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.

ROSINA

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

COSINA

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

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.


400,000 Stars | The Memory Palace

March 25, 2014

A touchy short podcast about Edward Pickering’s staff of human computers.

http://thememorypalace.us/2014/03/400000-stars/


Nice SOFIA video on YouTube

March 25, 2014

Inside NASA’s SOFIA Airborne Astronomical Observatory:

“They can see SOFIA operating [...] the next 10 to 15 years” – unfortunately NASA cut the funds so that operation will cease by September if no solution is found! Operation costs is about 85 Million USD and only 9 Million USD by DLR are left. Please. Get funding. Try to find a solution. Go and talk to ESA – Roskosmos – JAXA – whatever it takes.


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