"Backing" into orbit around a planet


#1

Whenever I drink too much coffee, I think about orbits. Either I need something stronger, or decaf. :cocktail:

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 the whole topic of orbits could probably take up 30 TMRO:Space episodes. So instead of that, just a few questions for discussion:

  • 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)?
  • Q4) I wonder if they will shift BFS into reverse to slide in backwards like this? Or just dive in head-first?

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.


#2

Not sure how much you’ve played with orbital mechanics, so I’ll try to explain visually:


The “backing in” you’re describing has to do with changes in the relative velocities of the craft vs the destination. It’s really just the craft being pulled in by the target planet’s gravity after the craft “gets ahead” of the planet in its orbit and “slows down”, then the planet “catches up” from behind. This “slowing down” requires no fuel, and is an effect of the conversion of kinetic energy into potential energy between the craft and the sun. The exchange is purely gravitational, like a ball slowing down and coming to a stop (before falling) when you throw it into the air.

When viewed relative to the destination planet, it can appear as though the craft is “backing in”, when really (from the sun’s perspective) it’s the planet “catching up”.


#3
  • A1 ) This is the only approach when going from an inner to an outer planet. Doing otherwise would involve moving faster than the planet, which would mean you either started from a higher orbit or you burned a lot of extra fuel in deep space (which is typically inefficient).
  • A2 ) Of course. Depending how the craft and the planet are oriented, you could use course corrections to be above, below, high or low in the planet’s orbit when it sweeps past you. This will influence what inclination your orbit around the planet has and whether that orbit is prograde or retrograde. It also can be used to change how close you get to the planet’s surface (altitude at closest approach). You can also adjust your course ahead/behind so that the time when the planet catches you will synchronize with a particular time-zone on the planet at closest approach. These adjustments are best done with trajectory correction maneuvers months in advance of arrival. Whether you’re at minimum velocity will depend far more on the relative positions of the planets when you first departed than on any course corrections you do mid-transfer.
  • A3 ) The situation is opposite for transferring from an outer planet to an inner one. The craft’s speed will be higher than the planet’s as it exchanges potential energy for kinetic energy from the sun’s gravitation. Thus, the craft will do the “catching up” to the planet.
  • A4 ) Regardless of the situation, from the craft’s perspective, it’s just approaching the planet: there is no sense of “forwards or backwards” just “towards and away”. It may do so in any orientation. If the craft wants to enter the atmosphere head first, it just needs to spin itself around. The same goes for back end first. Based on the presentation, the ideal orientation for a BFS is heat-shield first, so that would be the black under-belly first.

#4

I think what @Faulx describes is the normal Hohmann transfer which is the quickest way to get to Mars.

The idea that @Freda describes is similar to the Ballistic Capture AKA Low energy Transfer (LET) but its a bit more complicated than just “backing up” into a planet. It is safer, but it takes much longer… I can see space cruise liners using it in the future. It is also not restricted to the 26 months synchronization cycle so it can be used for cargo transport any time.


#5

https://www.researchgate.net/figure/Ballistic-capture-to-periodic-orbit-at-Europa-along-a-stable-manifold_fig5_269565774


#6

I’m reading my way through a paper on ballistic capture now. It’s much the same as a Hohmann transfer, but it makes use of 3-body physics instead of a capture burn to save on delta-V. It’s pretty helpful for a destination in high orbit of a body with no atmosphere, but doesn’t help much for landing on places where you can aerobrake (such as Mars). Also, although it allows flexibility on when you launch, it doesn’t really help you arrive sooner since typically you spend more time in transit. Both maneuvers require a planet to sweep past you (going from inner body to outer body), so the description of “backing in” is apt either way.


#7

Scott Manley shows the principal of using weak stability to get a reduced energy capture using the Principia mod in KSP: