91av

Between late November and mid-December, astronomers in California and
Puerto Rico are closely monitoring an asteroid about 5 kilometres across
called 4179 Toutatis as it passes within a few million kilometres of Earth.
They hope to obtain images of this small rocky object that are as good as
the snapshots of another asteroid, Gaspra, sent back by the Galileo spacecraft
earlier this year. But the images of Toutatis will be produced not by optical
telescopes or cameras, but by radar.

Radar systems have been in use for about fifty years. They work by bouncing
microwaves off solid objects and measuring the time it takes for an echo
to return. The first efforts to send microwaves beyond Earth and detect
the tiny return signal began in 1945. For the first time, meteorite showers
were detected and studied in broad daylight, when they are normally invisible.
The following year, the US Army Signal Corps bounced radar signals off the
Moon. But the limitations of early radio telescopes and the vast distances
to other planets made progress relatively slow, and it was 1958 before the
first echoes were received from our nearest planet, Venus. In the 1960s,
radar techniques established several important astronomical measurements,
including the distance from Earth to Venus, used in turn to determine the
distance from Earth to the Sun (the so-called astronomical unit), and the
rate of rotation of Venus.

Since then, radio telescopes have become more sensitive, and powerful
computers and sophisticated software have made it possible to analyse the
complex radar echoes that they receive. Using ground-based radar, astronomers
can now detect features as small as 120 kilometres across on Mercury, 70
kilometres on Mars and 1.5 kilometres on Venus.

Nearly 70 asteroids have been detected by radar since 1968. Steven Ostro
of the Jet Propulsion Laboratory in Pasadena, California, believes that
as more of these asteroids are discovered by modern optical search networks
such as the Spacewatch group in the US, astronomers will turn to radar for
information about their size, orbital characteristics and rotation rates.
Radar could even reveal what they are made of.

Ostro has already had some spectacular successes. After five years of
analysing data, his team last year announced some striking conclusions about
a 2.5-kilometre asteroid called 1986 DA, which came within 20 million kilometres
of Earth in 1986. It had the greatest radar brightness ever observed, reflecting
the radar waves so intensely that Ostro concluded it must be made of metal.
This was supported by observations from optical telescopes of its colour
and other spectral properties. Ostro believes this unusual object to be
the first near-Earth metal asteroid ever discovered. It is thought to be
made of iron, nickel and other metals such as platinum and gold – perhaps
containing 100 000 tonnes of precious metals. Three years ago, Ostro obtained
the most detailed radar images yet of an asteroid, using the world’s largest
radio telescope at Arecibo in Puerto Rico. This 3-kilometre chunk of rock,
now known as 4769 Castalia, turned out to be two irregular boulders welded
together like a pair of strung beads.

In their observations of Toutatis this month, Ostro and his team expect
to pick out details only 160 metres across. They hope from this to dis-cover
the asteroid’s shape, how fast it is spinning and what its surface is like.
Toutatis will pass much closer than 1986 DA – within a few million kilometres,
or about 10 lunar distances. It approaches Earth every four years and it
is considered by space scientists and astronomers to be a prime candidate
for a future flyby or rendezvous mission. Over a period of three weeks,
Ostro intends to use the large Goldstone antenna in California to transmit
a signal with a wavelength of 3.5 centimetres. Echoes are being received
by the 27 linked radio telescopes, each 25 metres across, which are spread
out over the high prairie of New Mexico to form the Very Large Array (VLA),
so providing an effective antenna diameter of 36 kilometres. From these
echoes, he hopes to produce a series of images that will make up the world’s
first asteroid movie.

MOVING TARGET

Before such observations can be made, radar astronomers have to locate
their target precisely, otherwise the narrow beam of the radar signal will
miss it. This is particularly important for asteroids. The intense microwave
beam that will ‘illuminate’ the asteroid can be transmitted for no longer
than it takes for the round trip. Echoes are received for about the same
time. Toutatis will be so close that the round trip time for the signal
will only be about 25 seconds, much shorter than for most radar observations.
To cope with this, the Goldstone system has been designed to switch quickly
from transmitting to receiving. Observations made from Arecibo are not expected
to be as accurate because the telescope is farther south than is ideal,
and cannot be turned like a traditional dish antenna.

