Hiatus

There is a going to be a pause now, a break in spacewordgame. Depending on a host of factors, it may resume shortly, or not.

Thanks!

Maria

Maria are the plural of mare. A dark, flat, large region on the surface of the Moon. The term is also applied to the less well-defined areas on Mars. Although maria literally means “seas,” watery regions do not exist on the Moon or Mars. Marias on the Moon and elsewhere may be evidence of past volcanic lava flows. 

Ablative

Ablative means 'able to be eroded or flaked away'. An ablative heat shield is one in which the shield material itself vaporises and takes the heat away with it as it goes. The Apollo command module used an ablative heatshield made of a resinous material held in an aluminum honeycomb, and most entry and reentry vehicles use some form of ablative aeroshell in the same way.

Catadioptric

Referring to an optical system which uses both reflection (mirrors) and refraction (lenses). This also describes the unwanted effect of lens elements reflecting images inside a compound camera lens. Undesirable catadioptric effects include ghosting and lens flares.

Cislunar

Cislunar comes from the Latin, and is just a handy way to refer to all the space between the Earth and the Moon.

Cislunar Transfer

Ullage

This is almost a second part to 'bladder'. Ullage is the volume in a closed container which is not occupied by the stored material, such as fuel. Its also sometimes used to refer to a manoeuvre deliberately to move floating fuel in a tank to the tank exit pipe. Dedicated ullage thrusters have been used during the Saturn IV-V era.

http://www.astronautix.com/craft/blo1s861.htm

Retrograde

Retrograde means contrary to the normal direction of motion or rotation. A retrograde orbit is one that goes in the opposite direction of the rotation of the body being orbited. A retrograde manoeuvre opposes the direction of travel. Weirdly, 2 of the planets in the solar system have retrograde motion (on their own axes): Venus and Uranus. Venus' axial tilt is 177 degrees, so its spinning almost exactly in the opposite direction to its orbital rotation.

Reseau

A reseau is a grid of points or marks placed in a photograph by a transparent plate very carefully and precisely aligned with the camera's optics. This provides a stable and reliable basis for measuring objects in a photograph and for detecting and correcting distortions in the photograph. The Hasselblad cameras used on the moon were fitted with reseau plates.

Reseau grid crosshairs just visible in the famous Aldrin shot

Bladder

Bladders ought to be a fairly familiar concept, a stretchy membrane for containing fluids/gases. Bladders are relevant to space technology because they are used inside fuel tanks for at least 2 good reasons. Firstly, they confine the fuel into a volume roughly equal to the volume of fuel remaining, which prevents it from sloshing around and causing disturbances to the attitude control or GNC systems. Also, in a microgravity environment, it means you can keep the fuel in a partially filled tank close to the exit valve so you can use it. Easy really. Check out this page from Astrium about their bladder tanks.

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.

Gegenschein

Gegenschein is a phenomenon that can be seen in places with very dark skies. A faint spot of light appears on the ecliptic at a point directly opposite the Sun's current position, caused by sunlight reflecting from dust in space.

Roche limit

This exotic-sounding term refers to the closest distance an orbiting body (held together only by its own gravity) can get to its parent planet before being torn apart by tidal forces. The Roche Limit is approximately 2.45 times the radius of the parent body. Also known as the tidal stability limit, but I think we can get more mystery and alternative meanings out of the name given it by the French astronomer Édouard Roche.

Swingby

Swingby is the colloquial term for 'gravitational/gravity assist manoeuvre' or 'sling-shot'. The basic principle is to use the relative motion of a massive body (a planet usually) to steal some momentum in order to accelerate or decelerate a spacecraft. This sounds like cheating, but it really works. The velocity relative to the Sun can be changed, but the speed relative to the swingby planet stays the same. Optimising swingby plans is a fun challenge in astrodynamics. 

You can read up on Gravity Assists in wikipedia, or elsewhere, including many excellent books.

As a real example of a swingby, check out this page for a good description and some vector diagrams of the Nozomi Earth Swingby manoeuvre. However the really classic example of this comes from the Voyager 2 Grand Tour:

Spacecraft speed with spikes at each planet caused by swingby
(note how the Jupiter swingby gave Voyager2 enough speed to escape the gravity of the Sun!)

Tunguska

Tunguska is a region within Siberan well known for being located under an asteroid that exploded in the atmosphere on 30 June 1908. The "Tunguska Event"  flattened a vast area (over 2100km²) of forest and was felt thousands of kilometers away. Check out the reports, explosion estimate and eye witness accounts here.

Zwicky

Fritz Zwicky was a Swiss astronomer who spent most of his professional life at Caltech making important theoretical and observational contributions. Mostly Zwicky is known for some of the more weird stuff in astronomy - supernovae, neutron stars, and galaxies acting as gravitational lenses. From 1961 to 1968 he and his colleagues published a 6-volume "Catalogue of galaxies and of Clusters of Galaxies", hence the "Zwicky Catalogue".

Fritz Zwicky (from Wikipedia)

Kick Motor

A Kick Motor is kind of 'single use' rocket engine used to put satellites into their (nearly) final orbit. After launch, satellites usually start in orbits that are limited by the performance of the launch vehicle, and also by principles of safe operations. Especially for moving into a geostationary orbit, a big single thrust is used to increase the size of the orbit (loosely speaking: the faster a satellite goes, the bigger the orbit - within limits). It is most energy-efficient to apply this thrust at the apogee (opposite of perigee) of the initial orbit, so the engine used is often called an Apogee Kick Motor (AKM) because of the timing of the thrust, and the single-use sudden change in orbit which kicks the satellite higher. 

Ylem

The leading theory for why any of the stuff we see around us exists, is the 'Big Bang'. The leading questions about the Big Bang are - what was there before, and how did it start? Ylem is the answer: the name for the original matter that existed before the formation of the elements we know today. It was used by George Gamow (Physical Review, April 1st, 1948) as a term for the stuff that normal matter condensed out of. Ylem is therefore some kind of superhot, superdense, superdark plasma. The name comes from a transliteration of the Greek for "fundamental matter" as described by Aristotle way back when.


 (Ylem is also a much less interesting album from the German band Dark Fortress).

Nadir

Nadir is the celestial mechanics slightly longer word, borrowed from Arabic, for "down". Its strictly more technical than that, since 'down' is something referenced to perception and gravity, while nadir relates to geometry on the celestial sphere. I find it somewhat easier to think about nadir as being the opposite of Zenith. Zenith is the point directly overhead at any given location, orthogonal to a local horizontal reference plane. Nadir is along the same line, in the other direction.


Apart from a contraction of "s/he's not here" (s'nadir) - what else can we make it mean?

Jansky

Our galaxy, the Milky Way, is a colossal bunch of energy and matter. Karl Jansky is credited with first discovering that it emits strong radio emissions. The measurement of electromagnetic flux density is named after him, the Jansky, with symbol Jy. It is most naturally defined as an integration over the whole solid angle subtended by the source. For extended surfaces/sources (like the whole galaxy), a per-solid-angle measurement is used (IRAS mission products use MJy/sr). Check this page for more discussion of what the Jansky means over the spectrum.

Jansky's Rotating Antenna (source linked below)

A wonderful historical note by John Kennewell and Malcolm Wilkinson is well worth reading: "The First 50 Years of Radio Astronomy". In other media, the excitement in Sagan's novel and the derivative movie "Contact" (1997) is about a strong encoded ('intelligent') radio source of well over 100 Janskys. 

Heliopause

Heliopause is a fabulous word. The heliopause is the boundary defined by the limit of our Sun's influence. Obviously, this is a pretty fuzzy boundary. Its the outermost reasonable definition for the limit of the Solar System - where the solar wind (that's for another day) meets the interstellar gas and general muck. The stuff inside the heliopause is in the heliosphere, the stuff beyond is really just the wide open nothing filled with radiation, dust and  gas. The heliopause is around 100AU away.

Fairing

Starting with a fairly dull one this week - Fairing. The fairing is the "nose cone" of a rocket, the protective structure that protects payloads (spacecraft) from the atmosphere during ascent. Release of the fairing is the final stage of a launch before the payload is released into its initial orbit. In the picture below the fairing has 2 halves and you can see a satellite inside during final assembly.

Perigee

Perigee is the point of an orbit closest to the central body, so for a satellite in orbit around Earth, its the closest approach to Earth (the central body). The 'gee' part is related to 'geo' meaning Earth, so when the central body is somewhere else (say, Mars), perigee is usually known by a more general term 'periapsis'.


For a good tutorial about orbital mechanics, see the Basics of Space Flight.


New meanings for either perigee or periapsis are welcome as comments.

Gravity Turn

The Gravity Turn is a manoeuvre used to optimise the efficiency of rockets during launch. Also sometimes called a zero-lift manoeuvre, but should not be confused with swingby or gravitational slingshot. By using gravity to change orientation of the vehicle, the trajectory control system maintains a zero angle of attack. In turn, this minimises lateral aerodynamic loads, allowing optimisation of structural elements. Excessive aerodynamic loads can damage or destroy vehicles. Similar principles apply to landing also.


Image shows gravity turn manoeuvre of a DeltaII launcher during the 2004 launch of Messenger.

Capcom

CAPCOM is a callsign used in space operations of human spaceflight, and is a heritage item from the early days, being an abbreviation of 'Capsule Communicator'. Capcom is today the primary communicator between a ground control segment and astronauts in flight. The Capcom is generally an experienced astronaut themselves.

Oort

as in "Oort Cloud", named after Jan Hendrik Oort. The Oort cloud is a large 'reservoir' of comets and similar small bodies around 2,000 to 5,000 AU (but there are estimates that put the outer limit much, much further away - beyond a lightyear). It essentially the junk left over from the vast pile of matter that collapsed to form the Sun. More info from Wikipedia, or images via Google. The Oort cloud is a convenient and reasonable solution to various conundra that supports observations about the solar system today.

What else is it?

Frangibolt

TiNi Aerospace's Frangibolt family of actuators is designed for deployment of a wide variety of stowed appendages including solar panels, antennae, cover doors, and various experimental payloads. The great benefit of the Frangibolt is that it requires no special handling during integration, can be tested, and in use it doesn't impact large shocks or release contaminating materials. It is a non-explosive bolt for the situations in which you really need an explosive bolt, but would prefer not to have one. Read more here...