IT WAS a science story the media could not resist: the detection of house-sized comets pelting the Earth at the rate of one every few seconds, each dumping tonnes of water onto our planet. Around the world, newspapers and magazines carried the astonishing image of one such object breaking apart 15 000 kilometres above the Atlantic, snapped by a camera aboard NASA鈥檚 Polar satellite.
The space agency鈥檚 press release this May put the discovery in context, quoting Louis Frank of the University of Iowa, the designer of Polar鈥檚 imaging system: 鈥淭he images show that we have a large population of objects in the Earth鈥檚 vicinity that have not been detected before.鈥�
But these weren鈥檛 the usual courtesy quotes granted by the agency to some back-room scientist whose efforts have finally borne fruit. Within the astronomical community, Frank is very well known; indeed, notorious is perhaps the appropriate term. For Frank has been claiming for over a decade that the Earth is being bombarded by mini-comets, and he has been bombarded with scorn in return. Now, it seems, he has proved the scoffers wrong.
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It鈥檚 a wonderful tale of a scientific David beating reactionary Goliaths. Or so it seems. But since the day in May that NASA released the images, researchers with expertise in everything from detection of faint objects to planetary atmospheres to the physics of hyper-velocity impacts have been pouring scorn on the Polar results. Their message is that something is-must be-terribly wrong with the satellite data. There is, they claim, absolutely, positively no way at all that so many objects can strike the Earth鈥檚 atmosphere so often, dump so much water and yet have remained undiscovered until now.
The current furore marks the rekindling of a bitter scientific controversy that has surrounded Frank and his claims for over a decade. Its origins date back to 1982, when John Sigwarth, one of Frank鈥檚 students at Iowa, was analysing images of the Earth sent back by a pair of NASA satellites collectively named Dynamics Explorer. Like Polar, they were designed to study the Earth鈥檚 auroral displays, and carried an ultraviolet camera capable of studying the effects of solar particles smashing into the upper atmosphere.
To Sigwarth鈥檚 frustration, many of the images were spoilt by tiny black dots, which at first they thought was the result of faulty data. But careful study by Sigwarth, Frank and John Craven revealed regular motions which strongly suggested that instrumental noise was not the cause. Instead, they appeared to be patches of UV-absorbing material 50 kilometres across that were being created in the atmosphere every few seconds.
The Iowa team realised that the only commonly occurring substance capable of producing the observed absorption is water. The implication was clear: 鈥渟nowballs鈥� containing tens of tonnes of water are hitting the Earth every few seconds.
When Frank and his colleagues submitted their first papers on the mini-comet claim to Geophysical Review Letters, both referees recommended rejection, but the then editor Alex Dessler, now at the University of Arizona, decided to publish the paper anyway. Even before its appearance in 1986, the paper and its claims came under attack. The suggestion that so many impacts and so much water could have been missed for so long was-and is-regarded as simply preposterous.
One obvious explanation is that the Dynamics Explorer results really were just instrumental noise. According to Frank, that was always a weak argument, and now Polar鈥檚 far better images have killed it off entirely. Frank calls the objects 鈥渉oles鈥� because they absorb light that would normally make it to Earth-so they appear as dark spots, or holes in the sky. 鈥淭he Polar images confirm the previous observations of atmospheric holes,鈥� he says. 鈥淭his includes the dimensions of the holes, their frequency of appearance, and their east-to-west motion across the sunlit atmosphere. The Polar detections are approximately several thousand per day and give a global rate in the range of 5 to 20 per minute.鈥�
Frank also insists that the explanation of the holes as huge clouds of water molecules has been confirmed by Polar. 鈥淥ne of the cameras for visible wavelengths was used to verify that the objects contain large amounts of water by viewing the intensities of hydroxyl radical (OH) emissions,鈥� he explains. Hydroxyl radicals could form when hydrogen atoms are torn away from water molecules as the mini-comets hurtle into the atmosphere. If this is happening, then the bursts of hydroxyl emissions should appear at the same rate as the mini-comets themselves-and they do. 鈥淭he frequency of the OH trails was very similar to the occurrence frequency of atmospheric holes,鈥� he confirms.
Beyond dispute
Some former critics of Frank now believe that something really odd is going on high above our heads. 鈥淭he Polar results definitely demonstrate that there are objects entering the Earth鈥檚 upper atmosphere that contain a lot of water,鈥� says Thomas Donahue, an atmospheric physicist at the University of Michigan in Ann Arbor, and at one time one of Frank鈥檚 most vociferous critics. 鈥淭he time has come for me and my colleagues to try to account for the Polar images with a robust model.鈥�
Others would be more willing to join with Donahue but for the truly astonishing implications of the Polar results: up to a thousand house-sized objects breaking up high above the Earth鈥檚 atmosphere every hour. Surely such large objects would be seen by even the most casual observer as 鈥渟hooting stars鈥� tearing across the night sky.