The pattern of the returned signals is being recorded on magnetic tape.
The echo’s frequency is changed or ‘Doppler shifted’ by the movement of
the asteroid relative to the radar’s ‘line of sight’, so either the transmitter
or the receiver must be continuously retuned. The echo is also ‘Doppler
broadened’ by the asteroid’s rotation, and this can reveal information about
its size and rate of spin. The echoes vary according to the nature of the
surface. Single back-reflections usually indicate surface features that
are much larger than the radar wavelength. There may also be multiple reflections
or single scattering from features such as buried rocks that are closer
in size to the wavelength of the signal. By picking out these components
from the complex reflected signal, astronomers can work out the roughness
of the surface, its general shape and, occasionally, its composition.

Mapping slow-moving, distant planets provides a different challenge.
By the time the radar beam reaches its target many millions of kilometres
away it has spread out enough to cover the entire ‘visible disc’ of the
planet. The strongest return signal is usually obtained from the ‘subradar
point’ – that is, the point directly between the radar source and the centre
of the planet or asteroid. The intensity of the echo drops off sharply away
from this point. A rapidly rotating target will also smear the image, just
as in a photograph, so special techniques are needed to compensate, unless,
like Venus and Mercury, the planet is rotating very slowly.

Fixing the location of a feature on the surface requires precise analysis
of the time delay between transmission and reception and its Doppler shift.
Signals from the edge of the planet’s visible disc arrive slightly later
than those reflected from the centre; the amount of frequency shift due
to planetary rotation varies with latitude and longitude. Scientists can
map the planet by dividing it into ‘cells’ of a particular distance, or
‘range’, and frequency shift. Such traditional range-Doppler techniques
use the same antenna to transmit and receive. With this arrangement it is
not possible to distinguish between two cells located symmetrically on either
side of the Doppler equator, because these cells have the same range and
the same frequency shift. Only by comparing images obtained from different
viewing angles or by combining two or more telescopes and separating the
transmitter from the receiver can this be overcome – a technique known as
bistatic radar.

Radar maps of Mercury made using both techniques provided some surprises
last year. Mercury is one of the most inhospitable planets in the Solar
System. The Sun blazes down on its barren, cratered landscape, raising the
temperature to 430 °C, twice as hot as a typical domestic cooker. It
hardly seems the sort of place to find water ice. But this is just what
American scientists said they had found in August 1991, when a team led
by Duane Muhleman of the California Institute of Technology succeeded in
obtaining the first detailed images of Mercury’s lesser-known hemisphere,
which at that time was almost directly facing Earth. Most of our knowledge
of Mercury is derived from data sent back almost 20 years ago by the Mariner
10 spacecraft, and these pictures are of only one hemisphere. Until recently,
little was known about the planet’s other side.

Muhleman’s team illuminated Mercury continuously for eight hours with
a 500-kilowatt signal at a wavelength of 3.5 centimetres using the 70-metre
dish at Goldstone. The faint echoes were picked up by the VLA. The radar
images, which revealed detail as small as 150 kilometres across, were dominated
by an apparently elliptical feature between 300 and 600 kilometres across,
quite close to Mercury’s north pole. A comparison of these images with similar
studies of the ice on Mars and of Jupiter’s satellites led the researchers
to the conclusion that these prominent echoes were coming from a very cold,
highly fractured layer of water ice.

Additional studies by the same team, in March 1991 and March this year,
using the Arecibo observatory, indicated a similar, though smaller, feature
close to the planet’s south pole. According to Martin Slade of the Jet Propulsion
Laboratory, this radar-bright patch corresponds in Mariner 10 pictures
to a crater 150 kilometres in diameter called Chao Meng-Fu. Slade is the
first to agree that polar icecaps are not very likely on Mercury. ‘If the
ice could be continually replicated, there would be no difficulty in explaining
the presence of the ice, but it has to be an almost permanent feature. For
this to be the case, there needs to be a temperature of around -160 °C,’
he says.

ICE ON MERCURY

But some astronomers believe such low temperatures could exist at Mercury’s
poles. The planet spins on an axis almost at 90 degrees to its orbital plane,
so at its poles the Sun never appears far above the horizon. Where the planet’s
surface is rough, a zone of permanent shadow might be created, particularly
on the floor of a large crater such as Chao Meng-Fu. Calculations by David
Paige and his colleagues at the University of California at Los Angeles
suggest that temperatures in such regions may plunge low enough to maintain
an icecap.

Another possibility is that the substance that is concentrated at the
poles is sodium. Ions of sodium have been detected on Mercury, and it is
conceivable that they could have been precipitated onto the poles along
magnetic field lines. But Slade and his colleague John Harmon of the National
Astronomy and Ionosphere Center at Arecibo say it is doubtful whether there
is enough sodium for this to have happened.

