[HN Gopher] Juice rerouted to Venus in first lunar-Earth flyby
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Juice rerouted to Venus in first lunar-Earth flyby
Author : janpot
Score : 65 points
Date : 2024-08-21 10:15 UTC (12 hours ago)
(HTM) web link (www.esa.int)
(TXT) w3m dump (www.esa.int)
| ninju wrote:
| Here's a deep link to an animated overview
|
| https://www.esa.int/ESA_Multimedia/Videos/2024/08/Juice_s_lu...
|
| (for those that are more visual)
| istultus wrote:
| Thanks!
|
| (everyone is more visual, though we have less aggregated data
| on how blind people use their visual cortex)
| JumpCrisscross wrote:
| "using the gravity of Earth to send it Venus-bound"
|
| While technically correct, this sentence is misleading. The ESA
| can do better.
|
| Passing by a body can deflect a spacecraft. So technically, the
| Earth's gravity sends the craft "Venus bound." But "the gravity
| of Earth" imparts no net delta-v and wouldn't on its own allow
| the craft to reach Venus.
|
| A "gravity assist around a planet changes a spacecraft's velocity
| (relative to the Sun) by entering and leaving the gravitational
| sphere of influence of a planet" [1]. The Earth's revolution
| around the Sun gets the craft to Venus, _not_ the Earth's
| gravity.
|
| [1] https://en.m.wikipedia.org/wiki/Gravity_assist
| jessriedel wrote:
| The gravity of the Earth absolutely changes the speed of the
| probe.
|
| In terms of the basic momentum transfers, non-propulsive
| gravity assists are essentially the same as elastic collisions
| with balls of non-equal mass. In particular, energy can be
| transferred, and that is mediated by the interaction forces: if
| a very heavy ball is rolling along at speed v and I place a
| tiny ball at rest in front of it, the tiny ball will bounce off
| at about 2v. We could certainly say "the atomic forces between
| the heavy ball and the tiny ball during the collision propel
| the tiny ball to its new destination". This is true even though
| the tiny ball's speed is constant in the center-of-mass frame.
| JumpCrisscross wrote:
| > _non-propulsive gravity assists are essentially the same as
| elastic collisions with balls of non-equal mass_
|
| From the Earth's frame of reference there is no change in
| delta-v other than a change in direction. It's only from the
| Sun's frame of reference that there is velocity added in the
| speed component (v_infinity, commonly). If you can find a
| _single_ measurement to the contrary, that's novel enough to
| be worth publishing.
|
| That's why you can't gravity assist around the Sun to get
| around the Solar System faster.
| saagarjha wrote:
| You've retreated to a different argument nobody was making.
| JumpCrisscross wrote:
| > _You've retreated to a different argument nobody was
| making_
|
| Respectfully, I have a background in aerospace
| engineering. This is the number one popular misconception
| about gravity assists/slingshot maneuvres.
| nick238 wrote:
| If jessriedel hadn't said
|
| > The gravity of the Earth absolutely changes the speed
| of the probe.
|
| but instead
|
| > The gravity of the Earth absolutely changes the
| _characteristic energy with respect to the sun_ of the
| probe.
|
| Would you be happy?
| JumpCrisscross wrote:
| No. The gravity of the Earth doesn't do any work on the
| probe. It's a coupling mechanism. The work that is done
| is by Earth's orbital energy around the Sun. That is the
| energy that is less after the spacecraft encounters the
| Earth than before. The Earth is "dragging" the spacecraft
| along with it around the Sun. Not trebucheting it.
| jessriedel wrote:
| The gravity of the Earth absolutely does net work done on
| the probe. That's an unambiguous mathematical statement.
| It's just that this work is frame dependent. It sounds
| like you want it to frame-independent and to be given by
| the value in the center-of-mass frame. But it's not.
| bgirard wrote:
| > From the Earth's frame of reference there is no change in
| delta-v other than a change in direction.
|
| But no one is talking about the Earth's frame of reference.
|
| > But "the gravity of Earth" imparts no net delta-v and
| wouldn't on its own allow the craft to reach Venus.
|
| That's statement is untrue. Gravity assists with planets
| can provide net delta-v allowing spacecrafts to reach other
| planets. See the Voyager 2 gravity assists for one example.
| JumpCrisscross wrote:
| > _Gravity assists with planets can provide net delta-v
| allowing spacecrafts to reach other planets. See the
| Voyager 2 gravity assists for one example_
|
| If you were on Jupiter measuring Voyager's speed before
| and after it interacted with Jupiter, you wouldn't
| measure a net effect. It's only if you're standing on the
| Sun (or somewhere else where you can see Jupiter
| revolving) that you see Jupiter "pull" the spacecraft
| along, thereby imparting velocity.
