Quaternion

In case you remember or use complex ("imaginary") numbers, then quaternions are just a generalisation of complex numbers into 3 dimensions. They are enormously useful for attitude dynamics and anything which flies because they form a continuous representation without singularities. They were properly described by Hamilton in the 1840s, but have links with earlier work by Euler. A few decades later, people began to realise that vector analysis was much easier to think about and visualise, despite being less powerful. Vectors mostly won, and today quaternions are mostly used in spacecraft control, computer graphics, robotics, and signal processing.


There is no point trying to explain how quaternions work here. If you care, start with wikipedia, then build you own quaternion calculator in any language/system/tool you want. If you want to know why they are useful, then the principal argument is gimbal lock - (see previous entry about gimbals) - the loss of one degree of freedom in gimbal control.

Gimbal

Gimbal is a fabulous and friendly word. A gimbal is a pivoted support, allowing rotation of a system around an axis. A pair of gimbals allows 2 axis rotation, usually configured to keep something locally horizontal. On boats, they keep galley stoves and deck compasses level. Rockets use gimbals to point their nozzles, and powered gimbals are found in control moment gyroscopes.

Encke Gap

The Encke Gap (or previously, the Encke Division) is a gap in Saturn's ring system. It occurs in the so-called outermost 'A'-ring, and is about 325km wide. The gap is caused by the presence of the very small moon Pan. While its named after Johann Encke, he is not known to have observed it himself, he gets the honour for his general contributions to the observations of the rings.

Kuiper belt

The Kuiper Belt is somewhat related to the Oort Cloud, which featured in the second ever post here. The Kuiper Belt is somewhat closer, and is a disk full of comets and other junk that lies between 30 and 50 AU from the Sun. This is close enough to include the former planet Pluto.

Leptons

Leptons are elementary particles in the standard model, meaning they have no apparent further internal structure. The family of leptons includes electrons, muons, tau-lepton, a neutrino associated with each of these, and the set of anti-particle equivalents (but there is some fuzziness about antineutrinos... don't ask). They are named after the fractional parts of Greek currency from way-back-when. Technically, they have baryon number 0, unity electrical charge (+1, 0, or -1 e),  and zero colour charge. Leptons are involved in all types of physical interactions except the strong nuclear force. 

Hypergolic

Hypergolic propellant refers to rocket propellants that spontaneously ignite when they come into contact with each other. Normally we call them the fuel and the oxidiser. While hypergolics are obviously very dangerous and require special handling on the ground, they are extremely useful in space because no ignition system is needed, which in itself would be complex, expensive, and take up mass  budget. The typical example in space applications is hydrazine, used together with nitrogen tetroxide.

Dayside Reconnection

Dayside reconnection is exciting, unusual, and complex. To begin with, it is reconnection that occurs on the day side of the Earth (and presumably other planets with strong magnetic fields). But what is reconnection? It is the process by which magnetic fields are rearranged causing release of energy in the form of heat and kinetic energy, by accelerating local matter. Reconnection applies when magnetic fields from different sources (domains) become spliced together. This is really special because it seems to violate conservation laws in plasmas.
Its interesting to know about this from a space point of view for different reasons:

• scientifically, reconnection occurs between the interplanetary magnetic field (IMF) and the Earth's magnetic field: on the dayside energy and particles arrive at low altitudes and contribute to the aurora, on the tail side it releases energy from the tail field and can cause aurora substorms.
• technologically, devices have been made that determine the triggering of reconnection and its relationship to magnetospheric storms (NASA's THEMIS)
• and for engineers, the ESA Cluster spacecraft have been watching all this stuff happen and have demonstrated that the physics is weird, it works, and can be beautiful.

Cluster

The wikipedia reconnection page and this article about flux transfer events are good starting points for further reading.

Quasar

Quasar is more fun name for QSO - Quasi-Stellar Object. These are distant objects very highly redshifted so that they are bright in both optical and infrared bands, but also strong X-ray emitters. Quasars are unimaginably bright things, outshining galaxies. They are potentially close to supermassive black holes at the centres of very remote galaxies. Quasars are often found to have bright 'jets' of radio emissions 'focussed' by magnetic fields, such as illustrated below. Read more about quasars here.

NGC4261 Artist's Impression

Hoop Conjecture

The Hoop Conjecture is a notion proposed in the early 1970s by Kip Thorne. The conjecture supposes that an imploding object forms a black hole when, and only when, a circular hoop with a specific circumference could be placed around the object and rotated. The critical circumference is 2 times Pi times the Schwarzschild radius for the object. This sounds rather arcane, but the idea is quite simple. What Thorne did was figure out the effects of gravity on different shapes, and decided that the only way gravity could make a black hole was if the object could be compressed in all 3 directions. Its somewhat akin to a magician claiming their floating assistant is not held up by wires, and demonstrating as much through a hoop conjecture: if the hoop can pass all the way over the assistant and be rotated at the ends, there are no wires. While nobody's made a hoop big enough to spin around a star so the conjecture remains a conjecture, most illusionists are also just illusionists and the pretty frock distracts you from the incompleteness of the demonstration. 
In short - objects only collapse into black holes when they are small enough in all directions (an infinite cylindrical star won't).

Terminator

The terminator is the imaginary line that forms the boundary between the day side and the night side of a planet. In reality, it is a bit fuzzy depending on surface topology and atmospheric properties. The day/night terminator on Earth is an approximately circular line with radius approximately the same as that of Earth. There are a bunch of factors that drive exactly where the line falls and what shape it is.

The 1984 science fiction action film by James Cameron is not about nearly circular lines forming the boundaries of shadows on planetary surfaces.

Nozzle

It is possible that you already have a good idea of what a nozzle is, but since it is a word with two 'z's in it, it deserves its place here. A nozzle is a mechanical device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber via an opening. They have different meanings and applications in a wide variety of technologies, but the principal use of nozzles in space industry is in optimising the expansion ratio of exhaust products from the combustion chambers of rocket engines. The concepts apply equally to solid and liquid fueled engines. Nozzles come in a wide variety of sizes, usually integrate active cooling components, and are often mounted on gimballed mechanisms to aid guidance controls.

Rocket nozzle

Syzygy

Syzygy is a somewhat redundant word, covering both conditions of opposition and conjunction. Thus, syzygy occurs when the Sun, Earth, and also the Moon or any planet, lie along the same line. It would also apply to cases where any 3 bodies lie on the same line, but being Humans, we usually only care if two of bodies concerned are the Sun and the Earth.


Its's also a 1996 episode of The X-Files television series.

Interstellar

Interstellar is a word made of two Latin words - inter, meaning 'between', and stellar, pertaining to stars. Interstellar simply means between, or among, stars. This is a bit too straightforward however, since Earth is clearly between the Sun and any other star. The Earth is not really in interstellar space however. Rather, we define a really very large region of space around each star which describes its region of influence, in terms of gravity, radiation, environment, etc. Interstellar space falls between all the distinct regions of influence of all the stars. 

Analemma

The analemma is not a preposterous medical treatment. It is the figure-eight shape that the Sun traces in the sky if it were to be photographed at the same time every day for a year. More technically, the analemma describes the curve representing angular offset of one body relative to the celestial sphere, as seen from another one. Usually it refers to the Sun as seen from the Earth. Don't believe it? Grab a camera, clear your schedule at the same time each day, and come back in a year...

Solar Wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It mostly consists of ionised hydrogen and helium with very high energy travelling very quickly. Some small part of the solar wind arrives at Earth, where it interacts with our satellites, and the magnetic field of the Earth in the polar regions to produce the aurora.

Artist image of solar wind interacting with Earth's magnetosphere

Nebula

A nebula is an immense cloud of gas and dust and spare junk (mainly hydrogen) floating in interstellar space, put there to look spectacularly beautiful. Run a Google Images search for nebula for some examples. The remnants of very old stars, the birth places of new stars, and all so far away we'll never fly through them and have trouble with the engines.


Nebula is also a stoner rock band, or an alien villain in Marvel Comics.

Flame Nebula and Horsehead Nebula in Orion

Ecliptic

The ecliptic is the name given to the plane of the Earth's orbit around the Sun. From the surface of the Earth, its the path followed by the Sun in the sky - and approximately, that of all the planets too. The ecliptic plane is about 23.5 degrees from the equator, defining the tropics of Cancer and Capricorn. The relative position of the Moon with respect to the ecliptic defines the criteria for eclipses to occur.

Apogee

Apogee is the opposite of perigee: it is the point in its orbit where a satellite is at the greatest distance from the Earth. Around any other body, apogee is generally known as apoapsis. For eccentric orbits, apogee occurs in the part of the sky where the satellite appears to 'hang' for the longest time - apogee dwell. This was put to good effect during the cold war by the USSR's Molniya satellites that spent a good many hours alternately 'over' North America and the Soviet Union.