OUR last visit to Jupiter was a suicide mission. Five years ago, the Galileo
probe parachuted into the planet鈥檚 stormy atmosphere, and for about an hour it
floated downwards, studying the alien sky around it. Finally, 150 kilometres
below the cloud tops, the intense heat and pressure turned it to scrap.
Next time around it could all be different. A new type of probe could spend
months or even years skimming through Jupiter鈥檚 clouds. Cruising at supersonic
speeds, it would circle the planet in just four days, taking in all the most
interesting spots en route. This single visit could unravel the atmospheric
secrets of the entire planet.
Such a mission would be impossible using a conventional rocket engine, as no
craft could carry enough fuel. Instead, the probe will suck in gas from the
planet鈥檚 atmosphere, heat it, and spew it out of the tail to create thrust, like
an ordinary jet engine. There鈥檚 just one difference: this engine will run on
nuclear power.
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Right now, NASA isn鈥檛 keen to launch a craft carrying nuclear material for
fear of the public鈥檚 reaction. Yet some scientists鈥攂oth inside and outside
the agency鈥攁re convinced that nuclear-powered rockets are the way ahead.
鈥淭here are a lot of people in NASA who are interested in this,鈥� says John Cole,
manager of space-transportation research at Marshall Space Flight Center in
Huntsville, Alabama. 鈥淪ome of the missions require more energy density than we
can get out of chemical propellants.鈥� If nuclear propulsion comes good, it could
soon be blasting missions to the distant corners of the Solar System far faster
than ever before.
But engineers working on nuclear propulsion first have to prove that the
technology works. Enter the 鈥渘uclear flyer鈥濃�攆unded by NASA鈥檚 Institute for
Advanced Concepts, it鈥檚 a paper project that tests many of the key ideas needed
to get a nuclear craft into space.
The unique engine that powers it is the brainchild of James Powell, a 40-year
veteran of Brookhaven National Laboratory in Upton, New York, and former head of
its nuclear reactor systems division. While he was at Brookhaven, Powell and
other colleagues, including aeronautical engineer George Maise, became involved
in the effort to create a dependable, lightweight nuclear rocket.
Nuclear-powered rockets have some impressive theoretical advantages over
conventional designs. Rocket engines such as those that blast the space shuttle
into orbit use a chemical reaction to create hot gases. Expelling these gases
from the exhaust nozzle generates thrust. But even fuels such as liquid hydrogen
contain very limited amounts of energy, so rockets have to carry an awful lot of
the stuff to cover any great distance.
Nuclear engines, in contrast, use just a few kilos of fissile uranium to
generate huge amounts of heat. The thrust comes from a scorching jet of gas,
heated by the uranium and fired out of the back of the rocket. The propellant
doesn鈥檛 ignite the way it would in a conventional rocket鈥攊t simply expands
very rapidly and forces its way out, like steam whistling from a kettle. The
result is an engine with bags more 鈥渙omph鈥� than the most powerful chemical
rocket.
In the early days of the cold war, the US looked at nuclear engines as a way
to power missiles carrying nuclear warheads. Later, when costs escalated and the
space race began, the programme concentrated on using them to launch missions to
the Moon and Mars. It culminated in NERVA, the 鈥渘uclear engine for rocket
vehicle applications鈥�. But in 1972, the US abandoned the research after
$7 billion had been spent on the project.
The launch of the Strategic Defense Initiative in 1982 saw nuclear engines
revived, this time to power rockets designed to intercept incoming missiles. The
old NERVA engines were too heavy for the job, so researchers at Brookhaven
started developing a smaller engine called a particle bed reactor. But towards
the end of that decade the collapse of the Soviet Union and the end of the cold
war meant that funding dried up.
