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MILLIONS of tiny comets may be striking the Earth every day, according
to a researcher in the US. But the comets may be disappearing in a puff
of water vapour long before they ever reach the ground.

John Olivero of Pennyslvania State University believes that the disintegration
of ‘minicomets’ could explain some puzzling observations he has obtained
of the upper atmosphere. While observing a particular region, he noticed
that roughly every three days a large amount of water vapour suddenly appeared.

Olivero’s observations support a controversial theory proposed in 1986
by Louis Frank of the University of Iowa. Frank found mysterious ‘holes’
while examining ultraviolet images of the upper atmosphere. He claimed that
about 10 million minicomets – giant ‘snowballs’, each containing about 100
tonnes of ice – were hitting the Earth every day.

According to Frank, such minicomets would be very fragile indeed. Long
before they could leave a visible trail in the sky, they would break up,
dumping water vapour in the upper atmosphere (New Å®ÉúСÊÓƵ, Science 12
May 1988).

Olivero was not looking for minicomets when he made his measurements
of water vapour in the upper atmosphere. In the course of his study, however,
he found evidence of this. Olivero made 22 000 separate observations, each
of which lasted 20 minutes. It was during 111 of the measurement intervals
that he noticed something odd: water appeared suddenly.

The water vapour anomaly occurred about once every three days. Frank
had predicted that an impact in a region the size that Olivero observed
should occur about once every two days – a close match.

Many astronomers are sceptical of Frank’s theory. Alex Dessler of Rice
University, in Houston, Texas, compares Frank’s proposal to cold fusion,
and points to ‘a vast amount of contrary evidence’. For example, he says
that seismometers left on the Moon by Apollo astronauts have not detected
impacts of minicomets.

In addition, Dessler says that if only a few kilograms of water were
dumped in the upper atmosphere, it would make the ionosphere – a charged
layer – disappear locally. Although such an effect should be seen easily
from the ground, no one has detected it. Finally, Dessler says that any
incoming objects would have been picked up by satellites used for military
surveillance. He concedes that a few small comets might exist in the Solar
System, but he says they would be very much rarer than Frank’s estimate.

Olivero does not think his study will cause a ‘great shift’ to the minicomet
theory. However, he says that ‘trying to disprove the theory is like trying
to prove that leprechauns do not exist’.

Dessler says he is ‘reserving judgment until Olivero does some crucial
tests and can certify that he has checked everything’. Only then, he says,
will he be convinced that Olivero has not obtained a spurious result.