Rosetta Glossary

APXS (Alpha X-ray Spectrometer)
This instrument on the Philae Lander is lowered to within 4 cm of the ground to detect alpha particles and X-rays, which provide information on the elemental composition of the comet's surface.

Astronomical unit: the mean distance between Earth and the Sun.
Average Earth-Sun distance equal to 1.4959787 1011 m or 214.94 Solar radii.

The smallest component of an element having the chemical properties of the element.

Bow Shock
The point at which the solar wind is suddenly slowed down from supersonic to subsonic speeds as it confronts the comet’s coma. The solar wind is forced by the bow shock to bend around the coma and then to regain speed, shaping the comet’s ion tail.

This instrument on the Philae Lander consists of six identical micro-cameras that will take panoramic pictures of the nucleus’s surface. A spectrometer studies the composition, texture and albedo (reflectivity) of samples collected from the surface.

The process of making divergent or convergent rays closer to parallel.

The atmosphere that develops around the nucleus of a comet as it approaches the sun
The cloud of diffuse material surrounding the nucleus of a comet

The outer boundary of the cometosphere, which consists mostly of cometary ions or plasma.

CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission)
This instrument on Rosetta probes the internal structure of the nucleus by studying radio waves that are reflected and scattered by the nucleus. Radio waves from CONSERT on the orbiter travel through the nucleus and are returned by a transponder on the Philae Lander.

COSAC (Cometary Sampling and Composition experiment)
This is one of two evolved gas analysers on the Philae Lander. It detects and identifies complex organic molecules from their elemental and molecular composition.

COSIMA (Cometary Secondary Ion Mass Analyser)
This instrument on Rosetta analyzes the characteristics of dust grains emitted by the comet, including their composition and whether they are organic or inorganic.

Daughter Molecules
New molecules created by the dissociation of parent molecules by solar photons.

Dust Tail
The luminous tail of a comet containing dust particles that are rushing away from the nucleus with the solar wind and illuminated by the sunlight reflecting off of them.

A stable elementary particle contained in the atoms of all elements and described as having a negative charge

Electron Beam
A stream of electrons.

Field-Aligned Electron Beams
A stream of electrons aligned along a magnetic field.

GIADA (Grain Impact Analyser and Dust Accumulator)
This instrument on Rosetta measures the number, mass, momentum and velocity distribution of dust grains coming from the nucleus and from other directions (reflected by solar radiation pressure).

Gravity Assist
A planet orbiting the sun has a great deal of angular momentum. That's what keeps it circling the sun instead of falling into it. If a spacecraft comes up behind the planet in just the right way, it will be pulled along by the planet's gravity and gain some of that momentum without being captured by the planet. The spacecraft can use the maneuver to change its trajectory at the same time. It can then continue on its way at a greater speed, while the planet pulls away and continues its orbit.

Inner Shock
The boundary in the comet’s coma that separates the ionosphere from the area close to the nucleus in which neutral molecules still hold sway, having not yet been ionized by solar radiation, ions in the solar wind, or the interplanetary magnetic field.

Interplanetary Magnetic Field (IMF)
Part of the sun’s magnetic field that is carried into interplanetary space by the solar wind.

An atom or molecular fragment that has a positive electrical charge due to the loss of one or more electrons or a negative electrical charge due to the gain of one or more electrons. The simplest ion is the hydrogen nucleus, a single proton.

The ionised plasma at the outer edge of the comet’s coma, produced by the dissociation of atoms.

The ionopause is a cavity without a magnetic field that contains only cometary ions and is separated from the solar wind by a sharp discontinuity

Ion Sputtering
High-energy ions impacting the surface of a solid cause ejection ("sputtering") of ions, atoms, and/or molecules from the surface.

Ion Implantation
When charged particles strike a surface and change its properties.

Ion Tail
Another tail that forms on a comet. Unlike the dust tail, the ion tail consists of solar and cometary ions being swept around the comet’s nucleus to dark side of the comet, opposite the sun and the solar wind.

An atom with the same number of protons in its nucleus as other varieties of the element but with a different number of neutrons.

Long-Period Comets
Comets with an orbital period of greater than twenty years around the Sun.

Mass Loading
Mass entering an area per unit of time
Retardation of solar wind mass loading by the substantial population of comet ions.

Instrument for measuring magnetic fields. Fluxgate magnetometers measure components of the magnetic field (three combined give its strength and direction) Rubidium-vapor and similar magnometers measure only the strength of the field, but their reading is absolute, related to atomic constants.

The boundary of the magnetosphere, lying inside the bow shock. The location in space where the comet’s magnetic field balances the pressure of the solar wind.

The region between the bow shock and the magnetopause, characterized by very turbulent plasma.

