快猫短视频

CLIFF FLEMING is no stranger to hair-raising stunts. As a helicopter pilot in the film industry, Hollywood often calls on him to stage a police helicopter shoot-out, or fake a catastrophic crash. In last year鈥檚 Vietnam epic We Were Soldiers starring Mel Gibson, for instance, he staged a crash by sending his aircraft diving into a spin and wobbling out of control, leaving the special effects folks to cook up the final explosion.

But one day in September next year, Fleming will take time out from the Hollywood high life to go on a special assignment for NASA 鈥� to fly up and grab a little piece of the sun. NASA鈥檚 Genesis spacecraft has been soaking up wisps of the solar atmosphere since December 2001, and will return to Earth in September 2004. To minimise the risk of contamination with material from Earth, the spacecraft will drop a canister containing the solar sample into the atmosphere. And, fingers crossed, Fleming鈥檚 helicopter will gently catch it in mid-air before the precious cargo can crash to the ground. 鈥淚t鈥檚 kind of exciting,鈥� says Fleming. 鈥淚t鈥檚 not often that you find yourself flying at 10,000 feet to snag something right out of the sky.鈥�

It will be an exciting day for science as well. When researchers have their piece of the sun safely back in the lab, they will be able to use state-of-the-art techniques to find out exactly what our nearest star is made of. The sun is a giant ball of hydrogen and helium, but it also contains a peppering of other, heavier elements that scientists know precious little about. Next year, with a soup莽on of sun at their fingertips, they hope to put that right.

This isn鈥檛 just about setting the record straight, though. Astronomers hope the Genesis sample will shed light on the birth of the sun and planets. A staggering 99 per cent of all the material in the solar system is held in the sun, and although the chemical make-up of its core constantly changes due to the nuclear reactions that make it shine, the outer layers are thought to preserve the exact composition of the giant interstellar cloud that gave birth to the solar system 4.6 billion years ago.

By studying its chemistry, scientists hope to resolve some of the mysteries about how that big bland cloud transformed itself into the spectacularly diverse bunch of planets, moons and icy comets here today. 鈥淕enesis will return a small but precious amount of data crucial to our knowledge of the sun and the formation of our solar system,鈥� says the mission鈥檚 head scientist, Donald Burnett of the California Institute of Technology in Pasadena.

The mission began in August 2001, when a spacecraft blasted off on a 3-month journey to a region 1.5 kilometres away from the Earth in the direction of the sun. Since December 2001, the spacecraft has been sampling little specks of the solar wind using five different collector arrays.

Each array is a round panels about 1 metre across, and all five are tiled with 55 palm-sized hexagons of exotic materials such as pure sapphire, diamond, gold and silicon. The materials were selected for their ability to trap specific elements. Experiments on the moon during the late 1960s captured solar particles, but the Genesis plans are far more ambitious. 鈥淭he Apollo experiments gave good measurements for just two elements 鈥� helium and neon 鈥� but we鈥檙e going for the whole periodic table,鈥� says Burnett. For instance, they are hoping their slab of silicon will play host to other noble gases, including argon, krypton and radon.

Three of the arrays will bare themselves to the sun only in certain solar weather conditions. Depending on the mood of the sun, the stream of particles that constantly blows outwards from its atmosphere varies in speed from about 300 to 800 kilometres per second. One Genesis array will catch this solar wind when it blows slowly, another will catch the faster particles. A third array will trap particles in so-called coronal mass ejections, giant blobs of ionised gas that the sun burps out a few times each month. While the composition of solar samples from these arrays will vary slightly, scientists say they can work out the exact chemistry of the sun鈥檚 outer layers by looking at all three.

Because the solar wind is so thin, the spacecraft has to sunbathe for more than 2 years to collect a useful amount of particles. 鈥淲e鈥檙e talking about just a few ions per cubic centimetre 鈥� this is really, really low-density stuff compared to anything on Earth,鈥� says Donald Sweetnam, project manager for the Genesis mission. By April 2004, the arrays will have trapped only about 20 micrograms of solar material other than hydrogen and helium 鈥� the weight of a few grains of salt.

That tiny amount could still be enough to revolutionise our view of the sun. To date, our best estimates of the make-up of the sun come from detailed studies of the light it emits and from spacecraft that have analysed the solar wind. In terms of mass, hydrogen and helium make up about 98 per cent of the sun, with other elements making up the rest.

Previous studies of the solar spectrum have identified the fingerprints of over 60 elements. But measuring their abundances isn鈥檛 so easy and most elements come with an error of at least 10 per cent. For some, astronomers confess they can鈥檛 measure the amounts at all.

Even worse, the solar spectrum says nothing about the various different isotopes of a particular element in the sun. That鈥檚 because the light an atom emits or absorbs depends on the energy levels of its electrons 鈥� not on the number of neutrons in its nucleus, which is what varies according to the isotope. Spacecraft measurements of isotope ratios often have uncertainties of 40 per cent or more.

