## 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 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!