The area above the ionosphere dominated by the comet’s magnetic field and bounded by the magnetopause. The magnetosphere contains ionized plasmas from two sources: solar wind particles (mostly protons and electrons) flowing from the sun, and ionized atoms) knocked loose from the comet’s nucleus? These plasmas swirl in complex patterns throughout the various regions of the magnetosphere.

The region of the magnetosphere containing field lines stretched away from the sun.

MIDAS (Micro-Imaging Dust Analysis System)
This instrument on Rosetta studies the dust environment around the asteroids and comet. It provides information on particle population, size, volume and shape.

MIRO (Microwave Instrument for the Rosetta Orbiter)
This instrument on Rosetta is used to determine the abundances of major gases, the surface outgassing rate and the nucleus subsurface temperature.

This is the second gas analyzer (along with COSAC) on the Philae Lander. Modulus Ptolemy is an evolved gas analyzer that obtains accurate measurements of isotopic ratios of light elements.

The smallest physical unit of an element or compound, consisting of one or more atoms.

MUPUS (Multi-Purpose Sensors for Surface and Subsurface Science)
These sensors on the Philae Lander's anchor, probe and exterior measure the density, thermal and mechanical properties of the nucleus’s surface.

Neutral Sheet
An area of no electrical charge separating two magnetic lobes of opposite polarities.

Noble Gases
The volatile gases: helium, neon, argon, krypton, xenon and radon.

OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System)
This imaging system on Rosetta includes a wide-angle camera and a narrow-angle camera to obtain high-resolution images of the comet’s nucleus and the asteroids that Rosetta passes on its voyage to Comet C-G. It will help in identifying the best landing sites for Philae.

The splitting of a molecule by a photon.

The quantum of electromagnetic energy, generally regarded as a discrete particle having zero mass, no electric charge, and an indefinitely long lifetime.

The electrically conductive fourth state of matter from solid, liquid, and gas, consisting of ions and electrons. Plasma is a partly or wholly ionised gas whose particles respond collectively to magnetic and electric fields.

A subatomic particle found in the nucleus of every atom. It has a positive electrical charge equal and opposite to that of the electron.

The transfer of energy by means of a particle (including photons).

Differently directed field lines link up, enabling the magnetic field to change topologically, determining the patterns in which plasma flows, and converting magnetic energy to kinetic and thermal energy of the plasma.

Remote-Sensing Instrument
An instrument that acquires and measures data or information about a property or properties of a phenomenon, object, or material without being physically in contact with the target of its surveillance.

ROLIS (Rosetta Lander Imaging System)
This is a CCD camera on the Philae Lander that will obtain high-resolution images during descent and stereo panoramic images of areas sampled by other instruments.

ROMAP (Rosetta Lander Magnetometer and Plasma Monitor)
This instrument on the Philae Lander is a magnetometer and plasma monitor for studying the local magnetic field and the comet/solar-wind interaction.

RPC (Rosetta Plasma Consortium).
In this set of Rosetta instruments (which includes IES), five sensors measure the physical properties of the nucleus, examine the structure of the inner coma, monitor cometary activity, and study the comet's interaction with the solar wind.

SD2 (Sample and Distribution Device)
This device on the Philae Lander drills more than 20 cm into the nucleus’s surface, collects samples, and delivers them to different ovens or for microscope inspection.

SESAME (Surface Electrical, Seismic and Acoustic Monitoring Experiments)
These are three instruments on the Philae Lander that will measure properties of the comet's outer layers. The Cometary Acoustic Sounding Surface Experiment measures the way in which sound travels through the surface. The Permittivity Probe investigates its electrical characteristics, and the Dust Impact Monitor measures dust falling back to the surface.

A discontinuity in pressure, density, and particle velocity moving through plasma.

Solar Wind
The high temperature of the plasma near the sun causes it to expand outwards against gravity, carrying the solar magnetic field with it. It starts at the sun as a hot, dense, slowly moving plasma but accelerates outwards to become cool, rare and supersonic.

The process of an ice becoming a gas without passing through liquid form. On Earth, we have an excellent example of this in “dry ice.”

Become a gas without passing through liquid form. On Earth, we have an excellent example of this in "dry ice."

Ultraviolet (UV)
Radiation that has wavelengths shorter than visible light but longer than X-rays—around100 nm.

VIRTIS (Visible and Infrared Thermal Imaging Spectrometer)
This Rosetta instrument maps and studies the nature of the solids and the temperature on the surface of the nucleus. It also identifies comet gases, characterizes the physical conditions of the coma, and helps to identify the best landing sites for Philae.

Wake Effects
The turbulence that occurs behind a fast-moving object. On Earth, we observe that wake behind a boat or airliner. For an asteroid, wake effects would include those atypical wakes following an asteroid’s galumphing rotation in space as it plows through the solar wind.

The distance that a wave of electromagnetic radiation will travel during one oscillation.

Electromagnetic radiation with a wavelength of about 100 picometers.