With the Genesis sample safely back in the lab, astronomers will be able to analyse it with the most sophisticated equipment to hand. This will improve their measurements of solar elements by a factor of 3, and will also allow them to pin down isotope ratios with errors of just 0.1 per cent, making them tens or hundreds of times more accurate than before.

Having such a precise list of raw ingredients for the solar system might help astronomers understand all the inexplicable differences between elements on Earth, the moon and Mars. For decades researchers believed that the isotope ratios of different elements 鈥� such as the ratio of carbon-14 to carbon-12 鈥� are the same everywhere, from the sun to the outer planets. But following the Apollo missions, it became clear that things are not that simple.

Between 1969 and 1972, astronauts on Apollo missions to the moon planted aluminium sheets on sticks into the powdery lunar surface to trap solar-wind particles. Back on Earth, scientists extracted helium, neon and argon from the metal, and found that the ratio of neon-20 to neon-22 was inexplicably 38 per cent higher than in the Earth鈥檚 atmosphere.

Even on the moon鈥檚 surface, the ratios of different nitrogen isotopes in a rock vary depending on its age. One possible explanation is that the nitrogen content of the solar wind has gradually changed over time, although how the sun could arrange that is a mystery.

The most dramatic variations in isotope mixes are for oxygen. The ratio of oxygen-18 to oxygen-16 is completely different in moon rocks and terrestrial rock, and in meteorites from Mars and the asteroid belt. 鈥淎s time has gone by, we have found more and more differences, as measurements get more precise,鈥� says Burnett. 鈥淭here are now a whole string of elements whose isotopes are known to vary in the moon, meteorites and in the Earth, and no one really knows why these variations exist.鈥�

The Genesis sample could help resolve some of these puzzles by filling in gaps in our picture of the sun鈥檚 chemical make-up, and how that differs from the make-up of the Earth, Mars and the asteroids. There could be underlying patterns in this 鈥渕ap鈥� that reveal why all these bodies came to be so different. That鈥檚 assuming nothing goes badly wrong on the sample鈥檚 return to Earth.

In April 2004, the spacecraft will seal the arrays into a capsule and set off on a 5-month return journey. The last leg of the trip will be a detour of about 1.5 million kilometres beyond the Earth, so that the spacecraft can position itself correctly to drop the capsule into the atmosphere above Oregon in daylight. From there it will head south-west across Nevada towards the deserts of Utah, and two parachutes will slow its descent to about 18 kilometres per hour. Fleming鈥檚 helicopter will criss-cross its path, and he will aim to catch the capsule by one of the parachutes using a hook and winch attached to the landing skid. Then it will be reeled in and flown back to a hangar. Its final home will be at NASA鈥檚 Johnson Space Center in Houston, Texas, which will dispatch solar samples to different labs for analysis.

Sweetnam says the day the Genesis capsule returns will be nerve-racking. 鈥淚 have great confidence in the flight team鈥檚 abilities, but it鈥檚 difficult to do and things can go wrong,鈥� he says. 鈥淚t鈥檚 like the reverse of a launch 鈥� you have a lot of things happening in a very short period of time.鈥�

One worry is that the capsule鈥檚 parachutes might fail to open, in which case it will plummet to the ground and shatter. Or the spacecraft might drop the capsule into the atmosphere on the wrong trajectory. The trajectory needs to be absolutely spot-on for the capsule to land in the right place. 鈥淕etting through the dinky little 鈥榢eyhole鈥� that you have to go through to get to the right spot in Utah is technically difficult,鈥� says Sweetnam.

But the most spectacular task is the mid-air retrieval by helicopter. The US military is well practised in this, having retrieved reconnaissance drones and film canisters from spy satellites as far back as the 1960s. 鈥淚n the old days, they used round parachutes and enormous helicopters or aeroplanes, and the odds of capture were sometimes as low as 50:50,鈥� says Sweetnam. 鈥淲ith new technology and really good pilots, we鈥檝e managed to make the likelihood virtually certain.鈥�

Fleming agrees, provided there are no unlucky mechanical failures. 鈥淣ormally, you can count on your car starting when you walk out to the garage in the morning, but there may be that one time that it just doesn鈥檛 for some reason,鈥� he laughs. To make sure mechanical failures don鈥檛 mess things up, a back-up helicopter and crew will also fly. It will hover at a lower altitude, so that if Fleming misses the Genesis capsule, the back-up helicopter will have a second chance.

Fleming has already practised catching a dummy capsule dropped by a plane, and everything went according to plan. He and the back-up crew will train again in the run-up to the real mid-air retrieval, so that he can be sure everyone will be confident on the day. 鈥淏y the time it happens, we鈥檙e going to have everything worked out.鈥