The Space Missions of 2018

2018 has definitely been a big year for space, and space exploration. I have managed to capture a few of the great moments like the launch of InSight, JAXA landing rovers on an asteroid, and the launch of the Parker Probe. There have been a few others that are notable mentions, and that is the point of this post, to talk about some great launched missions, and others that have finished their jobs, purposely or forced.

Bepicolombo

The British built Bepicolombo launched in October 20th, to begin its 7 year journey to visit Mercury. Currently one of the least explored planets in the solar system, Bepicolombo intends to change that. When it arrives in late 2025 it will endure temperatures of over 350 °C, and be there for at least a year, possibly for twice that. It is made up of two spacecraft, the Mercury Planet Orbiter (MPO) lead by ESA, and the Mercury Magnetospheric Orbiter (MMO) lead by JAXA. The aim is to measure the composition, atmosphere and magnetosphere of Mercury to understand its history. This could lead to understanding more about how other planets such as Earth formed. BepiColombo is named after Professor Giuseppe (Bepi) Colombo (1920-1984) from the University of Padua, Italy. He made big leaps in understanding Mercury, and suggested to NASA how to use a gravity-assist swing-by of Venus to place Mariner 10 into a solar orbit of Mercury.

Bepicolombo artists impression
Artist’s impression of the BepiColombo spacecraft in cruise configuration. The Mercury Transfer Module is at the bottom. The Mercury Planetary Orbiter is in the middle. The Mercury Magnetospheric Orbiter sits inside the sunshield, visible at the top. Credit: ESA/ATG medialab

InSight

Back in May I posted about how an Atlas V had just lifted the Mars Insight lander. In late November the $814 million lander it reached its target of the Elysium Planitia region of Mars, landing safely. The aim is for it investigate how the processes that shaped all the inner rocky planets more than 4 billion years ago worked. It uses two seismometers (one of which built by RAL space in the UK) and a number of other instruments to study the crust, mantle and core of the red planet. It works by measuring how much the area shakes when asteroids hit the planet. Also measuring the heat flow and precision tracking it is getting a glimpse of Mars we have yet to see. The launch also allowed for two cubesats, MarCO-A and MarCO-B to be the first to be launched into deep space. The first test of miniaturised cubesat technology being used on another planet. This mission will be one to watch for the near future.

There’s a quiet beauty here. Looking forward to exploring my new home. #MarsLanding pic.twitter.com/mfClzsfJJr— NASA InSight (@NASAInSight) November 27, 2018

Kepler

A bit sadder news is the end of the Kepler space telescope after 9 years service. It has collected a huge amount of data in its lifetime, finding the night sky is filled with billions of hidden planets, more planets than stars. This may seem obvious but is not easy to prove. During its time the planet hunter has found evidence of more than 2,600 planets outside our solar system, and left hints at many more, paving the way for future planet hunters and getting good engineering data on what works and what doesn’t. Telescopes such as ARIEL which will launch in the net decade will have better design due to Kepler. The space telescope had been running low on fuel for months, and struggled to point the correct way. After the 4 year mission it continued to work a different mission named K2. In October it was officially declared dead, left in orbit as it may have been dangerous for it to enter the atmosphere.

The Kepler Space Telescope mission, by the numbers
The Kepler Space Telescope mission, by the numbers. Credit: NASA/Ames/Wendy Stenzel

Parker Solar Probe

Back in august I wrote about the classic Delta IV heavy launching with the Parker Solar Probe aboard. The aim is to get closer to the sun than previously possible. Over the next seven years the probe will make 24 close approaches to the sun, with the aim of eventually getting within 3.8 million miles of the surface. The previous record (that Parker has now broken) was 26.6 million miles, set in 1976. It will revolutionise our understanding of the sun, and how the changing conditions can affect the solar system. It will use Venus’ gravity to slowly get closer to the sun. As a reference, we are 93 million miles away from the sun. It will eventually fly through the sun’s outer atmosphere, known as the Corona for the first time, getting brand new, in situ measurements. The spacecraft has a 4.5 inch thick carbon composite shield to protect it from the heat and radiation. The temperatures will reach over 1300 C.

