Geminga has intrigued astronomers since its discovery 20 years ago because
it emits intense gamma rays but virtually no radiation at other wavelengths.
In 1983, astronomers found a faint source of X-rays in the direction of
Geminga. It was close to a few very faint stars, one of which was bluer,
and so hotter, than the others. Although astronomers still had no firm idea
what Geminga was, they talked themselves into believing that it was this
star.

To find out more about it, Alain Smette of the European Southern Observatory
used the New Technology Telescope at La Silla, Chile, together with a sensitive
camera. The star is 100 million times fainter than anything visible to
the naked eye.

Smette recorded 10 exposures under good observing conditions. He then
processed the images to make a single composite photograph. At once, he
could see that Geminga was zipping across the sky relative to other stars
in the field of view. It was moving at 0.2 seconds of arc per year, which
is faster than any stars except a few near to the Sun.

Determining the distance to Geminga involve making a few assumptions.
Geminga has all the hallmarks of a neutron star, a collapsed star about
as massive as the Sun but just 20 kilometres in diameter. Astrono-mers know
the average velocity of isolated neutron stars from observations of these
objects at radio wavelengths. If Geminga is typical, Smette concluded, then
it must be 300 light years away. Certainly, the gamma ray intensity is roughly
that expected from a neutron star at such a distance.

So Geminga may be unusual only in that it is the closest neutron star.
At just 70 times the distance of the nearest normal star, it is a rich prize
for astrophysicists.

A cosmic gravitational lens occurs when light or radio waves from a
distant quasar are bent by the gravity of a galaxy between the quasar and
the Earth. This is a rare occurrence, requiring a near-perfect alignment
between the massive nucleus of a galaxy and a more distant quasar. When
this it happens, the image of the distant object can be transformed into
a ring or an arc or a complex multiple image. The first gravitational lens
was discovered by Jodrell Bank astronomers in 1979, but until last year
only 10 were known.

Then the Manchester team, led by Alok Patnaik, embarked on a systematic
search for new lenses. First, they selected radio sources which appeared
point-like from a ‘quick look’ survey by the Very Large Array of radio telescopes
in Socorro, New Mexico. This narrowed down their list of candidates from
900 quasars and active galaxies to 14.

The team then used Merlin to bear down on each object in the short list.
Merlin is an array of seven radio dishes scattered across the English Midlands
and connected permanently to form a single instrument that can see detail
as fine as a telescope 230 kilometres across would see.

Under Merlin’s piercing gaze, three compact sources shattered instantly
into blob-like mirages. One of them, B0218+357, has two core images, both
of which vary their brightness, and a broken ring of emission. Through optical
telescopes this source appears only as a faint reddish object at an unknown
distance.

Another of the new lenses, B1422+231 consists of four compact images
of a quasar about 15 billion light years away. The third lens, B1938+666,
has a peculiar arc structure plus a jet and four compact components. The
Manchester team announced these results in the latest issue of Monthly Notices
of the Royal Astronomical Society.

A distorted image can yield the mass of an intervening galaxy. According
to the Manchester team, lensing can be caused by all intervening matter,
whether it is luminous or dark. Lensing offers the best hope of detecting
this dark matter, which makes up at least 90 per cent of the matter in the
Universe.

The researchers also hope to measure the Hubble constant, which is linked
to the size of the Universe. ‘It can be determined for a lensed quasar if
the radio luminosity varies, as is often the case,’ they say. Time delays
have so far been seen in only one lensed system. In the latest trio the
Jodrell Bank astronomers pick out B0218+357 as an ideal candidate.

David Rabinowitz and his colleagues at the University of Arizona have
been carrying out a survey of small asteroids as part of their Spacewatch
programme. They announced their latest results this month at the Second
International Planetary Science Conference held in Munich.

It is the advent of charge-coupled device (CCD) that has enabled astronomers
to spot chunks of rock so near to home. Attached to the back of a telescope,
these electronic detectors can pick out objects that would not register
on photographic plates. For the Spacewatch survey, Rabinowitz and his colleagues
have been using a relatively small 92-centimetre telescope which is more
than 70 years old.

Prior to the latest observations using CCDs, most asteroids in our vicinity
had been found on photographic surveys carried out at Palomar Mountain in
California. However, no object smaller than about 300 metres across had
been spotted.

