The site for humanity’s first-ever attempt to land a spacecraft on a comet has been chosen.
Philae, the tiny lander being carried by the Rosetta probe, will attempt on Nov. 12 to anchor itself to an area on the smaller of comet 67P/Churyumov-Gerasimenko. The location is known as Site J.
“Now that we know where we are definitely aiming for, we are an important step closer to carrying out this exciting – but high-risk – operation,” Fred Jansen, ESA’s Rosetta mission manager, said in a statement released Oct. 15.
Rosetta itself is now about 10 kilometers away from the comet. It will launch Philae, if all goes according to plan, when it is about 22.5 kilometers from the comet’s center.
Philae’s journey to the comet will take about seven hours. Communication confirming a successful landing on the comet can be expected to arrive at Earth during the early- to mid-morning hours in North America.
Rosetta’s mission team will confirm that the probe is on the correct trajectory and that Philae is ready to receive computer commands before the final authorization to send the tiny lander to comet 67P/Churyumov-Gerasimenko is transmitted.
Philae will not be idle during its movement from Rosetta to the comet. It will study the comet’s dust, gas, and plasma while moving slowly through the short distance in space.
Mission planners are not sure how much time Philae will have, once anchored on the comet, to study its nucleus and other features. The lander has a solar-powered battery, which will provide more or less energy depending on the degree to which the solar power receiver is covered by dust from the comet.
ESA said in a press release that Philae is not expected to be able to conduct scientific observations of the comet past March 2015.
Researchers have chosen five possible landing sites for a spacecraft that will attempt in November to become the first ever to land on a comet.
The European Space Agency will consider several factors in making the final choice of a landing site on comet 67P/Churyumov-Gerasimenko for the Philae lander, including whether it has obstacles to landing and whether the amount of solar energy appropriate for the lander’s power array is available.
“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,” Steven Ulamec, the lander manager at the German Aerospace Center in Cologne, Germany,, said. “Of course, every site has the potential for unique scientific discoveries.”
One of the most basic factors that will influence the choice of a landing site is whether it is one to which the lander can be accurately guided. That challenge is magnified somewhat by the unusual shape of comet C-G, which has been likened to a rubber duck.
“Depending on where you try to make the lander land, the errors associated with everything can actually vary,” Dr. Daniel Scheeres, a professor of aeronautical engineering at the University of Colorado at Boulder and an expert in the navigation of space vehicles, said. “There will be regions on the surface where they can be more accurate than others.”
Another problem for mission planners is whether a potential site impedes Philae’s ability to anchor itself to the surface and remain upright. Rocky sites pose hazards to the spacecraft, but there’s no guarantee that a smooth site would be one to which the lander could be easily guided.
“There’s no reason why the easiest spot is also going to be the smoothest spot,” Scheeres said. “That’s where the challenge is going to be. They’ll have to make some sort of compromise.”
Yet another consideration will be the degree of solar illumination that predictably hits the landing site. Philae depends on batteries rechargeable by sunlight for its power.
“The real issue is what happens if the sun is blocked,” Scheeres explained. “Then you lose the power. And the comet is rotating. Even if you put it in the right spot, in a half a day it’s going to be in the dark.”
Because the movement of the comet will prevent Philae’s batteries from being constantly re-charged, it will be important for the landing site to be fully illuminated when that area of the comet faces the sun.
“If it lands somewhere where it’s shadowed, once the batteries are dead it won’t get any more power,” Scheeres said.
ESA faces a mid-September deadline to choose Philae’s destination on the comet. It will continue to gather information about the five potential landing sites for several more weeks.
“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,” Fred Jansen, the Noordwijk, The Netherlands-based Rosetta mission manager, said.
Three of the possibilities are located on the comet’s smaller lobe, while two are on the larger lobe.
Each of the sites must accommodate the one square meter-sized Philae lander, which will examine the chemical and physical structure of the comet and obtain close-up images of the comet’s nucleus.
Actual landing of Philae will occur when Rosetta and comet C-G are about 450 million kilometers from the sun.
Researchers want to make the attempt before the comet gets too close to the sun. Too much solar energy would make landing, and operation of the lander once on the surface of the comet, more difficult.
Rosetta is currently in orbit around the comet. It will continue efforts to study the comet through 2015, following it through an orbit around the sun.
The spacecraft Rosetta has met up, after a decade-long journey, with a comet orbiting the Sun between Mars and Jupiter, becoming the first vessel to do so.
Comet 67P/Churyumov-Gerasimenko will have its Earthly companion for about the next year as it moves around our star.
“After ten years, five months and four days traveling towards our destination, looping around the Sun five times and clocking up 6.4 billion kilometers, we are delighted to announce finally ‘we are here’,” Jean-Jacques Dordain, the director-general of the European Space Agency, said in a statement.
In May Rosetta began the process of slowing and positioning itself for the rendezvous. The spacecraft, which is now about 100 kilometers from the comet, will now prepare for an effort to land a probe on the comet’s surface.
That probe, called Philae, is to be launched in mid-November.
Rosetta is likely to help scientists learn about the comet’s nucleus and coma.
Launched on March 2, 2004, Rosetta has flown by Earth three times, by Mars once, and by two asteroids on its way to Comet 67P/Churyumov-Gerasimenko.
