March 24, 2015

Questions about Rosetta mission

Why those names Rosetta and Philae (Lander name)?

The Rosetta stone and the obelisk of Philae were used as complementary bases to the deciphering of the hieroglyphs by J.F. Champollion, in 1822. The Rosetta orbiter and the Philae lander will be used as complementary bases for the comprehension of comets, vestiges of the solar system.

What are the expected results?

The ideal conditions of observation of the nucleus (at short distance, low speed and during a long duration), as well as the surface analyses, will make possible to confirm or will oblige to change the models and theories currently used concerning the formation of the solar system, comets and their possible link with the appearance of the life on Earth.

Why is this mission so important for Europe?

The Giotto mission had placed, in 1986, Europe in the world first place in cometary science. By carrying out those two world "first" (orbiting around a comet nucleus and landing on its surface), the Rosetta mission will give again this place to our scientific community.

What is the importance of the Lander for this mission?

The Philae Lander will reach the "ground truth", that is soil components before they are broken by solar radiation after sublimation. They will make possible to calibrate the Orbiter's instruments. In that way, the Lander and the orbiter are complementary.
Thus we can consider that approximately 50% of the scientific return will be due to the Lander.

What were the impacts of the launch delay from January 2003 to February 2004?

The launch delay resulted in finding a new target satisfying the scientific goal but also the technical constraints. After examination of the various possibilities, the comet Churyumov Gerasimenko was finally selected.
As a consequence, the mission profile had to be re-examined and thus the cruise phase lasts now 10 years and needs 4 gravitational assistances (3 times Earth, 1 time Mars). Arrival date was postponed from 2012 to 2014.
At last, the new comet being larger, therefore probably more massive, we have been led to re-examine the sequence of separation, descent and landing of the lander as well as improving its landing gear to make it more stable.

Why does the comet have this name?

Churyumov Gerasimenko corresponds to the names of the 2 Ukrainian astronomers who discovered it in 1969. It is the 67th known periodic comet.

Why choose this comet?

The choice of the target comet is constrained by technical requirements coming from the spacecraft (min and max distances to the sun, fuel), of the launcher (conditions on the orbit injection and mass of the spacecraft) but also by the scientific interest. In the considered time scale (launch in 2003/2005), only 2 candidates were possible: the Wirtanen comet, initially retained for the launch in 2003 and the comet Churyumov Gerasimenko, for a launch in 2004 or 2005. The mission towards the comet Churyumov Gerasimenko was finally selected because it allows a launch by ARIANE 5 and offered a second launch opportunity in 2005.

Why did the launch took place during a limited launch window?

Conditions of rendez-vous with the comet and planets flybies for gravitational assistances are very accurates. It is thus necessary to await an ideal relative positioning between the Earth, Mars and the target comet, which defines the launch time period between the 26/02/04 and the 17/03/04. Furthermore, it is necessary, in this time period, to wait the hour which is optimal according to the considered launch pad and the rotation of the Earth.

Why is the journey so long?

The current techniques of propulsion do not make possible to take a more direct and shorter trajectory. Thus it is necessary to take a run-up from the gravity fields of the Earth and Mars to modify "naturally" the trajectory. This strategy forces to make several orbits around the sun, which lengthens the mission.

How can you be sure that the spacecraft will still be active after 10 years?

Durations of 10 years for a space mission are now very current (a telecommunication satellite usually has a higher lifetime).
To increase the reliability probability of correct operation of the spacecraft, it was put in dormant mode when it was possible, tested in flight every 6 months in average (platform and payload check-outs). Each planet and asteroid fly-by was an opportunity for calibration or scientific operations. In 2011-2013 period when power on board was minimal, the spacecraft was placed in hibernation mode. After exit of this mode, a new commissioning phase takes place comprising refreshing of on board software.
Lastly, to store the necessary information for the operations, a specific plan of knowledge preservation has been put in place for the scientific, engineering and operational teams.

Why is this mission difficult?

To land on a 4 km diameter comet at ½ billion km from the Earth represents the same performance as to land on a precise point of a coin of 1 euro cent located in Berlin when you are in Paris...
This mean size is even not precisely known. Lander and orbiter were designed without knowing the target body contrarily to all other space projects.

Which were the technical difficulties that had to be solved?

For the main satellite (the orbiter), the 2 main technical difficulties come from the distances. From the sun (a little less than one billion km): solar cells and specific elements of thermal control had to be developed for this mission. From the Earth: the communications are very critical, in volume and data rate as in travel times (more than 40 minutes), and thus require a very autonomous satellite.
For Philae, the major difficulty was to design landing gear, harpoons, separation mechanism and lot of units making a landing and a functioning possible whatever a broad range of characteristics of the nucleus (hardness, period of rotation, outgassing...).

What happened after the launch?

A lot of activities were performed until the arrival to the comet: the first ones consisted of the commissioning of the platform and its 12 instruments in 2004. The cruise phase followed with its 4 planet flybies. Crossing for the first time the main asteroid belt the probe flybied asteroid Steins on 5 of September 2008. The second crossing allowed to pass over asteroid Lutetia on 10 of July 2010. From 8 of June 2011 to 20 of January 2014 the probe hibernated (that is controlled its temperature only). In October 2012 Rosetta was at its highest distance to Sun and since then comes back towards it approaching the comet. Rosetta will be in close vicinity in August 2014.

What are the main activities in 2014?

Once the probe is slowed down by a series of braking in May and June to achieve a relative velocity of around 1 m/s, orbiter scientific instruments are used to determine the main characteristics of the nucleus and its outgassing.
Indeed these parameters are indispensable to ensure a safe landing of Philae. Rotation, mass, shape, relief, ..., have to be known as accurately as possible before the landing planned on 11 of November.
Other scientific measurements are planned but not considered with the same priority.

How was the landing site be chosen?

The orbiter approaching the nucleus very cautiously, choice of the landing site is scheduled in 3 phases. First 5 possible zones were preselected for mid-August on criteria as the possibility of a safe descent trajectory, a good Sun illumination, a good orbiter visibility. On 10 of September 2 of them were chosen, one is the nominal, the other one is the back-up. On 10 of October after a checking that boulders will not endanger the landing the final choice was done. From this time pre-delivery and post-delivery orbits are designed.

How long is the Philae lifetime on the comet?

Once the primary batteries are discharged lander survival depends on 2 parameters, energy and temperature, that are closely connected. They are both coming from the solar illumination of Philae. Just after the landing at 3 AU (450 millions of km to Sun) the received energy is low and keeping the internal lander temperature is challenging. But the lander approaching the Sun with the comet energy becomes excessive and its internal temperature will exceed the limits for electronics parts. Philae will dye of too much heat, we consider a lifetime between 4 and 6 months after landing.

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