text: Compare two proportional quantities, the free-fall time (time to fall to a point mass from rest)
  (17.7% of the orbital period in a circular orbit)
  and the time to fall to a point mass in a radial parabolic orbit
  (7.5% of the orbital period in a circular orbit)
  The fact that the formulas only differ by a constant factor is a priori clear from dimensional analysis.
  Energy
  The specific orbital energy () is negative, and
  Thus the virial theorem applies even without taking a time-average:
  the kinetic energy of the system is equal to the absolute value of the total energy
  the potential energy of the system is equal to twice the total energy
  The escape velocity from any distance is  times the speed in a circular orbit at that distance: the kinetic energy is twice as much, hence the total energy is zero.
  Delta-v to reach a circular orbit
  Maneuvering into a large circular orbit, e.g. a geostationary orbit, requires a larger delta-v than an escape orbit, although the latter implies getting arbitrarily far away and having more energy than needed for the orbital speed of the circular orbit. It is also a matter of maneuvering into the orbit. See also Hohmann transfer orbit.