The tiltrotor aircraft lands and takes off like a helicopter, using horizontal rotors. Once airborne, the rotors tilt forward like the propellers on a conventional plane. The swivelling propellers are quieter than helicopters’ horizontal rotors and allow the aircraft to fly twice as fast as a helicopter of a comparable size.

Two aircraft manufacturers, Bell Helicopter Textron and the Boeing Company’s helicopters division, are building a nine-passenger commercial version of the aircraft, known as the Bell-Boeing 609. The vehicle will be tested in July 1999, according to officials at NASA’s Ames Research Center in Mountain View, California, which developed a prototype vehicle called the XV-15 on which the 609 is based.

The Bell-Boeing 609 will be the world’s first commercial tiltrotor, says John Zuk, head of civil tiltrotor studies in the Advanced Tiltrotor Transport Technology Office at Ames. Bell and Boeing are already building a version for the US Navy, known as the V-22 Osprey. The US Marines has ordered 16 V-22s and may eventually order hundreds more, says Boeing.

Despite this recent success the tiltrotor has had a chequered history. The US defence secretary tried to cancel the Osprey in 1989, but Congress overruled him and forced the project to continue. Two prototypes have crashed during testing. A crash in 1992 near Washington DC killed seven people, but an investigation concluded that the tiltrotor mechanism was not to blame (Technology, 15 August 1992, p 18).

Once the tiltrotor is in service, Zuk says its impact will be felt first around major US airports. Passengers often arrive at these airports from distant locations by jet and then take smaller, propeller aircraft to surrounding cities. The growth of these “commuter” airlines has created competition for takeoff and landing slots on runways. Because a tiltrotor does not need a runway it will eliminate this rivalry and may eventually replace the propeller commuter craft, says Zuk. The tiltrotor is also more flexible than a propeller-driven aeroplane because it can land in a business park or on top of a building to pick up or drop off passengers.

Tiltrotor aircraft could prove a boon for developing nations that have not yet built an infrastructure of major airports, he says. These nations could use tiltrotors to set up a point-to-point transport network without need of massive airports. “I truly believe that it will revolutionise air transportation around the world,” Zuk says.

Sadly, Europe’s own tiltrotor project, called the European Future Advanced Rotorcraft, has not got beyond the drawing board stage.

GIANT chunks of ice have been caught in the act of plunging into Earth’s
atmosphere from space, vindicating a scientist who has spent the past decade
trying to persuade fellow astronomers that this happens.

Louis Frank of the University of Iowa announced the discovery this week at a
meeting of the American Geophysical Union in Baltimore. At the same time, NASA
released pictures of the giant ice cubes taken by its Polar satellite.

The satellite also captured images of small dark holes in the ultraviolet
emissions that radiate upwards from Earth. The holes are apparently caused when
the ice from the minicomets melts, generating small clouds of water vapour that
briefly block ultraviolet emissions from below.

Frank estimates that the chunks of ice are the size of a house and can weigh
as much as 20 tonnes. The minicomets do not pose any threat to life on Earth,
however, because they melt while they are still hundreds or thousands of
kilometres above the planet.

Thomas Donahue of the University of Michigan at Ann Arbor, who has been one
of Frank’s most vocal critics in the past, says that Frank has now proved his
case. “All in all, the observational evidence is overwhelming,” he says. Two
separate instruments on the satellite, including one that Frank does not
operate, have registered the holes, and the ice chunks “move as they should
move”, he says.

Frank caused a furore in 1986, when he claimed that NASA’s Dynamics Explorer
satellite had spotted icy chunks plunging into the atmosphere at the rate of 20
a minute. After a vigorous scientific debate, Donahue and virtually all other
astronomers dismissed his claims as a misinterpretation of the satellite’s
observations.

Even though Donahue now believes in the “packages of water”, as he prefers to
call them, the discovery still leaves atmospheric scientists and astronomers
with some puzzles to solve. For example, atmospheric scientists think they have
already explained the levels of water vapour in the upper atmosphere, but if
extraterrestrial water is regularly dumped there, this would upset their
theories.

And if ice chunks are reaching Earth, they must also be hitting the
Moon—where they should generate measurable seismic tremors and leave
telltale impact craters. So far, there has been no evidence for either, says
Donahue. “It’s an extraordinarily interesting discovery.”

Although it was known that the eyes of infants can change colour, the new
study is the first to show clearly that adult eye colour is not fixed, says
Laszlo Bito of Columbia University in New York, one of the authors of the study
published in the May issue of Archives of Ophthalmology (vol 115, p
659).

Eye colour is determined by the concentration in the iris of melanin, the
pigment that determines the colour of skin and hair. Given this, Bito says it
should not come as a complete surprise that eye colour can shift. “It’s not much
different than greying of the hair.”

The results also suggest that shifts in eye colour are under genetic control.
Identical twins tended to change eye colour in the same way. But for fraternal
twins, who are no more similar genetically than other brothers or sisters, it
was more common for one to develop darker eyes while the other’s became
lighter.

