FOGBANK

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“The material is classified. Its composition is classified. Its use in the weapon is classified, and the process itself is classified.”

FOGBANK is the codename of a material used in modern nuclear warheads such as the W76 used in Trident submarine-launched ballistic missiles, the W78 used in silo-launched Minuteman III intercontinental ballistic missiles and the W80 used in air-launched cruise missiles and the Tomahawk cruise missile.

Exactly what FOGBANK is, and what it does, is unknown. It has been suggested that it is an aerogel-like substance that transfers energy from the fission stage of a thermonuclear (fission-fusion) device to the fusion stage; preventing the fission stage from destroying the fusion stage before it has time to react.

In 1996, during the refurbishment process for the aforementioned warheads, it was discovered that detailed records of the manufacturing process for FOGBANK did not exist, and the facility used to manufacture it had been mothballed.  Uncertain whether alternate materials would suffice, the National Nuclear Security Administration spent twenty-three million dollars on research and new facilities to recreate FOGBANK.

Unfortunately, the Mark II FOGBANK did not work correctly. Eventually it was discovered that this was due to the presence of an impurity, accidentally incorporated into original batches of FOGBANK, that was not included in the second manufacturing process. This impurity was included in the new formula as an additive and the refurbishment process was successful.

The Torino Scale

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The Torino Scale is a system for categorising the risk presented by near earth objects (NEOs) such as asteroids and comets. On the Torino Scale NEOs are rated on a scale from zero to ten, based on a combination of the probability of an object striking earth and the kinetic energy of that object.

Because orbits are unstable and can change the scale only applies to potential objects less than one hundred years in the future. The diagram below shows the different Torino Scale categories, with a logarithmic scale on both axes and an approximate indication of the diameter of the asteroid on the kinetic energy axis.

torino-scale

The Torino Scale is separated into five categories:

  • White (Torino Scale 0) – No hazard; “the likelihood of a collision is zero, or is so low as to be effectively zero”.
  • Green (Torino Scale 1) – Normal; “a routine discovery in which a pass near the Earth is predicted that poses no unusual level of danger … new telescopic observations very likely will lead to re-assignment to Level 0”.
  • Yellow (Torino Scale 2-4) – Meriting attention by astronomers; “current calculations give a 1% or greater chance of collision capable of localised [Level 3] or regional [Level 4] destruction”.
  • Orange (Torino Scale 5-7) – Threatening; at its most extreme “a very close encounter by a large object, which if occurring this century, poses an unprecedented but still uncertain threat of a global catastrophe”.
  • Red (Torino Scale 8-10) – Certain collision; “a collision is certain, capable of causing localized destruction [Level 8] … unprecedented regional devastation for a land impact or the threat of a major tsunami for an ocean impact [Level 9] … or capable of causing global climatic catastrophe that may threaten the future of civilisation as we know it, whether impacting land or ocean [Level 10]”.

As new data about an NEO becomes available the Torino Scale rating for an object can jump suddenly: the Chelyabinsk meteor had a kinetic energy of 0.4 megatons TNT equivalent, giving it a Torino Scale rating of zero, but had it been only a little bit more massive or slightly faster (one megaton) it would have suddenly jumped to an eight.

Currently NASA’s Jet Propulsion Lab’s Sentry system lists only one NEO with a non-zero Torino Scale rating. 2007 VK 184 has a Torino Scale rating of one, but the earliest possible collision date is in June 2048, so we don’t have to start worrying just yet.

The Trestle

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The Trestle (or more formally the Air Force Weapons Lab Transmission-Line Aircraft Simulator) is a unique structure built by the US government in the Albuquerque desert and which was used to test aircraft’s resilience against the electromagnetic pulses created by nuclear weapons.

trestle

The Trestle is three hundred metres long and nearly two hundred metres tall and made entirely from wood and glue. The presence of any metal would distort readings from EMP testing and therefore The Trestle does not even use metal nails or braces. It was built from more than fifteen thousand cubic metres of Douglas Fir and Southern Yellow Pine and was strong enough to support the weight of a fully loaded two hundred tonne B-52 Stratofortress strategic bomber.

trestle1

Source

The Trestle was equipped with a two hundred gigawatt, ten megavolt Marx generator and was used to test bomber, fighter and transport aircraft and even long-range missiles. The Trestle programme was shut down in 1991 when computer simulations became good enough to simulate the effects of EMPs and the dried-out, creosote-soaked wood now poses a serious fire hazard.

