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Accelerometer Mode

a mode in which one test mass is forced to follow the spacecraft as it jiggles around with minute forces from the thrusters. The forces are applied by electrodes similar to those that sense the test mass position capacitively.

Albert Einstein

the German-American physicist who developed the Special and General Theories of Relativity, which along with quantum mechanics is the foundation of modern physics. Awarded the Nobel prize for physics in 1921.

Albert Michelson

the Prussian-American who developed the interferometer, and the first to apply interferometry to astronomy. Awarded the Nobel prize for physics in 1907.


the smallest particles of an element that can exist either alone or in combination, considered a source of vast potential energy.


single-celled microorganisms that live in soil, water, organic matter, or the bodies of plants and animals.

Black Hole

a region in space where gravity is so strong that not even light can escape from it. Black holes in our galaxy are thought to be formed when stars more than ten times as massive as our Sun end their lives in a supernova explosion. There is also evidence indicating that supermassive black holes (more massive than ten billion Suns) exist in the centers of some galaxies.

Caging System

a mechanism that keeps the instrument from being jostled.

capacitive Measurement

a method where the capacitance depends on the separation of the test mass from the walls. By measuring the capacitance with specialized circuitry, the separation of the electrodes, and hence the position of the test mass, is inferred. All six surfaces of the box have electrodes, allowing measurement of the precise position and orientation of the mass.

Cosmic Radiation

Cosmic of or relating to the cosmos, the extraterrestrial vastness, or the Universe in contrast to the Earth alone. radiation of the cosmos left over from the Big Bang, a theory in which the expansion of the Universe is presumed to have begun with a primeval explosion.


the electrode of an electrochemical cell at which reduction occurs: a : the negative terminal of an electrolytic cell b : the positive terminal of a galvanic cell.


like a substance that consists of particles dispersed throughout another substance, such as a mixture consisting of a colloid together with the medium in which it is dispersed (smoke is a colloid).

Disturbance Reduction System

refined sensor, interferometry, and thruster technology that will enable the measurement of objects in weightlessness with 100 times greater precision than has been achieved before.


free of atmospheric influences. Drag-free is a term that was coined from missions in the 1970's when the biggest force on low-orbiting spacecraft was from atmospheric drag.


a core of magnetic material surrounded by a coil of wire through which an electric current is passed to magnetize the core.


relating to static electricity.

Feature Detection

Software that recognizes as interesting predefined images that have been identified by scientists. This software can be trained to recognize volcanoes, lava flows, volcano plumes, or other features.


equals 10-15——about one-tenth a Newton

General Theory of Relativity

the geometric theory of gravitation developed by Albert Einstein, incorporating and extending the theory of special relativity to accelerated frames of reference and introducing the principle that gravitational and inertial forces are equivalent. The theory has consequences for the bending of light by massive objects, the nature of black holes, and the fabric of space and time.


one of the fundamental physical forces of nature attracting two bodies to each other.

Gravitational Waves

"ripples in the curvature of space-time" produced by disturbances caused by the movement of massive objects in space.

Hubble Space Telescope (HST)

NASA's space telescope, which orbits 500 kilometers (310 miles) above Earth to observe the Universe. HST has taken more than 330,000 separate observations of the Universe, observed more than 25,000 astronomical targets, and to date has created a data archive of over 7.3 terabytes. HST is the first scientific mission of any kind that is specifically designed for routine servicing by spacewalking astronauts.


electromagnetic radiation at wavelengths longer than the red end of visible light and shorter than microwaves (roughly between 1 and 100 microns). Almost none of the infrared portion of the electromagnetic spectrum can reach the surface of the Earth, although some portions can be observed by high-altitude aircraft (such as the Kuiper Observatory) or telescopes on high mountaintops (such as the peak of Mauna Kea in Hawaii).


separated (a compound) into ions.


An instrument that uses the principle of interference of electromagnetic waves for purposes of measurement.


the use of interference phenomena of light waves to measure distances and angles between objects.

Laser Interferometer Space Antenna

the first dedicated space-based gravitational wave observatory.


the common term for electromagnetic radiation, usually referring to that portion visible to the human eye. However, other bands of the electromagnetic spectrum are also often referred to as different forms of light.

Micro Newton Thrusters

technology that has high stability dual electrode thrusters, a high stability propellant feed system, nanotube field emission, and high voltage converters.


a unit of force, equal to the weight of about 100 grams (about four ounces).

Photo Emission

the release of electrons from a usually solid material (as a metal) by means of energy supplied by incidence of radiation and especially light.


Refers to orienting the spacecraft or its instrument such as a telescope, camera ,or a sensor towards a specific target or coordinates.

Positively Charged

having a positive charge.


part of Einstein's Theory of General Relativity, that space and time are linked.


a continuum of color formed when a beam of white light is dispersed (as by passage through a prism) so that its component wavelengths are arranged in order. All forms of electromagnetic radiation can be dispersed to form a spectrum.

Solar Arrays

equipment that provides power to the spacecraft by converting sun light into electrical energy.

Star Tracker/Star Mapper

Recognizes star patterns in the sky to determine the spacecraft pointing. Many star trackers look for a certain star of predetermined brightness. Any shift of this star in the field of view of the tracker registers as the spacecraft or instrument motion.

Sun Sensor

mechanism used to detect the Sun's rays.


Path on which the spacecraft flies.


electromagnetic radiation of shorter wavelength than visible light. UV cannot be seen by the eye, and much of it is absorbed by ozone, a molecule of oxygen, at altitudes of 30-40 km above the Earth. Satellite telescopes, however, can and do view stars, galaxies, and the Sun in UV.

Visible Light

electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers).


the distance between adjacent peaks in a series of periodic waves. Also see electromagnetic spectrum.


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