‘I applaud the Spacewatch efforts,’ says Eleanor Helin, who runs part
of the Palomar search. ‘There are far more very small objects than had been
predicted. Their discovery is really exciting because up until now we’ve
known nothing about such small objects near the Earth.’

Paolo Farinella of Pisa University in Italy says the near-Earth asteroids
cannot be relics from the origin of the Solar System, because such debris
would have been swept up by the Earth long ago. He suggests that catastrophic
collisions between bodies in the asteroid belt between Mars and Jupiter
may be responsible for a steady stream of rubble. He has simulated millions
of collisions between thousands of known asteroids, and has found that
plenty of fragments are flung our way.

Reassuringly, catastrophic impacts that cause global change are rare;
the last was possibly 65 million years ago. Nevertheless, asteroids measuring
between 30 and 100 metres across may hit the Earth once a century on average.
Paul Thomas of the University of Wisconsin and his colleagues have used
a computer to simulate the impact of a stony asteroid of the kind seen
by Spacewatch. He finds that such an object would disintegrate as an ‘airburst’
about 10 kilometres above the ground. An explosion of this type is thought
to have occurred above Tunguska, Siberia in 1908.

The unexpected abundance of near-Earth objects may also explain the
origin of meteorites, according to the Spacewatch scientists. They say
small asteroids in our vicinity may be important vehicles for bringing meteorites
to Earth.

The Spacewatch team now aims to find all asteroids that pose a definite
threat to life on Earth. If one is found, a way will have to be found to
destroy or deflect it.

The galaxy, called 10214+4724, was only matched with an intense source
of infrared light last year by Michael Rowan-Robinson of Queen Mary and
Westfield College, London. It shines with the light of 100 000 billion Suns,
and is 12 billion light years away. ‘It’s an absolutely fabulous galaxy,’
says Eric Becklin of the University of California at Los Angeles.

Usually, galaxies that are prodigiously bright are assumed to harbour
giant black holes deep within their nuclei. The energy these galaxies emit
is therefore gravitational energy released by matter as it falls into the
black hole. But in the case of 10214+4724, there is good evidence that the
ordinary nuclear reactions inside stars are sufficient to explain the galaxy’s
emission.

Phil Solomon of the State University of New York, Stony Brook observed
radio emission from carbon monoxide (CO), a gas commonly found in giant
molecular clouds, where stars are born. He found that the galaxy emits a
very strong radio signal, and from its strength concluded that it contains
the equivalent of 100 billion times the mass of the Sun in molecular gas,
the raw material needed to make stars. This is equivalent to the entire
mass of a normal spiral galaxy. In ordinary galaxies, the interstellar matter
accounts for between 1 and 10 per cent of the its mass.

Star formation is a self-regulating process in normal galaxies such
as the Milky Way. But in galaxies which emit copious infrared light, star
making is a runaway process. ‘Star formation is proceeding without birth
control,’ says Solomon.

The huge quantities of interstellar gas in 10214+4724 mean that nuclear
reactions inside new stars alone can create all the energy that the parent
galaxy is radiating. The light is absorbed by dust and reradiated, emerging
as infrared light.

Robert Brown of the National Radio Astronomy Observatory in Charlottesville,
West Virginia, also detected enormous quantities of carbon monoxide in 10214+4724.
His results indicate that the molecular gas in the galaxy is even greater
– 10 times as massive as the entire Milky Way.

The results point to 10214+ 4724 being a ‘protogalaxy’ of a kind that
astronomers have long wished to study. Such galaxies were producing energy
and heavy elements at a furious rate by the time the Universe was one-fifth
its present age.

The intense luminosity of this primeval galaxy means that astronomers
will be able to study it with a battery of ground based telescopes across
a broad range of wavelengths.

Most craters on Venus are either volcanic in origin or were caused by
the impact of meteorites, but the Magellan team reported finding plenty
of craters with strange shapes. Some seemed to have hardly any central crater
at all, but consisted of concentic discs which appeared dark in the orbiter’s
radar images. In contrast, a typical crater caused by the impact of a meteorite
is surrounded by bright discs of debris.

Kevin Zahnle of NASA Ames Research Center, California, has modelled
the motion of meteorites travelling hypersonically through the thick Venusian
atmosphere. He presented the results at the 23rd Planetary Science Center
in Houston, Texas.

