Hohmann transfer - Revision history

RichardWSmith: /* Free return trajectory */ improved grammar

2024-09-17T04:31:04Z

Free return trajectory: improved grammar

RichardWSmith: /* Free return trajectory */

2024-09-15T13:48:23Z

Free return trajectory

RichardWSmith: Added a Free Return Trajectory.

2024-09-15T13:47:08Z

Added a Free Return Trajectory.

Sdubois: Added Earth-Mars transfer time (8.5 months)

2019-08-05T17:36:15Z

Added Earth-Mars transfer time (8.5 months)

Sdubois: Minor formatting changes

2019-07-19T19:13:57Z

Minor formatting changes

Sdubois: Added categories

2019-07-17T14:29:05Z

Added categories

Sdubois: Adjusted gif visibility and formatting

2019-07-16T21:35:11Z

Adjusted gif visibility and formatting

Sdubois at 18:31, 8 July 2019

2019-07-08T18:31:04Z

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2019-07-08T18:30:12Z

Sdubois at 18:28, 8 July 2019

2019-07-08T18:28:59Z

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 ===Free return trajectory===
-If greater impulse is given when the space craft leaves Earth, it will go farther out than Mars, but get there more quickly.  This has the disadvantage that there is extra speed that must be lost at Mars.  But you spend less time in transit.  Many Mars missions do this, taking about 6 to 7 months for transit to the Red Planet.
+If greater impulse is given when the space craft leaves Earth, it will go farther out than Mars, but get to Mars more quickly.  This has the disadvantage that there is extra speed that must be lost at Mars.  But you spend less time in transit.  Many Mars missions do this, taking about 6 to 7 months for transit to the Red Planet.
 However, if the ship is given enough thrust to reach Mars in 6.5 months, and for some reason the people decide NOT to land on Mars, then they will travel past Mars, move further away from the sun (to the inner edge of the asteroid belt) then fall back to Earth's orbit.  When they again reach Earth's orbit, exactly 2 years will have passed since the launch, and <b>Earth will be there to greet them</b>.  This means that if they abort the mission, they will come back to Earth safely.  This is a strong reason to pick a travel time of 6.5 months.  This is a type II trajectory described above.

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 If greater impulse is given when the space craft leaves Earth, it will go farther out than Mars, but get there more quickly.  This has the disadvantage that there is extra speed that must be lost at Mars.  But you spend less time in transit.  Many Mars missions do this, taking about 6 to 7 months for transit to the Red Planet.
-However, if the ship is given enough thrust to reach Mars in 6.5 months, and for some reason the people decide NOT to land on Mars, then they will travel past Mars, move further away from the sun (to the inner edge of the asteroid belt) then fall back to Earth's orbit.  When they again reach Earth's orbit, exactly 2 years will have passed since the launch, and <b>Earth will be there to greet them</b>.  This means that if they abort the mission, they will come back to Earth safely.  This is a strong reason to pick a travel time of 6.5 months.
+However, if the ship is given enough thrust to reach Mars in 6.5 months, and for some reason the people decide NOT to land on Mars, then they will travel past Mars, move further away from the sun (to the inner edge of the asteroid belt) then fall back to Earth's orbit.  When they again reach Earth's orbit, exactly 2 years will have passed since the launch, and <b>Earth will be there to greet them</b>.  This means that if they abort the mission, they will come back to Earth safely.  This is a strong reason to pick a travel time of 6.5 months.  This is a type II trajectory described above.
 ===Low-thrust relative transfer===

← Older revision		Revision as of 13:47, 15 September 2024
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	These advantages come at the cost of increased energy expenditure: not only does the spacecraft have to accelerate more at the departure point, it also must spend extra fuel decelerating at the destination in order to match the target orbit<ref name=":2" /><ref name=":4" />. For a trip from Earth to Mars, decreasing travel time by 10% necessitates twice as much fuel, while cutting travel time in half requires ten times as much; although these diminishing returns seem undesirable from an efficiency standpoint, they may prove worthwhile when considering factors such as decreased exposure time to radiation for crewed missions or the possibility of arriving in time for a return launch window which the Hohmann transfer would otherwise miss.<ref name=":4" />		These advantages come at the cost of increased energy expenditure: not only does the spacecraft have to accelerate more at the departure point, it also must spend extra fuel decelerating at the destination in order to match the target orbit<ref name=":2" /><ref name=":4" />. For a trip from Earth to Mars, decreasing travel time by 10% necessitates twice as much fuel, while cutting travel time in half requires ten times as much; although these diminishing returns seem undesirable from an efficiency standpoint, they may prove worthwhile when considering factors such as decreased exposure time to radiation for crewed missions or the possibility of arriving in time for a return launch window which the Hohmann transfer would otherwise miss.<ref name=":4" />
		+
		+	===Free return trajectory===
		+	If greater impulse is given when the space craft leaves Earth, it will go farther out than Mars, but get there more quickly. This has the disadvantage that there is extra speed that must be lost at Mars. But you spend less time in transit. Many Mars missions do this, taking about 6 to 7 months for transit to the Red Planet.
		+
		+	However, if the ship is given enough thrust to reach Mars in 6.5 months, and for some reason the people decide NOT to land on Mars, then they will travel past Mars, move further away from the sun (to the inner edge of the asteroid belt) then fall back to Earth's orbit. When they again reach Earth's orbit, exactly 2 years will have passed since the launch, and <b>Earth will be there to greet them</b>. This means that if they abort the mission, they will come back to Earth safely. This is a strong reason to pick a travel time of 6.5 months.

	===Low-thrust relative transfer===		===Low-thrust relative transfer===

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 ===Type-I and Type-II Trajectories===
 [[File:TypeIandIItrajectories.png|right|250px|thumb|Potential Earth-Mars alternatives to the Hohmann transfer (represented by the thick, black semicircle). The green dotted line represents a trajectory which leaves sooner, traveling beyond the target orbit before encountering it on its second intercept. The red dashed line leaves and arrives later on its first intercept.<ref name=":4" />]]
-As discussed above, the Hohmann transfer involves a trajectory carrying the spacecraft precisely 180° around the Sun. This method requires the lowest expenditure of energy, but does come with the disadvantages of extended travel time and launch window limitations. These disadvantages can be compensated for to a certain degree by increasing fuel expenditure and altering the time of departure.
+As discussed above, the Hohmann transfer involves a trajectory carrying the spacecraft precisely 180° around the Sun. This method requires the lowest expenditure of energy, but does come with the disadvantages of extended travel time (around 8.5 months<ref>Stern, D. P. (2004, December 12). Flight to Mars: How Long? And along what path? Retrieved August 5, 2019, from <nowiki>http://www.phy6.org/stargaze/Smars1.htm</nowiki></ref>) and launch window limitations. These disadvantages can be compensated for to a certain degree by increasing fuel expenditure and altering the time of departure.
 The resultant trajectories will arrive at the target after proceeding less than 180° around the Sun (Type-I Trajectory) or more than 180° (Type-II).<ref name=":1" /> Both trajectory types carry the spacecraft beyond the target orbit and therefore cross it two times—by timing the arrival of the target with one of these two intersections, the spacecraft can still reach its destination, allowing more flexibility in terms of launch times and/or transit duration.

