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The Rosetta Mission was a 10-year long project by the European Space Agency (ESA) to place a lander on a comet. It succeeded with the placement of the Philae lander on the Comet 67P/Churyumov-Gerasimenko (AKA Chury) on November 12, 2014.

The mission was originally aiming for a much smaller comet, 46P/Wirtanen1, but difficulty with the Ariane 5 engine postponed liftoff from January 2003 to March 2004, causing Rosetta to miss the launch window for Wirtanen. Chury was identified as a good backup target, and the mission was back on.

The goal of the mission was to find clues about the formation of the Solar System. Chury's composition should reflect the composition of the pre-solar nebula out of which the Solar System formed over 4.6 billion years ago. The ESA will also be looking at the isotopic profile of the cometary ice to see what percent of the water in Earth's oceans may have come from early comet impact, and will check for organic molecules that may have affected abiogenesis.

The media make a point of telling us that Rosetta traveled 6.4 billion kilometres (42.78 AU) to reach Chury, but this is potentially misleading. Chury is not nearly this far out2, and the distance traveled includes four trips around the sun, with three gravity assists from Earth (in 2005, 2007 and 2009) and one from Mars (2007). But any way you spin it, ten years of weaving through the inner solar system to hit a target the size of Cruithne is impressive.

Rosetta spent about nine months gathering data on the comet through photographs and sampling the various off-gassings. Finally, it moved within 10 km of the comet and released the Philae lander3, a clunky cube about the size of a washing machine, which was intended to land gently on the comet and take some serious rock samples and high-resolution photos. Chury is an odd shape, often compared to a rubber duck, consisting of a larger and a smaller lobe connected by a thick 'neck'. Philae was targeted to land on a large open area on the smaller lobe.4

In order to land on the comet in microgravity, Philae was supposed to rely heavily on a harpoon anchor system, but this failed to fire. The lander bounced off the surface, arching a full kilometer up and a kilometer to the side before coming back to the surface, bouncing lightly once more, and finally landing in shadow and at an odd angle. All systems continued to work properly, but Philae is solar-powered, and will run out of power shortly if it can't be moved. Fortunately, the drills were still able to take subsurface samples despite the tilt of the lander, and all of the planned initial experiments were completed successfully.

The lander has run out of power, but the solar panels have been re-positioned, and it is possible that it may be able to power back up at some point in the future. There is not any great expectation in this area, however.

In the meantime, here is the ESA's photo gallery.


1. Wirtanen is much smaller than Churyumov-Gerasimenko, with an estimated diameter of 1.2 kilometres v. Chury's 4.1. We know comparatively little about it, but it will be coming quite close to Earth in December of 2018, passing within a mere 0.0777 AU (Chury will only come within 3.3 AU of Earth, but we will only have to wait until 2015). We may yet pay Wirtanen a visit; NASA's proposed Comet Hopper mission would like to make comet-fall in the early 2020s, if funding comes through. It doesn't seem very likely at this point.

2. Aphelion of 5.6829 AU; Perihelion of 1.2432 AU. As the crow flies, Rosetta reached a max distance of 6.68 AU from the Earth (a point reached in 2012), before its orbit moved back in to align with Chury's.

3. Named after an island in the Nile region of Egypt, the place of discovery for the Rosetta stone.

4. The original landing site, originally known as Site J, was renamed Agilkia as the result of an international naming competition. Ironically, it is named for Agilkia Island, the island that many buildings, including the famous Temple of Isis, were moved to when Philae flooded during the building of the Aswan dams in the 1960s. This would be a very appropriate name for an unfortunate, unplanned landing site, but odds are that the new landing site will get some other title.

pH is the standard measure of the acidity or alkalinity of a solution1. It relates to the concentration of free protons in a solution – a large number of protons corresponds to a low pH, which is to say an acidic solution. If a particularly small number of protons is available, the pH is high and the solution is alkaline.

If that sounds a bit abstract, bear in mind that it's just a physical description of a very familiar chemical property – acidity is what makes things taste sour; some languages, like German, have the same word for 'sour' and 'acidic'. It's also one of the first things anyone ever learns about chemistry, so it's interesting that it's so difficult to pin down a clear physical description of the nature of pH – what do any of the well-known properties of acids have to do with protons?

How Acids Work

Besides tasting sour, acids also have the power to 'burn' or dissolve some materials. They do this with a kind of two-pronged attack. Every molecule of a standard acid is made of at least one hydrogen atom attached to one or more other atoms; the hydrogen breaks off from the rest of the molecule when it dissolves in water, leaving its sole electron behind2. Without its electron, a hydrogen atom is just a proton3. A proton has a positive charge, while the other part of the acid4 will have a negative charge – hydrochloric acid, for example, divides into positive hydrogen ions and negative chlorine ions.