So where could the ice have come from? Volatile substances such as water
could be driven from the rocks as they boil under the midday sun. Such gases
would then either escape into space or settle as ‘snow’ in the cold polar
regions. Further supplies of water and carbon dioxide ice could be brought
at regular intervals by comets and asteroids, which vaporise on impact with
the planet. Such ice might remain intact for a long time, according to Slade.
‘Since radar can see through loose surface debris, it is possible that the
ice is buried,’ he explains. A blanket of material, perhaps 50 centimetres
thick, could prevent the icecap from melting and from being eroded by interstellar
radiation.

The radar maps also reveal areas which are thought to be huge impact
basins. Two of them have diameters of about 800 kilometres and lie close
to the same longitude (350 degrees West) but on opposite sides of the equator.
A similar feature is found at 13 degrees North between 230 degrees and 250
degrees longitude. There is also a large kidney-shaped patch near the equator
at about 300 degrees West which seems to coincide with a plateau 2.5 kilometres
high. Radar images from Arecibo show it to be heavily cratered, whereas
a lower region to the east has a smooth appearance.

The planetary surfaces invisible to radar can be almost as revealing.
The first detailed radar maps of Mars were obtained in 1988 by Muhleman’s
team using Goldstone and the VLA. The polar icecaps and volcanoes of the
Tharsis and Elysium regions show up clearly but there is a huge blank on
the maps which displays no noticeable echo because almost all of the signal
is absorbed. Appropriately christened ‘Stealth’ by Muhleman’s team, the
region starts in the Tharsis region and extends about 2000 kilometres to
the west. It seems to be covered in loose windblown ash from the nearby
volcanoes, which absorbs the radar signal. Several other ‘stealthy’ regions
have also been uncovered by radar, all in the southern hemisphere.

TITANIC REFLECTIONS

The greatest challenge for radar astronomers is Titan, Saturn’s largest
moon (see ‘A moon with atmosphere’, 91av, 28 September 1991). Its
surface is permanently hidden at visible wavelengths by a dense covering
of organic smog. Because it is so far away, the VLA can only pick up a return
echo of 10-22 watts, which is close to the limits of detection. Nevertheless,
radar observations with Goldstone and the VLA over the past three years
have led Muhleman to conclude that Titan does not always keep the same hemisphere
towards its home planet, as had previously been believed. One very radar-bright
region consistently appears 15 hours earlier than expected, suggesting that
its rotational period is 49 minutes shorter than its orbital period of 15.945
Earth days. Muhleman and his team say this could be an effect of Titan’s
large orbital eccentricity if its interior is not strong enough to support
a permanent bulge.

Even more significant are the variations in radar reflectivity – some
areas reflect a lot of signal, others less – which give the first indications
of surface conditions on Titan. Results from instruments on the Voyager
spacecraft in the 1980s suggested that there might be a global ocean of
liquid ethane. However, Muhleman’s group report that the most that will
be found by the European-built Huygens probe-scheduled to land on Titan
early next century after a journey aboard the Cassini spacecraft – are a
few patches of liquid. The moon’s surface seems to be covered mainly by
icy continents, perhaps coated in tars or other hydrocarbons. Muhleman hopes
eventually to map some surface features to help with the design of the probe.

Closer to Earth, a powerful radar system called AMOR was unveiled earlier
this year by a team of astronomers led by Jack Baggaley of the University
of Canterbury in New Zealand. AMOR is the first radar system designed to
produce detailed orbit information about individual meteoroids down to a
size of 100 micrometres. The 20-kilowatt instrument sends pulses every 2.5
milliseconds which bounce off the ionisation trails of meteors as they burn
up 100 kilometres above the Earth. Using a network of receiving stations,
the team has already been able to calculate the velocity and direction of
travel of more than 200 000 meteors, and 1500 new orbits are computed each
day.

Radar astronomers at Goldstone and Arecibo want to improve the performance
of their own instruments to a level that will produce images with a resolution
of less than 50 metres for at least one or two near-Earth asteroids every
year. ‘Radar promises a revolution in astronomy,’ Ostro declares. ‘Such
research is equivalent to what the people in the lead ships in Columbus’s
fleet experienced when they saw new land.’

Peter Bond is a freelance writer based in Sheffield who specialises
in space exploration and astronomy.