Parker Solar Probe in the Fairing
Parker Solar Probe in the Fairing, ready to be put on the rocket in the clean room. Credit: NASA/Johns Hopkins APL/Ed Whitman

TESS

Back in April I posted about the launch of the TESS exoplanet hunter by a Falcon 9. I have already talked about exoplanets and planet hunters, and this is a big part of that plan. TESS stands for Transiting Exoplanet Surveying Satellite, and it does what it says on the tin, it is surveying the sky for potential exoplanets. Basically it is looking for exoplanets that could harbour life. The expectation is that it will catalog thousands of planet candidates and vastly increase the known number of exoplanets. Approximately 300 are expected to be Earth-sized and super-Earth-sized exoplanets that can then use the future more complex telescopes such as JWST to look at in more detail. The satellite will look at the sky for two years by breaking it up into 26 sections, and looking at each one for 27 days at a time. Unlike Kepler and K2 TESS will be looking at brighter stars, meaning ground based observatories can corroborate the observations.

the TESS telescope
The TESS satellite before launch, the four cameras can be seen on the top of the spacecraft; Credit: NASA.

Dawn

In September I posted about the Dawn spacecraft and the rise of Ion Engines. With the loss of the Dawn mission around the same time as Kepler, they ran out of fuel within two days of each other. The 11 year Dawn mission racked up a few very important records. It is the first spacecraft to orbit two different celestial bodies, and the first to orbit any object in the main asteroid belt between Mars and Jupiter. It is also a record breaker for electric speed. Travelling over 25,700 mph. Visiting Ceres and Vesta, it found out some very important scientific data that tells us a huge amount about the formation of our solar system. With a large proportion of the meteorites hitting Earth coming from these two bodies, Dawn showed the difference between the potential dwarf planets. One of the early uses of ion engines, it also showed the potential of the efficient form of travel, and now many more satellites are using them.

Dawn prior to encapsulation at its launch pad on July 1, 2007. Credit: NASA/Amanda Diller

Mars Rovers

This is a mixed bag, we have already had great news about the InSight lander, with it recording sounds of Martian winds, the rovers also have big news this year. In June the Curiosity rover found Organic matter in the Martian soil. The samples, taken from 3 billion year old mudstone contained complex hydrocarbons. This along with its detection of methane changes in the atmosphere are one step along the way to finding evidence of life on other planets. There have also been many more photos from the red planet, with Curiosity taking a few more selfies. See here how the car sized rover achieves the great pictures. On the other side of it there was a huge Martian storm that may have killed the Opportunity rover by covering the solar panels in dust. Although there are still hopes the rover can start communications again, we will have to see.

Curiosity in a dust storm
An image shared by Seán Doran on Sunday of the Mars Curiosity in the middle of a dust storm reported to cover an area the size of the US and Russia Combined. CredIt: NASA/JPL/Seán Doran.

Asteroid Rovers

In late september, another great story came out, that JAXA (the Japanese space agency) successfully landed a number of rovers on an asteroid. Still to launch all of the four onto the surface, there are already great images from the surface of an asteroid. The little rovers use a hopping mechanism to get around, as the gravity on the asteroid is so small a wheeled rover just wouldn’t work. The spacecraft will also be attempting to collect samples to return to Earth in the coming years. The Hayabusa 2 probe is a follow up to the Hayabusa probe which was not a sample return. The second launched on December 3rd 2014 and rendezvoused with the near-earth asteroid 162173 Ryugu on the 27th of June 2018. Currently in the process of surveying the asteroid for a year and a half, it will depart in December 2019, returning to Earth in December 2020.

 MINERVA-II image
First pictures from a MINERVA-II-1 rover that landed on the asteroid. Credit: JAXA.

Thank you for reading, take a look at my other posts if you are interested in space, electronics, or military history. If you are interested, follow me on Twitter to get updates on projects I am currently working on. Most of all, thank you for taking the time to read my posts this year! So all have a Happy New Year, and here’s to a great 2019 in space!

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Operation Mincemeat: The Imaginary Man

Major Martin Identity card
One of the identity cards on Major Martins body when it was found.

The 1st of May 1943, the height of the second world war. Early in the morning on a spanish beach, a fisherman found a waterlogged corpse. Not an uncommon thing to find at this time it looked like he had washed ashore during the night. The dead body was a man dressed in British military uniform wearing a life jacket and suspiciously he had a briefcase chained to his arm. Reported as a casualty of an airplane accident at sea the body was moved to a local port where Nazi officials in the city of Huelva took possession of it. From the items on the body they identified him as Major William Martin, a temporary captain acting major in the British Royal Marines. The German intelligence organisation (the Abwehr) decided to break open the briefcase to examine the contents, but did contemplate letting it go on intercepted. Along with personal effects they found personal correspondence between Lt. Gen. Sir Archibald Nye, vice chief of the Imperial General Staff, and Sit General Harold Alexander, the British commander of North Africa. The letters described key details of plans to invade Nazi Held territory. It seemed like Germany now had the upper hand, but all was not as it seemed. All part of the British Operation Mincemeat, this is not a christmas story.

Within days the news of the body being found in Spain got to the British military. The body was returned and buried with full military honours in Huelva. The spanish took longer to return the documents though. The British admiralty demanded their return, with emphasis on discretion due to their sensitive nature. The Spanish government had to respond as they were technically a neutral party, but they were sympathetic to the Nazi cause. Eventually the documents were returned to the British military, but not before the German Abwehr agents had teased open the sealed letters, photographed the entire contents of the briefcase, and then resealed the envelopes. The photographs made their was to Berlin to be carefully analysed. The German intelligence were wary of a ruse, and examined the other effects in great detail. His possessions included many normal items like a photograph and love letters from his fiance, a set of keys, recently used stubs for the theatre and a hotel bill. After the close inspection they believed the items were likely genuine. This indicated the letters he was transporting were also authentic. There was another letter from the Chief of Combined Operations to the Commander-in-Chief in the Mediterranean that indicated that Major Martin was carrying a letter too sensitive to be sent through normal channels. This was the apparently reason that he was flying, acting as a courier. An image of his body can be found here.

Major Martin's fiancee, but really an MI5 officer.
The image of Major Martin’s fiancee found in the briefcase.

By all appearances the Axis powers had stumbled upon some extremely valuable intelligence, and they thought that the Allies were unaware. This, a letter indicating the exact beaches that the Allies were planning to use to invade, beaches the Axis powers could divert troops and reinforcements. The plan in the letters was described as “Operation Husky”, a secret plan to invade Nazi controlled Europe via Sardinia, Corsica and Greece. It also described a false attack upon Sicily, to draw German forces away from the “true” invasion site. Up to this point the Germans expected the Allies would invade via Sicily. Upon learning of the letter, Adolf Hitler took action. On May 12th he sent an order: “Measures regarding Sardinia and the Peloponnese take precedence over everything else.” This order diverted significant defences away from Sicily to the landing points indicated in the letter. These defences included an extra Waffen SS brigade, several Panzer divisions, patrol boats, minesweepers and minelayers. The thing is, the attacks never came to Sardinia, Corsica or Greece. The German intelligence had been duped by an elaborate deception designed to draw the Nazi defences away from the true target: Sicily. Major Martin, the dead man with the briefcase never existed.

Adm. John Godfrey
Adm. John Godfrey, the British director of naval intelligence, crafted the idea for Operation Mincemeat with the help of Lt. Cmdr. Ian Fleming. When Fleming went on to create the world of James Bond, it was rumored that the character M was based off of Godfrey. Credit: Imperial War Museum

The idea to plant false military documents on a dead man, who then fell into German hands was conceived by Lt. Cmdr. Ewen Montagu at British Naval Intelligence. He had built on an earlier idea proposed by Flight Lt. Charles Cholmondeley of the counter-intelligence service MI5. The original plan was to place a wireless radio on a dead soldier whose parachute was rigged to look like it had failed. The radio would then be a channel to provide disinformation to the enemy. This plan was deemed impractical, so the death at sea ruse conceived by Montagu was used instead, dubbed operation mincemeat. The Montagu team quietly procured the body of a 34 year old homeless man who had recently died of pneumonia. As his lungs already contained fluid like a drowned man’s would, it was perfect. As the body was waiting in storage his new identity was fabricated. MI5 had an operation known as the Twenty Committee, who had expertise in counter espionage. They were known as the XX, witch is the roman numeral for 20, but also refers to “double cross”. They gave the corpse identification, keys, personal letters, and other possessions. They attempted to show that Major Martin was an absent minded yet responsible chap, so as to explain the fact he had chained himself to the briefcase. They planted evidence such as overdue bills, and a replacement ID card to achieve this.

transporting Major Martin
Charles Cholmondeley and Ewen Montagu on 17 April 1943, allegedly transporting the body to Scotland.

On April 2th 1943, the new Major Martin was placed on the submarine HMS Seraph in a special steel canister packed with dry ice. The crew set of the coast of Spain, where a citizen of the Axis-aligned country would locate the body and report it to the authorities. After two days at sea the sub surfaced about a mile off the coast of Spain at 4:30 in the morning. The plan was so secret that the crew of the submarine believed the canister contained meteorological equipment, carrying it on deck. Then everyone went below, apart from the officers. There in the dark, Lt. Norman L.A. (Bill) Jewell, the commander of Seraph explained the plan, and the contents of the canister, swearing the men to secrecy. They then removed Major Martin from the canister, onto the deck. They then fitted a life jacket and chained the briefcase to him. They read the 39th Psalm and committed the body to the sea, where the tide could take him to shore. When discovered, the British requesting the swift return of the briefcase helped the illusion that the contents were important. To complete the illusion, Major Martin was even mentioned in the British Casualty list in the Times. When the British got the documents back they found tell tale signs that the letters were opened. They also intercepted German transmissions indicating the Nazi’s were moving forces towards Greece and Corsica. The news prompted a brief cable to Winston Churchill with the words “Mincemeat Swallowed Whole”.

Officers of HMS Seraph
The officers of HMS Seraph, the submarine selected for the operation, on board in December 1943. Credit: Royal Navy

On July 9th 1943 the Allied forces launched the real attack, Operation Husky. The plan struck the southern tip of Sicily, and the swiftly conquered the island. For the following two weeks the Germans still anticipated the landings in Sardinia and Greece, but they never came. By the time they realised of the trick, there was no time to regroup, so the forces retreated to Messina. It took a month to take control of the entire island. In the years afterwards there have been speculation of the true identity of Major William Martin. In 1996 an amateur historian, Roger Morgan wrote the book the Man Who Never Was. The book theorised that the body was of Glyndwr Michael, a Welsh vagrant. His official cause of death was of chemical pneumonia due to ingesting rat poison. The markings at his burial place in Heuva have been updated to show Glyndwr’s name on the tombstone. That being said, not everyone is convinced, as some pieces of the story don’t fit such as the time between his official death and the execution of Operation Mincemeat. Also the HMS seraph took a long detour before heading to the Spanish coast, which leads to the possibility the body was picked up elsewhere. There are theories that it could be a victim of an accident onboard the HMS Dasher. Due to the nature of the operation, and the efforts to protect the true identity it is unlikely we will ever find out who Major Martin was.

Thank You for reading, take a look at my other posts if you are interested in space, electronics, or any other sort of history. Alternatively follow me on Twitter to get updates on projects I am currently working on.

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Creating RAF Harwell

Some time in early 1935 there was a knock at the door of the bungalow stables near Chilton. Leonard Cundall opened the door to two uniformed men, with only one thing to say. “Mr Cundall there’s going to be a war”. Within 6 months the Air Ministry had compulsory purchased the land for £11,6500. The land would be used for one of many new airfields being built as part of a national war effort. The John Laing engineering company won the contract to build 66 airfields for the RAF expansion period prior to World War Two. Reportedly the area south of Harwell village had been used as an emergency landing ground for night flying since WW1. There has also been talk of the area being used as a glider training ground around this time as well. The ground is surprisingly flat, and a big open space, perfect for air training. The John Laing engineering company was also famous for building the Mulberry Harbour system, which was the artificial harbour built for the Normandy landings. They also build the Royal Ordinance factory in Sellafield, Windscale nuclear power plant, the M1, and the reconstruction of Coventry cathedral. Of the 66 airfields Laings company built, Harwell was the first to be constructed. The work started on the airfield in June of 1935. 