|
| Gravity doesn't do any work. The gravitational potential
| energy of the Voyager-Jovian system is entirely conserved
| in a flyby. Jupiter's orbital energy about the Sun is
| what's stolen.
|
| This is a common misconception when it comes to gravity
| assists. It's why I think that language could be tighter.
| nick238 wrote:
| I don't know what you mean by "gravity assist around the
| Sun":
|
| 1. Using the perihelion in an orbit "around the sun" as a
| gravity assist?: spacecraft usually care about their speed
| relative to the sun (characteristic energy, C3), and a
| (free) gravity assist around the sun won't do much.
| Dropping close to the sun to perform a _powered_ bi-
| elliptic transfer could be a thing if you wanted to travel
| extreme distances (e.g. put a telescope at 500 AU to use
| the solar gravitational lens)
|
| 2. Using other bodies that are "around the sun" to get a
| gravity assist?: spacecraft do this _all the time_.
|
| Also "get around the solar system faster":
|
| 1. Decrease the orbital period (lower orbits orbit faster):
| This is exactly what Messenger and Parker Solar Probe is
| doing flying by Venus/Mercury. They're 'bouncing' off of
| the planets, trading orbital energy and raising the
| planets' orbit around the sun while dropping their own.
|
| 2. Get to places faster: This is what outer planets probes
| (Voyagers 1/2, Cassini, New Horizons) do. If Jupiter wasn't
| there, these missions might not be possible.
| JumpCrisscross wrote:
| > _If Jupiter wasn 't there, these missions might not be
| possible_
|
| If Jupiter weren't there _and moving relative to the
| destinations_. The gravity isn't the critical piece, it's
| the relative motion.
|
| Just having a massive object does nothing because gravity
| isn't doing any work, it's just coupling you to a moving
| object.
| nick238 wrote:
| Oh, sorry, I didn't specify which Jupiter I was referring
| to, the real, moving Jupiter that orbits the sun vs. the
| stationary, straw-man Jupiter that jumps out at you in
| bad faith retorts.
| JumpCrisscross wrote:
| > _I didn 't specify which Jupiter I was referring to,
| the real, moving Jupiter that orbits the sun_
|
| This is why I said you can't gravity assist around the
| Sun to travel around the Solar System. The Sun is moving
| around the galaxy at a terrific speed. But so is the
| Solar System. Dropping into and out of the Sun's gravity
| well does nothing other than change your trajectory.
| jessriedel wrote:
| Your first paragraph is exactly what I already said in my
| comment.
| adolph wrote:
| > The gravity of the Earth absolutely changes the speed of
| the probe.
|
| Wait! Wouldn't Earth's gravity take away when departing just
| as much as given when arriving? However, the probe's
| direction could change based on how close it passes Earth.
|
| As the probe passes Earth, a mass proportionate amount of
| Earth's velocity would be shared to the probe. I have a
| distant grade-school memory of an analogy of two people on a
| roller-skate rink. The passing and passed persons link hands
| and some of the passed person's velocity is emparted to the
| passing person's velocity.
| mgsouth wrote:
| The magnitude of the probe's "average" velocity relative to
| the Earth-object barycenter remains the same. If the
| universe was just the Earth and it, then you'd see the
| object making nice ellipical orbits around the Earth, and
| the Earth wobbling a bit.
|
| However, the barycenter is moving relative to Venus.
| Imagine just the three things--the Earth, Venus, and this
| little object. Now imagine the object is coming almost
| directly from Venus, loops in a tight ellipse around the
| Earth, and goes shooting back almost directly towards
| Venus. The velocity relative to Venus changes enormously.
| Even if you're just concerned with the magnitude, some of
| the Earth - Venus relative motion gets added to the probe.
| Think bouncing a rubber ball against a wall that's moving
| towards you. The wall slows down a tiny amount, and almost
| all of the wall's velocity is added to the ball when it
| shoots back towards you.
| adolph wrote:
| That is a nice explanation. However, it fails to answer
| the question posed to the assertion regarding gravity:
| >> The gravity of the Earth absolutely changes the speed
| of the probe. > Wait! Wouldn't Earth's gravity
| take away when departing just as much as given when
| arriving?
|
| As I understand your contribution, it is congruent with
| Earth's velocity altering the speed of the probe, not
| Earth's gravity.
|
| _The flyby of Earth reduced Juice's speed by 4.8 km /s
| relative to the Sun, guiding Juice onto a new trajectory
| towards Venus. Overall, the lunar-Earth flyby deflected
| Juice by an angle of 100deg compared to its pre-flyby
| path._
|
| I can see how your characterization of "goes shooting
| back" doesn't mean 180deg change, but a change relative
| to where Venus will be as the probe interacts with Earth
| and arrives back to Venus.