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 ===Type-I and Type-II Trajectories===
 [[File:TypeIandIItrajectories.png|right|250px|thumb|Potential Earth-Mars alternatives to the Hohmann transfer (represented by the thick, black semicircle). The green dotted line represents a trajectory which leaves sooner, traveling beyond the target orbit before encountering it on its second intercept. The red dashed line leaves and arrives later on its first intercept.<ref name=":4" />]]
-As discussed above, the Hohmann transfer involves a trajectory carrying the spacecraft precisely 180° around the Sun. This method requires the lowest expenditure of energy, but does come with the disadvantages of extended travel time and launch window limitations. These disadvantages can be compensated for to a certain degree by increasing fuel expenditure and altering the time of departure. The resultant trajectories will arrive at the target after proceeding less than 180° around the Sun (Type-I Trajectory) or more than 180° (Type-II).<ref name=":1" /> Both trajectory types carry the spacecraft beyond the target orbit and therefore cross it two times—by timing the arrival of the target with one of these two intersections, the spacecraft can still reach its destination, allowing more flexibility in terms of launch times and/or transit duration. These advantages come at the cost of increased energy expenditure: not only does the spacecraft have to accelerate more at the departure point, it also must spend extra fuel decelerating at the destination in order to match the target orbit<ref name=":2" /><ref name=":4" />. For a trip from Earth to Mars, decreasing travel time by 10% necessitates twice as much fuel, while cutting travel time in half requires ten times as much; although these diminishing returns seem undesirable from an efficiency standpoint, they may prove worthwhile when considering factors such as decreased exposure time to radiation for crewed missions or the possibility of arriving in time for a return launch window which the Hohmann transfer would otherwise miss.<ref name=":4" />
+As discussed above, the Hohmann transfer involves a trajectory carrying the spacecraft precisely 180° around the Sun. This method requires the lowest expenditure of energy, but does come with the disadvantages of extended travel time and launch window limitations. These disadvantages can be compensated for to a certain degree by increasing fuel expenditure and altering the time of departure.
 ===Low-thrust relative transfer===
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 ===Three-impulse maneuver===
 [[File:3impulse.png|250px|right|thumb|Diagram showing the locations of each engine firing in a three-impulse maneuver. This maneuver is also called a bi-elliptic transfer in reference to the two half-elliptical orbits of which it consists.]]
-When traveling exceptionally long distances between orbits (specifically, when the radius of the outer orbit is over 12 times greater than the inner one), a three-impulse maneuver proves more efficient than the Hohmann transfer.<ref name=":6" /> This maneuver involves 1) expanding ones orbit beyond that of the target, 2) creating a second elliptical orbit which matches the target orbit, 3) decelerating in order to match the final target orbit. Although such a maneuver has limited applications in comparison to the Hohmann transfer due to the required ratio of distances as well as the much longer travel time, it does offer efficiency increases given the proper parameters.<ref name=":6" />
+When traveling exceptionally long distances between orbits (specifically, when the radius of the outer orbit is over 12 times greater than the inner one), a three-impulse maneuver proves more efficient than the Hohmann transfer.<ref name=":6" /> This maneuver involves 1) expanding ones orbit beyond that of the target, 2) creating a second elliptical orbit which matches the target orbit, 3) decelerating in order to match the final target orbit.
 [[File:venusflyby.png|upright=0.95|left|thumb|Proposed Mars mission consisting of a modified Hohmann/Type-I Trajectory en route to Mars (blue line) followed by a Venus-assisted return.]]
 ===Gravity assist===
-A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />
+A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref>
 ===Ballistic capture===

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	==References==		==References==
	<references />		<references />
		+	[[Category:Mars Spacecraft/Robotic Missions‎]]
		+	[[Category:Mission Planning]]
		+	[[Category:Orbital Mechanics‎]]

