On a comet

AS Tony Phillips, production editor of Science@NASA, explains in a NASA video, interplanetary space missions come in three categories: “difficult, more difficult and ridiculously difficult”.

Spacecraft fly-bys, in which satellites travel hundreds of millions of kilometres towards a distant target—planet, moon or asteroid—and fly past it at 30-50,000 kilometres/hour, clicking thousands of pictures and collecting as much data as possible during the brief rendezvous, are “difficult”.

Getting a satellite into orbit around one such object is “more difficult”. It requires the spacecraft to approach the target along a precise trajectory, apply brakes by retro-firing its thruster rockets to the right amount so that it is captured by the gravitational pull of the object into an orbit around it at the desired altitude.

Landing a lander or a rover on a planetary object is “ridiculously difficult”. It calls for getting the mother spacecraft into orbit around the object first and then release the lander/rover to retrorocket and parachute slowly for a touchdown at a designated location on the surface of the object.

Over five decades or so, world space scientists and engineers have even succeeded in a dozen or so such “ridiculously difficult” missions. But on November 12, the European Space Agency (ESA) achieved what could only be termed as the most extreme of “ridiculously difficult” missions by successfully dropping a three-legged robotic lander from its mother spacecraft Rosetta on to a comet called Churyumov-Gerasimenko (also designated as 67P) that is only about 4 km across (see box), as it is whizzing towards the sun at a speed of about 66,000 km/hr.

This is the first-ever successful landing on a comet. About a dozen missions have been sent to visit comets to date, but they were all high-speed fly-bys that provided fleeting glimpses into the comet structure and composition.

Complex, inter-planetary route

Rosetta is perhaps one of the most complex and ambitious missions ever undertaken. Its main objective is to rendezvous with, and enter into an orbit around, the comet C-G and deploy a lander on its surface to observe its nucleus and coma. For this purpose, the payloads of the orbiter spacecraft Rosetta and the lander named Philae that it carried comprise two separate suites of instruments—11 in Rosetta to study the comet’s global and local environment, surface and sub-surface features, and 10 in Philae to investigate the comet’s local environment, surface and sub-surface composition. The orbiter measures 2.8 m x 2.1 m x 2.0 m with two 14 m solar wings; the lander is a cubic structure (before deployment of the landing gear) of one-metre side. The launch mass of the spacecraft was three tonnes, of which the orbiter Rosetta weighed 2.9 tonnes, carrying 165 kg of science payload, and the lander weighed 100 kg with 26.7 kg of science payload.

Launched on March 2, 2004, Rosetta had its first rendezvous with the comet C-G on August 6 after a decade-long complex interplanetary travel that covered a distance of 6.55 billion km, which included three earth fly-bys, one Mars fly-by, encounters with two asteroids Steins and Lutetia, followed by 957 days of hibernation before it woke up on January 20 to head towards the comet in its unique mission. On November 12, it successfully deployed the lander Philae on the comet’s surface.

Rosetta will also follow the comet studying its increasing activity under the influence of increasing intensity of the sun’s radiation as it travels towards its perihelion (186 million km from the sun) and then loops back towards the colder and outer reaches of the Solar System with decreasing activity as it moves away from the sun towards its aphelion (850 m km from the sun), close to the orbit of Jupiter. Rosetta will reach its perihelion on August 13, 2015.

The Rosetta mission is designed to last until December 2015. Its mission is to study the comet in detail, to monitor its transformation during the course of more than a year, and put all earlier cometary observations in a comprehensive perspective. According to the ESA, the mission was conceived in the 1980s, even before its Giotto mission, which flew by the famous comet 1P/Halley and took the first detailed picture of a comet nucleus. It became a reality 20 years down the line and was scheduled to be launched in January 2003 aboard the French Ariane 5 launcher. But in December 2002, another Ariane 5 launch failed. Pending investigations into the failure, the €1 billion Rosetta mission had to be postponed though it meant a huge loss.

The mission’s original target was the comet called 46P/Wirtanen. Searching for alternative comets for Rosetta to visit and study, the scientists settled on the comet C-G, a somewhat more massive comet. This would mean a faster landing speed for Philae, which called for a strengthening of the lander’s legs.

At launch, Rosetta was at a distance of 585 m km from the comet. To reach the comet, because of several technical considerations, Rosetta could not head straight for it. Instead, it executed a series of loops around the sun so that it could come back for the earth and Mars fly-bys. The swings past these planets were to extract gravitational energy from them so that the velocity and the trajectory of the spacecraft changed suitably to travel towards the comet. The Mars fly-by was a critical operation as Rosetta’s new trajectory towards C-G required it to fly past Mars at just 250 km above the surface and 24 minutes in Mars’ shadow. For the C-G mission the spacecraft had to be reprogrammed totally as the earlier target, Wirtanen, did not require Rosetta to be in Mars’ shadow.

Problems along the way

In 2006, the engineers discovered that the spacecraft had developed a leak in the Reaction Control System, because of which the fuel tanks could not be pressurised further. But the decade-long cruise helped the engineers to learn how to handle Rosetta’s thrusters at slightly reduced efficiency. There were other problems, for instance, with two of the four reaction wheels, which are used to orient the spacecraft so that the instruments can point to the comet, the solar arrays to the sun, and the main antenna to the earth. An alternative software had to be developed and uploaded so that the spacecraft could function with just two reaction wheels operating at lower than the designed speeds.

The en-route encounters with the two asteroids gave scientists the opportunity not only to gain experience with the instruments and the navigation system but also to study these objects. In the process, however, the team discovered that the OSIRIS science camera and the navigation cameras did not function properly when Rosetta flew past the asteroid Steins. However, the engineers managed to fix the problems, and the next asteroid encounter (with Lutetia) in July 2010 was trouble-free. In fact, the mission has obtained a wealth of complex geological data about the asteroid, which are being studied.

In July 2010, the spacecraft was travelling at a speed of 54,000 km/hr. But even at that speed, since there were four more years to the comet rendezvous, and also since the spacecraft was too far from the sun to be able to energise even its advanced technology-based exceptionally efficient solar cells, it meant that Rosetta had to be put to sleep for two years, seven months and 12 days. So everything, except its on-board computer, was sent into hibernation until January 20, 2014.

Rendezvous with C-G

The rendezvous with C-G took place when the spacecraft was at a distance of 100 km from the comet. The gravitational field of the comet, whose mass is estimated to be 10 trillion tonnes, is extremely weak: it is about a hundred thousandth of the earth’s gravitational pull. So from a 100-km distance the spacecraft had to be manoeuvred down to much lower heights for it to be captured into an orbit around the comet. This was done through a sequence of trajectory manoeuvres called “pyramid trajectories”. The final orbit the spacecraft achieved was at an altitude of 30 km from the comet’s surface.

The comet’s weird shape, revealed by the on-board instruments in the weeks prior to the rendezvous date, meant that the weak and complex gravitational field had to be characterised very precisely to enable a correct trajectory manoeuvre. (Characterising the comet meant determining the shape, rotation rate and orientation, gravity field, reflectivity, surface features and surface temperature of the nucleus. It also required measuring the outgassing and quantifying the density and velocity distribution of particles in the coma, the envelope of gas and dust surrounding the comet.)

Because of negligible surface gravity, a sophisticated system with critical operations formed part of the lander to ensure that Philae was secured to the ground. At the moment of touchdown, the probe’s three-legged landing gear absorbs the impact. Several coordinated actions, which are critical, should follow at once. Ice screws in each footpad begin drilling immediately on contact; harpoons shoot into the ground and lines retract to pull the probe down to the surface; and, the upward-firing thruster ignites to counter the recoil caused by the launch of the harpoons.

By August 24, on the basis of the data collected, five candidate sites had been identified for further analysis, three on the head and two on the body and none on the neck. After a detailed analysis of Philae’s capabilities and the site characteristics over just six weeks, the site J, situated on the smaller lobe or the head, was identified as the best choice. This was later christened Agilkia following a naming competition. As a back-up to the primary site, C on the larger lobe or body was also chosen.

