快猫短视频

MARK BIDWELL lay on his back on a plank, arms outstretched. Only a metre
separated his plank from the one and a half tonnes of highly explosive fuel in
the Intelsat 702 communications satellite above his head. Bidwell鈥檚 precarious
position was unexpected鈥攏o one had ever mentioned it during the planning
meetings . . .

Putting small experimental satellites into space as piggyback riders with
much larger payloads can be wonderfully cost-effective. But, as Bidwell and his
colleagues were discovering, the complications just seemed to keep on
coming.

The team, from the Defence Evaluation Research Agency in Farnborough,
Hampshire, had built two small satellites the size of TV sets to ride with the
Intelsat payload. Now, only two weeks before launch, the team supervising
integration of the spacecraft into the nose faring of the Ariane 4 rocket had
discovered an unforeseen glitch with the potential to destroy the
multimillion-dollar mission.

Several months earlier, mock-ups of Intelsat 702 and DERA鈥檚 two hitchhikers
were joined to a mock-up of the launch structure to check their fit. 鈥淭he test
was to ensure there was sufficient clearance between the antennas and the base
of the main payload,鈥� says Nigel Wells, spacecraft manager for one of the two
satellites鈥攌nown as Space Technology Research Vehicles (STRVs) 1a and
1b.

The check showed that the short antennas on the STRVs were too close to the
thermal blankets on the underside of the main satellite. Because of trapped air,
thermal blankets tend to swell as pressure drops during a launch, and if the air
doesn鈥檛 escape fast enough, the blankets could crush the antennas. So
Arianespace, the company that launches Ariane rockets, decided the antennas had
to be moved. The DERA team duly moved them roughly two centimetres so that they
were well clear of the thermal blankets in the mock-up.

But when the real spacecraft were mated to the launch vehicle, the DERA team
got a nasty shock. The mock-up of Intelsat 702 had been inaccurate and the team
discovered they had unwittingly moved the antennas to an even worse position
than before. When the problem was discovered at the launch site there was time
only to reduce the length of the masts supporting the antennas. A 40 per cent
reduction was needed, and that would give a clearance of a mere 8
millimetres.

Back in Farnborough, more than 7000 kilometres away, the British team
scrambled to find out what the effects of the antenna alteration would be. They
worried that the antenna would now be so close to the body of the STRV that
reflections might alter or block its signals. If its signals were too weak,
there was no option but to take the spacecraft off the launcher and wait an
uncertain time for another suitable ride into space. There was no question of
delaying the Ariane 4 launch to accommodate DERA鈥攖he interests of the
owners of the main payload always take precedence over piggyback riders.

DERA calculated that the mission would be feasible as long as the team had
regular access to the powerful antennas of NASA鈥檚 deep space network (DSN).
These dishes would need to take over the task of receiving the STRV data when
their signals became too weak to be picked up by the ground antenna at DERA
Lasham. Fortunately, the spacecraft were carrying experiments for the US
Ballistic Missile Defense Organization, which had already negotiated access to
the DSN as a backup.

A pint of beer

That is how Bidwell came to be strapped to the plank. He was reaffixing the
antennas to the two small satellites, having cut away 40 per cent of their
supporting masts. Despite the last-minute problems, STRVs 1a and 1b were
launched in the summer of 1994 at a cost to DERA of 拢8 million, a bargain
by the standards of space research.

The STRV vehicles proved to be highly successful in their study of radiation
in space, and demonstrated the flexibility and lower financial risks made
possible with small satellites. Small satellites can be designed, built and
launched more quickly and cheaply than large satellites, and the losses
associated with a launch failure are lower.

Like so many good ideas, the STRVs were first conceived over a pint in the
local pub. DERA鈥檚 young engineers, including Wells and Neil Wallace, the
spacecraft engineer for STRV 1a, were fresh from university and had been
recruited to build technologies that could work in space. Why, they asked
themselves, shouldn鈥檛 they build a satellite to carry their experimental
technologies into space. The launch could be cheap, they reasoned, because their
small satellites could hitch a ride into orbit.

There is almost always spare room on a rocket. Primary payloads such as
Intelsat 702 rarely fill the space in the nose cone entirely. And because a
rocket鈥檚 lift capacity is increased incrementally by the addition of
solid-rocket boosters, the total thrust is often more than is required. Often a
dummy mass must be added to the final stage of the rocket to prevent it
overshooting its intended orbit. This spare mass and volume can be taken up by
small hitchhiker satellites. Arianespace has even designed a special structure
to carry them.

Radiation belt

Another important aspect of the Intelsat launch was its planned geostationary
orbit at an altitude of 36 000 kilometres. At this height, a single orbit takes
24 hours so the satellite remains permanently above the same spot on the Earth.
Geostationary orbits are ideal for large communications satellites relaying huge
amounts of data, and for some types of Earth-observing satellites.

