The study of asteroids's orbit croissing planetary orbit is handled with astrometry, acquisition and exploitation of databases dedicated to observations and dynamical modelisation. In this three areas, the maximum of precision is required and is a guarantee of reliability that can be applied to specific studies. In the first part, I will present a state of the art of our knowledge of near-Earth asteroids. I will present the different dynamical families known, discovery statistic as well as the kind of observations available. Then, a section will be dedicated to the dynamical model used, presenting the main forces acting on those asteroids. At the end, I will deal with orbit determination and propagation. Next, I will present a dynamical and geometrical analysis of the close encounter problem towards the example of the asteroid (99942) Apophis and its close encounter with the Earth in 2029. In this chapter, I will devellop the senstive side of its orbit and I will introduce the terminology of "keyholes" necessary to our understanding and interpretation of the risk of collision with the Earth. Then, I will analyse the effect of new ground-based observations on its orbit focusing on the error propagation. The last part will the dedicated to the Gaia mission and its contributions to Apophis's orbit using its very accurate future data. After presenting the mission, I will analyse the effect of space data on the current and long term uncertainty of Apophis's orbit. Then, a chapter will the dedicated to orbit determination for poor observed asteroids, specially in the case of the Gaia mission where only two observations will be available. The error propagation will be analysed and I will also study the synergy of space and ground-based data for near-Earth asteroids.
