By Duncan Steel 11/06/2019 2


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Comet Grigg-Skjellerup was one of the first such celestial bodies to be visited by a spacecraft, the Giotto probe which was sent on to encounter it in mid-1992 after having first visited the famous Comet Halley in 1986. Comet Grigg-Skjellerup was discovered about a century ago, independently by a New Zealander (John Grigg) and an Australian (Frank Skjellerup). The younger brother of the latter (George Skellerup) moved to New Zealand in 1902, and established the iconic NZ rubber company. 

It being my birthday, I thought that the occasion should be marked with a blog post. But about what? My quandary was solved when my partner Jan presented me with a splendid pair of gumboots. And here they are:

 

Now, perhaps (like the BBC) the Science Media Centre might eschew any advertising, but it happens that the brand of the gumboots shown above is pivotal in the short story I am going to relate. It’s about how comets and gumboots – and generally, the Skellerup rubber products – are linked.


Back in 1987-88 I was working for the European Space Agency (ESA), and based at Lund Observatory in the south of Sweden. One of the things I was doing was trying to provide a calibration of the dust impact detection system (DIDSY) carried on the Giotto spaceprobe, which in 1986 had flown close by Comet Halley. During the nearest approach the spacecraft was being hit by up to 100,000 dust grains per second. These impacts were monitored by the shield that was protecting the rest of the satellite, so far as was possible, from the damaging effect of such a bombardment. In essence that shield had sensors attached to it recording ‘pings’ as each struck.

Fundamentally the calibration we were attempting in Sweden involved dropping small ball-bearings onto a duplicate dust shield, and seeing how the ‘pings’ depended on the momentum and kinetic energy of these metallic spheres. The actual cometary dust grains were tiny, typically a ten microns or so across, whereas the ball-bearings may have been small (a few millimetres in diameter) but still they had far greater masses than the dust. The dust from the comet, though, was hammering into the Giotto probe at a far higher speed, almost 70 kilometres per second. In the end, the laboratory testing we attempted never really led to a useful calibration. Such is life, and science.


A few years later it was realised that the Giotto probe could be directed to fly close by another comet, this one not as famous as Halley, but nevertheless well-known to astronomers. This was Comet Grigg-Skjellerup.

It happened (not by chance) that in 1990 the spaceprobe would make another fly-by of the Earth, and so our gravity could be used to divert the satellite to direct it to encounter Comet Grigg-Skjellerup in July 1992. To make this happen it was necessary for the trajectory to be very carefully adjusted in order to miss the Earth at just the right distance and just the right angle, and whilst there was adequate rocket fuel left to make this happen, there was nothing like enough to make the probe travel to this second comet unless Earth’s gravity could be leveraged.

In the event this ‘slingshot’ passage by the Earth was successful, and so it was known that the Giotto satellite would approach its second comet in mid-1992. This was called GEM: the Giotto Extended Mission.

There was a problem, though. During the encounter with Comet Halley many of the instruments had been badly damaged by that intense dust bombardment. In particular the optical camera had been put out of action. This meant that the spacecraft was flying blind, with two repercussions: (1) We would not get close-up images of Comet Grigg-Skjellerup (though other instruments, including the dust detectors, could return useful data); and (2) The camera system could not be used for navigation.

One could think of this as being like trying to thread a strand of cotton through a needle whilst blindfold. We knew where the cotton was (the spacecraft, from radio tracking) but we did not know where the tiny slot in the needle (the comet) was. And to get the most-valuable data we needed to get Giotto within a few hundred kilometres of the cometary nucleus – whilst it was 214 million kilometres away from Earth.

Colour-coded image of Comet Grigg-Skjellerup observed in May 1992. There are no really good photographs of the comet from the 1992 apparition.

At this time (1991-92) I was working at Siding Spring Observatory, near Coonabarabran in New South Wales. In 1990 I had begun the first southern hemisphere project aimed at discovering near-Earth objects: asteroids and comets with orbits bringing them close by our planet. This project I had entitled the Anglo-Australian Near-Earth Asteroid Survey, or AANEAS. (Whilst learning Latin at school I had translated part of Virgil’s Aeneid: the epic poem telling the legend of Aeneas, a Trojan hero said to have wandered the Mediterranean before founding Rome. Along the way he had dallied with the Queen of Carthage, hence Purcell’s opera Dido and Aeneas.)

In order to get their spaceprobe to fly close past Comet Grigg-Skjellerup, ESA project managers (and in particular the controllers at the European Space Operations Centre (ESOC) at Darmstadt in Germany) needed a far better knowledge of the forthcoming path of that comet than what was available at the time. One of the problems with comets is that the ices (H2O, CO2, CO, CH4, C2H6, NH3, and many others) that sublimate from the proverbial dirty snowballs that are cometary cores impose a jetting force on the nuclei, so that not only are dust grains swept outwards, but also the nucleus itself is pushed around a bit. We call these non-gravitational forces (e.g. see this item).

