Nov 122017
 

On this date in 2014 the lander module Philae detached from the Rosetta space probe built by the European Space Agency and landed on comet Churyumov–Gerasimenko (a.k.a. 67P) at 15:33 UTC.

Rosetta was set to be launched on 12 January 2003 to rendezvous with the comet 46P/Wirtanen in 2011. This plan was abandoned after the failure of an Ariane 5 carrier rocket during Hot Bird 7’s launch on 11 December 2002, grounding it until the cause of the failure could be determined. In May 2003, a new plan was formed to target the comet 67P/Churyumov–Gerasimenko, with a revised launch date of 26 February 2004 and comet rendezvous in 2014. The larger mass and the resulting increased impact velocity made modification of the landing gear necessary.

After two scrubbed launch attempts, Rosetta was launched on 2 March 2004 at 07:17 UTC from the Guiana Space Centre in French Guiana. Aside from the changes made to launch time and target, the mission profile remained almost identical. Both co-discoverers of the comet, Klim Churyumov and Svetlana Gerasimenko, were present at the spaceport during the launch.

To achieve the required velocity to rendezvous with 67P, Rosetta used gravity assist maneuvers to accelerate throughout the inner Solar System. The comet’s orbit was known before Rosetta’s launch, from ground-based measurements, to an accuracy of approximately 100 km (62 mi). Information gathered by the onboard cameras beginning at a distance of 24 million kilometers (15,000,000 mi) were processed at ESA’s Operation Centre to refine the position of the comet in its orbit to a few kilometres.

On 25 February 2007, the craft was scheduled for a low-altitude flyby of Mars, to correct the trajectory. This was not without risk, as the estimated altitude of the flyby was a mere 250 kilometers (160 mi). During that encounter, the solar panels could not be used since the craft was in the planet’s shadow, where it would not receive any solar light for 15 minutes, causing a dangerous shortage of power. The craft was therefore put into standby mode, with no possibility to communicate, flying on batteries that were originally not designed for this task. This Mars maneuver was therefore nicknamed “The Billion Euro Gamble”. The flyby was successful, with Rosetta even returning detailed images of the surface and atmosphere of the planet, and the mission continued as planned.

The second Earth flyby was on 13 November 2007 at a distance of 5,700 km (3,500 mi).] In observations made on 7 and 8 November, Rosetta was briefly mistaken for a near-Earth asteroid about 20 m (66 ft) in diameter by an astronomer of the Catalina Sky Survey and was given the provisional designation 2007 VN84. Calculations showed that it would pass very close to Earth, which led to speculation that it could impact Earth.[73] However, astronomer Denis Denisenko recognized that the trajectory matched that of Rosetta, which the Minor Planet Center confirmed in an editorial release on 9 November.

The spacecraft performed a close flyby of asteroid 2867 Šteins on 5 September 2008. Its onboard cameras were used to fine-tune the trajectory, achieving a minimum separation of less than 800 km (500 mi). Onboard instruments measured the asteroid from 4 August to 10 September. Maximum relative speed between the two objects during the flyby was 8.6 km/s (19,000 mph; 31,000 km/h). Rosetta’s third and final flyby of Earth happened on 12 November 2009.

On 10 July 2010, Rosetta flew by 21 Lutetia, a large main-belt asteroid, at a minimum distance of 3,168±7.5 km (1,969±4.7 mi) at a velocity of 15 kilometers per second (9.3 mi/s). The flyby provided images of up to 60 meters (200 ft) per pixel resolution and covered about 50% of the surface, mostly in the northern hemisphere. The 462 images were obtained in 21 narrow- and broad-band filters extending from 0.24 to 1 μm. Lutetia was also observed by the visible–near-infrared imaging spectrometer VIRTIS, and measurements of the magnetic field and plasma environment were taken as well.

In May 2014, Rosetta began a series of eight burns. These reduced the relative velocity between the spacecraft and 67P from 775 m/s (2,540 ft/s) to 7.9 m/s (26 ft/s). In 2006, Rosetta suffered a leak in its reaction control system (RCS). The system, which consists of 24 bipropellant 10-newton thrusters, was responsible for fine tuning the trajectory of Rosetta throughout its journey. The RCS operated at a lower pressure than designed due to the leak. While this may have caused the propellants to mix incompletely and burn ‘dirtier’ and less efficiently, ESA engineers were confident that the spacecraft would have sufficient fuel reserves to allow for the successful completion of the mission.

