Mars’s natural satellites are very small. Phobos has an ellipsoidal shape (28x20 km) and is in a very low orbit, with an average altitude of about 6,000 km. Its surface is riddled with craters. The biggest of them, Stickney, is about 10 km in diameter, which is about one-third of the satellite’s largest dimension. A series of striations emanating from Strickney appear to be fractures resulting from the impact which created the crater.
It is generally thought that Phobos and Deimos are asteroids which were “captured” by the planet’s gravitational field when they wandered too close to Mars. Another theory is that both moons are the results of the accretion of debris ejected in Mars orbit from the impact of an asteroid on the mars surface. However, the regoliths (a layer of rock and dust) on both satellites seem quite different from each other.
More accurate on-site observation of Phobos’s surface composition, in addition to meticulous analysis of the samples brought back to Earth, would have provided an answer. Furthermore, there is a good chance rock which was kicked up from the planet’s surface during asteroid impacts landed on Phobos. These rocks should have a different signature from the primal material.
The Phobos-Grunt mission was designed to use its significant payload to carry out on-site observations and should have operated on the surface for one year. Furthermore, it would have collected soil samples (about 200 g) which were to be sent back to Earth for detailed analysis.
The mission’s primary objectives focused on the following issues:
- Phobos’s (and most likely Deimos’s) origins, through the study of its primordial material,
- its evolution in relation to Mars,
- the effect of asteroid impacts on planet formation and on the evolution of their atmosphere, their crust, and their volatile elements inventory.
The mission was planned as follows:
- spacecraft launch and injection using a Zenit-3F rocket;
- Earth-Mars transfer, with 3 interplanetary trajectory corrections;
- deceleration upon approach of Mars, to enter the initial 3-day orbit;
- Yinghuo-1 (YH-1) satellite separation;
- step-by-step alignment of the satellite’s orbit with Phobos’s, landing on Phobos, and sample collection on the moon’s surface;
- take-off from Phobos and flight on the satellite’s base orbit, dual-burn transfer to the satellite’s pre-launch orbit;
- acceleration from pre-launch orbit, injection into Mars-Earth transfer with 5 trajectory corrections, atmospheric re-entry and landing on Earth of the capsule, capsule recovery.