NASASpaceFlight.com Forum
General Discussion => Q&A Section => Topic started by: freda on 11/12/2018 02:37 pm
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I was looking at http://cristianopi.altervista.org/as/ and selected the Juno probe.
One of the graphs shows Juno approaching Jupiter from the "Sun" side, then "backing" into the eventual orbit. I think I heard that some Mars probes have done that too, rather than approaching from the "outer" side and going head-first directly into the eventual orbit.
This is a fun detail to think about, and might fit into this Q&A thread:
Q1) What are the trade-offs in using either approach method? Direct orbit entry verses "backing" into the orbit.
Q2) Could an approaching probe/capsule/lander even optimize the backing" maneuver so as to meet the planet at almost no (or minimum) relative velocity, hence reducing the heat-shield requirement needed to enter the atmosphere?
Q3) The "backing" maneuver seems to be used when approaching an outer planet from an inner planet (eg: probe is going from Earth to Mars). What are the issues in a probe/capsule returning to an inner planet (eg: Mars to Earth)?
Image from the referenced site showing Juno approaching Jupiter is attached: "Sun"-side is to the right. You can see Juno (green color) approaches, slows, and "backs" into Jupiter to be captured.
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Q1 - "Backing" in to the orbit is the more energy efficient one, since you are doing the opposite of a typical momentum transfer slingshot - but instead the planet helps you slow down. That will reduce the delta-v needed to capture into an orbit of the planet, BUT it will tend to leave you in a retro-grade orbit. (Bad if you plan on reaching the moons)
Q2 - No, there is no such trajectory. If there was, the opposite trajectory would also exist, where you could leave a planet with no or minimum relative velocity.
The best way to learn about this stuff is to play Kerbal Space Program. :)
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There are also 'low power' aerocaptures.
Several Mars satellites have done this.
Start off in the lowest energy transfer orbit, and then do a ~800m/s burn to put you in a week period orbit or so with a minimum distance of just over what you think the worst case atmosphere is.
Gradually lower your orbit until you start touching the atmosphere - gently enough that you can use solar panels as drag surfaces, and then carefully continue slowly braking down each orbit for a few months till you're in low orbit.
This is a much kinder gentler approach than hitting the atmosphere at 10km/s and relying on your atmospheric model being Juuust right.
(https://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Odyssey_summary_br.jpg/1024px-Odyssey_summary_br.jpg)