Paratroops of 22 Independent Parachute Company, British 6th Airborne Division, waiting to board the Armstrong Whitworth Albemarle Mk V that will drop them over Normandy, RAF Harwell, 5 June 1944. H 39065 War Office official photographers.

In RAF tradition all airfields have pretty much the same layout. This makes sense when you think about it as if a new pilot flies in from another group they know where to go to find the mess hall or where to get paid or where to find the Commanding Officer. Another tradition is that every building is numbered in the order of construction. No. 1 was the guardroom at the entrance to the site. After initial construction the site had 202 individually numbered buildings, and then almost another 100 houses on top of this for people to live. The hangars had a slightly different numbering system, the four hangars were originally named H1-H4, but at the outbreak of war they were renamed H7-H10. At the other end of the numbering B199 was a Gymnasium that doubled up as a chapel. There were no buildings west of Hangar 10 when the site was built. There was a gunnery range in this area that was there to realign the aircraft guns, but this was subsequently occupied by the B220 radio chemical facility, the first major building built by Chivers post war. Two civilian houses were demolished to make way for a a deep underground fuel store, building B3. It contained 6 large cylindrical tanks, with excavated dirt piled on top to produce bomb proof mounds. The second building was pulled down to become the main runway, which is now Frome Road. Aldfield farm to the north and Upper farm to the south escaped demolition. Upper farm stables were requisitioned though. The long building with large doors was perfectly sized to be turned into an aircraft cleaning shed. Go in one side dirty, come out the other side clean!

The Harwell site taken in 1944 with the A34 running from left to right at the top. According to the markings it was taken in April 1944.

The naming of the airfield comes with an interesting story. The 800 acre site sits mainly in the parish of Chilton. A third of the site is in East Hendred parish, with the smallest section being in Harwell. Up until the Commanding Officer got there, it likely had some sort of code name or number. When the CO arrived the subject of the name had to be handled. He lived in the largest of the houses in the northern end of the site in South Drive. He declared that the airfield should be named after whichever parish his house resided in. That parish happened to be Harwell, therefore RAF Harwell it was. As it was a very early airfield it had many flaws. Even worse, it was a bomber training station, with new and inexperienced pilots everywhere. The first big issue was that the three runways intersected in the middle (almost). This is a huge issue, as it would only take a single bomb to take out all three runways. Plus there is a much larger risk of the new pilots crashing in the centre when trying to launch at the same time. There was also a hugely complex system of taxi tracks. There were also over 120 dispersal pads for the planes/gliders scatted around the site. These pads are there to disperse parked aircraft, so if there is an air raid it will minimise the damaged caused overall. Also called hardstands, they also stopped damage to other planes if there was an accident while “bombing up”. These pads tended to be 150 ft (46 m) in diameter and at least 150 ft from the funnel track, this was actually a rule for all buildings on the site.

The pathfinders of the 22nd Independent Company and the Advance parties disembark from their trucks at RAF Harwell alongside the Albemarles of 295 Squadron that will carry them to Normandy. Copyright: Imperial War Museum.

One big thing that they overlooked in the design was the location of the munitions dump. They placed it at the end of the shortest runway, now known as Severn Road. The other issue with this particular runway is that it had a slight hill. When going along the runway you couldn’t actually see the other end over the brow of the hill. There are many stories of young pilots with huge bomb payloads barreling down the runway to then realise they did’t have enough speed to make it. They would have to ‘prang’ the aircraft into the field between the end of the runway and the bomb store fence. There was no reports of any explosions caused by this though. One other notable point is the building materials used in the construction of many of these buildings and tracks. The runway of a bomber station was often up to 9 inches (23 cm) thick, and took 18,000 tons of dry cement and 90,000 tons of aggregate, then covered with a layer of asphalt. In later airfields the hardcore used was often from destroyed buildings, and were carried by train to the sites. Harwell initially opened in February 1937 with a grass airfield but got replaced with concrete runways between July and August 1941.

