Rosetta and Philae

Image description: View of the disturbed surface of the Comet Churyumov–Gerasimenko taken by Rosetta's camera from an altitude of a few kilometers. It is understood that on this chaotic surface, the landing gear had to deploy correctly, the thruster had to perfectly press Philae onto the ground, and the harpoons had to function synchronously to secure the robot to the ground. Image source: European Space Agency - ESA.

The Philae Robot

Philae, the robot carried by the Rosetta probe, landed on comet 67P/Churyumov–Gerasimenko on November 12, 2014. Released at 9:35 AM Paris time, at a distance of 22.5 km from the center of the comet, the unpropelled robot touched the comet's sandy surface 7 hours later at a speed of about 1 m/s. The site where Philae landed is named Agilkia. This is the first cometary landing ever attempted by humans.

To reach Earth, the signal confirming this historic landing took about 40 minutes and arrived at 16:03 UTC. The confirmation was relayed by the Rosetta orbiter and simultaneously captured by the ESA ground station in Malargüe, Argentina, and the NASA station in Madrid, Spain. The first data from the lander's instruments were transmitted to the space agency's navigation center in Toulouse, France.

Landing on such a small comet is a real feat given the object's extremely low gravity. The 100 kg Philae lander robot weighs only a few grams on the surface of "Tchouri". For ten years, since March 2, 2004, Rosetta and Philae traveled together to reach their destination. Since August 6, 2014, Rosetta has been orbiting the comet, analyzing its surface to choose a landing site for Philae.

The Landing Site

The landing site is located on the head of the two-lobed object and was chosen based on images and data collected at distances of 30-100 km from the comet, only six weeks after arrival. At the end of a very long journey of 6.4 billion km, on November 12, 2014, Rosetta and Philae separated.

The main objective of the mission was to reach the periodic comet Churyumov–Gerasimenko (one passage every 6.59 years), then to land the 100 kg Philae robot on its surface. The ESA's Rosetta probe spent a lot of time (the second half of October 2014) in orbit around the comet, less than 10 km from its surface. Magnificent and unprecedented images taken by Rosetta's navigation camera reached us during this period. They show us this disturbing and mysterious world that has imprisoned Philae for eternity.

The Rosetta Comet

The comet is extremely dark, much darker than in the image, darker than coal, but the camera has enhanced the light and shadows of this ancient relic of our solar system. These first images revealed a world strewn with rocks, cliffs, pits, and impressive precipices.

Rosetta will reach its closest distance to the Sun on August 13, 2015, at about 185 million km between the orbits of Earth and Mars. Rosetta will follow the comet and monitor Philae throughout 2015, moving away from the Sun, then Rosetta and Philae will fall asleep in the icy space of the solar system.

N.B.: The Rosetta Stone, discovered in 1799, is an inscribed stele from ancient Egypt on which the same text is written in three versions, which allowed the deciphering of hieroglyphs at the beginning of the 19th century. At the top of the stone, the text is in Egyptian hieroglyphs, in the center, the same text in Demotic script, and at the bottom in ancient Greek.
The Rosetta Stone measures 112.3 cm in height, 75.7 cm in width, 28.4 cm in thickness, and weighs ≈760 kg.

N.B.: Philae was an ancient Egyptian city on the banks of the Nile that housed one of the best-preserved temples of Isis. Since the commissioning of the High Dam of Aswan in 1970, Philae is only a rock emerging from the lake. All the constructions placed on the granite soil of Philae, stone by stone, have been transported to another granite islet named Agilkia.

What is the Weight of Philae on the Comet Churyumov–Gerasimenko?

Philae, despite the malfunction of the anchoring system, remained on the comet's surface. The Philae robot landed well on the comet Churyumov–Gerasimenko at the location intended by the scientists, but the harpooning that was supposed to secure Philae to the ground went wrong. The harpoons were supposed to be fired by an explosive system. But Philae, under the thrust of the shock absorbers, bounced. This malfunction, which could have ejected the robot from the comet forever, was not catastrophic; Philae landed on soft ground. According to astrophysicist Francis Rocard, if the ground had been hard, Philae would have experienced a violent rebound. Indeed, the system only sank 4 cm, while the maximum amplitude of the shock absorber is 20 cm.

Then, according to Stephan Ulamec (lander manager), Philae made "a huge leap" of about one kilometer. Due to the low gravity, this leap occurred in slow motion and lasted nearly two hours. Fortunately, the landing ended well, but Philae ended up in an uncomfortable position in the shade and with one leg in the void, which will pose a problem for recharging its batteries.

Calculation of Philae's Weight on the Comet

To calculate the weight of Philae on Churyumov–Gerasimenko, i.e., the force of gravity on the comet, several parameters must be used: the gravitational constant G, the acceleration due to gravity g, the mass of the comet M, the mass of Philae m, and the radius of the comet R, or rather R min and R max since it is not a spherical object.

• G = 6.67384 × 10−11 N·m2·kg-2
• g = G (M/R2) in m/s2
• M = 1×1013 kg
• m = 100 kg
• R min and max = ≈0.65 km and ≈2.05 km (dimensions of the comet's lobes) (4.1×3.2×1.3 km) + (2.5×2.5×2.0 km).

All that remains is to calculate the gravitational force exerted by the comet on Philae, where F=G ((Mm)/R2) newtons or m·kg·s-2. The gravitational force is ≈0.01588 N at the farthest from the comet's center and 0.1579 N at the closest. The weight of Philae is therefore ≈1.5 g at the farthest from the comet's center and ≈15.7 g at the closest. Scientists must be very delicate in piloting Philae's instruments. A very small uncontrolled push would be enough to satellite the robot and lose it forever.

N.B.: Mass and weight should not be confused, although on Earth mass and weight are often confused. The weight of a body is the force exerted by the ambient gravitational field. It therefore varies depending on the location. Mass corresponds to a quantity of matter (number of atoms); it does not change depending on the location; it is the same throughout the Universe. Weight is measured in newtons (N), mass in kilograms (kg). Mass and weight are different quantities but related to each other by the relationship: weight = mass × g (g represents the acceleration or intensity of gravity). Acceleration is therefore the change in speed in meters per second every second or meters per second squared (m/s2). Example: if a car goes from 0 to 100 km/h in 5 s, it undergoes an acceleration of (100 km/h)/(5 s) = 20 (km/h)/s = 5.6 m/s2 ≈0.56 g.