Editing Interplanetary spaceflight (section)

==Economical travel techniques==
[[File:MESSENGER interplanetary departure.webm|thumb|250px|View of Earth from ''[[MESSENGER]]'' as it performs a [[Flyby (spaceflight)|flyby]] to reach [[Mercury (planet)|Mercury]] via gravity assist.]]

One of the main challenges in interplanetary travel is producing the very large velocity changes necessary to travel from one body to another in the Solar System.

Due to the Sun's gravitational pull, a spacecraft moving farther from the Sun will slow down, while a spacecraft moving closer will speed up. Also, since any two planets are at different distances from the Sun, the planet from which the spacecraft starts is moving around the Sun at a different speed than the planet to which the spacecraft is travelling (in accordance with [[Kepler's laws of planetary motion|Kepler's Third Law]]). Because of these facts, a spacecraft desiring to transfer to a planet closer to the Sun must decrease its speed with respect to the Sun by a large amount in order to intercept it, while a spacecraft traveling to a planet farther out from the Sun must increase its speed substantially.<ref name=curtisbook>{{cite book |last=Curtis |first=Howard |date=2005 |title=Orbital Mechanics for Engineering Students |edition=1st |publisher=Elsevier Butterworth-Heinemann |isbn=978-0750661690 |page=[https://archive.org/details/orbitalmechanics00curt/page/n273 257]|title-link=Orbital Mechanics for Engineering Students }}</ref> Then, if additionally the spacecraft wishes to enter into orbit around the destination planet (instead of just flying by it), it must match the planet's orbital speed around the Sun, usually requiring another large velocity change.

Simply doing this by brute force – accelerating in the shortest route to the destination and then matching the planet's speed – would require an extremely large amount of fuel. And the fuel required for producing these velocity changes has to be launched along with the payload, and therefore even more fuel is needed to put both the spacecraft and the fuel required for its interplanetary journey into orbit. Thus, several techniques have been devised to reduce the fuel requirements of interplanetary travel.

As an example of the velocity changes involved, a spacecraft travelling from low Earth orbit to Mars using a simple trajectory must first undergo a change in speed (also known as a [[delta-v]]), in this case an increase, of about 3.8&nbsp;km/s. Then, after intercepting Mars, it must change its speed by another 2.3&nbsp;km/s in order to match Mars' orbital speed around the Sun and enter an orbit around it.<ref name=marsdeltavs>{{cite web|title=Rockets and Space Transportation |url=http://www.pma.caltech.edu/~chirata/deltav.html |access-date=June 1, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20070701211813/http://www.pma.caltech.edu/~chirata/deltav.html |archive-date=July 1, 2007 }}</ref> For comparison, launching a spacecraft into low Earth orbit requires a change in speed of about 9.5&nbsp;km/s.

===Hohmann transfers===
[[File:Hohmann transfer orbit2.svg|frame|right|Hohmann Transfer Orbit: a spaceship leaves from point 2 in Earth's orbit and arrives at point 3 in Mars' (not to scale).]]
For many years economical interplanetary travel meant using the [[Hohmann transfer orbit]]. Hohmann demonstrated that the lowest energy route between any two orbits is an [[ellipse|elliptical]] "orbit" which forms a [[tangent]] to the starting and destination orbits. Once the spacecraft arrives, a second application of thrust will re-circularize the orbit at the new location. In the case of planetary transfers this means directing the spacecraft, originally in an orbit almost identical to Earth's, so that the [[aphelion]] of the transfer orbit is on the far side of the Sun near the orbit of the other planet. A spacecraft traveling from Earth to Mars via this method will arrive near Mars orbit in approximately 8.5 months, but because the orbital velocity is greater when closer to the center of mass (i.e. the Sun) and slower when farther from the center, the spacecraft will be traveling quite slowly and a small application of thrust is all that is needed to put it into a [[circular orbit]] around Mars. If the manoeuver is timed properly, Mars will be "arriving" under the spacecraft when this happens.

The Hohmann transfer applies to any two orbits, not just those with planets involved. For instance it is the most common way to transfer satellites into [[geostationary orbit]], after first being "parked" in [[low Earth orbit]]. However, the Hohmann transfer takes an amount of time similar to ½ of the orbital period of the outer orbit, so in the case of the outer planets this is many years – too long to wait. It is also based on the assumption that the points at both ends are massless, as in the case when transferring between two orbits around Earth for instance. With a planet at the destination end of the transfer, calculations become considerably more difficult.