A slightly better quality photograph of RAF harwell, you can clearly see the dispersal pads and the concrete runways. 

The buildings themselves were made from the classic red brick but the food preparation and storage areas were made from a brick named ‘grano’. Short for granolithic stone it was a mixture of sand cement and granite dust. The resulting stone was hard wearing and was perfect for the constant cleaning and wiping of food storage areas. The issue found later was that the high granite content meant measurable amounts of natural uranium/thorium. The food areas were slightly radioactive. They were also found in the walls of offices in the aircraft hangers and in the BISO, the aimens mess! Plus most dials on compasses and equipment were painted with radium, another radioactive material. Turns out Harwell always had some radioactive heritage.

A Short Stirling Mark IV (LK115, ‘8S-Z’) of No. 295 Squadron RAF, taking off from Harwell, Oxfordshire (UK), towing an Airspeed Horsa glider. Credit: Royal Air Force/ Imperial War museum

By the end of the war over 500 airfields had been built, and most of these issues were ironed out in later designs. The original site was only designed to last 10 years, but even today a huge amount of buildings on that Harwell site still remain in some state. There are a few sections of runway still left, and even a few of the dispersal pads are still visible underneath the undergrowth. 

Thank You for reading, take a look at my other posts if you are interested in space, electronics, or any other sort of history. Alternatively follow me on Twitter to get updates on projects I am currently working on.

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How to Define a Kilogram

So I recently read a great twitter thread by Max Fagin about the redefinition of the kilogram. It was on the news, and I saw a few TV segments about it, but I don’t think most appreciate the enormity of changing this basic building block of science. I learnt because if this story that there is a whole branch of science dedicated to the study of measurement, metrology. Metrologists ask and have answered the question, what actually is a unit of measurement? A weird question because we all probably have an idea in our head of what it means. If you are in the UK (or most of the world) then you would use a metre to measure distance, liters to measure volume, and kilograms to measure weight. In some parts of the world such as the USA then you may use gallons, feet, inches and pounds, but they all have to reference something. There are 2.2 pounds in a kilogram, 2.54mm in an inch but which one defines the other? Is it just a tangled mess of  interconnecting reliance?

It turns out it is not. All official units used in science (even the imperial units used in America) are defined in relation to the SI (Système International) unit definitions. They were established and have been maintained by the Bureau International des Poids et Mesures (BIPM) in France. It all starts with seven base units, and from them every other unit of measurement in existence can be defined. They are:

  • Kilogram, kg (mass)
  • Metre, m (distance)
  • Second, s (time)
  • Kelvin, K (temp)
  • Ampere, A (electric current)
  • Candela, cd (luminous intensity)
  • Mole, mol (quantity)

Every unit you have ever used is “officially” derived from these seven units. Ever used Watts? defined as 1 kg*m^2/s^3. 5 volts is officially 5 kg*m^2/(s^3*A). What about a gallon? Officially it is 0.003785 m^3. One atmosphere of pressure would seem simple enough until you realise it is 101325(kg/(m*s^2) of pressure.

Difference between the SI base units and the SI units that are derived from it. Image from @usnistgov

There is no standard foot, pound or gallon sitting in a vault in Washington or London, although they was at one point. All units are defined (also known as traceable) through the SI base units. But how do we define the 7 base units themselves? Well historically they are all based off of a thing, or artifact. For instance in the 1940’s 1 second was defined as 1/86400 the time it takes the Earth to rotate once. Then in the 1950’s it was redefined as 1/31556925.9747 the time it takes the Earth to orbit the sun. It took until the 1960’s for technology to advance far enough to redefine the second to something that isn’t an artifact (in this case the Earth). The time it takes the Earth to orbit the sun changes over time, it may be a long time, but it does change slightly. If the thing you are defining from changes, the thing you are using changes. If the Earth takes slightly longer to orbit the sun then the second becomes slightly longer. Although most of us don’t care, some very important science is based off of this very specific value.  Now it is based off of a fundamental property of the universe, something that doesn’t ever change. The fundamental property defining the second is the time it takes an electron in a cesium-133 atom to oscillate 9,192,631,770 times. As current science believes that the oscillations of electrons are constant properties of the universe the definition is now good forever.

