Asteroid 99942 is probably the world’s most famous asteroid with its nickname Apophis. It was once considered the most dangerous astroid ever detected, but subsequent measurements ruled out the possibility of impact. Nevertheless, by observing are orbiting the sun, it is known that it will enter the Earth’s sphere of influence in 2029.  We know the speed at which it will enter the Earth’s system and the minimum gap between the center of the Earth and the axis that the asteroid followed before being influenced by the earth.

#### Data

Earth’s standard gravitational parameter: 398 600 km3 s-2

Speed at infinity: Vinf =5.85km/s

Minimal Earth-axis difference: B=47,630 km

#### Calculations

One can calculate the semi-major axis through speed at infinity and follow with eccentricity.

With geometric data, it is possible to calculate the most important data in this case, la distance minimale lors du survol de la terre. This minimal distance to the center of the Earth is the periastre radius.

This is reassuring, considering that the value is greater than the radius of the earth (6,380km) and sufficiently above the atmosphere (100km more) not to risk a devastating collision. However, it will pass at a distance below the communication satellite in geostationary orbit (42,000km from the center of the earth).  We can also calculate the speed at which Apophis will move to this perigee.