The work wasn鈥檛 dead, however. Powell and Maise saw the potential for this
kind of rocket engine in long-distance space exploration, and developed an even
smaller version, which they named the 鈥渕iniature reactor engine鈥�, or MITEE. 鈥淭he
thing about MITEE is that it鈥檚 very compact and lightweight,鈥� says Maise. 鈥淚t
can be as light as 50 kilograms.鈥� In 1997, they set up a small company called
Plus Ultra Technologies to develop MITEE鈥檚 potential.
The MITEE reactor consists of a bundle of cylindrical fuel elements which
each generate thrust. These elements are built from rolled-up metal sheets,
impregnated with particles of uranium oxide fuel. Each element is wrapped with a
moderator made of lithium hydride and beryllium, and the bundle of elements is
encased in a pressure vessel (see Diagram).
Three control rods in the core are rotated to switch the reactor on. As the
nuclear chain reaction begins and the fuel heats up, hydrogen is pumped into one
end of the pressure vessel, where it flows through tiny holes cut in the layered
elements. Heated to almost 3000 掳C, this super-hot gas races down the hollow
core and emerges explosively from the exhaust nozzle at the end of each
element.
In Powell and Maise鈥檚 plan, MITEE measures about a metre long and weighs
between 50 and 200 kilograms. Although its maximum thrust is less than a tenth
that of NERVA, it is much lighter, which makes it ideal for space
exploration.
MITEE wouldn鈥檛 be used to blast directly from Earth into space. Instead, a
conventional rocket would launch a MITEE-powered craft into orbit. Once there,
the reactor would be switched on and the mission would begin.
So what鈥檚 the pay-off for going nuclear? Consider a mission to Jupiter, for
example. Propelled by a conventional rocket, the Galileo probe took six years to
reach the planet, since it had to rely on Venus and Earth for gravity-assisted
鈥渟lingshots鈥� en route. A craft powered by MITEE, on the other hand, could set a
direct course and arrive in just two years.
But the MITEE engine has one drawback. The hydrogen propellant also cools the
reactor, so when the engine runs out of hydrogen, the fuel elements overheat and
melt. This isn鈥檛 a problem if all the craft needs is a single 鈥渂urn鈥� to take it
where it鈥檚 going, such as a fast fly-by of a distant planet, for example. If a
mission requires two burns鈥攐ne to get the rocket onto a trajectory and one
to slow it down once it arrives鈥攖he craft would have to be built with two
reactors in separate stages that can be jettisoned when their jobs are done.
But what if MITEE had an inexhaustible source of propellant gas, from the
atmosphere of a planet, for example? Drop a MITEE-powered craft into its clouds
and the engine would have almost limitless heat and propellant gas to cool the
engine and to provide the thrust鈥攅verything it needs to run on and on.
So the physicists at Plus Ultra Technologies have designed a MITEE-powered
probe to explore Jupiter. To ensure that their engine would have a sufficient
supply of high-pressure gas, they decided to operate MITEE like a simple type of
jet engine called a ramjet. Conventional jet engines squeeze incoming air with
compressor blades before it is mixed with fuel and burnt to create a hot blast
of exhaust gases. Ramjets, however, have no compressor blades. They rely on
their supersonic speed and a specially shaped air inlet to compress the air as
it rushes into the engine.
With virtually unlimited energy and propellant, the nuclear flyer would
cruise through the atmosphere like a shark in the ocean. And with no moving
parts apart from control surfaces on the wings, such a probe could operate for
years.
The design calls for a probe shaped rather like a small jet fighter. Just 1.9
metres long with a wingspan of 1.7 metres, the whole thing will weigh about 220
kilograms. It will carry a guidance system and communications instruments. This
kind of craft should be manoeuvrable enough to dive in and out of the clouds of
ammonia and water ices that cloak Jupiter. It could check out the chemistry of
Jupiter鈥檚 alien skyscapes and even swoop down into the Great Red Spot to chart
the structure of this giant storm, where wind speeds can reach several thousand
kilometres per hour.