The new caesium fountain atomic clock NPL-CsF3 becomes operational, allowing continuous operation of a primary clock at NPL. Work continues on the next generation of optical atomic clocks at NPL, which should achieve accuracies equivalent to losing or gaining one second in the age of the universe. Credit: NPL

The metre had a similarly long journey. British and BIPM spelling is metre, and American is meter. First it was defined as 1/10,000,000 of the distance from the equator to the north pole, but then they realised that the Earth surface is not consistent, and is definitely not constant. The Earth’s shape will change over time. So then they defined it as the distance between two marks on a platinum-iridium bar in France, literally an artifact. The bar was 90% platinum and 10% iridium, and was measured at the melting point of ice. This original bar is still kept under the conditions specified at it’s creation in 1889. In the 1980’s timing equipment became precise enough to move away from this artifact based definition. The constant of the universe used is the speed of light in a vacuum, 299,792,458 m/s (roughly 300 million). Basically a meter is defined as the distance light travels in 1/299,792,458 of a second. As it is constant it can be measured literally anywhere in space and time and it would be exactly the same, so a good definition. It also relies on time being measured using a constant of the universe too.


Closeup of National Prototype Metre Bar No. 27, made in 1889 by the International Bureau of Weights and Measures (BIPM) and given to the United States, which served as the standard for defining all units of length in the US from 1893 to 1960

All of the other base SI units have been defined in some way based on constants in the universe, with some being more complex than others. That is all but the kilogram, the stubborn one that has up until this point eluded redefinition. For 129 years the kilogram has been defined as the mass of an artifact stored in a vault in France. It is called the International Prototype Kilogram. Made like the metre, it is 90% platinum and 10% iridium. Unlike time and distance there is no easy way to precisely measure mass. So every attempt up until this point of redefining the kilogram has not met the precision of just taking the IPK out of the vault and giving it a good measure every once in a while. This is somewhat frustrating to metrologists as over the past 100 years there is evidence that the IPK has actually changed in mass by losing some material. It has changed by about 50 micro grams compared to it’s replicas. This is an odd paradox because it was the only thing in the universe that cannot not be a kilogram. So if some technician or scientist dropped it or chipped it then the weight we define atoms would literally go up. The IPK always weighs 1 kg. This is equally annoying because so many other measurements are based on the kilogram. The Newton for instance is defined as the force needed to accelerate one kilogram by 1 m/s^2.

Mass drift over time of national prototypes K21–K40, plus two of the IPK’s sister copies: K32 and K8(41). All mass changes are relative to the IPK.

The new way to define the kilogram is very complex, and I honestly don’t understand the details, there are better sites out there to get the details of this. Essentially it is based on the Planck constant, a fundamental property of the universe. Basically it relates the energy of a photon to its frequency. If Because you can know the energy of the photon, you can know the mass, and this is then directly related to the frequency of that photon. This means the kilogram can be defined in terms of the metre, second and a few constants of the universe. This has been hypothesized for some time, but until this point the technology has not been good enough to measure the Planck constant to a  sufficient accuracy. The best way that we currently use to do this measurement is using something called a Kibble balance, previously known as a watt-balance. It uses the electric power needed to oppose the force of the kilogram. As current, and electrical potential are already defined by constants of the universe the Kibble balance, with extensive calibration, can define 1 kilogram in terms of current. When I say extensive I do mean it, there needs to be an extremely precise measurement of gravity at that point. This extra complexity does mean that most countries are unlikely to invest in such devices, for the moment. But this is an important time, all measurements we currently know of are now based on actual constants of the universe. Gone are the days where we have to take something out of a vault to calibrate our measuring devices. 

Thank You for reading, take a look at my other posts if you are interested in space, electronics, or any other sort of history. Alternatively follow me on Twitter to get updates on projects I am currently working on.

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