Eye colour is currently listed on US driving licences. Bito and his
team—who tried, unsuccessfully, to conduct their research from driving
licence records before turning to the Kentucky twins—say that their
results show that it is of little value as an identifying feature.

IT’S not just milk that turns sour when lightning strikes. A geologist in
Athens, Georgia, has found that acid rain becomes nastier during thunderstorms,
when lightning generates reactive chemicals that speed up acid production in the
atmosphere.

Bruce Railsback of the University of Georgia discovered the relationship
while studying how the acidity of rainfall changes during a storm. “Studies of
rainfall pH have typically used sampling intervals of one day or
longer,” he says. “Variation of pH within rainfall events has received
little attention.”

Last year, during 14 storms in northeastern Georgia, Railsback gathered 132
samples of falling rain at intervals as short as 90 seconds apart. Many were
collected when thunder was rumbling in the sky, showing that lightning must have
flashed within about 25 kilometres of the sampling site, whether it was visible
or not.

In the current issue of The Science of the Total Environment (vol
198, p 233), Railsback reports that the samples he collected while he could hear
thunder had an average pH of 3.63. That was sharply more acidic than
the 4.05 average for rain that fell when the skies were silent.

Sulphur dioxide pollution from power stations, cars and lorries is the main
source of acidity in rain. When the gas meets highly reactive chemicals such as
ozone and hydroxyl ions, it is oxidised to form sulphur trioxide. Combined with
water, this gas forms sulphuric acid. Nitrogen oxide pollution from cars and
lorries also creates acid rain, when it is oxidised to form nitric acid. Both
acids fall in rain and snow.

Railsback says that the reason rain becomes more acidic during thunderstorms
is probably that lightning can increase the production of the reactive chemicals
that are needed to oxidise the pollutants. This fits in with earlier studies
that have suggested that lightning generates high levels of ozone. “Lightning
presumably contributes to the lower pH of summer rainfall in the
eastern US,” Railsback adds.

Railsback also noticed that the pH of samples collected during the
day was on average lower than for those collected during the night. This must be
because sunlight, which also speeds up the production of oxidants, makes rain
more acidic.

Railsback adds that in a single storm, the pH of rainwater could
change over periods of an hour or less. “There seems to be some process going on
that is faster than anything that we presently recognise,” he says. “I’ll be the
first to admit that I don’t know what it is.”

Sleep apnoea is caused by excess tissue in the palate, nose or tongue, which
blocks the throat during sleep, waking sufferers repeatedly during the night.
Previously, surgeons cut or burnt away the tissue, but now Nelson Powell, a
surgeon at Stanford University Medical Center, has developed a technique that
uses radio waves. A needle-like electrode is stuck into the offending tissue.
Here, says Powell, it emits oscillating radio waves that heat the tissue,
denaturing the proteins in it and killing the area.

During healing, compact scar tissue replaces the original flesh and, after
repeated treatments, enough shrinkage may occur to leave the airway open during
sleep.

The procedure worked well in tests on 19 pigs’ tongues. One shrank by 26 per
cent 10 days after an electrode administered 35 kilojoules of energy in one
20-minute treatment. Powell and colleagues reported their results in the May
issue of the journal Chest (vol 111, p 1348).

Powell is now testing the technique in 23 human subjects who snore deeply
during the night because of partly blocked airways but who have not yet
developed sleep apnoea. Powell says that early tests prove the procedure can be
carried out in 20 minutes under local anaesthetic: there have been few
complaints of pain or bleeding, either during the procedure or afterwards.

Obstructive sleep apnoea afflicts 4 per cent of American men and 2 per cent
of women. It has been linked to memory loss, high blood pressure, heart attacks
and strokes.

On 33 separate occasions in the 1950s and 1960s, researchers released
powdered zinc cadmium sulphide into the atmosphere to see how it would be
dispersed under a variety of meteorological conditions. The powder was chosen
because its particles are about the same size as bacteria used in biological
weapons.

When details of the experiments were widely publicised several years ago,
they caused a furore. At the insistence of Congress, the US Army asked the
academy to find out whether anyone had been injured by the experiments. The
academy’s report, released last week, concludes that zinc cadmium sulphide is
probably harmless. Even if the compound were as carcinogenic as pure cadmium,
the report says that human exposure to the chemical was so low that there is no
cause for alarm. In St Louis, Missouri, which experienced the highest levels of
contamination, the rate of excess cancers among children living in areas where
the chemical fell most thickly would be only 1.2 per million people—too
low to show up in health statistics.

The US Army restricted the study’s remit to health issues, so the thorny
question of whether the experiments were ethical is not addressed in the
report.

The experiments apparently revealed the most effective method for spreading
dangerous bacteria via biological weapons. But that method is still a military
secret. “That was the one piece of information the Army didn’t give us,” says
Rogene Henderson of the Lovelace Respiratory Research Institute in Albuquerque,
New Mexico, who chaired the study.

