Cape Town - Scientists have taken another key step in the unfolding drama of what is being hailed as one of the great scientific achievements of all time: sending a spacecraft to catch a comet and land a probe on its surface as the comet continues its headlong rush towards the Sun.

The spacecraft Rosetta caught Comet 67P/Churyumov-Gerasimenko earlier this month and will send its lander, Philae, down on to the comet’s icy surface on November 11 – if everything keeps going according to plan.

This week, less than three weeks after Rosetta arrived at the comet after a mind-boggling journey of 10 years, five months and four days in which it looped around the Sun five times and clocked 6.4 billion kilometres, mission controllers identified five candidate landing sites after extensive but rapid mapping of the comet.

The comet, which some observers suggest is shaped like a duck with two big sections joined by a narrow neck – technically, it has a “bi-lobate” form – has a nucleus about 4km across and each elliptical landing site covers about 1km2.

“Choosing the right landing site is a complex process,” said the European Space Agency (ESA), which is the lead player in the Rosetta mission that is planned to achieve several historic scientific firsts.

“That site must balance the technical needs of the orbiter and lander during all phases of the separation, descent and landing, and during operations on the surface with the scientific requirements of the 10 instruments on Philae.”

There have been previous comet fly-bys to collect data, but Rosetta will be the first to orbit and track a comet at close range before, during and after perihelion – the point in the comet’s orbit at which it is nearest the sun, in August next year, as well as being the first to send a lander on to a comet (if successful).

It will study the comet at close range “as it transforms from a quiet nugget of ice and rock, frozen solid by years spent in deep space, to a sun-warmed dynamo spewing jets of gas and dust into a magnificently evolving tail”, Dr Tony Phillips, production editor of Nasa Science News, said earlier this year.

Rosetta was launched on its 11-year journey by an Ariane 5 rocket from Kourou in French Guiana in March 2004. During its long, lonely journey, it travelled around the Sun five times and picked up energy during three gravity-assist fly-bys of Earth and one of Mars, in order to line it up with its rendezvous point with Comet 67P/Churyumov-Gerasimenko.

In June 2011, for the coldest leg of its mission as it travelled out towards the orbit of Jupiter, Rosetta was put into a record 957 days of deep space hibernation mode.

Then, with its destination finally in sight – but still 9 million kilometres away – it was “woken” on January 20 this year and its cameras turned on and its 11 science instruments and 10 lander instruments were reactivated and readied for science observations.

Ten orbital correction manoeuvres were carried out between May 7 and August 6 when it arrived at the comet, reducing the spacecraft’s velocity with respect to the comet from 775 metres per second to 1 m/s, equivalent to walking pace, the ESA said.

“Each of these manoeuvres was critical: if any had failed, no rendezvous would have been possible.”

When the odd couple paired up, they were 405 million kilometres from Earth, about halfway between the orbits of Jupiter and Mars, and were rushing towards the inner solar system at nearly 55 000 km/h, the ESA said.

Scientists calculated that the average temperature of the comet – comprising rock and ice; comets are often described as “dirty snowballs – to be about -70°C.

This week, Stephan Ulamec, Philae lander manager at the German Aerospace Centre in Cologne, said it was the first time landing sites on a comet had been considered.

“The candidate sites that we want to follow up for further analysis are thought to be technically feasible on the basis of a preliminary analysis of flight dynamics and other key issues.

“For example, they all provide at least six hours of daylight per comet rotation and offer some flat terrain.

“Of course, every site has the potential for unique scientific discoveries.”

For each possible landing zone, important questions had to be asked, the ESA said. Will the lander be able to maintain regular communications with Rosetta? How common are surface hazards such as large boulders, deep crevasses or steep slopes? Is there sufficient illumination for scientific operations and enough sunlight to recharge the lander’s batteries beyond its initial 64-hour lifetime without causing overheating?

“The process of selecting a landing site is extremely complex and dynamic; as we get closer to the comet, we will see more and more details, which will influence the final decision on where and when we can land,” said Fred Jansen, Rosetta’s mission manager from the ESA’s Science and Technology Centre in Noordwijk, The Netherlands.

“We had to complete our preliminary analysis on candidate sites very quickly after arriving at the comet, and now we have just a few more weeks to determine the primary site. The clock is ticking and we now have to meet the challenge to pick the best possible landing site.”

Rosetta is being manoeuvred to within just 50km of the comet to allow a more detailed study of the proposed landing sites, and the final selection is likely to be taken by September 14.

The landing of Philae is expected to take place in mid-November when the comet is about 450 million kilometres from the Sun, the ESA said.

“This will be before activity on the comet reaches levels that might jeopardise the safe and accurate deployment of Philae to the comet’s surface, and before surface material is modified by this cometary activity.”

Sylvain Lodiot, the ESA’s Rosetta spacecraft operations manager, said: “Arriving at the comet is really only just the beginning of an even bigger adventure, with greater challenges still to come as we learn how to operate in this uncharted environment, start to orbit and, eventually, land.”

The mission is due to finally shut down at the end of next year.

l Rosetta is an ESA mission with contributions from its member states and Nasa.

For more, see www.esa.int/rosetta

Sunday Argus