‘Some meteorites explode because of frictional heating,’ says Zahnle.
‘A spherical blast wave expands, and I’ve worked out what happens when that
hits the ground.’ Zahnle says the blast wave, which is equivalent to that
from a million megaton nuclear explosion, shocks the surface rocks to a
depth of a kilometre and to a radius of between 15 and 30 kilometres. ‘Everything
is shattered to fine rubble,’ says Zahnle. Because this does not reflect
Magellan’s radar, this explains why the Venus orbiter registers a dark disc.

Magellan has found about 400 of these young ‘airblast’ craters. Zahnle
thinks that Venus probably has more than 1000. Further study of them could
also sharpen up estimates of how frequently objects of this kind hit the
Earth, he says.

Zahnle has included various kinds of incoming objects in the models:
dead comets, asteroids, and metallic meteorites. His studies have caused
him to speculate on the nature of the so-called Tunguska event of 1908,
in which an object entered the atmosphere above Siberia. It was too small
to have been a comet, and it would have hit the ground if it were a metallic
object, says Zahnle. ‘It was probably a dense stony object, such as an asteroid,’
he concludes. ‘It probably entered the atmosphere at low velocity and exploded
10 kilometres above the ground.’

Images of Mars taken by spaceprobes clearly show dry riverbeds and canyons.
The features strongly suggest that water once ran freely across the surface.
Today the Martian atmosphere contains only tiny amounts of water, so where
did all the water go? The most likely explanation, say planetary scientists,
is that most of Mars’s ancient oceans now lies beneath the surface of the
planet.

Gibson and his colleagues extracted a water drop from slices of six
meteorites obtained from museum collections. They presented the results
of their analysis of the water at the 23rd Planetary Science Conference
in Houston.

The six meteorites, all examples of a rare type known as SNC meteorites,
are believed to come from Mars because they contain noble gases in precisely
the same ratio as rocks on the Martian surface which were analysed by NASA’s
Viking lander.

Gibson’s team heated the meteorite samples – no more than few grams
– gently to extract any water trapped in micropores of the rock. They were
careful not to heat the rock too fiercely in case this forced water within
the silicates to break free of its chemical bonds. A few milligrams of water
were released, and was analysed

The ratios of oxygen isotopes in the water showed that it could not
have come from the silicates in the meteorites, because they display a different
isotopic ‘signature’. According to Gibson, the drop of trapped water contained
more of the isotope oxygen-17 than the water in silicates.

Gibson and his colleagues suggest three possible sources for the water
trapped in the meteorites. It could have come from a Martian ocean, from
water vapour in the Martian atmosphere, or from an icy comet that struck
the surface of Mars. Gibson says that the comet theory is ruled out because
the water is present in all six meteorites. An ancient Martian ocean is
the best explanation, he says (Science, vol 255, p 1409).

In another presentation at the Planetary Science Conference, French
space scientists outlined their plans to send robot explorers to Mars. These
would crawl over the surface up to 500 kilometres from a base station, and
one of their tasks would be to set up a seismic experiment to detect any
ice below the surface.

Gene Byrd of the University of Alabama was prompted to investigate NGC
4622 after seeing a fuzzy photograph in a textbook. He recognised a single
leading arm winding into a ring near the centre of the galaxy. The two outer
arms are normal.

Byrd’s colleagues, Ron Buta and Deborah Crocker, used a 1.8 metre telescope
in Chile to observe NGC 4622 at infrared wavelengths. In his part of the
spectrum the leading arm shows up as a clear pattern in the stars. The galaxy
is like a lawn sprinkler spraying water in two directions at the same time.

One possible cause of a leading spiral could be a tidal interaction
with a nearby galaxy. It might also happen if a high proportion of the mass
of NGC 4622 is in an invisible spherical halo, surrounding the galactic
disc.

The new pulsar was detected by the Compton Gamma-Ray Observatory, launched
into Earth orbit in April 1991. Although the pulsar also produces radio
and X-ray pulses, most of its energy is emitted as high-energy photons.

The two previous gamma-ray pulsars were discovered in 1969 and 1975.
Each emits two pulses of gamma-rays for every rotation of the pulsar, but
the new gamma-ray object, known as the Circinus Pulsar, gives out only one
burst per rotation.

Theorists now need to explain how the pulsar’s magnetosphere can generate
such photons with such high energy. ‘Observations are expected eventually
to reveal pulsed energy at even higher energies,’ says Robert Wilson of
the Marshall Space Flight Centre.