RUSSIA has finally has coughed up some of its share of the money for the
International Space Station. But Moscow’s tardiness will still delay the start
of scientific research on the station.

Representatives of the various national space agencies collaborating on the
project approved a new construction schedule for the station last week. There
were fears that the launch of the first portion of the station would be delayed
until October 1998, but following assurances from Russia the first launch is to
take place in June next year.

The delays affect the station’s service module, which is being built by the
Russian company Khrunichev Industries to provide living space for astronauts,
and a rocket system to keep the station from falling into the atmosphere.

According to Randy Brinkley, NASA’s space station project manager, the new
schedule means that large-scale scientific research will begin in September
2001, seven months later than planned, once a space shuttle delivers several
racks of scientific equipment to the station.

Brinkley says that NASA will try to compensate by conducting micrograviy
experiments aboard two shuttle flights. This cuts little ice with Martin
Glicksman, a materials scientist at the Rensselaer Polytechnic in Troy, New
York, who follows NASA’s microgravity research programme for the National
Academy of Sciences. “It isn’t much,” he says.

Even if the service module suffers no more delays, Russia’s shortage of cash
may affect science on the station in another way. From next year, Russia is
supposed to finance construction of three research modules to be attached to the
space station in orbit, and Brinkley admits he has no guarantee that these
modules will be completed on time. “I’m just trying to get through 1997,” he
says.

AMATEUR rocket scientists have joined the space race. Last Sunday, the L5 Society of Huntsville, Alabama, part of the National Space Society, fired a 2-metre rocket to an altitude to 70 kilometres. “This is the edge of space,” says Gregory Allison, the project leader.

The rocket, called High Altitude Lift Off (HALO), was launched from a balloon to take advantage of the low air resistance at high altitude. It was a “hybrid” design, in which the solid fuel and oxidiser are mixed only at the moment of combustion. Hybrid rockets are safer than those in which the fuel and oxidiser are stored together.

Balloon-launched rockets, otherwise known as “rockoons” can reach greater heights than would be possible with the same rocket launched from the ground. HALO’s organisers had intended the balloon to rise to about 32 kilometres before the rocket was released. Unfortunately, at about 18 kilometres, a camera on the rocket revealed that one of the balloon’s seams had ruptured.

Ground controllers immediately fired the rocket, which reached around 70 kilometres above the Earth before plunging into the Atlantic Ocean. If the balloon had not come apart, the rocket should have reached an altitude of 119 kilometres—unequivocally in space.

Rockoons were once the cutting edge of space technology. But by the mid-1950s the US and the Soviet Union had developed intercontinental missiles that could double as space rockets. Allison says the HALO project may revive rockoons as a cost-effective way for researchers to study space. NASA seems interested, and has contracted the HALO team to conduct two rockoon launches from the Gulf of Mexico.

GPS satellites broadcast two signals. One is available to anyone, and the other is encrypted so that it is accessible only to the military. By measuring the time delay between signals from different satellites, civilian GPS receivers can generally calculate location to within 100 metres—although if signals are rebroadcast from a fixed receiver at a known position, it is possible to narrow this down to a few metres. According to the Pentagon, military GPS receivers are accurate to within about 20 metres, even without this refinement, known as differential GPS.

However, Stan Huntting, the author of a program called SA Watch, which estimates the error in the GPS signals, says that on 20 April civilian GPS receivers were providing locations with an accuracy of as little as 2 metres without the help of rebroadcasting.

The US Air Force Space Command, which operates the GPS satellites, confirms that the usual intentional inaccuracy in the civilian signal—a feature known as “selective availability”—was deactivated on 20 April. But Don Miles, a lieutenant-colonel and spokesman for the space command, refuses to say why.

One strong possibility, however, is that the satellites were being used to aid the search for an A-10 ground attack aircraft that disappeared during a training exercise in Arizona earlier in the month. Both civilian and military planes were scouring a mountainous region of Colorado for wreckage, and giving all of the aircraft involved access to the same high-quality satellite navigation signal would have helped speed the search.

Indeed, Huntting notes that shortly after the publicly available signal was returned to its usual level of accuracy the Pentagon announced that the wreckage had finally been located.

The Pentagon, which has had to rely in part on “fuzzy” civilian GPS receivers because of a shortage of equipment that can receive the military broadcasts, also switched off selective availability at the start of the Gulf War to improve the civilian signal. “In all the major conflicts we’ve had to date, they’ve turned it off,” says Laurence Adams, a former president of the aerospace company Lockheed.

Adams also chaired a National Research Council panel which argued in 1995 that selective availability should be permanently deactivated (This Week, 10 June 1995, p 8). President Bill Clinton’s administration has promised to do this, but has not yet set a date.

Civilian GPS users were eagerly discussing the 20 April incident last week on the Internet, and are arguing that the Pentagon should be more forthcoming about the technical capability of its navigation system.