Juice rerouted to Venus in first lunar-Earth flyby
esa.int
esa.int
https://www.esa.int/ESA_Multimedia/Videos/2024/08/Juice_s_lu...
(for those that are more visual)
(everyone is more visual, though we have less aggregated data on how blind people use their visual cortex)
- 2nd and 3rd Earth flybys 9/2026 and 1/2029
- 7/2031 arrives Jupiter; Jupiter orbit insertion and apocentre reduction with multiple Ganymede gravity assists
- 1/2032 .. 11/2034 Reduction of velocity with Ganymede–Callisto assists. Increase inclination with 10–12 Callisto gravity assists.
- 12/2034 enter Ganymede orbit for its close-up science mission
- 12/2035 will impact on Ganymede when runs out of propellant
summarizing https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer#Sum...
"Rerouted" means that the route was changed, not that the trajectory was changed. The route was planned before launch, and that hasn't changed. The headline makes it seem like there was an unplanned change.
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.
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.
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.
Respectfully, I have a background in aerospace engineering. This is the number one popular misconception about gravity assists/slingshot maneuvres.
> 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?
> The gravitational coupling of the probe to the Earth absolutely changes the characteristic energy with respect to the sun of the probe.
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.
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.
Again that's not something I'm claiming.
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.
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.
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.
And as you say, an unpowered planetary flyby is a gravity assist.
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.
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.
>> 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?
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 100° compared to its pre-flyby path.
I can see how your characterization of "goes shooting back" doesn't mean 180° 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...
Here's a nice short-ish web page from NASA with nice details [1].
Yeah, "back" seems a little wonky. Maybe somebody got confused by earlier plans? Your second link, to the ESA video, is from 2017 and evidently isn't what they wound up with. According to that video the Vensus flyby was supposed to have been last October, and next encounter would be Mars. Or maybe they're thinking "back down to Venus's orbit."
[1] https://science.nasa.gov/learn/basics-of-space-flight/primer...
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.
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.
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.
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/pre...
https://academicguides.waldenu.edu/writingcenter/grammar/art...