Editing Gravity assist (section)

==Limits==
[[File:Voyager Path.svg|thumb|The trajectories that enabled NASA's twin Voyager spacecraft to tour the four giant planets and achieve velocity to escape the Solar System]]

The main practical limit to the use of a gravity assist maneuver is that planets and other large masses are seldom in the right places to enable a voyage to a particular destination. For example, the [[Voyager 1|Voyager]] missions which started in the late 1970s were made possible by the "[[Planetary Grand Tour|Grand Tour]]" alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century. That is an extreme case, but even for less ambitious missions there are years when the planets are scattered in unsuitable parts of their orbits.{{fact|date=May 2022}}

Another limitation is the distance of closest approach to the planet. The magnitude of the change in velocity depends on the spacecraft's approach velocity and the planet's escape velocity at the point of closest approach. The closer to the center of the planet that approach is, the greater the achievable change in velocity. The atmosphere, if any, of the available planet will set a limit to the approach distance; for bodies with no atmosphere, like the moon, the closest approach is set by the constraint that the trajectory must not intersect the surface.  For planets with atmosphere, as a spacecraft gets deep into the atmosphere, the energy lost to drag can exceed that gained from the planet's velocity. (On the other hand, this drag can be used to accomplish a different delta-V maneuver, [[aerobraking]]). There have also been theoretical proposals to use [[lift (force)|aerodynamic lift]] as the spacecraft flies through the atmosphere. This maneuver, called an [[aerogravity assist]], could bend the trajectory through a larger angle than gravity alone, and hence increase the gain in energy.<ref>Jon A. Sims, James M. Longuski, and Moonish R. Patel (1995). "Aerogravity-assist trajectories to the outer planets," ''Acta Astronautica'', Volume 35, Supplement 1, pp. 297-306. https://doi.org/10.1016/0094-5765(94)00195-R</ref>

Interplanetary slingshots using the Sun itself are not possible because the Sun is at rest relative to the Solar System as a whole. However, thrusting when near the Sun has a related effect, the [[Oberth effect]]. This has the potential to magnify a spacecraft's thrusting power enormously, but is limited by the spacecraft's ability to resist the heat.{{fact|date=May 2022}} For planetary gravity assists, a thrust applied near the closest approach (a "powered periapsis maneuver") can add the Oberth effect to the gravity slingshot effect, producing a larger change in orbital velocity than either effect by itself.

A [[rotating black hole]] might provide additional assistance, if its spin axis is aligned the right way. [[General relativity]] predicts that a large spinning {{not a typo|mass produces}} [[frame-dragging]]—close to the object, space itself is dragged around in the direction of the spin. Any ordinary rotating object produces this effect. Although attempts to measure frame dragging about the Sun have produced no clear evidence, experiments performed by [[Gravity Probe B]] have detected frame-dragging effects caused by Earth.<ref name=PRL>{{cite journal |title=Gravity Probe B: Final Results of a Space Experiment to Test General Relativity |journal=Physical Review Letters |first=C. W. F. |last=Everitt |display-authors=etal |volume=106 |issue=22 |at=221101 |date=June 2011 |doi=10.1103/PhysRevLett.106.221101 |bibcode=2011PhRvL.106v1101E |arxiv=1105.3456 |pmid=21702590|s2cid=11878715 }}</ref> General relativity predicts that a spinning black hole is surrounded by a region of space, called the [[ergosphere]], within which standing still (with respect to the black hole's spin) is impossible, because space itself is dragged at the speed of light in the same direction as the black hole's spin. The [[Penrose process]] may offer a way to gain energy from the ergosphere, although it would require the spaceship to dump some "ballast" into the black hole, and the spaceship would have had to expend energy to carry the "ballast" to the black hole.{{fact|date=May 2022}}