The Rosetta mission countdown: dread, hope, excitement

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When you're a landing junkie, you know the stages.

First, there are the endless checklists and the reassurance that comes with sticking to the plan. Then, the pre-emptive declaration that everything that has been done can be done. What more could anyone ask for?

With one day to go, there emerges a creeping realization that all human plans and contingencies pale before a supremely powerful and indifferent cosmos. The likelihood of failure ceases to be an abstraction. The prospect that years of effort and millions of dollars worth of flight hardware could vanish in one nauseating instant now looms large.

Yet there is also hope and excitement, fuelled by the irresistible allure of an alien world and the chance, however slim, of being the first to reveal its secrets.

All of these emotions and more were at play on Tuesday at the European Space Operations Centre in Germany and at other locations where the scientists behind the Rosetta mission have gathered on the eve of their daring attempt to land a small probe on Comet Churyumov-Gerasimenko, a historic first.

The lander, called Philae, will either drop down toward the comet's surface for its scheduled touchdown on Wednesday around 11 a.m. Eastern Time, or it will be held back at the last moment because of a technical glitch.

But, however the day unfolds, there is no question that something very ambitious is being undertaken here – something that will require everyone involved with the mission to be at the top of their game.

"I think this is the key week of our professional careers," said Andrea Accomazzo, flight director for Rosetta. "I don't think we will land on a comet another time."

Like everyone else associated with Rosetta and Philae, Mr. Accomazzo understands all too well that by the time another such mission can be brought to an equivalent point, he will no longer be at the controls, just as many of those who were first involved in conceiving the mission are now witnessing its culmination from retirement.

"It really is an intergenerational piece of science," said Mark McCaughrean, senior science adviser at the European Space Agency.

And, if it works, it will transport all of us to someplace entirely different.

There have now been numerous landings of unmanned probes around the solar system and more than a few unsuccessful attempts. But the number of different kinds of places where spacecraft have landed still comes down to mere handful:

• The 1960s and early 1970s saw the first successful landing on the moon, initially with un-crewed probes and then by human astronauts.

• From 1970 to 1981, the Soviet Union added Venus to the list with a succession of landings in spite of the planet's corrosive and crushing atmosphere.

• In 1976, U.S. National Aeronautics and Space Administration finally beat the curse of the Red Planet and successfully placed its two Viking landers on the rust-coloured surface of Mars, followed more recently by a fleet of rovers and landers.

• In 2001, flight controllers behind NASA's NEAR-Shoemaker asteroid mission scored an unexpected milestone. At the end of a planned one-year reconnaissance, they brought their spacecraft so close to the surface of the asteroid Eros that it came to rest without crashing. However, NEAR-Shoemaker was not built to be a lander and there were no surface measurements taken and no pictures looking out across the asteroid's boulder-strewn terrain.

• In 2005, Europe's Huygens probe detached from the Cassini spacecraft orbiting Saturn and drifted on a parachute down through the murky atmosphere of Titan, the planet's largest moon. It captured a breathtaking surface photo from a dry stream bed at -180°C where liquid methane apparently once tumbled over and rounded off broken chunks of ice.

What all of these feats have in common is that they have shown us landscapes that are utterly alien to human experience, yet inherently relatable because they show features on a human scale.

If Philae survives on Wednesday, it will be able to add its view of a new landscape – possibly the strangest one yet – to those already seen by Earth's various robot emissaries on the moon, Mars and elsewhere.

Close-ups of the landing site, selected last month, show a broad, well-lit plain with relatively few hazards compared with other areas of the comet.

The lighting is crucial because Philae will be using solar panels to recharge its batteries in hopes of continuing surface activities beyond its first couple of days on the comet.

But small uncertainties in the position and velocity of Rosetta at the time of separation, expected around 4 a.m. ET on Wednesday, will mean that the lander may set down anywhere within a 500-metre radius of its target, and there are plenty of boulders and steep slopes where it would be disastrous for Philae to land.

Over all, scientists say, about three-quarters of the area looks safe, but this does not take into account whether the surface will be solid and stable enough for Philae to land. Rosetta's measurements have shown that the bulk of the comet is less than half the density of water, Dr. McCaughrean cautioned, which means that its surface may be more powdery and less firm than close-up images suggest.

"Intuition fails us constantly on an object like this," he said.

As the landing approaches, team members and colleagues on Rosetta have been joined by members of an elite club of landing experts and veterans who are here to offer support and to observe.

Among them is Miguel San Martin of NASA's Jet Propulsion Lab in Pasadena, Calif., who played a key role in designing the landing of the Curiosity Rover on Mars in 2012, but who now says his interests have turned toward the challenge of landing on small irregular bodies such as asteroids and comets.

When asked what drew him to attend the landing in person at Rosetta's mission control instead of conferring with European colleagues online, Mr. San Martin did not hesitate in his answer. "It's because you know that history will be made – just not what kind of history it will be."