Efforts are being made to have the Trestle site declared a National Historic Landmark, but these efforts are being hampered by the fact that The Trestle is located on Kirtland Air Force Base. Kirtland houses a number of Top Secret units such as the US Air Force Nuclear Weapons Centre, the 498th Nuclear Systems Wing and the Air Force Research Laboratory and therefore access to the site is highly restricted.

Leap smear

For reasons I have discussed before it is occasionally necessary to add* a leap second to the time, in order to keep the time on Earth in line with Earth’s inconsistent rotation.

Many systems require an accurate time to function correctly and the addition of a leap second can cause these systems to malfunction. In June 2012 the addition of a leap second caused a number of major websites such as Reddit, FourSquare, Yelp, LinkedIn, Gawker and StumbleUpon to malfunction and crash, but Google came up with a unique workaround – the Leap Smear – that prevented this from happening.

Google, like many others, uses the Network Time Protocol (NTP), to synchronise time across a network.† In order to cope with the leap second problem they configured their NTP servers to gradually add a small fraction of a second over a long period of time (in this case one day) so that at the end of this period their NTP servers’ time would have caught up with the adjusted time.

Google used the following algorithm:

t \left(\textnormal{Google}\right) = t + gain \left( 1 -cos \left( \pi \left( \frac{t}{window} \right) \right) \right)

Where t(Google) is the time according to Google’s NTP servers; t is the actual UTC time; gain is the desired amount of gain time (in this case one second); and window is the time over which this gain should happen (in this case twenty-four hours).

The effect of using the cosine function is such that the time offset is small at first (in the first hour only four milliseconds are added) and gradually increases (to sixty-five milliseconds per hour at most) before decreasing again towards the end of the window.

leap-smear-offset

This prevented servers and devices connected to Google’s NTP servers from “noticing” that something was wrong and applying their own corrections.

As they say in their blog post,

The leap smear is talked about internally in the Site Reliability Engineering group as one of our coolest workarounds, that took a lot of experimentation and verification, but paid off by ultimately saving us massive amounts of time and energy in inspecting and refactoring code. It meant that we didn’t have to sweep our entire (large) codebase, and Google engineers developing code don’t have to worry about leap seconds.

I wouldn’t be at all surprised to see others employing Google’s Leap Smear technique in the future.

* There are also provisions to subtract a leap second, but this has never yet happened.

† The NTP does contain a “leap indicator” but Google decided to force their NTP servers not to apply this.

The Roche limit and planetary rings

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As a satellite orbits around an object (a primary), the gravitational force on the side closest to the object is greater than that on the side opposite the object. This difference in gravitational attraction gives rise to a tidal force (so-called because it is what causes the tides on Earth). As a satellite approaches closer to the body it orbits this tidal force will eventually become greater than the gravitational forces holding the satellite together. The point at which this occurs is known as the Roche limit (named for Édouard Roche who first calculated it).

The Roche limit d depends on the radius of the primary RM, the density of the primary ρM and the density of the satellite ρm.

d = 2.44\; R_M \sqrt[3]{\frac {\rho_M} {\rho_m}}

If we take our Earth-Moon system as an example, with the radius of Earth being 6370 kilometres, the Earth’s density as 5520 kilograms per cubic metre and the Moon’s density as 3350 kg/m3 this gives us a Roche limit of 18 400 km. The Moon’s actual orbit is 385 000 km, so luckily we don’t have to worry about the Moon breaking up any time soon.

earth-moon-roche
The Earth-Moon system, to scale. The area beyond the Roche limit is shaded red.

When an orbiting object passes through the Roche limit it begins to break up, with the material closest to the object moving faster than the material behind it. This eventually leads to the formation of rings.

roche-rings