Although December 25 is celebrated the birth of Newton few people know that we have a satellite in the space that bears his name.
The XMM-Newton is a European satellite of elliptical orbit (that is, with orbit in the shape of an egg) destined to capture the important x-rays.
The curiosity of this satellite is not only for what it does but its orbit, very eccentric.
As you will remember of geometry, the ellipses have the particularity that the sum of the distances to two of their points called foci, is constant. This gives them that egg shape, having two semi-axes of different size. The difference between the semi-major axis and the semi-minor axis is called eccentricity (mathematically it is not so, but it is the distance between the distance of one of its focal points to the half-axis major divided by the distance from said focus to the center).
So we have that an elliptical orbit is more eccentric when this division gets closer to 1 and, therefore, will seem like “flatter”.
Returning to XMM-Newton, this satellite has been commissioned, thanks to its 50 mirrors, to the NASA observatory Chandra to detect a neutron star (a pulsar, as you recall) through its X-rays and how it curved.
It was also decisive in imaging when the Deep Impact spacecraft collided with 9P / Tempel 1, a comet that “passes” every 5 ½ years.
This satellite, XMM-Newton is so important in x-ray images that its life has been extended several times, the last until 2020 without known replacement.