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.
The RPC’s instruments are (first hyperlink jumps to the section in this article, second link to the institute):
- Langmuir Probe by the Swedish Institute of Space Physics (LAP)
- Ion and Electron Sensor by the Southwest Research Institute, USA (IES)
- Ion Composition Analyser by the Swedish Institute of Space Physics (ICA)
- Fluxgate Magnetometer by the Institute of Geophysics and Meteorology, Technical University of Braunschweig, Germany (MAG)
- Mutual Impedance Probe by the Laboratoire de Physique et Chimie de l’Environnement, France (MIP)
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”.
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 are arriving the probe. Energy is directly related to the velocity of the particle.
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.
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!).
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.
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!
That’s it for now. Hope you had as much fun reading it as I had writing it!