- #1
emma149
- 3
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how do space ships come to rest when they get into space?
Landing a Spacecraft: Understanding Gravity in Space
- Context: High School
- Thread starter emma149
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- Gravity Space Spacecraft
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Discussion Overview
The discussion revolves around how spacecraft come to rest or achieve stable orbits when entering space, particularly in relation to celestial bodies like the Earth and the Moon. It includes considerations of orbital mechanics, propulsion methods, and the nature of motion in space.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- Some participants question the concept of "rest" in space, suggesting that spacecraft are always in motion relative to the bodies they orbit.
- One participant mentions the need for relative velocity and the use of retro-rockets or other propulsion systems to shed excess velocity when entering orbits around celestial bodies.
- Another participant discusses the use of gravity assists and elliptical orbits to minimize fuel consumption, indicating that careful trajectory planning can significantly enhance payload capacity.
- There is a clarification regarding geo-stationary satellites, noting that they are in orbit but appear stationary relative to the Earth's surface due to their synchronized rotation.
- One participant specifically addresses maneuvers related to the Moon, highlighting the use of backward rocket firing to reduce speed and achieve circular orbits.
Areas of Agreement / Disagreement
Participants express differing views on the concept of "coming to rest" in space, with some emphasizing the continuous motion of spacecraft while others focus on the mechanics of achieving stable orbits. The discussion remains unresolved regarding the nuances of these concepts.
Contextual Notes
There are limitations in the discussion regarding the assumptions about what "rest" means in the context of space travel, as well as the specifics of orbital mechanics that may not be fully explored.
- #2
Vagn
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emma149 said:
Rest with respect to what?
- #3
Nik_2213
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Are you thinking of geo-stationary orbits ? Or 'entering' the L4/L5 halo positions ? Taking up Mercury or Mars orbit ??
To 'get there', you need relative velocity, then you must shed that excess velocity with Retro-rockets or whatever you want to call the payload's final propulsion system. To minimise fuel use, spacecraft may use gravity assists, a diminishing succession of elliptical orbits with sparing thrust at critical time, ion thruster or aero-braking...
A really careful trajectory that uses planetary gravity to sling-shot probes can easily double or treble the available payload...
To 'get there', you need relative velocity, then you must shed that excess velocity with Retro-rockets or whatever you want to call the payload's final propulsion system. To minimise fuel use, spacecraft may use gravity assists, a diminishing succession of elliptical orbits with sparing thrust at critical time, ion thruster or aero-braking...
A really careful trajectory that uses planetary gravity to sling-shot probes can easily double or treble the available payload...
- #4
Filip Larsen
Gold Member
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emma149 said:
The short answer is that they don't really come to rest when in space. Like roller coasters without brakes they are (by intention of the designers) always moving (actually, free falling) with fairly great speed "around" the body they orbit so that they won't "fall into" the body.
In case you are thinking of geo-stationary satellites, as Nik mentions, you should know that such satellites are in fact orbiting the Earth, they just do so at exactly the same rate as the Earth rotates, so when you view one from the surface of Earth the satellite appears to just hang there (see [1]). Its like a child sitting on a merry-go-round watching her (fairly athletic) dad running along on the ground just outside with his video camera keeping up with the rotation of the merry-go-round; the man is not moving relative to the view of his child even though he is running like mad over the ground.
[1] http://en.wikipedia.org/wiki/Geostationary_orbit
- #5
emma149
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- 0
with respect to the moon.
- #6
tiny-tim
Science Advisor
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welcome to pf!
hi emma! welcome to pf!
the usually fire their rockets "backwards" on the way to the Moon, to reduce their speed relative to the Moon, and then again on the other side of the Moon, to make the orbit circular
see eg http://en.wikipedia.org/wiki/Apollo_8#Lunar_sphere_of_influence"
hi emma! welcome to pf!
emma149 said:
the usually fire their rockets "backwards" on the way to the Moon, to reduce their speed relative to the Moon, and then again on the other side of the Moon, to make the orbit circular
see eg http://en.wikipedia.org/wiki/Apollo_8#Lunar_sphere_of_influence"
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