Across the interval of interest, Comet Grigg-Skjellerup would be in the southern sky. AANEAS was operational. There were few other suitable teams in the southern hemisphere. At Mount John Observatory, Alan Gilmore and Pam Kilmartin were (as ever) doing sterling work on asteroid and comet tracking. At Siding Spring, though, we had better weather, and we were using the 40-inch (okay, about a metre) telescope regularly for comet and asteroid tracking using a new-fangled device called a CCD detector (rather than film). The 40-inch was the first telescope installed at Siding Spring, by Bart Bok in 1963.

Please could you track the comet for us?” ESOC asked. Yes, of course. It required some mechanical gymnastics, because the comet was very low in the sky, just above the horizon near daybreak, and we had to manoeuvre the rather large telescope to place it above the polar axis (a forbidden process) because otherwise the wall supporting the dome was in the way, but Rob McNaught and I managed it on every possible morning. The outcome was that the ephemeris for the comet was far-better determined, and the Giotto probe was flown to within 200 km of the comet’s nucleus (see the graphic here).

The orbit of 26P/Grigg-Skjellerup in the in the inner solar system (from the NASA-Jet Propulsion Laboratory Solar System Dynamics website).

Comet Grigg-Skjellerup was one of the very first comets to be encountered by a spaceprobe, then. Why might that be of interest in New Zealand?

Well, part of its name comes from John Grigg, who discovered it in 1902, from Thames, on the southwestern end of the Coromandel Peninsula. (Notice that I wrote that Grigg discovered it; I did not say that he was the first person to spot it.) If you want to know more about Grigg, see this article by my friend Wayne Orchiston, a prolific historian of NZ (and other) astronomy.

Back in those days, more than a century ago, news of celestial discoveries took some time to propagate around the globe. The comet was not well-observed in 1902, and so its orbit was by no means certain. In 1922 it was re-discovered by Frank Skjellerup, an Australian who was at the time working in South Africa, and living near Cape Town. Orchiston has also described Skjellerup’s astronomical career.

The comet, therefore, is named Grigg-Skjellerup. Actually, astronomers generally reference it as ’26P/Grigg-Skjellerup’ – the 26th object in the list of periodic comets (which return at least once every 200 years); the first in the list is ‘1P/Halley’.

I wrote above that Grigg was not the first to observe this comet which bears his name, and that of Skjellerup. In 1987 was it was realised that the comet had been seen in 1808, though only for a few days, by French astronomer Jean-Louis Pons. Nevertheless it carries the name of a New Zealander (John Grigg), and an Australian (Frank Skjellerup). There is, though, another NZ connection.


Frank Skjellerup was born in Cobden, Victoria, the tenth of 13 children. His father, a Dane (hence the surname), died whilst Frank was young. Obviously-enough, he was not as young as the 13th child, George Waldemar Skjellerup. Skjellerup senior, a farmer, died before George was three years old. When he was 18 George started working in Melbourne for the Dunlop Pneumatic Tyre Company.

In 1902, just before he turned 21, George sailed for New Zealand, and initially worked in a bicycle shop in Dunedin. Within a few years he was living in Christchurch, and his Melbourne employers helped him get a job with Dunlop there.

In 1910 George established his own concern, the Para Rubber Company. Within a few years he had dropped the ‘j’ from his name, so he was thereafter George Skellerup. (In Danish the beginning of the surname would be pronounced somewhat like ‘shell’; in Swedish it is a far more difficult letter construction to say, and the best way I can describe it is that it is like saying ‘shoo’ whilst vibrating the tongue against the roof of your mouth. By dropping the ‘j’, everyone started saying a hard ‘k’ sound, as in ‘scowl’ or ‘school’, which just shows that spelling can be a poor guide to pronunciation.)

Skellerup’s company went through some hard times, and some good ones, manufacturing a variety of rubber products and also branching out into other commercial activities. The multifarious branches were gathered under the heading of Skellerup Holdings from 1948. It is not an accident that the company symbol is the silhouette of a Viking ship, reflecting the original source of the name.

George Skellerup died in 1955 in Somerset, England, it happens six days before I was born in the same county.

Nowadays, of course, the Skellerup company is almost proverbial in New Zealand for a wide range of rubber goods. In terms of footwear these range from gumboots to bandals.


What is the connection between comets and rubber then? Well, Comet Grigg-Skjellerup was discovered by one New Zealander (John Grigg), but also an Australian (Frank Skjellerup) whose younger brother (George Skellerup) become an adopted kiwi and established an iconic New Zealand company, the Skellerup Group, best-known on the high-street for its rubber products.


2 Responses to “Connecting comets and rubber”

  • My old school mate Duncan (aka: Dr. Steel), certainly knows how to have an intense birthday comet celebration – as for me; I had an all day BBQ with friends dropping in to eat, drink & make merry with me! Happy Birthday Duncan.