= 1.2, CC-BY-SA-3.0-migrated; Source http://commons.wikimedia.org/wiki/File:Massenspektrum_Tetrachlordibenzofuran.svg

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.

## Rosetta: Expecting first light from OSIRIS

March 24, 2014

Just to remind everyone, for today it’s scheduled to let OSIRIS have a first look at 67P:

24 March – Pending successful re-activation, OSIRIS will take a first look in the direction of the comet. The comet will be too far away (around 5 million kilometres) to resolve in these first images and its light will be seen in just a couple of pixels. These images will be acquired regularly for navigation purposes and to already start planning the trajectory corrections planned for  May.

OSIRIS is a camera with wide- and narrow-angle optics along with quite a few filters for different spectra and was covered in part 2 of the “All eyes on Rosetta” series.

In the meantime, here an older picture OSIRIS took before going to sleep:

Colour composite of the Orion nebula M42, obtained with the OSIRIS NAC during commissioning; Keller et al. 2007, “OSIRIS – The Scientific Camera System Onboard Rosetta”; Figure 39; Page 39

I can’t wait for OSIRIS’ first light after ending hibernation.

## All eyes on #Rosetta part 3: #RPC

March 23, 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 third installment of this series about the scientific instruments on board of Rosetta and Philae! For now I’d like to present you the Rosetta Plasma Consortium (RPC) collection of instruments. This is not a single, but actually five instruments; all of them built by different institutes around the world. Three of the five instruments share a common electronic housing, the RPC-0 box; so the institutes who build the individual instruments had also the great challenge during system integration by working together very closely!

This article was created during kind of a writing binge and I hope I managed to keep it understandable (and correct at the same time).

The instruments will study the solar wind and radiation, the electric- and magnetic fields in and around 67P’s coma. From the Steins and Lutetia flybys – RPC was active back then – we already know that the solar wind (ions, means protons and electrons) and radiation (photons, means light & X-rays) exert a force, hence a pressure, on the comet’s halo. For one, the particles of the halo are being electrostatically charged; the magnetic field get’s distorted, in turn influencing the velocity and direction of charged particles (ions & electrons); so, all these influences affect the shape and other properties of the coma. We already know that in the tail regions of lower density, charge and pressure occur as these depend on the forces from the sun. While Rosetta will fly through the coma and around the comet, it will be recorded how exactly the halo looks like by measuring charge, electrical impedance (~resistance), the velocity and direction of the ions and the distribution of the different types of ions (atoms stripped off their electrons – examples are Protons, Helium, Iron, etc.). And not only that: Although the forces from the sun producing the pressure are tiny, they influence the comet’s spin around it’s axis! Rosetta actually experienced it during it’s hibernation phase; the solar wind and radiation gave the spacecraft a spin and when it woke up, it had to look around, orientate itself, fire it’s reaction control systems so that the antenna points to earth and the solar panels to the sun.

Now let’s have a look at the individual experiments.

### LAP – Langmuir Probe

While Rosetta travels through space, it’s floating in a low-density plasma. This plasma is electrically charged and the Langmuir Probe is a device measuring the electrical currents flowing around the spacecraft. From these readings the density, velocity and the temperature of the plasma can be derived. The Swedish Institute of Space Physics calls it a “space weather station”.

Langmuir Probe (LAP); Swedish Institute of Space Physics; Source: http://www.space.irfu.se/rosetta/

What will be interesting to know how the plasma flows around the comet and how it behaves in the coma itself – can’t wait to see the first charts! And doesn’t it look a little like a magic wand? ;-)

### Ion and Electron Sensor (IES)

The IES is a sensor measuring the flux, direction and energy of charged particles like protons and electrons. Flux and direction means, how many particles per $cm^2$ are arriving the probe. Energy is directly related to the velocity of the particle.

Figure 1. (a) Solar wind interaction with a bare nucleus at large heliospheric distances (d > 3 AU). (b) Plasma environment of an active comet near perihelion; Burch, J. L., et al. “RPC-IES: The ion and electron sensor of the rosetta plasma consortium.” Space science reviews 128.1-4 (2007): 697-712.; Source: http://www.researchgate.net/publication/226646695_RPC-IES_The_Ion_and_Electron_Sensor_of_the_Rosetta_Plasma_Consortium/file/32bfe50db32d34a882.pdf

If you measure all three parameters while travelling around the comet and the coma you get a more refined picture of the plasma in and around the coma, complimentary to the information from LAP.

Ion and Electron Sensor (IES); Southwest Research Institute; ©2004 Southwest Research Institute. These images may be used by the public and the media for educational and informational purposes only; Source: http://www.swri.org/press/rosetta.htm

The IES has a Field of View of 90°× 360° so it covers quite something in the direction of travel.

### ICA – Ion Composition Analyser

The Ion Composition Analyser (ICA) meaures positive ions, like protons (hydrogen), helium, oxygen and even other molecules. It gives the distribution of these ions – means how much of everything – and also tells the velocity and direction from whence it came. Although designed by the Swedish Institute of Space Physics, it was manufacured by the Southwest Research Institute, which also designed and made the IES.

It consists of three filters; the first, the electrostatic arrival angle filter, measures the direction from where the ion arrives. The electrostatic energy filter then can derive the energy and hence the apparent mass; the magnetic momentum filter, lastly, works like a little NMR spectroscope (although technically speaking it’s none, as I understand it so far); it gives you a pretty good idea what exactly this particle could be, if it’s a simple proton or even something more complex like methane (oh boy, that’d be cool!).

