The Ion and Electron Sensor (IES) is one of five instruments that make up the Rosetta Plasma Consortium (RPC) suite. The suite weighs only 7 kg, and consumes less than a quarter of the power of a light bulb. Scientists will use the RPC to characterize the electromagnetic forces that drive the high energy and complex environment of the comet’s coma, which develops when the comet approaches the sun. In the past, we have been able to sample the comas of comets during only a few brief flybys. This time, however, the RPC will have several months to investigate the development of the coma as comet C-G approaches the sun.
We know that as a comet approaches the sun, a gigantic cloud of particles and gas, called the coma, begins to emerge from what was previously a cold, quiet nugget of ice and rock. As the comet approaches the sun, this growing cloud envelops a dynamic complex of chemical and physical reactions. Neutral molecules outgassing from the comet’s nucleus are impacted by solar radiation, with many becoming ionized into cometary ions and others interacting with incoming solar ions from the solar wind. Dust particles may be fragmented by photons. Proton transfers occur. There’s a lot going on.
The neutral molecules are arcing through the coma, while the charged particles (ions and electrons) are spiraling through the coma in one direction or the other depending on whether they are negatively or positively charged. In this growing maelstrom, the particles collide with each other or repel each other depending on their relative charges. Some particles recombine into an astonishing array of 'daughter' molecules . To learn more about the interrelationships between the comet coma and the solar wind, see How Does a Comet Work? To see how charged and uncharged particles interact and move through magnetic fields, click here.
In this growing maelstrom, ALICE and MIRO will observe the uncharged particles, and IES will observe the charged particles. IES will investigate the coma’s developing layers or boundaries, focusing especially on the "inner shock" layer. In this layer, high-velocity particles spewing from the nucleus smash into lower-velocity particles from the coma's middle layer that have been slowed by the solar wind. IES will also monitor those ions and electrons in the solar wind that manage to find their way through the coma into the inner regions of the comet’s atmosphere. The coma can grow to a million times the size of the comet’s nucleus.
Scientists want especially to study the chemical interactions within the inner shock layer, paying particular attention to the complicated hydrocarbon and organic chemistry, key processes that occur on Earth as well.
Unlike the remote-sensing Alice and MIRO instruments, IES cannot measure charged particles from afar. It is an in situ (in place) instrument, as is its companion, the magnometer; they must be where the particles can actually strike them. For IES and the magnometer to map the comet C-G’s inner-shock region, Rosetta will vary its altitude from very near the nucleus to roughly 1000 km out.
En route to C-G, IES has already sampled ions and electrons in the magnetosphere of Mars, during its flyby of that planet, and has produced a spectrum for Earth’s magnetosphere as well: see Mission Science Results.
During Rosetta's flybys of asteroids Steins and Lutetia, IES was used to detect whether or not the solar wind bends around the asteroids, indicating that they have developed magnetic fields. The solar wind is a flow of fast-moving charged particles (plasma) emanating from the Sun. If the asteroids have developed magnetic fields, they would deflect the solar wind, creating magnetotails. If an asteroid does exhibit a magnetic field, scientists can learn more about the asteroid’s thermal history from it. By detecting the asteroid’s thermal history, scientists can better understand where the comet has been in space because of the temperatures it has experienced.
IES was provided by the Southwest Research Institute, San Antonio, Texas, under a contract with NASA. Its principal investigator is James Burch.