By Duncan Steel 28/09/2019

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The All Blacks are currently resting up in the Japanese spa town of Beppu, awaiting their next game. Like Rotorua and several other spa towns spread around the globe, Beppu has an impact crater on asteroid (951) Gaspra named for it. 

Perusing the intellectual pages (i.e. the sports section) in The Press this morning whilst sipping coffee at Yaza! in Montgomery Square, the buzz of the Nelson Saturday Market ongoing outside, I noted the following headline:


Now, Beppu might not mean much to you (apart from being the town where the All Blacks are resting up and/or training prior to their match against Canada in Ōita on Wednesday night), but because I was born near the city of Bath in England I knew Beppu to be a spa town. That makes it a great place for rugby players to soak their aching bodies in the warm waters before their next game.

This blog series is about outer space, you might now say, so what is the connection here? Well, the learned might know that the planet Uranus was discovered in 1781 by (Sir) William Herschel from his observatory in Bath, which was rather less plagued by city lights in those days – or should I write “nights”? – and so a viable location for a keen astronomer to set up his/her telescope, as Herschel did in collaboration with his sister Caroline. Their house is now the lovely little Herschel Museum of Astronomy, just a short walk from the Roman Baths and Bath Abbey, and much to be recommended. Indeed, if astronomy is not really your thing then whilst perambulating westward from the city centre (and the tumult of tourists) you might prefer to stop by the Bath Royal Literary and Scientific Institution instead; there is generally an attractive free exhibition about some aspect of natural history in the main foyer, plus regular talks and field trips.

There is another major ‘space’ connection here, though, and it also involves New Zealand, through Rotorua; yes, another spa town. Beppu, you see, is twinned with both Bath and Rotorua. But these all being spa towns, they are also names given to impact craters on the surface of main-belt asteroid (951) Gaspra.

Other craters on Gaspra have titles including Baden-Baden (in Germany), Bad Gastein (Austria), Aix-les-Bains and Bagnoles (France), Spa (Belgium), Helwan (Egypt), Saratoga (New York State, USA), and Moree (NSW, Australia).

Why spa towns? Because asteroid (951) Gaspra is itself named for a spa town, in Crimea.

The photo at the head of this post is the highest-resolution picture we have of (951) Gaspra; in fact it’s a mosaic composed from several images obtained in 1991 as NASA’s Galileo satellite whizzed by on its way to Jupiter. The pair of images below show you true- and enhanced-colour mixes obtained with the Galileo camera. This asteroid (or minor planet) was discovered in 1916, and is about 18 km long and 10 km wide. It orbits the Sun in the main asteroid belt, between Mars and Jupiter.

True and enhanced colour mixes of images of main-belt asteroid (951) Gaspra, as photographed by the NASA spacecraft Galileo in 1991. This was the first asteroid ever to be visited (in a flyby) by a space mission. One of its craters is named Rotorua, while another is called Beppu, for the spa town in Japan where the All Blacks are currently preparing for their next game in the Rugby World Cup.

So, which crater is Rotorua, and which is Bath, and Beppu? The map below tells you the answers to such queries…

Map of (951) Gaspra built up from the best-resolution images collected by the NASA space probe Galileo as it flew past the asteroid in 1991. The black areas are regions not accessible (i.e. on the far side at that time) during the Galileo encounter; the blurry parts were more-distant from the spacecraft and so only lower-resolution images exist.

Whilst writing about a town and a crater named Beppu, I must also mention an asteroid called Bennu. The name comes from an ancient Egyptian deity, but it is also the appellation given to an Earth-crossing asteroid discovered in 1999, and now catalogued in our data banks as (101955) Bennu.

You will be hearing more about Bennu over the next few years, because it is the target of a NASA space probe named OSIRIS-REx. In fact OSIRIS-REx (I will leave you to look up the acronym rendering that letter sequence) was launched in 2016, and arrived near Beppu at the end of last year. I wrote “near” because the space probe is essentially flying alongside/around this asteroid (which is near-spherical with a diameter around half-a-kilometre, though it has many smaller rocks spread over it) for a couple of years as the complete surface is mapped in detail.

The OSIRIS-REx space probe near asteroid (101955) Bennu – artist’s impression, courtesy NASA.

When the mapping is done, OSIRIS-REx will be directed to drop down to the asteroid’s surface, and grab a sample. All being well that sample will be returned to Earth just less than four years from now.

Planetary scientists are particularly interested in this B-type asteroid (a rare sub-division of C-types, the C meaning carbonaceous), because it seems to have much the same composition as a carbonaceous chondrite meteorite: the most-primordial type of meteorite, having solidified early in the solar system’s history, as the planets were agglomerating. We think that such asteroids/meteorites (and comets) may have delivered much of the water and organic chemicals that made the development of life on Earth feasible.

The best imagery of asteroid Bennu available so far (courtesy NASA). The full width of the map (lower panel) is about 1.5 km. Whilst the brightness scaling used for clarity makes it appear as if Bennu is highly-reflective, in fact its surface absorbs almost 96 per cent of the sunlight that hits it, reflecting only about 4.4 per cent. It’s as dark as soot. 

There is another reason why we should be ‘interested’ in asteroid Bennu. According to our best knowledge of how its orbit will alter over the next 150 years, there is about a one-in-3,000 chance that it will impact the Earth sometime between 2175 and 2199. If it were to do so then it would release energy equivalent to about 100,000 times that of the Hiroshima atomic bomb.

On that sobering note, let me finish by saying something else about the All Blacks. The team wears, yes, all black. Thing is, Bennu is all black as well. It reflects little more than four per cent of the sunlight that hits it (i.e. its albedo is only about 0.04), that being one of the reasons it was not discovered until twenty years ago, despite being a goodly size and passing close by the Earth. Like the All Blacks’ uniforms, Bennu is darker than coal, and as reflective as soot.