viernes, 4 de abril de 2008

Meteor Storms

Comet 3D/Biela in February of 1846 (top), in a drawing by E. Weiss from Vienna, and on September 25, 1852, in a drawing by O. W. Struve at Pulkovo Observatory. North is up, west is to the right.

Each generation seems to get a chance, or two, to see a mind-boggling display of shooting stars one night. The most spectacular displays in my memory are the 1999 and 2001 Leonid storms. Before my time, observers swore by the 1966 Leonids, and could not stop talking about the spectacular 1933 and 1946 Draconid storms. Those were not quite as intense as the Leonids, but the Draconids moved so slowly that several were seen gliding across the sky at the same time.

In the 19th century, the most spectacular storms were the 1872 and 1885 Andromedids, which were almost as strong as the Draconids and also very slow moving. At the time, Chinese astronomers wrote: "shooting stars fell like rain." From the counts of meteors in the west, we now estimate that rates peaked around two per second.

Some years earlier, in 1846, a comet called 3D/Biela had been observed to break into at least two pieces. The pieces had drifted further apart in 1852. Based on the rate of that drift, the moment of breakup is thought to have been in either 1842 or early 1843, when the comet was far from the sun, near Jupiter's orbit. Because of their phenomenal intensity, the Andromedid storms were believed to be caused by Earth traveling through the debris created during that breakup.

So, did we pass through the breakup debris of comet 3D/Biela? Astronomer Jeremie Vaubaillon, now at Caltech, and I decided to investigate. We calculated where the debris from 1842/43 would have ended up in the comet orbit and discovered that a breakup far from the sun does not disperse dust easily. The dust tends to keep the same orbital period, returning back at the same time. The cloud of dust tends to hang around the position of the comet, only gradually lagging the comet as a whole due to the push of solar radiation.

We concluded that Earth has never encountered the debris created during the 1842/1843 breakup! So what caused the Andromedid meteor storms? Upon further reflection, we calculated that Earth had passed through the dust trails that were generated when the two fragments came back towards the sun in 1846 and 1852. Ejection close to the sun results in higher ejection speeds and thus a relatively large variation of orbital periods. After one orbit, some dust returns early, other dust late, and thus all dust spreads out along the comet orbit much more quickly. We found that the planets happened to steer those new dust trails into Earth's path every time when observers on the ground noticed unusual Andromedid rates in later years.

What if Earth would have crossed the dense cloud of debris of the broken comet shortly after 1843? The resulting meteor storm would have been a hundred times more intense, if not more. The impressive storms of 1872 and 1885 were caused by a tiny bit of dust, we estimate 30 times less than what came off during the initial breakup. Moreover, that dust had spread out quite a bit. Just imagine what could have been. We appear to have missed out on a truly spectacular sight.

Then again, the next generation may be in for a surprise. The Andromedids have moved on and now pass far from Earth's orbit, but other comets continue to break and create new meteoroid streams. One date to keep in mind is May 31, 2022, when Earth will pass very close to (but perhaps will just miss) debris created during the 1995 breakup of comet 73P/Schwassmann-Wachmann 3. And there may be others; we don't know yet what dormant comet nuclei may be out there that could break any time now.