In fact, they have; or maybe they haven鈥檛. It depends on whom you ask. In a paper published in Astronomy and Astrophysics in 1990, Frank and others reported seeing precisely the kind of 鈥渟treaks鈥� expected, during a search of the skies using the University of Arizona鈥檚 Spacewatch telescope at Kitt Peak. Others are convinced that the streaks, like the dots on the Dynamics Explorer images, are just instrumental noise. According to Alan Harris, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California, if mini-comets exist in anywhere near the numbers suggested by Frank, you wouldn鈥檛 need a telescope to see them: 鈥淵ou should see these things with a pair of binoculars in the evening sky at the rate of one every few minutes.鈥�
Frank disagrees. Like ordinary comets, he says, the mini-comets are pitch-black, and can only be seen when they lie outside the Earth鈥檚 shadow, just before sunrise and just after sunset. And although they shine about as bright as Venus for a second or so as they disintegrate, naked-eye observers could expect to see one only once in every 100 hours or so of continuous observation.
Then there is the question of where all that water goes to: on Frank鈥檚 estimates, a million tonnes of it every day. Again, it is a claim that has triggered near-apoplexy in some scientists. 鈥淭he water level in the stratosphere is well accounted for and the influx from these `comets鈥� would be greatly noticed,鈥� says one NASA astronomer. 鈥淲e fly the Kuiper Airborne Observatory and the Stratospheric Observatory for Infrared Astronomy there to enjoy the dryness.鈥�
Frank鈥檚 rejoinder is that the comets break up over 10,000 kilometres above the Earth, and that the amount of water they add to the lower atmosphere is pretty low. He calculates that the comets add enough water to provide an extra one-thousandth of a millimetre of rainfall each year: 鈥淭his will certainly not `wet鈥� the stratosphere.鈥�
But the biggest criticisms of the mini-comet hypothesis centre not on the Earth, but on the Moon. Although the Moon is much smaller than Earth and has a weaker gravitational field, there is no doubt that if mini-comets exist, they must strike the Moon once or twice each minute. And with a mass of around 30 tonnes and an impact speed of 30,000 kilometres per hour, the impact should have the explosive punch of 300 tonnes of TNT-easily detectable by the seismometers placed on the Moon by the Apollo astronauts. Yet those instruments detected only about 2000 impacts a year-a fraction of the number expected for mini-comets.
Frank says there is a major flaw in this argument: it assumes that mini-comets hit like rocks, and generate seismic jolts capable of being detected by the Apollo instruments. 鈥淪mall comets are like fluffy snowballs, not rocks,鈥� he objects. 鈥淚t is the seismometers鈥� lack of sensitivity to the impact of small comets that accounts for the discrepancy.鈥�
But what about all that water? At the predicted rate of impact, the Moon should be literally knee-deep in water after just 1000 years of bombardment. Again, Frank says that such supposedly knockout arguments ignore basic physics: 鈥淭he lunar gravity is so low that practically all the water vapour from the impact of small comets simply flies off.鈥�
Still they don鈥檛 believe
But Frank鈥檚 critics aren鈥檛 about to wave the white flag even if presented with unimpeachable evidence for lunar water. Despite Frank鈥檚 protestations, most see absolutely no reason to change their minds about the existence of mini-comets.
Dessler, who was instrumental in getting Frank鈥檚 original claims published, now sees his rejoinders as little more than special pleading. There remain a whole host of objections that he still has not answered to the satisfaction of his scientific peers, Dessler says. Why does ocean sediment not contain huge amounts of the chemical compounds expected to be found in comets? Why haven鈥檛 the blasts of steam created by mini-comet impacts erased delicate lunar features? And why, over the centuries of observations by amateurs and professionals alike, has no one reported seeing anything like these objects? The sheer number of negative findings, says Dessler, makes denial of the small-comet hypothesis compelling.
