ASTRONOMERS in Europe are now hoping to obtain most of the observations
they had originally planned from the Hipparcos satellite, even though the
satellite – designed to map the heavens in unprecedented detail – remains
marooned in the wrong orbit as a result of a booster failure during its
launch last month.
The scientists say that even in its present orbit, Hipparcos may be
able to measure star positions to about half the precision originally planned.
This is still considerably sharper than anything that telescopes on the
ground can achieve.
In August, an Ariane rocket launched Hipparcos, exactly as planned,
into a highly elliptical orbit that stretches from about 200 kilometres
above the Earth’s surface to around 35 900 kilometres. A small rocket on
board the satellite should have fired at the highest point, to boost Hipparcos
into a circular orbit with a period of 24 hours. From here, it would have
beamed its data continuously to astronomers at a ground station at Odenwald
in West Germany.
But this rocket failed, despite several attempts to ignite it (This
Week, 19 August). The European Space Agency has now abandoned attempts to
fire the rocket, and plans to operate Hipparcos in its current, highly elliptical
orbit.
Advertisement
The first problem was that at the lowest point of its orbit, Hipparcos
was running into the top of the Earth’s atmosphere. Friction with the gases
would have brought the satellite so low that it would have burnt up within
a few months. Fortunately, Hipparcos carries small thrusters, fuelled by
32 kilograms of hydrazine. In the original mission, controllers would have
used these thrusters to push the satellite to exactly the correct position,
and to alter its direction and its rate of spin. As it was, they were able
to raise the lowest point of the orbit to 500 kilometres, ensuring that
Hipparcos will stay in orbit, clear of the atmosphere, for several years.
Hipparcos currently orbits the Earth every 10 hours 37 minutes. The
antenna in West Germany that was intended to keep continuous contact with
the satellite can communicate with it for only one-third of the time. The
European Space Agency has now assigned another antenna, near Perth in Australia,
to track Hipparcos, so the craft is in communication with Earth for nearly
two-thirds of the time. The agency is currently negotiating with France
to use its antenna at Kourou in French Guiana to raise coverage to 81 per
cent. It is also thinking of using a fourth, mobile, antenna to fill the
gap.
The main effect of the elliptical orbit on the spacecraft itself is
to drag it into the Earth’s ‘radiation belts’. The charged particles in
these belts produce electric currents in the satellite’s electronic light
detectors, so making it difficult to distinguish the signals from faint
stars. The team working on Hipparcos has been measuring this ‘dark current’
and has found it to be less severe than expected. The best predictions suggested
that Hipparcos could observe stars only when it was more than 10 000 kilometres
above the Earth, but it now looks as though it will be able to operate at
altitudes down to 8000 kilometres. Margaret Penston, a member of the team
working at Britain’s Royal Greenwich Observatory, says that ‘ironically,
Hipparcos is providing us with new data about the radiation belts’.
The major problem, however, is that the particles in the radiation belts
damage the solar panels on which Hipparcos depends for power. The mission
controllers thought at first that the solar panels might last only a few
months. But the latest measurements suggest that the panels might last for
up to 18 months.
Although this is shorter than the 30 months of the mission as it was
originally planned, it will allow astronomers to complete one of the mission’s
most important objectives. This is to measure the distances to the stars
from their ‘parallaxes’ – the small shifts in their apparent positions caused
by the Earth’s yearly orbit around the Sun. Astronomers need to observe
a star’s position for at least a year to measure the parallax accurately,
and to separate this apparent motion from the actual, but slow, motion of
the stars across the sky.
Even on the pessimistic assumption that Hipparcos will last only until
February, when controllers must abandon measurements for a couple of months
as the satellite’s orbit takes it into the Earth’s shadow, it should be
able to measure the positions of 65 per cent of the intended 120 000 stars
with much higher precision than is possible from the Earth. In nine months,
it can measure 90 per cent of the stars. In neither case can it measure
parallaxes directly, although astronomers can derive some parallaxes indirectly
by combining the measurements from Hipparcos with observations from Earth.
But things are much better if the solar panels last 18 months. Hipparcos
will then be able to reveal the distances to stars up to 800 light years
away, as compared to 300 light years for observatories on Earth.