|
| I have a hard time understanding how the flyby fits in
| the overall plan however. This is the first Earth flyby,
| right? "Flybys en route: August 2024 Lunar-Earth, August
| 2025 Venus, September 2026 Earth, January 2029 Earth" [0]
| If so, the probe is not going "back" to Venus, because it
| hasn't been there yet. It has been on an orbit of a
| different ellipse than Earth and so this flyby is where
| it takes a left turn to head towards Venus for the first
| time.
|
| _As JUICE starts its first elliptical solar orbit, its
| distance to the Sun decreases. This results in an
| increase in speed--according to Kepler 's second law of
| planetary motion--and the spacecraft overtakes Earth._
| [1]
|
| 0. https://www.esa.int/Science_Exploration/Space_Science/
| Juice
|
| 1. https://sci.esa.int/web/juice/-/58815-juices-journey-
| to-jupi...
| jessriedel wrote:
| > Wouldn't Earth's gravity take away when departing just as
| much as given when arriving?
|
| Only in the Earth's center-of-mass frame. In the solar
| system rest frame, the probe leaves with a different speed
| than it entered with.
|
| > However, the probe's direction could change based on how
| close it passes Earth.
|
| Both the magnitude and the direction of the velocity vector
| change.
| wongarsu wrote:
| Everything you gain from gravity you have to give back on the
| way out. You can change the shape of your orbit that way, for
| example rotating it or making it more elliptical. But if we
| are talking about _gaining energy_ from a gravity assist it
| would be more accurate to describe that as stealing some of
| the rotational energy of the planet.
| jessriedel wrote:
| > Everything you gain from gravity you have to give back on
| the way out.
|
| This is true only in the center-of-mass frame, and that's
| true for all conservative forces.
|
| If I have a bowling ball covered in springs and I throw it
| at a marble at speed v, the marble will traveling at a
| speed 2v after the collision. It would be completely
| correct and not misleading to say "the springs on the
| bowling ball accelerated the marble to 2v" even though the
| marble's speed in the center-of-mass frame is the same
| before and after the collision.
|
| It's _also_ true that the energy gained by the marble comes
| from the bowling ball in the rest frame. That doesn 't make
| the first statement wrong or misleading. It just means that
| you like thinking about things in the center-of-mass frame.
| JumpCrisscross wrote:
| > _would be completely correct and not misleading to say
| "the springs on the bowling ball accelerated the marble
| to 2v"_
|
| I suppose this is where I disagree. The springs
| transferred momentum. Your throw (and the resulting
| motion of the bowling ball) did the work. (Tyres don't
| accelerate a car, its engine does.)
|
| You could slingshot around a moving body magnetically,
| the fundamental principles remain the same. The effect of
| a gravity assist comes from the motion of the body, _not_
| gravity _per se_.
| jessriedel wrote:
| "Work done" is the integral of a force.
|
| https://en.wikipedia.org/wiki/Work_(physics)
| verzali wrote:
| Common mistake, but it is just ESA, not "the ESA". Just as it
| is NASA and not "the NASA".
| adolph wrote:
| _Prepositions in English are highly idiomatic. Although there
| are some rules for usage, much preposition usage is dictated
| by fixed expressions._ [0]
|
| Prepositions are fascinating. Why is it that NASA and ESA
| don't get a "the" but the Sun, Earth, Moon and USA do? Why
| does it always feel wrong when Apple doesn't use "the" before
| iPod, iPhone, etc?
|
| 0. https://academicguides.waldenu.edu/writingcenter/grammar/p
| re...
| thomukas wrote:
| "The" is not a preposition but a definite article.
| adolph wrote:
| Thanks! The rules for definite articles are also odd.
| Only very specific categories of proper nouns are
| articled. USA gets one because "of" is in the name. I
| think the categories of articled proper nouns is probably
| missing sports leagues, like the NFL and the NBA.
|
| https://academicguides.waldenu.edu/writingcenter/grammar/
| art...
| anigbrowl wrote:
| I read 'send' as being about the change in direction rather
| than any change in velocity.
| xeonmc wrote:
| In other news, Lunchables may soon be meeting its demise:
| https://www.cbsnews.com/news/lunchables-lead-sodium-consumer...
| snapcaster wrote:
| Interesting article but i think posted on wrong thread
| istultus wrote:
| Isn't it time, as the process is becoming normalized (as with the
| Artemis project) to start using the term "(gravitational)
| slingshot" in headlines?
| munchler wrote:
| Agreed. The word "rerouted" makes it sound like this represents
| a change of plans, which it is not.
| nick238 wrote:
| No, it was rerouted around Earth because of the hyperspace
| bypass pending construction.
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