To achieve geostationary orbit, a launcher first places its satellite into a
highly elliptical orbit called a geostationary transfer orbit. This has an
apogee (the point farthest from the Earth) of at least 36 000 kilometres, and a
point of closest approach, the perigee, of only a few hundred kilometres. The
satellite then uses its own thrusters to change its orbit into a circular one.
In moving from perigee to apogee, the satellites must pass through the Van Allen
belt, where radiation levels are extremely high.

Most owners want to get their satellites out of this region of space as
quickly as possible because radiation can damage electronic equipment. But for
DERA鈥檚 engineers, the geostationary transfer orbit was exactly what they wanted.
It gave them two opportunities. First, they could map the radiation environment
of the Earth more accurately, thereby acquiring data to update computer models
of radiation belts. These models allow engineers to calculate the amount of
shielding required by electronic systems on spacecraft flying through the belts.
This orbit would also allow the team to expose prototype computer memories and
microprocessors to high levels of radiation and follow their performance and
survival under these harsh conditions. Not surprisingly then, the mission
attracted experiments and funding from civil and military backers, including
Britain鈥檚 Ministry of Defence, the US鈥檚 Ballistic Missile Defense Organization
and the European Space Agency.

The STRVs sent back lots of valuable data. It turns out, says Wells,
that engineers have overestimated the harshness of the radiation environment and
have probably overdesigned the electronics in some cases. The STRV vehicles also
gave DERA鈥檚 engineers an opportunity to hone their skills as systems engineers
while building satellites. 鈥淲e were in at the deep end with lead water wings and
had to learn how to swim,鈥� says Wells.

Each satellite weighed 55 kilograms and was shaped like a cube with sides of
45 centimetres and carried aloft 14 experiments altogether. All bar one worked
and provided valuable data. And that failure is another example of the things
that can go wrong at the last minute.

Highly charged

The experiment was designed to test an idea to reduce the electrostatic
charge that can build up on spacecraft. Some structures are made from insulating
materials, such as the glass that covers solar panels. If charged particles
strike these materials, the charge builds up, and if left unchecked it can
result in potential differences of kilovolts between different parts of the
vehicle. Eventually, arcing begins, often damaging delicate electronic
components and bringing a mission to a premature end.

DERA鈥檚 idea was to envelope the spacecraft with a cloud of ions and
electrons, a low-energy plasma. Any build-up of positive or negative charge on
the spacecraft would then be neutralised by either electrons or ions from the
cloud. To test the effectiveness of this method, the researchers planned to
measure the change in an electric field near a piece of insulating plastic film
by using two crystals that alter the polarisation of a beam of light depending
on the electric field applied to them. One crystal was shielded from the
electric field with foil. By comparing the two beams after they had passed
through the crystals, the scientists could measure the strength of the electric
field that was deforming the unshielded crystal.

Despite the ingenuity of the experiment, it never reached orbit intact. Just
before launch, Intelsat insisted that DERA give the STRVs a more rigorous set of
vibration tests. During the test, one of the crystals shattered. Although the
crystal was replaced, there was no time to recalibrate the instrument and
correctly align the crystal with the laser. Nobody yet knows whether satellites
can be protected from a build-up of charge in this way.

Despite frustrations such as these, Wells, Wallace and Angela Cant, who was
responsible for assembly, integration and testing of the spacecraft, enjoyed the
experience. To the amusement of their more senior colleagues immersed in work on
DERA鈥檚 large military satellites, the team took their satellites in the back of
a Ford Transit van into deepest Surrey late one night to test their radio link
with the receiver at DERA Lasham. The test did not take long, but their van got
stuck in a bog and took nearly 12 hours to extract.

Aborted launch

As the launch date approached, tension mounted. The team had already waited
nearly six months while Arianespace investigated the failure of a previous
flight. As the launch date approached that June, team members who had remained
in Farnborough gathered to watch a live video feed from French Guiana. Wallace
recalls looking steadfastly at the ground and paling as the countdown reached
zero and then moved inexplicably into positive numbers. The launcher remained,
steaming, on the pad. Martin Sweeting, of the University of Surrey and doyen of
the small-satellite community in the UK, said into the silence: 鈥淓ither it will
blow up, or nothing will happen.鈥�

The rocket did not explode, but the launch had to be aborted. The release
mechanism that should have freed the rocket from the launch pad had failed. Two
weeks later, the team gathered again, and the satellites were lofted
successfully at 07:07 Universal Time on 17 June 1994. 鈥淭here was no champagne or
cheering,鈥� says Cant. The team鈥檚 main concern was whether they would receive
telemetry from their shortened antennas. They did. Buoyed by the success of
their first small 鈥渢echnology demonstrator鈥� satellites, DERA is developing STRVs
1c and 1d. A launch aboard Ariane 5 is planned for mid-1999.