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.
Read more
about these calculations in the paper "3D/Biela and the Andromedids:
Fragmenting versus sublimating comets," by P. Jenniskens and J. Vaubaillon
(Astronomical Journal, 134, 1037-1045).
advertisement