To deliver this probe, a mother ship would carry it to Jupiter and drop it
from orbit into the planet鈥檚 atmosphere, where it would decelerate and jettison
its heat shield. Then, while still travelling at about 4000 kilometres per hour,
it would start its engine, using this forward speed to ram gas through the
reactor.
Carrying instruments similar to those mounted on the Galileo probe, it could
measure the atmosphere鈥檚 temperature, pressure and chemical composition, for
example. This information would be transmitted continuously back to the orbiter,
which would relay it to scientists on Earth.
鈥淚t would be excellent,鈥� says John Clarke, an astronomer at the University of
Michigan in Ann Arbor. 鈥淩ight now it鈥檚 very expensive to get any atmospheric
information. You put the probe in and you鈥檝e only got one sample.鈥滱 probe that
took measurements at different times and places would give us a vastly better
idea of the overall composition of the atmosphere, says Clarke. 鈥淚t would be
fascinating information,鈥� he adds.
Mission to Pluto
Since much of the research into nuclear engines has already been completed,
Maise believes that they could launch the nuclear flyer for about $800
million, and within just six years. Once the development work is completed,
MITEE could also find uses in all types of missions that are impossible with
chemical rockets. A flight to Pluto, for example, would take just five years,
compared with at least 10 for a conventional engine. Missions already at the
planning stage won鈥檛 even be able to decelerate and enter orbit. One, the
Pluto-Kuiper Express, will have to fly past at about 80,000 kilometres per hour.
A MITEE-powered craft could enter orbit and even drop a probe.
MITEE could also make a big difference on trips nearer to home. One of the
biggest concerns is that astronauts travelling between Earth and Mars might have
to spend a total of two and a half years in space, exposed to dangerous levels
of background radiation. Ironically, a nuclear rocket could halve the travel
time and reduce the dose that the astronauts receive.
Maybe the most revolutionary mission MITEE would make possible, Maise
believes, is a return trip to the outer planets鈥攁 probe that could land on
Pluto or a Jovian moon, for example. On the surface, it would take samples, use
the hydrogen bound up in surface ice to refuel, and then launch itself back
towards Earth.
Many scientists accept that regular, rapid trips around the Solar System are
almost impossible with current propulsion technology. 鈥淲e need better energy
sources. That鈥檚 all there is to it,鈥� says Bob Sackheim, assistant director for
space propulsion systems at the Marshall Space Flight Center. 鈥淚n my view,
anything past the Moon gets to be a problem with chemical rockets.鈥� Ed Weiler,
associate administrator for space science at NASA, agrees: 鈥淚f our children and
grandchildren are going to fulfil our destiny in space, there鈥檚 a simple law of
physics: you get more energy from nuclear reactions than you do from chemical
谤别补肠迟颈辞苍蝉.鈥�
Yet NASA may not launch a nuclear-powered craft for years. 鈥淚n the near term,
I鈥檇 say it鈥檚 very unlikely,鈥� admits Weiler. 鈥淭here鈥檚 a large concern in the
public with anything that has the word nuclear.鈥� The administration is only
willing to fund paper studies and tests that stop well short of going
nuclear.
Powell and Maise are applying for a further grant from NASA鈥檚 Institute of
Advanced Concepts that could give the project $500,000. After that it
will be a question of whether NASA gives them the thumbs up. No one is building
anything until the administration gets on board.
Maise argues that the public shouldn鈥檛 be worried about the idea of
nuclear-powered craft. The controversial planetary probe Cassini carried
highly radioactive plutonium, whereas MITEE would merely contain uranium fuel,
which is weakly radioactive and far less toxic. Only when the engine starts,
well clear of Earth, would it produce dangerous radionuclides, and the
likelihood is that these craft will only be used in regions that are already
racked with radiation.
Even if NASA backs the concept and can convince the public that nuclear
rockets won鈥檛 fall to Earth, it will still take a decade or more for a nuclear
rocket to be built. In the meantime, scientists like Powell and Maise continue
to keep the nuclear fire alight.