end effector

a device or tool connected to the end of a robot arm

fine-guidance sensor

the targeting devices aboard HST that lock onto “guide stars” and measure their positions relative to the object being viewed. Adjustments based on these precise readings keep Hubble pointed in the right direction.


the amount of something (such as radiation) passing through a surface per unit time


a spinning wheel mounted on a non-stationary frame that stabilizes and points a space-based observatory. This spinning wheel resists applied external forces and tends to retain its original orientation in space.

Hubble constant

a number that expresses the rate at which the universe expands with time. H0 appears to be between 60 and 75 kilometers per second per million parsecs. (One parsec is equal to 3.26 light-years and 3.085678 × 1013 kilometers, or approximately 18 trillion miles.)

Milky Way Galaxy

The Milky Way, a spiral galaxy, is the home of Earth, the Sun, and the rest of our solar system.

orbital debris

any man-made object, or portions thereof, in orbit about Earth which no longer serves a useful purpose

planetary nebula

an expanding shell of glowing gas expelled by a star late in its life

proto-solar system

matter that is beginning to come together to form a star and its collection of orbiting planets


the brightest type of active galactic nucleus, believed to be powered by a supermassive black hole. The word “quasar” is derived from quasi-stellar radio source, because this type of object was first identified as a kind of radio source.

ranging device

an instrument or instrument system for measuring the distance, for example, between two spacecraft as they approach one another

reaction wheel

one of four spinning wheels that work by rotating a large flywheel up to 3000 rpm or braking it to exchange momentum with the spacecraft which will make HST turn. The flywheels work together to make the observatory rotate either more rapidly or less rapidly toward a new target.

red shift

an apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by motion of the emitting object away from the observer

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