Nearly three years of quiet from a probe speeding toward a rendezvous with a distant comet ended Monday as engineers in Germany successfully reactivated the Rosetta spacecraft.
The solar-powered probe is designed to be the first to land on a comet. If all goes as planned, Rosetta will accomplish that feat in November of this year when an onboard lander alights on the head of comet 67P/Churyumov-Gerasimenko.
Rosetta will also follow the comet around the sun, another first for human space exploration.
The probe’s reactivation came at 18:18 Greenwich Mean Time (11:18 MST). The Twitter account set up for Rosetta announced it a few minutes later with a cheery “Hello, World!”
Rosetta was launched on March 2, 2004. During the 118 months that have since elapsed it has flown by the Earth three times and once around Mars. The probe was placed into hibernation at the command of engineers in Europe on June 8, 2011.
During the hibernation only the computers onboard and heaters necessary to prevent the spacecraft from freezing were functional. All other instrumentation, including that necessary to study the comet, was shut down.
The deactivation of Rosetta was necessary because the probe would travel too far from the Sun to obtain the solar energy needed to power its systems.
Reactivation occurred when a timer programmed into the spacecraft’s computer system reached the previously specified time of the event. Rosetta’s star trackers were the first instruments to come back online. Then the probe’s computer system slowed the spacecraft’s spin and oriented it toward Earth in order to transmit a signal.
“This was one alarm clock not to hit snooze on, and after a tense day we are absolutely delighted to have our spacecraft awake and back online,” Fred Jansen, the European Space Agency’s manager for the mission, said in a statement.
Now about nine million kilometers from comet 67P/Churyumov-Gerasimenko, Rosetta will enter an orbit around the comet in May.
In November, after Rosetta has had time to slow down enough time to synchronize its velocity with the comet, a smaller craft called Philae will land on it.
Philae, which has a mass of about 100 kilograms, is much smaller than the nucleus of the comet.
“The comet itself, it’s about four kilometers across,” Michael R. Combi, a professor in the University of Michigan’s Department of Atmospheric, Oceanic, and Space Sciences and a member of the Rosetta scientific research team, said. “The lander is very small. I don’t think it’s two feet across.”
Philae will contend with landing on a surface that exhibits very little gravity. Unlike Earth, where there is enough gravity to hold soil and rock in place, the environment on a small comet like 67P/Churymov-Gerasimenko does not have enough to hold all solid material in place.
“It’s fairly fluffy,” Combi said. “There’s almost no gravity at the surface of an object that small.”
The comet has about 1,000 times less gravity than does Earth.
The lander will study 67P/Churyumov-Gerasimenko’s nucleus, and determine the chemical composition of the materials found there, while being secured to the comet’s nucleus by two harpoons.
“We want to measure all we can, how much material is coming off the comet as it goes around the sun,” Combi said.
He explained that ice is the dominant material on a comet’s nucleus, and that it sublimates, or changes phase from a solid to a gas, as the comet heats up on approach to the sun. One of the mission’s goals is to understand the mechanics of this sublimation process.
Rosetta also aims to help scientists gain a better understanding of the specific chemical makeup of comets.
“Practically all seem to be dominated by water ice,” Combi said. “Then there’s this rocky, dusty material and some organic material. There’s a whole host of other volatiles – carbon dioxide, carbon monoxide, methanol. Most comets seem to have those materials in them, but the ratios do vary.”
One unique feature of the mission will be to allow the measurement of carbon dioxide in the nucleus, a task that is not possible from Earth.
“It’s very difficult to observe some of the chemical species when they’re far away and faint,” Combi said in a statement. “Carbon dioxide is probably the second most abundant species at most comets, but it’s not been observed in the thousands we’ve looked at from Earth.”
Another priority will be to study the solar wind, which can produce solar storms.
Composed of a stream of charged particles, the solar wind exhibits different velocities at different locations on the sun. Since Earth’s orbit takes it closest to the equator of the sun, it is difficult for scientists to study the dynamics of the solar wind at higher solar latitudes.
“But comets pass through all of it. With their help, we can study the fast solar wind,” Tomas Gombosi, a professor of engineering in UM’s Department of Atmospheric, Oceanic, and Space Sciences and another researcher on the Rosetta team, said in a statement.
Of course, as relics from the earliest days of the solar system, Rosetta should also provide significant insights about the distant past of our stellar neighborhood.
“Comets, more than most other objects in the solar system, preserve information about the solar system when it was formed,” Gombi explained.
The Rosetta mission was designed to replace a similar undertaking conceived by NASA called the Comet Rendezvous Asteroid Flyby during the 1980s. That proposed mission would have involved flying a probe alongside a comet for three years. It was cancelled in 1992 as a result of budget constraints facing the American space agency at that time.
The Rosetta mission will end in 2015 after completing the orbit of the sun alongside comet 67P/Churyumov-Gerasimenko.
The Rosetta spacecraft will come closest to the sun on Aug. 13, 2015, when it and the comet are about 185 million kilometers away from our star.
Artist’s conception of orbiter Rosetta and lander Philae at comet 67P/Churyumov-Gerasimenko courtesy European Space Agency.
Graphic showing dust composition of cometary nucleus courtesy University of Michigan; graphic by Valeriy Tenishev.
NOTE: This article also appears at Examiner.com.