The Trusty Servant

Anthony Wesley, an Australian amateur astronomer, started observing about 11pm last Sunday evening. By midnight the “seeing” had deteriorated and he was thinking of quitting, but at 12:10am decided to do something else for half-an-hour, leaving his camera running. He came back at 12:40am and noticed a dark spot rotating into view in Jupiter’s south polar region.

When first seen close to the limb (and in poor conditions) it was only a vaguely dark spot, I thought likely to be just a normal dark polar storm. However as it rotated further into view, and the conditions improved I suddenly realized that it wasn’t just dark, it was black in all channels, meaning it was truly a black spot.

My next thought was that it must be either a dark moon (like Callisto) or a moon shadow, but it was in the wrong place and the wrong size. Also I’d noticed it was moving too slow to be a moon or shadow. As far as I could see it was rotating in sync with a nearby white oval storm that I was very familiar with – this could only mean that the back feature was at the cloud level and not a projected shadow from a moon. I started to get excited.

It took another 15 minutes to really believe that I was seeing something new – I’d imaged that exact region only 2 days earlier and checking back to that image showed no sign of any anomalous black spot.

So he uploaded his report and emailed his contacts. Fairly soon, night-time astronomers everywhere were training their telescopes at Jupiter, whilst those in daylight impatiently waited for night-time. Wesley’s server went down under the pressure and he had to move his report onto something bigger.

He is at Murrumbateman, which is almost due north of Canberra, 10 hours ahead of GMT, or UTC (coordinated universal time) as astronomers like to call it. Pedants will note that UTC can be as much as 0.9sec different from GMT. So his video turned out to show the spot first coming into view around 14:02 Sunday 19 July, or 15:02 London time. Although much bigger, Jupiter rotates faster than Earth, in about 10 hours. So if Jupiter got hit whilst the surface was out of sight it was between about 10am and 3pm London time on Sunday. But it could have been earlier. Wesley saw that the spot was not present 2 days earlier. There has been plenty of hoopla, so if anyone has an image of the actual collision event, they would probably have come forward by now.

The Hubble Space telecope was being serviced, but it was hastily returned to service, and captured this image on Thursday:

Jupiter’s radius is about ten times that of Earth, so the dinky little black spot is a good deal bigger than it looks – estimating by eye, something like the North Atlantic in size.

It is just over 15 years since the better-known Shoemaker-Levy 9 impact, when a comet disintegrated in Jupiter’s gravitational field and collided with Jupiter as about two dozen pieces:

It took a surprisingly long time for people to grasp that the earth is under constant bombardment. One has only to look at the moon to see that it is covered with craters, and reports of smallish meteors have been constant, but earth’s ecosystem soon makes the craters from large impacts hard to see, so the overwhelming majority of scientists resisted the idea of such impacts for far too long.

The Barringer crater in Arizona and all the lunar craters were attributed to volcanoes. [The Barringer crater is now thought to have been formed by a meteorite about 25 metres in diameter which vapourized on impact.]

On 30 June 1908 a ball of fire apparently exploded 10 km above the ground in a wooded area of Siberia, near the Tunguska River, flattening trees over a wide area (about 50 km across). The traditional explanation has been that a meteorite just missed the Earth and exploded in the atmosphere, releasing energy equivalent to a 10-20 megaton nuclear weapon.

The details are still debated. In December 2007, Mark Boslough and others at Sandia released a slew of simulation videos (around 10MB each, for download rather than streaming), following extensive supercomputer work (full article: International Journal of Impact Engineering (2008) 35, 1441-8, not available on the internet without a password). I should explain that following the test ban treaties (partial 1963, and full 1996, not ratified by the USA), the US weapons labs have been carrying out extensive (and constantly refined) simulations of nuclear explosions in order to design “improved” bombs without actually testing them. They presumably adapted the software slightly to simulate asteroid impacts.

The videos look quite realistic. They claim to show that the explosion was only 3-5 megatons, because just as for nuclear weapons, airbursts are much more destructive than ground bursts, and the main damage is caused by shockwaves. The calculations are no doubt correct, but the premise may be wrong. Earlier in 1997, Italian scientists claimed that the Tunguska meteorite actually hit the Earth at what is now Lake Cheko, 8 km to the north.

The Hoba meteorite was found by a farmer in Namibia and written up in a journal in 1920. It weighs about 60 tonnes and is about 3 metres in diameter, but it is certainly big enough to make clear that not all meteorites are charming little stones that you can pick up and put in a display cabinet.

But despite all this evidence, the geophysicists refused to countenance major Earth impacts until the seminal paper of Luis Alvarez who used the iridium signature to demonstrate that the dinosaur extinction had been caused by one (Science (1980) 208, 1095; reprinted in Discovering Alvarez, ed Peter Trower, Univ Chicago Press 1987. There is an interesting account of Alvarez’ work in Nemesis, The Death Star, Richard Mueller, 1988. It explains at length why Alvarez picked Iridium. Even then it took a decade or two before enough detailed data was collected to convince those geophysicists who had not died off.

But astrophysics types realized the truth much earlier. Figuring out how to deflect approaching asteroids before they hit was first tackled as a student project at MIT in the 1960s (Project Icarus, MIT Press, 1968) – and popularized 30 years later in two Hollywood movies (Armageddon and Deep Impact, 1998).

The NEAT (Near Earth Asteroid Tracking) project, organized by NASA, with the help of hundreds of amateur astronomers, now works full-time trying to find dangerous asteroids. The current thinking is that the danger is fairly remote (eg from the NEAT site):

Fortunately for us, bodies that are large enough to make it through without slowing down or burning up are very rare. The threshold size is roughly 100 meters. A body of about this size exploded over the Tunguska forest in Siberia in 1908. It flattened about 700 square miles of trees. We believe that such events occur once a century on average, but this estimate is uncertain. Military sensors in orbit about the Earth, which watch for explosions that might indicate a violation of nuclear proliferation treaties, have detected dozens of high- altitude explosions each year. These are caused by meter-sized asteroids that have impact energies equivalent to tens of kilotons of TNT, comparable to the bomb dropped on Hiroshima …
The most dangerous asteroids, capable of a global disaster, are extremely rare. The threshold size is believed to be 1/2 to 1 km. These bodies impact the Earth only once every 1,000 centuries on average. Comets in this size range are thought to impact even less frequently, perhaps once every 5,000 centuries or so…
In the course of our search for Earth-crossing asteroids, we could find one that will hit not in the next year, or even in the next ten years, but might hit in the next hundred years. We believe that the chance that we will find such an object is only 1 in 1,000, even after a complete search.

But this could be misleadingly reassuring. If Sandia are correct, a quite modest sized asteroid exploding 10km above London would wreak huge blast damage, substantially worse than Hiroshima. The chance of such an explosion happening above a major city is fairly small, but the number of asteroid strikes goes up dramatically as size goes down. I have not seen a careful analysis. Some sort of bound is that there is nothing in recorded history over the last 20 centuries to suggest such airburst above a city. Also any big airburst, whether or not above a city, would have been detected in the last 50 years, or possibly rather longer.