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 {{Stefan}}
-[[File:Hohmann_transfer.gif|thumb|right|400px|Animated depiction of Hohmann transfer]]
+[[File:Hohmann_transfer.gif|frame|Animated depiction of Hohmann transfer]]
 In orbital mechanics, a Hohmann transfer describes the path taken to transit between two orbits of differing radii while traveling 180° around their common focus. Conventional analyses consider such a path to be the most efficient transfer method for traveling between circular orbits located on the same plane.<ref name=":0">Let’s Go to Mars! Calculating Launch Windows Activity. (n.d.). Retrieved July 5, 2019, from NASA/JPL Edu website: <nowiki>https://www.jpl.nasa.gov/edu/teach/activity/lets-go-to-mars-calculating-launch-windows/</nowiki></ref><ref name=":1">[2] Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref><ref name=":2">Hohmann Transfer Orbit & Total Travel Time. (2001). Retrieved July 5, 2019, from <nowiki>http://www.polaris.iastate.edu/EveningStar/Unit6/unit6_sub2.htm</nowiki></ref>
 ==Description==
-[[File:Transfer_orbits.png|left|thumb|The three relevant orbits of a Hohmann transfer from a lower orbit to a higher one: the orbit of the departure body (1), the destination body (3), and the Hohmann trajectory which connects the two (2).]]
+[[File:Transfer_orbits.png|right|thumb|The three relevant orbits of a Hohmann transfer from a lower orbit to a higher one: the orbit of the departure body (1), the destination body (3), and the Hohmann trajectory which connects the two (2).]]
 Named for Walter Hohmann, the German scientist who first proposed the transfer method in 1925,<ref name=":3">[4] Hohmann transfer orbit. (2019). In ''Wikipedia''. Retrieved from <nowiki>https://en.wikipedia.org/w/index.php?title=Hohmann_transfer_orbit&oldid=897743154</nowiki></ref> the Hohmann transfer consists of one half of an elliptical trajectory which connects two concentric orbits. Consider, for example, traveling from one planet (e.g. Earth) to another which is located farther away from the Sun (e.g. Mars). Applying the rules of land-based transit, one might assume that the most efficient way to travel between planets would be to wait until Earth and Mars were as close to one another as their orbits allow and then fly straight towards the target. However, both planets revolve around the Sun at tens of thousands of miles per hour<ref>Rocheleau, J. (2013, October 15). Orbital Speed of Planets in Order - Rotational Speed Comparison. Retrieved July 5, 2019, from Planet Facts website: <nowiki>http://planetfacts.org/orbital-speed-of-planets-in-order/</nowiki></ref> and travel is far from instantaneous over such a vast distance (even at their theoretically closest point, an expanse of 33.9 million miles still separates Earth from Mars<ref>Sharp, T. (2017, December 15). How Far Away is Mars? | Distance to Mars. Retrieved July 5, 2019, from Space.com website: <nowiki>https://www.space.com/16875-how-far-away-is-mars.html</nowiki></ref>).
 For this reason, spacecraft must target where Mars will be at the time of arrival, not where Mars is at the time of launch. By targeting an arrival location in Mars’ orbit 180° around the Sun from the launch position at Earth’s orbit, the spacecraft makes maximal use of the Earth’s orbital speed in allowing Mars to catch up to the spacecraft’s location. Just enough propellant is used to reach Mars’ orbit at the intercept location—any less and the spacecraft would fall short, with its elliptical trajectory carrying it back toward its departure point. The final Hohmann trajectory therefore intersects with the Earth’s orbit when closest to the Sun (launch), and with Mars’ orbit when at its farthest point from the Sun (arrival).<ref name=":0" /><ref name=":1" />
-[[File:reversehohmann.jpg|right|350px|thumb|A Hohmann transfer from a higher orbit to a lower one such as when traveling from Earth to Venus. The trajectory intersects both the departure and target orbits, beginning at its farthest point from the Sun (aphelion) and arriving at both Venus and its closest point to the Sun (perihelion) simultaneously.]]
+[[File:reversehohmann.jpg|left|350px|thumb|A Hohmann transfer from a higher orbit to a lower one such as when traveling from Earth to Venus. The trajectory intersects both the departure and target orbits, beginning at its farthest point from the Sun (aphelion) and arriving at both Venus and its closest point to the Sun (perihelion) simultaneously.]]
 The same principles apply in reverse for travel from a higher orbit to a lower one, such as would be the case in a return trip from Mars or one from Earth to Venus.<ref name=":1" /> Again, the idea is to use the minimal energy necessary to reach the desired orbit, traveling 180° around the Sun to arrive at the same time as the target.
