A commonly accepted theory is that the asteroid belt is the rubble left over from a “proto-planetary disk,” the dense ring of gas that surrounds a new-born star.

But the orbiting rocks have long been a source of deep curiosity. They are remarkably varied, ranging from mixtures of ice and rock to igneous rocks, which implies they have jumbled origins.

The answer to the mystery, according to a study published by the British journal Nature on Wednesday, is that a “significant fraction” of the asteroid population in fact comprises ex-comets.

Famously described as “dirty snowballs” of ice and dust, comets are lonely, long-distance wanderers of the Solar System whose elliptical swing around the Sun can take decades.

Researchers in France and the United States ran a mathematical model of the development of the early Solar System, when the planets were accreting from clustering masses of dust and gas.

According to this model, the nascent giant planets — Jupiter, Saturn, Uranus and Neptune — orbited at this time in a pretty compact configuration, between five and 15 astronomical units (AUs) from the Sun.

An AU is a standard unit of measurement for Solar System distances. It equals the distance from the Earth to the Sun, or around 150 million kilometres (93 million miles).

Beyond the giant planets was a disk-shaped mass of comets, known as trans-Neptunian objects, between 16 and 30 AUs from the Sun.

As the giants became bigger and bigger, their orbits became unstable.

Eventually, after around 600 million years, Uranus and Neptune were kicked out by gravitational jousting.

They rammed into the disk of comets and scattered its members throughout the Solar System, according to this model. Many of them were captured by the weak gravitational force of the asteroid belt, where they remain to this day.

“It’s a paradigm shift,” said Matthieu Gounelle of France’s National Centre for Scientific Research (CNRS) in a press release.

“The asteroid belt is not just a leftover from the formation of the Solar System, but also that of violent phenomena” including the great planetary migration.

If the model is right, it implies that the difference between the most primitive asteroids and comets is even slimmer than thought.

It would also shed light on the controversial origin of micrometeorites, or tiny extraterrestrial particles that survive the fiery passage through Earth’s atmosphere.

Micrometeorites are different from meteorites in composition and texture, and this could be explained if they derive from comets, which are richer in organic material and crumblier than native asteroids.

(Agencies)

The lab will kick-start the reaction by focusing 192 giant laser beams on a tiny pellet of hydrogen fuel.

To work, it must show that more energy can be extracted from the process than is required to initiate it.

Professor Mike Dunne, who leads a European venture that is also pursuing nuclear fusion with lasers, told BBC News that if NIF was successful, it would be a “seismic event”.

“It would mark the transition for laser fusion from ‘physics’ to ‘engineering reality’,” he said. The California-based NIF is the largest experimental science facility in the US and contains the world’s most powerful laser. It has taken 12 years to build. “This is a major milestone,” said Dr Ed Moses, director of the facility.

“We are well on our way to achieving what we set out to do – controlled, sustained nuclear fusion and energy gain for the first time ever in a laboratory setting.” Experiments will begin in June 2009, with the first significant results expected between 2010 and 2012. “We have an incredible amount to do and an incredible amount to learn,” added Dr Moses. Fusion is looked on as the “holy grail” of energy sources because of its potential to supply almost limitless clean energy. But the challenge of creating a practical fusion reactor has eluded scientists for decades. Now, however, they believe they are nearing their goal. “We are now very close to the culmination of 50 years’ effort,” explained Professor Dunne. There are currently several experimental facilities around the world aimed at demonstrating the building blocks of nuclear fusion. In this process, two heavier forms of hydrogen, known as deuterium and tritium, are fused together to form helium. Deuterium is commonly found in seawater, whilst tritium can be prepared from lithium, a relatively common element found in soil. When these isotopes are combined at high temperatures, a small amount of mass is lost and a colossal amount of energy is released. Fusion naturally occurs at the centre of stars where huge gravitational pressure allows the process to happen at temperatures of about 10 million Celsius. At the much lower pressures on Earth, temperatures to produce fusion need to be much higher – above 100 million Celsius.

NIF will focus on a process known as inertially confined fusion, in which these extreme temperatures are achieved using ultra powerful lasers.

“When all NIF lasers are fired at full energy, they will deliver 1.8 megajoules of ultraviolet energy to the target,” explained Dr Moses. NIF’s beams are intended to deliver more than 60 times the energy of any previous laser system. When fired, the pulse will last just a few nanoseconds (billionths of a second) but it will impart 500 trillion watts of power – more than the peak electrical generating power of the entire United States. This intense energy will be focused on a ball-bearing-sized pellet of fuel, ablating the surface and compressing the remaining material inwards.

“This process will create temperatures of 100 million degrees and pressures billions of times greater than Earth’s atmospheric pressure, forcing the hydrogen nuclei to fuse and release many times more energy than the laser energy required to spark the reaction,” said Dr Moses. This “energy gain”, as it is known, is key. If it works, NIF will release 10 to 100 times more energy than the amount pumped into the lasers to kick-start the reaction. Other experiments have shown that ignition is possible, but so far none has been able to demonstrate a net energy gain. “The world is looking to NIF to provide a clear, unequivocal demonstration that lasers can initiate fusion energy gain,” said Professor Dunne. “This would lay the fundamental physics question to rest, allowing the community to focus on harnessing this energy.”

Although NIF is only at the beginning of its experimental life, scientists are already planning its successor, a European project known as Hiper (High Power Laser Energy Research). “The technology of NIF allows the laser to fire every few hours,” explained Professor Dunne, director of Hiper.

“This is right for the demonstration of the physics ‘proof of principle’, but does not meet the requirement of a laser fusion power plant, which needs to operate a few times per second.” Hiper aims to lay the foundations of this continuous fusion cycle by showing it can ignite a steady stream of fuel pellets. “This means a fundamentally different laser technology, a new approach to fuel pellet production, and a suite for robotic handling capability,” said Professor Dunne.

In October 2008, Hiper received approximately 13m euros of funding to carry out a feasibility study. It also has access to European hardware and capability worth a further 50m euros. If all goes well, engineers will begin to build the Hiper facility towards the end of the next decade, bringing the vision of a commercial fusion reactor one step closer to reality. At approximately the same time, scientists will also get their hands on another mammoth fusion experiment, the International Thermonuclear Experimental Reactor (Iter), currently being built in Cadarache, France. Iter will attempt to initiate fusion using a different method, known as magnetic confinement, in which a super-heated volume of gas is constrained by magnetic fields in a doughnut-shaped vessel known as a tokamak. “We are entering a period when much of the technology development is common to both approaches,” said Professor Dunne.

“We believe that the two-track approach is essential given the scale of the problem, and the predicted impact on society.”