The comet’s increasing activity as it headed towards the sun meant that the lander had to be deployed before this activity rose to levels that could prevent a safe landing. On the other hand, it could not take place too early because of the need for sufficient sunlight to keep the lander’s solar cells energised in order to power the lander for weeks after the landing. Also, the surface temperature had to be suitable for the lander to operate. On October 14, the go-ahead to land at J on November 12 was given. The lander was programmed to touch down inside the “landing ellipse”, roughly a few hundred metres across.

Multiple bounces during descent

The descent, from a height of 22.5 km, took about seven hours, with Philae dropping slowly without propulsion or guidance, gradually gathering speed in the comet’s weak gravitational field, with its attitude stabilised via an internal gyroscope. Philae successfully reached the surface at roughly the walking pace of about one metre per second on its first touchdown, which occurred at 9 p.m. IST.

However, the landing did not turn out to be perfect as later communication with Philae revealed that despite a safe and precise first touchdown the lander had bounced twice nearly 1 km back into space. The final landing was at a location nearly 1 km from the target site. The second touchdown was almost two hours after the first contact. The second bounce was softer but did make Philae airborne. The final soft landing on the comet occurred about an hour later.

However, Philae’s final position is at a site where there is not enough light for the solar panels to operate. It seems to have been dangerously perched in the shadow of a cliff with one of its legs hanging in space instead of being grounded. But the lander, despite its precarious position orientation, was stated to be stable enough to carry out its scientific measurements. The multiple bounces seem to have occurred because of the failure of the harpoons to fire, which otherwise would have anchored the lander securely. The upward thruster countering the harpoon recoil also did not fire.

As a result, the lander’s solar panels were receiving half an hour of sunlight instead of six to seven hours, according to the ESA’s Stephan Ulemac, the project manager for the lander. One of the objectives of the lander mission was to drill into the comet to collect and analyse samples. Since this could affect Philae’s stability, this was postponed to the end of the 60-hour period. Philae’s final location is, however, not exactly known, though high-resolution images are being scrutinised to pinpoint the location. There was, however, steady communication between Philae and Rosetta. Scientific data, including drilling for sample, were gathered during its primary science mission, which lasted for 57 hours, according to the ESA. During this phase, Philae’s body was lifted by about 4 centimetres and rotated by about 35° in an attempt to gather more sunlight. But this did not improve matters much, as on November 15 the lander’s power rapidly depleted and Philae went into hibernation.

Unprecedented images

Despite the hitches, unprecedented images of the surroundings have been returned during Philae’s descent, according to ESA. The surface of the comet has been found to be covered by dust and debris ranging from millimetre- to metre-size material. Panoramic images have revealed layered walls of harder material. As part of the experiment called MUPUS, the probe was hammered into the comet surface, yielding data suggesting that the comet surface is as hard as ice.

As for the orbiter, even as it is ever ready to receive communication from Philae whenever its batteries get recharged, until February 2015 it will continue to stay close to the comet. From February onwards, depending upon the increased cometary activity, Rosetta will move out of the bound orbit and carry out a series of fly-bys, including one as close as within 8 km of the centre of the nucleus! This phase of the mission will focus on studying and monitoring the comet, including its surface and surrounding coma. If nothing goes wrong, Rosetta is expected to be operational in this phase until March 2015. In July 2015, it will be flown directly over an active region where jets of material are ejected from the comet’s surface. Meanwhile, Rosetta has been moved back into a 30-km orbit from the altitude of 22 km from where it deployed Philae. It will return to a 20-km orbit on December 6 and continue its mission to study the comet as it becomes more active on its way to its closest encounter with the sun on August 13, 2015. As the comet starts to cool again and the activity subsides, Rosetta will study C-G until the end of its nominal mission in December 2015 at least to see if the activity decreases in a symmetrical way to increase during approach towards the sun.

As Matt Taylor, the ESA’s Rosetta project scientist, remarked, the data collected by Philae and Rosetta are set to make the mission a game-changer in cometary science. Despite the unsatisfactory landing, this space mission was truly historic.