February 1, 2015

Wonder how difficult it would be to land a mosquito on a speeding bullet?  What do you think?  Well, that’s just about the degree of difficulty in launching, navigating and landing the PHILAE spacecraft on the comet 67P/Churyumov–Gerasimenko.  Like all comets, Churyumov-Gerasimenko is named after its discoverers.


It was first observed in 1969, when several astronomers from Kiev visited the Alma-Ata Astrophysical Institute in Kazakhstan to conduct a survey of comets.  Comet 67P is one of numerous short period comets which have orbital periods of less than 20 years and a low orbital inclination. Since their orbits are controlled by Jupiter’s gravity, they are also called Jupiter Family comets.  These comets are believed to originate from the Kuiper Belt, a large reservoir of small icy bodies located just beyond Neptune. As a result of collisions or gravitational perturbations, some of these icy objects are ejected from the Kuiper Belt and fall towards the Sun.

When they cross the orbit of Jupiter, the comets gravitationally interact with the massive planet. Their orbits gradually change as a result of these interactions until they are eventually thrown out of the Solar System or collide with another planet or the Sun.  Actually, the favored target for Rosetta was the periodic comet 46P/Wirtanen, but, after the launch was delayed, another regular visitor to the inner Solar System, 67P/Churyumov-Gerasimenko, was selected as a suitable replacement.


Philae (/ˈfli/ or /ˈfl/) is a robotic device designed and launched by the European Space Agency .  The mission was called Rosetta. In November 1993, the International Rosetta Mission was approved as a Cornerstone Mission in ESA’s Horizons 2000 Science Program.  Rosetta’s industrial team involved more than 50 contractors from 14 European countries and the United States. The prime spacecraft contractor is Astrium Germany. Major subcontractors are Astrium UK (spacecraft platform), Astrium France (spacecraft avionics) and Alenia Spazio (assembly, integration and verification).

The duration of travel was more than ten years after departing Earth. (Now do you see the complexity?  It’s a “tough putt” to land a small object on a rapidly moving object and after a ten-year launch.)  The Rosetta spacecraft is a work of engineering art in itself. It’s basically a large aluminum box measuring 2.8 x 2.1 x 2.0 meters with scientific instruments mounted on ‘top’ of the box forming the Payload Support Module while the subsystems are on the base or the Bus Support Module.

On one side of the orbiter is a 2.2-metre diameter communications dish with a steerable high-gain antenna. The Lander itself is attached to the opposite face.

Two enormous solar panel ‘wings’ extend from the sides. These wings, each 32 square meters in area, have a total span of about 32 meters tip to tip. Each assembly comprises five panels, and both may be rotated +/-180 degrees to catch the maximum amount of sunlight. A digital photograph of the Rosetta is given as follows:


On 12 November 2014, the probe achieved the first-ever soft landing on a comet nucleus. Its instruments obtained the first images from a comet’s surface. PHILEA is tracked and operated from the European Space Operations Centre (ESOC) in Darmstadt, Germany.  Several of the instruments on PHILEA made the first direct analysis of a comet, sending back data that will be analyzed to determine the composition of the surface.

The Lander is named after the Philae obelisk, which bears a bilingual inscription and was used along with the Rosetta Stone to decipher Egyptian hieroglyphics.  A very condensed version of the mission is given by the JPEG below:


An Ariane 5G+ rocket carrying the Rosetta spacecraft and PHILAE Lander  was launched from French Guiana on 2 March 2004, and travelled for 3,907 days (10.7 years) to reach the target–Churyumov–Gerasimenko. Unlike a Deep Impact probe, PHILAE is not an impactor. Some of the instruments on the Lander were used for the first time as autonomous systems during the Mars flyby on 25 February 2007.   One camera system returned images while the Rosetta instruments were powered down, while one system took measurements of the Martian magnetosphere. Most of the other instruments need contact with the surface for analysis and stayed offline during the flyby. An optimistic estimate of mission length following touchdown was “four to five months”.


Components of PHILAE are as follows:


A digital photograph of the Lander with the basic instrument packages is given below.



The results of the landing and the investigation are striking.  The comet’s surface, as Nicolas Thomas of the University of Bern has discovered, is surprisingly complex. It has 19 distinct regions, characterized by features such as pits, wide depressions and smooth, dust-covered plains. It even sports things that look like sand dunes.

The surface is also, according to Fabrizio Capaccioni of the National Institute of Astrophysics  in Rome, drier than expected and rich in organic compounds. That may excite those who wonder how the chemicals needed for life’s development arrived on Earth. The comet’s interior, meanwhile, says Holger Sierks of the Max Planck Institute for Solar System Research, in Göttingen, Germany, has only half the density of water. It is therefore probably porous and fluffy. And it ejects jets of material into space particularly from the neck that connects the two halves of the comet’s peculiar dumbbell shape.

The reason for that shape, though, remains a mystery. Possibly, Dr Sierks speculates, Churyumov-Gerasimenko is made up of two comets which have collided and joined together. Determining the truth of this will require further investigation.  A depiction of the comets configuration is given as follows:



Number one—we know now that navigation and impact can be accomplished.  With that being the case, maybe mining the subterranian riches for minerals might be possible for a great number of comets.  One greater “find” might be adding one piece to the puzzle as to whether or not there is life other places than Earth.  We are just becoming able to investigate that possibility with marvelous devices such as Rosetta and PHILAE.  Time will tell.

As always, I welcome your comments.


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