But according to Duncan Steel, at Spaceguard Australia, part of the international organisation that keeps watch on near-Earth objects, Frank鈥檚 critics rest their arguments on a questionable assumption. 鈥淭hey talk about the consequences of having these things striking constantly for millions or billions of years,鈥� he says. 鈥淣o one, including Frank, has stopped to think whether the phenomenon could be episodic.鈥� This, he points out, could reconcile mini-comets to known data: 鈥淚f we are talking about these objects having been produced relatively recently, then the figures are quite different.鈥�
Steel also questions the assertion that nothing like a mini-comet has ever been observed. He points out that over the past century the scientific literature has carried many reports of unexplained 鈥渄ark objects鈥� crossing the face of the Sun. Frank鈥檚 data from Polar imply that anyone observing the Sun for an hour has a 1 in 1000 chance of seeing a mini-comet passing in front of it, Steel calculates. 鈥淭hat sounds plausible,鈥� he says. 鈥淪uch observations would be infrequent.鈥�
But while Steel finds fault with the sceptics鈥� arguments, he is also critical of Frank: 鈥淲ith these atmospheric holes, he seems to have a phenomenon which needs explaining. But few are happy with his explanation, including me鈥�. Dessler is reluctant to go even that far: 鈥淢y opinion at the moment is that the holes are not real,鈥� he says. 鈥淔rank may well be once again showing us instrument artefacts.鈥�
With the origin of the Polar images clearly so controversial, why did the media seize on Frank鈥檚 claim that they represented solid evidence for mini-comets when they were released in May (This Week, 31 May, p 7)? Many astronomers hold NASA to blame. Its press release quoted Robert Hoffman, project scientist for Polar at NASA鈥檚 Goddard Space Flight Center, saying: 鈥淚 am pleased that Polar鈥檚 instruments were able to actually detect these objects streaking towards the Earth and disintegrating into clouds of water vapour.鈥�
Some have drawn comparisons between the mini-comet story and NASA鈥檚 previous media coup, the discovery of supposed evidence for life on Mars, announced last August. They suggest the space agency hyped the story to gain coverage for an otherwise fairly mundane mission. Polar鈥檚 avowed aim is to study the Earth鈥檚 auroras; Frank鈥檚 observations are a spin-off from one experiment aboard the satellite. 鈥淧erhaps NASA鈥檚 news releases should be published with a small-print notice at the top stating `paid advertisement鈥�,鈥� said a senior researcher at one of NASA鈥檚 own laboratories.
Hoffman rejects the accusation: 鈥淣ASA has not endorsed the interpretation of the images in any way whatsoever.鈥� He adds, however, that the invective heaped on the Polar results does no credit to the scientific process: 鈥淚t would be refreshing to hear an expert say `I鈥檓 not going to comment before I read the paper鈥�.鈥�
Independent review
There are in fact four papers on Polar鈥檚 results currently being refereed, and whether any of them will convince the sceptics is unclear. What Frank鈥檚 detractors are demanding is yet more evidence from other, independent sources: for example, images of bursts taken by American spy satellites. Frank is trying to obtain such evidence.
The question some are asking is just how much more experimental data Frank should be expected to collect. The vigorous, sometimes acrimonious debate over mini-comets highlights the problems facing any scientist with unorthodox ideas. To some, the continued rejection of Frank鈥檚 apparently impressive satellite data and the mini-comet theory by which he explains them seem more like obstinacy than a justifiable demand for rigour.
But Harris thinks that scepticism is entirely proper, and sees the controversy as an object lesson in the way science arrives at the truth. 鈥淭he main objection is not to his data,鈥� says Harris. 鈥淚t is the enormous leap Frank has made from what he observes to what he thinks is the explanation for his observations.鈥� Mini-comets have been discredited on so many grounds, 鈥渢hat I believe we are justified in expecting Frank to look elsewhere for an explanation鈥�, says Harris.
Frank himself feels it is his critics鈥� arguments that have been 鈥渢horoughly discredited鈥�. And he is convinced it is now their turn to face up to the implications of rock-solid data: 鈥淭he verification of the existence of atmospheric holes is completely secure,鈥� he insists. 鈥淭he startling conclusions come from trying to account for the absorption of the ultraviolet dayglow over areas of 50 to 100 kilometres in diameter. Water is the only common gaseous substance in the solar system that can do it efficiently. No one has ever offered an alternative mechanism.鈥�
Donahue, once an arch-critic, believes the time has come to adopt a middle way. 快猫短视频s should not deny the reality of the Polar results, he says, but find ways of squaring the data with astronomical facts: 鈥淧reviously, there were two problems-detection and interpretation. Under those circumstances, it behoved Frank to bolster the observational evidence.鈥� Donahue accepts that this is exactly what Frank has done, and that this puts the ball in his critics鈥� court. 鈥淚t is up to us to reconcile what he observes with the Earth and planetary system as we know it. If we have to change some strongly held positions, so be it.鈥�
