From doomsday to fact: Science lifts veil on comets

For millennia, the sight of a comet filled humans with awe or dread.

The birth of Jesus, the assassination of Julius Caesar, the Great Plague of London, the coming of war or peace, bountiful harvests or famine... all thought to be portended by cosmic herald.

Bit by bit, mysticism about comets has been replaced by fact as scientists discover more about these epic and ancient travellers of the skies.

As it turns out, comets may be more extraordinary than even the deepest superstition could imagine.

They may even have brought life to our planet, according to some theories.

For decades, astrophysicists have debated whether, at the dawn of our Solar System, comets peppered Earth with some of the chemical essentials for life as we know it.

The answer may be within reach.

On Wednesday, a European robot lab, Philae, will make the first-ever landing on a comet.

It will carry out experiments on the comet's back as the pair hurtle toward the Sun, escorted by Philae's mother ship, Rosetta, in orbit.

"Comets are the most ancient objects in the Solar System," Francis Rocard of France's National Centre for Space Studies (CNES), told AFP.

"We want to know if comets played a part in providing Earth with water and carbon. There is no doubt that the results from Rosetta and Philae will shake up what we know."

Dubbed "dirty snowballs" by US astronomer Fred Whipple, comets are deemed to be clusters of primeval carbon and ice, typically a few kilometres (miles) across.

Blacker than coal, they were formed shortly after the Sun flared into life in a halo of dust and gas -- the stuff that eventually formed the planets and other bodies.

Today, the Solar System is considered a pretty quiet place compared to 4.6 billion years ago, when it was a shooting gallery.

The nascent Earth would have been whacked over and over again by comets and asteroids, swelling the planet in size and depositing ice, which on some theories became today's oceans.

"This would have created a wonderful culture medium -- a liquid enabling highly rich carbon material to react and create prebiotic chemistry," said Rocard.

"It would have led to the first membranes and ultimately the first cells -- life itself," he said.

The key to confirming this theory lies in the ratio of hydrogen and deuterium on the comet, which Rosetta and Philae will measure with their instruments, to be compared to the chemical make-up of water on Earth.

Doomed to orbit the Sun in elliptical circuits, comets undergo thermal and gravity stress as they near the star.

Some of their ice is melted and transformed into gusts of gas, the bright "coma" around a comet's head.

- Hairy star -

The gassy wake, and dust loosened by the melting ice, creates a spectacular tail reflected in the Sun's rays and may stretch across millions of kilometres.

The word for comet comes from "stella cometa" -- Latin for "long-haired star".

The best-known is Halley's Comet, named after 17th-century English astronomer Edmond Halley, who was the first to show that comets orbit the Sun and return regularly.

Just over 5,000 comets have been observed since the first recorded sighting by Chinese skygazers around 240 BC.

But some experts believe there could be as many as a trillion out there, the European Space Agency says.

"Short period" comets take less than 200 years to complete an orbit. They are believed to return to the Kuiper Belt, just beyond the orbit of Neptune.

"Long period" comets return to the Oort Cloud, a vast, frigid, dark place on the Solar System's fringes.

Some of these last swung by more than 100,000 years ago, when Homo sapiens ventured out of Africa, at the start of our conquest of the planet.

Highlights of unmanned space exploration
Paris (AFP) Nov 08, 2014 - In 1942, the Nazis' V-2 rocket became the first man-made object to touch the fringes of space.

Since then, humankind has sent scouts around the Solar System to explore its central star, planets and other bodies.

If all goes well, another milestone will be reached next Wednesday when Europe lands a robot lab on a comet.

Here are other firsts in unmanned space exploration:

SPUTNIK 1

The first artificial satellite was launched by the Soviet Union on October 4, 1957, ushering in the space age and the Cold War tussle for the cosmos.

The beachball-sized, aluminium sphere took 98 minutes to orbit the Earth, and sent the first-ever message received from space.

Another pioneering satellite is NASA's Hubble Space Telescope, placed in near-Earth orbit in 1990, which has provided dazzling pictures of objects in deep space.

LUNA 2

Another Soviet record, this probe was the first man-made craft to reach another celestial body, crashing into the Moon in 1959 and scattering Soviet pennants on its surface.

Its successor Luna 3 sent back the first-ever picture of the far side of the Moon later that same year, and Luna 9 made the first soft landing on the Moon in 1966.

VENERA 3

This Soviet lander was the first to touch the surface of another planet -- Venus, in 1966. A landing capsule was released successfully, but contact with it was lost and no scientific data was returned.

The first soft landing on a planet, also Venus, was achieved by Venera 7, four years later.

Venera 7 transmitted the first signals from another planet, and revealed that Venus, far from being a home from home, would be lethal for humans.

PIONEER 10

In 1973, the NASA spacecraft carried out the first flyby of Jupiter, swinging past the biggest planet of the Solar System at a distance of 130,000 km (80,000 miles).

In 1983, it became the first spacecraft to travel past the orbit of Neptune, the outermost of the acknowledged planets.

Pioneer 10 and its sister, Pioneer 11, carry aluminium plaques with the drawing of a man and a woman along with information indicating where the probes came from.

VOYAGER 1 AND 2

Launched in 1977 to explore further afield than ever before, Voyager 1 returned detailed photographs of Jupiter and Saturn before becoming, in 1998, the most distant human-made object.

In 2012, it entered interstellar space, the region between the stars.

Like its companion Voyager 2, which in 1986 became the first spacecraft to fly past Uranus, the vessel carries a 30-cm gold-plated copper disc.

The record includes a "greeting to the universe" in 60 languages, music and 115 images of Earth -- complete with a stylus with which to play it.

GALILEO

Launched in 1989, this NASA mission became the first to go into orbit around a gas giant planet, Jupiter, in 1995. It carried a range of science instruments and an atmospheric probe.

It found evidence for liquid water oceans under the icy surface of Jupiter's moon Europa.

MARS PATHFINDER

An innovative airbag cocoon cushioned the landing of this spacecraft on the Red Planet in 1997, from which emerged the first-ever wheeled rover, dubbed Sojourner, to explore another planet.

NEAR Shoemaker

The first landing on an asteroid happened about 355 million km from Earth in 2001, touching down at a gentle 1.5 metres per second.

Four years later, Japan's Hayabusa spacecraft was the first to land on, take a sample, and take off from an asteroid, Itokawa, and send the dust it collected back to Earth.

CASSINI

A joint project of NASA, the European Space Agency (ESA) and the Italian ASI, this explorer in 2004 became the first to enter the orbit of Saturn, from where it has closely studied the giant planet's magnificent rings.

In 2005, Cassini sent down a probe, Huygens, to Saturn's largest moon Titan -- a strange world with lakes of liquid methane.

STARDUST

In 2004, this NASA mission was the first to collect samples from the wake of a comet, dubbed 81P/Wild, as it shaved by at a distance 236 km. The particles were returned to Earth in a capsule in January 2006 for analysis.

Sources: NASA, ESA