If you put a satellite in just the right orbit, it will appear to be parked in the sky.
Near and Far
Today marks the point in Earth’s orbit where it is closest to the Sun, known as perihelion. It might seem odd for those in the northern hemisphere for this to occur in the middle of winter, but that’s because the seasons are due largely to the tilt of Earth’s axis relative to its orbital plane. The variation in Earth’s distance from the Sun is minor in comparison, because Earth’s orbit is almost (but not quite) a perfect circle.
Delta-V
Of all the inner planets, only Mercury hasn’t had a probe land on it. It likely won’t for quite some time. The reason isn’t because of lack of desire, or worthy science to be done, but because of a simple thing known as delta-v.
Forever Alone
A plutino is an asteroid-sized body that orbits the Sun in a 2:3 resonance with Neptune. They are named after Pluto, which also orbits the Sun twice for every three orbits of Neptune. It is thought that Pluto and the other plutinos were clustered into this resonance during the migration of the outer planets during the early solar system.
Twist and Shout
Orbital dynamics, that is, the motion of planets and stars about each other, is deeply dependent on computational modeling. The basic motion of one planet or star about another (the so-called two body problem) is fairly simple, and can be summarized by Kepler’s laws of planetary motion, the motion of multiple planets and stars is extraordinarily complex. In fact while the two-body problem is almost trivial to solve, the three-body problem has no exact general solution. As soon as you have three or more masses in your system, the motion can be highly chaotic.