-Because the Hohmann transfer relies on precise timing and positioning of the trajectory, a mission must wait for the two planets to reach the proper alignment before launching. Departing too soon or too late will still result in the spacecraft passing through the destination orbit, but the target itself will not be present for rendezvous upon arrival.
+Because the Hohmann transfer relies on precise timing and positioning of the trajectory, a mission must wait for the two planets to reach the proper alignment before launching. Departing too soon or too late will still result in the spacecraft passing through the destination orbit, but the target itself will not be present for rendezvous upon arrival. [[File:Launch_window_miss.gif|right|frame|Launching too late will mean the planet will pass through the intended contact point before the spacecraft can arrive]]
 In the case of an Earth-Mars mission, these opportunities occur only once every 25-26 months<ref name=":1" /><ref>Kayton, M. (n.d.). Hohmann transfer orbit diagram. Retrieved July 5, 2019, from <nowiki>http://www.planetary.org/multimedia/space-images/charts/hohmann-transfer-orbit.html</nowiki></ref>, adding considerable pressure to launch timelines: if a spacecraft finds itself unprepared for launch during the appropriate window, it will have to wait two years for another chance. Furthermore, a separate set of launch windows exist in the reverse direction, so a mission wishing to return to Earth from Mars using a Hohmann transfer in both directions must be capable of sustaining itself on the red planet for roughly 1.5 Earth years before an opportunity to return home becomes available.<ref name=":4">How long is the trip? (n.d.). Retrieved July 5, 2019, from <nowiki>http://www.polaris.iastate.edu/EveningStar/Unit7/unit7_sub3.htm</nowiki></ref> Accommodating for this extended stay in manned missions constitutes a central feature of Robert Zubrin’s [[Mars Direct]] plan.<ref name=":5">Zubrin, R. (2011). ''The Case for Mars''. Simon & Schuster.</ref>
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 [[File:3impulse.png|250px|right|thumb|Diagram showing the locations of each engine firing in a three-impulse maneuver. This maneuver is also called a bi-elliptic transfer in reference to the two half-elliptical orbits of which it consists.]]
 When traveling exceptionally long distances between orbits (specifically, when the radius of the outer orbit is over 12 times greater than the inner one), a three-impulse maneuver proves more efficient than the Hohmann transfer.<ref name=":6" /> This maneuver involves 1) expanding ones orbit beyond that of the target, 2) creating a second elliptical orbit which matches the target orbit, 3) decelerating in order to match the final target orbit. Although such a maneuver has limited applications in comparison to the Hohmann transfer due to the required ratio of distances as well as the much longer travel time, it does offer efficiency increases given the proper parameters.<ref name=":6" />
+[[File:venusflyby.png|upright=0.95|left|thumb|Proposed Mars mission consisting of a modified Hohmann/Type-I Trajectory en route to Mars (blue line) followed by a Venus-assisted return.]]
 ===Gravity assist===
 A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />
 ===Ballistic capture===

← Older revision		Revision as of 18:31, 8 July 2019
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	===Gravity assist===		===Gravity assist===
	A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />		A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />
		+

← Older revision		Revision as of 18:30, 8 July 2019
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	===Gravity assist===		===Gravity assist===
	A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />		A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />
		+
		+

← Older revision		Revision as of 18:28, 8 July 2019
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	===Gravity assist===		===Gravity assist===
	A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />		A planet or other astronomical object can accelerate or redirect the trajectory of a passing spacecraft without the use of additional propellant. Probes including Voyager 1 and 2, Galileo, and Cassini have all utilized gravity assists from various planets in order to boost or alter their trajectories without increasing the amount of propellant required.<ref>Basics of Space Flight - Solar System Exploration: NASA Science. (n.d.). Retrieved July 5, 2019, from NASA Solar System Exploration website: <nowiki>https://solarsystem.nasa.gov/basics/chapter4-1/</nowiki></ref> Mission architecture including a flyby of Venus to return a manned crew from Mars to Earth offers the potential for a return launch window which would reduce total mission time (i.e. departure, stay on Mars, and return) by roughly 30%, but much of this time would be spent in the return transit near the Sun where spacecraft suffer increased exposure to solar heating.<ref name=":5" />
		+