===Gravitational slingshot===
{{main|Gravity assist}}

[[File:Voyager 2 velocity vs distance from sun.svg|thumb|right|Plot of ''Voyager 2''{{'}}s heliocentric velocity against its distance from the Sun, illustrating the use of gravity assist to accelerate the spacecraft by Jupiter, Saturn and Uranus. To observe [[Triton (moon)|Triton]], ''Voyager 2'' passed over Neptune's north pole resulting in an acceleration out of the plane of the ecliptic and reduced velocity away from the Sun.<ref>{{cite web|author=Dave Doody|url=http://www2.jpl.nasa.gov/basics/bsf4-1.php |title=Basics of Space Flight Section I. The Environment of Space |publisher=.jpl.nasa.gov |date=2004-09-15 |access-date=2016-06-26}}</ref>]]
The gravitational slingshot technique uses the [[gravity]] of planets and moons to change the speed and direction of a spacecraft without using fuel. In typical example, a spacecraft is sent to a distant planet on a path that is much faster than what the Hohmann transfer would call for. This would typically mean that it would arrive at the planet's orbit and continue past it. However, if there is a planet between the departure point and the target, it can be used to bend the path toward the target, and in many cases the overall travel time is greatly reduced. A prime example of this are the two crafts of the [[Voyager program]], which used slingshot effects to change trajectories several times in the outer Solar System. It is difficult to use this method for journeys in the inner part of the Solar System, although it is possible to use other nearby planets such as Venus or even the [[Moon]] as slingshots in journeys to the outer planets.

This maneuver can only change an object's velocity relative to a third, uninvolved object, – possibly the “centre of mass” or the Sun. There is no change in the velocities of the two objects involved in the maneuver relative to each other.  The Sun cannot be used in a gravitational slingshot because it is stationary compared to rest of the Solar System, which orbits the Sun.  It may be used to send a spaceship or probe into the galaxy because the Sun revolves around the center of the [[Milky Way]].

===Powered slingshot===
{{main|Oberth effect}}
A powered slingshot is the use of a rocket engine at or around closest approach to a body ([[periapsis]]). The use at this point multiplies up the effect of the delta-v, and gives a bigger effect than at other times.

===Fuzzy orbits===
Computers did not exist when [[Hohmann transfer orbit]]s were first proposed (1925) and were slow, expensive and unreliable when [[Gravity assist|gravitational slingshots]] were developed (1959). Recent advances in [[computing]] have made it possible to exploit many more features of the gravity fields of astronomical bodies and thus calculate even [[Low energy transfer|lower-cost trajectories]].<ref>{{cite web | url=https://www.discovermagazine.com/the-sciences/gravitys-rim | title=Gravity's Rim | publisher=discovermagazine.com | access-date=2023-04-12 | archive-date=2019-10-22 | archive-url=https://web.archive.org/web/20191022002414/http://discovermagazine.com/1994/sep/gravitysrim419 | url-status=live }}</ref><ref>{{cite book |last=Belbruno |first=E. |date=2004 |url=http://www.pupress.princeton.edu/titles/7687.html |title=Capture Dynamics and Chaotic Motions in Celestial Mechanics: With the Construction of Low Energy Transfers |publisher=Princeton University Press |isbn=9780691094809 |access-date=2007-04-07 |archive-url=https://web.archive.org/web/20141202074354/http://www.pupress.princeton.edu/titles/7687.html |archive-date=2014-12-02 |url-status=dead }}</ref> Paths have been calculated which link the [[Lagrange points]] of the various planets into the so-called [[Interplanetary Transport Network]]. Such "fuzzy orbits" use significantly less energy than Hohmann transfers but are much, much slower. They aren't practical for human crewed missions because they generally take years or decades, but may be useful for high-volume transport of low-value [[commodity|commodities]] if humanity develops a [[space-based economy]].

===Aerobraking===
[[File:Apollo cm.jpg|thumb|Artist's rendition of an [[Apollo Command Module]] aerobraking ]]
[[Aerobraking]] uses the [[Celestial body atmosphere|atmosphere]] of the target planet to slow down. It was first used on the [[Apollo program]] where the returning spacecraft did not enter Earth orbit but instead used a S-shaped vertical descent profile (starting with an initially steep descent, followed by a leveling out, followed by a slight climb, followed by a return to a positive rate of descent continuing to splash-down in the ocean) through Earth's atmosphere to reduce its speed until the parachute system could be deployed enabling a safe landing. Aerobraking does not require a thick atmosphere – for example most Mars landers use the technique, and [[Mars#Atmosphere|Mars' atmosphere]] is only about 1% as thick as Earth's.

Aerobraking converts the spacecraft's [[kinetic energy]] into heat, so it requires a [[heatshield]] to prevent the craft from burning up. As a result, aerobraking is only helpful in cases where the fuel needed to transport the heatshield to the planet is less than the fuel that would be required to brake an unshielded craft by firing its engines. This can be addressed by creating heatshields from material available near the target.<ref>{{Cite web |url=https://www.nasa.gov/pdf/744615main_2011-Hogue-Final-Report.pdf |title=NASA.gov |access-date=2016-05-13 |archive-date=2016-06-02 |archive-url=https://web.archive.org/web/20160602012046/https://www.nasa.gov/pdf/744615main_2011-Hogue-Final-Report.pdf |url-status=dead }}</ref>