Monthly Archives: January 2013

Whipple shielding

Space is full of micrometeroids and debris whizzing around at incredible speeds, thousands of metres per second. At these speeds even tiny objects have enormous amounts of kinetic energy and can cause serious damage on impact.


The image above shows what happens to a eighteen centimetre-thick aluminium plate when struck by a two-and-a-half gram twelve millimetre-wide aluminium ball travelling at nearly seven kilometres per second . You can see the large crater created, and spalling beginning to occur on the opposite side of the plate as the impact shockwave reflects off it.

To prevent damage to spacecraft thick and heavy shielding as shown above is clearly unsuitable; this is where the Whipple shield comes into play.

In its simplest form a Whipple shield is simply two thin layers of shielding separated by a gap. Impact with the first layer of shielding causes the projectile to vaporise, preventing it from penetrating the second layer. In this way two light and thin layers can have a much better shielding effect than one heavy and thick layer. (Other types of Whipple shielding also exist, using multiple layers or “stuffed” layers containing a substance like Kevlar.)

whipple-shield(L-R) A multi-layer and twin-layer Whipple shield.

In the example on the right-hand side it is easy to see the shielding effect in play – the projectile has punched a tiny hole in the first layer but failed to penetrate the second layer, causing only minor heat damage (from the plasma produced on impact).

Gravel Gertie

A Gravel Gertie is a structure specially designed for use when handling nuclear weapons. It is not designed to contain the force of a nuclear explosion, but rather to reduce the damage and contamination caused by a non-nuclear explosion – for example if the explosive lenses used in a compression-type thermonuclear weapon detonated prematurely during inspection or maintenance.


A Gravel Gertie has thick reinforced walls, but is “open” at the top with the roof being a seven-metre layer of gravel, held back by a thick waterproof membrane. In the event of an explosion expanding gas vents out through the gravel, but this gravel also acts to trap radioactive contaminants. In tests at Sandia National Laboratories a Gravel Gertie reduced airborne contamination after an explosion by a factor of ten.

View Larger Map

Four Gravel Gerties are visible in this aerial map of the Royal Ordnance Factory in Burghfield where the UK’s nuclear weapons are assembled; ROF Burghfield is part of the UK’s Atomic Weapons Establishment.

GPS in space

The global positioning system (GPS) run by the US Department of Defense makes it very easy to find your position anywhere on Earth, but what if you’re not on Earth? What if you’re in space? At the moment researchers rely on simple time of flight calculations using antennae spaced out across the Earth’s surface to work out where their probes are, but this method becomes increasingly inaccurate as the distance to objects increases. (As an example, the error in the position of an object at the orbit of the most distant planet, Neptune, is about 120 kilometres.)

A group led by Professor Werner Becker from the Max Planck Institute for Extraterrestrial Physics in Germany think they might have a way to make a “GPS for Space” using X-ray pulsars.


A pulsar (“pulsating star”) is formed when a massive star explodes in a supernova, compressing the star’s core into a highly magnetised fast-rotating neutron star that emits a beam of electromagnetic radiation. The period of rotation of a pulsar is very short (milliseconds to seconds), and the the invariance of the period of rotation of some pulsars rivals atomic clocks in their accuracy.


Because each pulsar has a characteristic period of rotation it is easy to identify a signal as having been received from one specific pulsar. By mapping the position of hundreds or thousands of highly accurate pulsars relative to each other, and comparing the signals received at a spacecraft’s location with the data for a reference location it should be possible to map an object’s position anywhere within our galaxy to an accuracy of five kilometres, an incredible degree of accuracy. This is a little bit like being able to measure your position on the surface of Earth to within the width of an atom or two!

More information:

  • Becker et al, “Timing X-ray Pulsars with Application to Spacecraft Navigation”, arXiv:1011.5095.
  • Becker et al, “Autonomous Spacecraft Navigation Based on Pulsar Timing Information”,  arXiv:1111.1138.

The Bufo test

Modern pregnancy testing is very easy: an immunoassay is prepared which contains a special pigmented antibody that binds to human chorionic gonadotropin (hCG), a hormone produced by the developing placenta and blastocyst. When the assay is exposed to a substance (usually urine) containing hCG it changes colour, clearly indicating pregnancy. These modern tests are accurate between six and ten days after fertilisation, but there is another test (the rosette inhibition assay) on blood, cervical mucus or amniotic fluid that can detect fertilisation within 48 hours.

It was not always so easy.

In the 1930s, when reliable pregnancy testing was invented, a pregnancy test involved injecting the woman’s urine underneath the skin of a female mouse or rabbit. A few days later the mouse or rabbit would be euthanised and dissected, and if found to be ovulating this indicated pregnancy. This test, the Aschheim-Zondek test, was time consuming and very unfortunate for the mouse/rabbit in question.


A female Bufo viridis frog. Other species of frog were used later.

The Bufo Test, introduced by Lancelot Hogben, used a frog (of the genus Bufo) in place of a mouse or rabbit. The frog was injected with a woman’s plasma or urine and if it produced eggs within twenty-four hours, the woman was pregnant. The Bufo test didn’t require the frog to be killed and it was possible for a frog to be “re-used”; it was the standard test until the development of radioimmunoassay tests in the 1960s and monoclonal antibody immunoassays in the 1970s.

Optimal stopping

Imagine a conveyor belt in front of you, on which are placed one hundred various-sized piles of money. You are allowed to stop the belt at any point and take the pile of money in front of you, but you cannot take any pile that has already passed you. Which pile should you take?

There is a mathematical solution to this problem, (sometimes called the Sultan’s Dowry Problem or the Fussy Suitor Problem) which is quite elegant.

  1. Wait until 37% of the piles have gone past you. (The figure of 37% is the reciprocal of e, the base of the natural logarithms.)
  2. Pick the next pile that is better than all the other piles so far.

Here are one hundred randomly-generated piles of money under £100:

£52.33, £80.83, £27.39, £84.75, £63.87, £1.66, £96.82, £76.51, £22.77, £90.94, £24.08, £60.41, £10.38, £95.59, £92.98, £46.80, £85.86, £21.96, £92.22, £29.19, £59.08, £72.22, £45.08, £63.39, £16.38, £71.49, £29.59, £78.62, £30.05, £97.98, £70.95, £3.79, £19.22, £77.52, £1.78, £48.74, £48.71, £35.95, £79.48, £11.50, £47.33, £32.83, £99.19, £3.23, £10.59, £58.22, £21.15, £61.37, £42.78, £25.27, £58.86, £32.82, £91.75, £13.04, £21.76, £72.29, £85.48, £58.81, £8.70, £91.63, £93.30, £23.00, £13.49, £11.67, £95.27, £21.37, £67.27, £90.99, £50.88, £77.22, £9.51, £10.63, £28.23, £63.94, £89.51, £90.12, £68.53, £76.98, £76.83, £92.04, £19.21, £73.82, £71.31, £99.94, £26.96, £86.92, £33.94, £8.25, £13.70, £74.44, £60.08, £11.54, £42.75, £78.67, £41.92, £92.36, £8.25, £92.89, £37.31 and £36.62.

The “best” value from the first thirty-seven piles is £97.98, so you should proceed through the remaining piles until you reach a value greater than this. This means stopping at the 43rd pile: £99.19.

Looking at the data, this strategy doesn’t actually yield the best value – waiting until the 84th pile would yield £99.94, which is slightly more. For large numbers of piles the 37% rule yields the perfect result in only 37% of cases, but this is a greater percentage than any other solution and it usually results in a very good result (i.e. one close to the perfect result).