Ion Composition Analyzer ICA; Swedish Institute of Space Physics; Source: http://www.irf.se/?dbsec=P3&docid=52

More information is available at the Imperal College’s Rosetta page and on the IRF’s project page.

## MAG – Fluxgate Magnetometer

Altough personally I find the MAG the most interesting apparatus from the RPC collection, there’s not much information available about this one; most information is hidden behind paywalls of different outlets like Springer &c., so I’ll just give a verbatim copy of the abstract from the paper Glassmeier, Karl-Heinz et al. “RPC-MAG The Fluxgate Magnetometer in the ROSETTA Plasma Consortium.” Space Sci Rev 28 May 2007 : 649–670:

The fluxgate magnetometer experiment onboard theROSETTAspacecraft aims to measure the magnetic field in the interaction region of the solar wind plasma with comet 67P/Churyumov-Gerasimenko. It consists of a system of two ultra light (about 28 g each ) triaxial fluxgate magnetometer sensors, mounted on the 1.5 m long spacecraft boom. The measurement range of each sensor is ±16384 nT with quantization steps of 31 pT. The magnetometer sensors are operated with a time resolution of up to 0.05 s, corresponding to a bandwidth of 0–10 Hz. This performance of the RPCMAGsensors allows detailed analyses of magnetic field variations in the cometary environment. RPC-MAG furthermore is designed to study possible remnant magnetic fields of the nucleus, measurements which will be done in close cooperation with theROSETTAlander magnetometer experiment ROMAP.

MAG ultra light triaxial fluxgate magnetometer; The Imperial College Rosetta Project; Source: http://www.sp.ph.ic.ac.uk/Rosetta/members.html#mag

MAG Project Homepage at IGEP Braunschweig (German)

### MIP – Mutual Impedance Probe

Basically the MIP is an antenna. The goal is to measure the velocity and density of the plasma; the plasma in the coma forms waves, driven by solar radiation and wind. To measure the velocity and wavelength of this literal “waves” of plasma, it sends out a radio signal in the 7 kHz to 3.5 MHz spectrum. Another pair of antennas receives the signal and by observing how the impedance and therefore the phase of the sent signal has shifted, you can figure out the characteristics of the plasma waves. This thing is sheer magic!

MIP sensor (structural model); ESA Science & Technology; Source: http://sci.esa.int/sre-fi/35972-mip/

That’s it for now. Hope you had as much fun reading it as I had writing it!

## All eyes on #Rosetta part 2: #OSIRIS

March 22, 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, this is the second posting of this series and I’d like to talk about the 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 before time runs out :-)

From ESA’s OSIRIS website:

OSIRIS (Optical, Spectroscopic, and InfraRed Remote Imaging System):
is a telephoto and wide-angle camera, which will take high-resolution images in different spectral bands for the characterisation of the nucleus and its surrounding environment.

Principal Investigator (PI): Holger Sierks, Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany.

Co-Investigators: Stubbe Hviid, Jörg Knollenberg, Ekkehard Kührt, Institute of Planetary Research, German Aerospace Center, Berlin, Germany.

And that’s how the beauty looks like:

The Optical, Spectroscopic, and Infrared Remote Imaging System; ESA; Copyright ESA/AOES Medialab; Source: http://www.esa.int/spaceinimages/Images/2002/11/The_Optical_Spectroscopic_and_Infrared_Remote_Imaging_System

OSIRIS has wide- and narrow-angle optics and will be used to take high-resolution pictures of the comet in the visible and infrared-spectrum. This camera will become very important as soon as it comes to finding a proper landing site for Philae.

Keller et al. 2007, “OSIRIS – The Scientific Camera System Onboard Rosetta“, has all the juicy details about this shiny piece of technology, but don’t be disappointed: The resolution is “only” 2048 x 2048 pixels (4 Megapixels) but take this into account: Rosetta is now in space for 10 years and back, when OSIRIS was built, this was a huge deal! Also note that the CCD-sensors in space have to cope with a lot of nasty radiation, stuff which your average consumer camera will never encounter on earth. This puts constraints and limits on how you build the sensors; a broken pixel can’t be repaired and so you better go with a sturdy, boring, conservative system which is known to be working than with something shiny new which was probably never deployed to space before. I know, I wouldn’t take the risk :-)

Anyway, Rosetta will get really close to 67P in the final phase, so 4 MP with a 2.20 x 2.22° Field of View from the narrow-angle camera will give a pretty decent picture – one pixel on the picture will present 2 x 2 cm!

But it’s not only the size, what matters, but also what’s else in this camera. It comes with lot’s of different filters of various colors to improve contrast and with several IR- and UV-filters to lock at specific features only visible in these spectra. It’s often so that if you only want to see the infrared-portion of an image that you need to block out all other colors because they’re so much brigther (in intensity) than what you really want to look at (photographers call this “oversaturation“).

So. Lots of features to look at – lots of filters. I won’t go too much into detail, the people from the Planetary Society already did a really great job at explaining all these.

OSIRIS already caught first light in the commissioning phase before hibernation, but we still have to wait for the first pixels from 67P. In the meantime just look at this picture of M42, taken with the narrow angle camera (NAC).

Colour composite of the Orion nebula M42, obtained with the OSIRIS NAC during commissioning; Keller et al. 2007, “OSIRIS – The Scientific Camera System Onboard Rosetta”; Figure 39; Page 39