Rosetta’s reaction wheels also showed higher than expected friction levels, though testing during the deep space hibernation period revealed the system could be operated safety at much slower speeds reducing the bearing friction noise. Before hibernation, two of the spacecraft’s four reaction wheels began exhibiting increased levels of “bearing friction noise” and one was turned off after the encounter with Lutetia to avoid possible failure. Engineers turned on all 4 wheels after the spacecraft awoke from Deep Space Hibernation in January 2014, ran them at lower speeds and elevated the control settings on the bearing heaters using an On-board Control Procedure to help reduce the level of bearing friction noise seen on 2 of the Reactions Wheels prior to Deep Space HIbernation. These changes allowed all 4 Reaction Wheels to be used throughout the period Rosetta was in orbit around 67P/Churyumov–Gerasimenko. Additionally, new software was uploaded which would allow Rosetta to function with only two active reaction wheels if necessary.

In August 2014, Rosetta made a rendezvous with the comet 67P/Churyumov–Gerasimenko and commenced a series of maneuvers that took it on two successive triangular paths, averaging 100 and 50 kilometers (62 and 31 mi) from the nucleus, whose segments are hyperbolic escape trajectories alternating with thruster burns. After closing to within about 30 km (19 mi) from the comet on 10 September, the spacecraft entered actual orbit about it.

The surface layout of 67P was unknown before Rosetta’s arrival. The orbiter mapped the comet in anticipation of detaching its lander. By 25 August 2014, five potential landing sites had been determined. On 15 September 2014, ESA announced Site J, named Agilkia in honour of Agilkia Island by an ESA public contest and located on the “head” of the comet, as the lander’s destination.

Philae detached from Rosetta on 12 November 2014 at 08:35 UTC, and approached 67P at a relative speed of about 1 m/s (3.6 km/h; 2.2 mph). It initially landed on 67P at 15:33 UTC, but bounced twice, coming to rest at 17:33 UTC. Confirmation of contact with 67P reached Earth at 16:03 UTC. On contact with the surface, two harpoons were to be fired into the comet to prevent the lander from bouncing off, as the comet’s escape velocity is only around 1 m/s (3.6 km/h; 2.2 mph). Analysis of telemetry indicated that the surface at the initial touchdown site is relatively soft, covered with a layer of granular material about 0.82 feet (0.25 meters) deep, and that the harpoons had not fired upon landing.

After landing on the comet, Philae had been scheduled to commence its science mission, which included:

Characterization of the nucleus

Determination of the chemical compounds present, including amino acid enantiomers

Study of comet activities and developments over time

After bouncing, Philae settled in the shadow of a cliff, canted at an angle of around 30 degrees. This made it unable to adequately collect solar power, and it lost contact with Rosetta when its batteries ran out after two days, well before much of the planned science objectives could be attempted. Contact was briefly and intermittently reestablished several months later at various times between 13 June and 9 July, before contact was lost once again. There was no communication afterwards, and the transmitter to communicate with Philae was switched off in July 2016 to reduce power consumption of the probe. The precise location of the lander was discovered in September 2016 when Rosetta came closer to the comet and took high-resolution pictures of its surface. Knowing its exact location provides information needed to put Philae’s two days of science into proper context.

Researchers expect the study of data gathered will continue for decades to come. One of the first discoveries was that the magnetic field of 67P oscillated at 40–50 millihertz. A German composer and sound designer created an artistic rendition from the measured data to make it audible. Although it is a natural phenomenon, it has been described as a “song” and has been compared to Continuum for harpsichord by György Ligeti. However, results from Philae’s landing show that the comet’s nucleus has no magnetic field, and that the field originally detected by Rosetta is likely caused by the solar wind.

The isotopic signature of water vapor from comet 67P, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. That is, the ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it very unlikely that water found on Earth came from comets such as comet 67P, according to the scientists. On 22 January 2015, NASA reported that, between June and August 2014, the rate at which water vapor was released by the comet increased up to tenfold.

On 2 June 2015, NASA reported that the ALICE spectrograph on Rosetta determined that electrons within 1 km (0.6 mi) above the comet nucleus — produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier — are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.

I don’t have any great ideas for food recipes to celebrate a module landing on a comet, but I do have two ideas for recipes in a wider sense. Once is a “recipe” for making a comet, or a simulacrum of a comet made out of common items, most of which are available in the kitchen. If you go on YouTube and search for “comet recipe” you will find any number of videos of people replicating the structure of comets using household items.  Here’s one:

That recipe does not produce something edible, however. On the other hand, there are quite a few recipes for cocktails called “comet.” They are all quite different from one another, and none, in my opinion, evokes comets in any way. I don’t drink alcohol any more, but when I did I had some memorable experiences with blackcurrant vodka, so this one struck a chord:

Comet Cocktail

Ingredients

30ml Smirnoff Double Black vodka
10ml blackcurrant cordial
60ml pineapple juice
lemon wedge

Instructions

Shake the vodka, blackcurrant cordial, and pineapple juice in a cocktails shaker.  Pour over cracked ice in a glass and garnish with a lemon wedge.

 

 

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