Usually, anything with an excess charge like this will take any opportunity to lose it, and if a couple of those protons manage to grab a pair of electrons from somewhere they'll immediately make off with them to form a nice stable hydrogen molecule. Similarly, the negative ions will off-load their extra electrons at the first chance they get. One way to do this is by finding a positive ion to form a compound with; metals are handy for this, because they are essentially made of positive ions held together by free electrons. So the negative part of the acid (the anion) takes a positively charged atom out of the metal at the same time as the positive part (the proton, a cation) grabs an electron to maintain the overall charge. The metal dissolves into the liquid, and hydrogen bubbles escape.

How Bases Work

The flip-side of this is basicity (or alkalinity - an alkali is a base that dissolves in water). Bases remove protons, usually by giving them a hydroxide ion to react with5. Oxygen is very attractive to electrons6, so when hydroxide breaks away from a larger molecule, it takes an extra electron with it and leaves the remainder of the base with a positive charge. Often what's left behind is something that wasn't particularly keen on keeping that electron in the first place – the alkali metals can hardly shed them fast enough7, which is why they react so dramatically with water.

Since acids are proton donors, while bases are proton acceptors, they neutralise each other, producing water (OH- add H+ gives H2O) and a salt. Table salt (sodium chloride) is the salt you get from reacting hydrochloric acid with sodium hydroxide, but chemists define a salt as any compound that can be produced in this way.

Alkalis can also burn or dissolve various things, but the really interesting thing they do is to turn fats and oils into soaps. This process, known to humans for thousands of years, is called saponification, and it's the reason alkali solutions feel slippery on our fingers – our natural skin oils are changed instantly into detergents. The hydroxide breaks up the fat molecule into glycerol and fatty acids, while the other bit, for example the sodium or potassium, reacts with the fatty acid to produce soap.


1Note that acidity of a substance can also be measured by its dissociation constant, Ka. This describes a chemical, rather than its solution, so unlike pH it doesn't change with the strength of the solution, and also work with things that aren't solutions
2Technically, I'm only talking about acids by the Arrhenius or Brønsted-Lowry definitions here; according to the Lewis definition an acid is a substance that can accept a pair of electrons to form a covalent bond
3Actually, a lone proton is so reactive that it will invariably attach itself to one or more nearby water molecules, making H3O (hydronium) or more likely something like H5O2 or H9O4
4Which, confusingly enough, now counts as an alkaline – the conjugate base of the acid
5The early Arrhenius theory of acidity only recognised compounds with hydroxide as alkalis, but this has now been superseded by the Brønsted-Lowry theory, in which any proton acceptor is a base, and any soluble base is an alkali
6That is, it is highly electronegative
7That is, they have very low electronegativity

Contact resistance is a hindrance to the flow of thermal, electrical, or kinetic energy at an interface between conductive materials. Such surfaces of contact can be formed by welding, soldering or simple mechanical contact.

Contact resistance is very important in the design, construction, testing and operation of electrical systems and thermal systems, because it causes energy loss and inefficiency in device operation and inaccuracy and unreliability in measurement.

Contact resistance can occur at the interface between different phases of the same material, where two pieces of the same material touch, or where two pieces of dissimilar materials touch. The resistance depends on many factors, including the actual surface area of contact (constriction resistance), the presence of oxides or other products of chemical reaction, absorption or adsorption, the cleanliness and the flatness of the two contact surfaces, differences in the conductivity of the materials, and tunneling resistance in thin-films.

One practical example of the important role of contact resistance in the operation of electrical circuits is in the operation of circuit breakers. A circuit breaker is like a switch that physically interrupts the flow of current in a circuit, usually to protect equipment and wiring systems against overcurrent. The ignition system of some internal combustion engines have contact breaker points that open and close rapidly in synchronization with the rotating engine crankshaft to deliver current with the right timing to a spark plug. The points can become fouled by oil or dirt, which acts as an insulator between the points and increases the contact resistance. The surface of the points can also be roughened by metal transfer from one surface to the other as a result of current arcing. Arcing may also cause formation of oxide layers on the point surfaces. Changes in the contact resistance of the points over time can cause mistiming of the ignition in the combustion chamber and poor engine performance. This is one reason that breaker-point ignitions have largely been replaced by electronic ignition systems in automobiles.

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