Juice probe scores epic fuel save after snapping selfies with Earth and Moon ESA's Juice spacecraft has had close-ups with the Moon and Earth and is on its way to Venus, having snapped images with its monitoring cameras and collected scientific data as it passed. The ultimate destination of the spacecraft is Jupiter, but to save fuel, the probe is using the gravity of the Earth, Moon, and Venus to send …
While, given the scope of the mission, the 150 kg savings might be considered fairly small, it does mean the mission payload can larger and heavier. But I reckon the real win is the ability to do this, and possibly even more intricate gravity assists in future missions to get to places both faster and with less fuel than had previously been considered essential.
The thing that concerns me is that the indirect route they're taking means that the satellite is going to take six years to get to Jupiter, which means it has much longer for components to die as it flies in circle after circle around the sun before hitting its final destination.
I'm not a rocket surgeon, and this has almost certainly been taken into account - but it feels a bit excessive to have 6 flybys to get to the fourth closest planet, spread across six-odd years.
I understand the relationship between the fuel saved and the mass of the craft itself, but was thinking more about the total mass of the system that launched it, which dominates initial propulsion calculations, and why the 150 kg fuel was replaced by scientific kit.
As I said, I think the main innovation may well turn out to be the ability to use non-propulsive means to accelerate craft.
"As I said, I think the main innovation may well turn out to be the ability to use non-propulsive means to accelerate craft."
Hardly an innovation, though this one was particularly interesting with the moon/earth double tag.
That 150kg saving was probably worth about a ton on the first stage...
150kg (330lb) of fuel is an absolute ocean.
Consider that's about 3,000kg of booster to get that into space.
The Shuttle would have cost $8,250,000 at $25K/lb.
A Falcon 9 is roughly $1,200/lb so that would be somewhere around $400,000.
And that's just to get it to low Earth orbit... not to Jupiter.
See the animation on https://www.esa.int/Science_Exploration/Space_Science/Juice/Where_is_Juice_now.
The acceleration is not due to the gravitation of the Moon, Earth and Venus, but due to their orbital velocity.
Gravity assists like this are made possible by the Oberth effect, increasing V (from mv^2) at a higher base velocity causes a much stronger increase in Kinetic Energy.
The increased V comes from converting Gravitational Potential Energy to Kinetic Energy by being deeper in a celestial body's gravity well.
I might be missing something here, I read your comment as "due to [the celestial body's] orbital velocity" rather than "due to [the probe's] orbital velocity".
The bit you are missing is the point of view. If you are sitting on the Moon watching Juice approach and recede the motion is symmetrical about the point of closest approach. Although Juice accelerates on its way to the Moon it decelerates exactly the same amount as it leaves.
Now jump off the Moon and walk over to the sun and watch from there. From this point of view the Moon is moving. Juice's approach and departure are not symmetrical. The difference is up to the velocity of the Moon in either direction depending on which side Juice passes. This free velocity change did not come from nowhere. The Moon's velocity changed too to preserve momentum. As Juice has far less mass than the Moon the Moon's velocity changed far less than Juice's.
If you want to imagine this in more familiar terms, transform Juice into a ping pong ball and the Moon into a bat. While sitting on the bat you see the ball approach at speed and recede with the same speed in the opposite direction. When sitting on the table you see the bat move and the ball returns faster than it arrived because it picked up velocity from the bat.
Oberth effect is due to powered thrust being used at some point (not just for course correction).
Gravity assist is when energy is exchanged between 2 bodies, so if the probe slowed down, the larger body was accelerated, if the probe sped up, the larger body was decelerated.
See Wikipedia for more info.
If I were the controller I don't think I would be doing that about the trajectory. Science data take perhaps. The trajectory was set after the first course correction. I would have checked the orbit several times after that though to make damn sure it didn't need another burn. Now I would be doing the checking several times again to see if it needed another course correction before getting to the right point in space next to Venus. If they don't need another burn before that I will be very impressed.
Ah, but remember the problems they had at the start, and the serious velocity changes needed once the destination is reached and they still have the relative velocity vectors from where they set off. Not much point in arriving at Jupiter if you still have the direction and speed of the Earth :-)
"Some very complex routing the better to approach Venus and sling from there to Jupiter?"
Doc: "You aren't thinking four dimensionally!"
Marty: "yeah, I have a real problem with that"
Orbital mechanics can be very non-intuitive since other than being stuck to the ground, we don't have to factor it into how we plan a driving holiday.
Yes it’s all about momentum change, a gravity assist doesn’t just ‘speed up’ a spacecraft but also changes its trajectory. I think the best, grounded analogy I ever heard was this.
Imagine I stand at the side of a railway track, in my hand I hold a tennis ball and with the best will in the world I could throw this ball at, say 10 metres per second, a train is approaching say moving at 100 metres per second. At a critical point I throw the ball at the front of the train, no directly, not head-on (I would need to be standing on the track to do that), but at an angle. The ball hits the front of the moving train and rebounds away.
Now from the perspective of the train driver, they see the ball approach, quite fast, depending on how fact the train is going, how fast I can throw the ball, and the angle, the ball hits and the driver sees it rebound away at a different angle but apparently the same speed - so nothing has happened!
But from my perspective, the ball bounces off the front of the train in a totally different direction but moving a lot faster. If I could carefully measure the trains speed, I would find that it has slowed down, but as the train is, what, millions of times more massive than the ball, this is just not relevant.
Replace the tennis ball with a space probe, replace the train with a planet, replace the driver with someone standing on the surface of that planet with a really good telescope, and most importantly, replace the person standing by the rail track with the sun! Now, obviously the probe doesn’t literally ‘bounce’ off the planet, it’s a gravitational interaction, the closer you can get to the centre of the planet, the greater the effect will be. So from the point of view of someone standing on the planet, the probe approaches, getting faster and faster and then swings by the planet and gets slower again as the planets gravity acts to pull it back. But from the viewpoint of someone standing on the sun (yes, OK, I know), the probe accelerates towards the planet, swings by and departs, in a totally different direction, and much faster. The planet ever so slightly slows down, but it’s completely imperceptible.
And you can go the same in reverse, and ‘lose’ speed (with respect to the sun) - in this analogy, you can throw the ball faster than the train is going and you throw it at the back of the train as it passes, in this case the ball slows down (with respect to you), and transfers momentum to the train.
Something mentioned above was the Oberth effect. Imagine you have a spacecraft in an elliptical orbit, so at one point it is close to its primary (say the Earth) and at the other point it is far away. You want to raise your orbit, well at least raise the apogee, in effect ‘add energy’ to the space craft. You have fuel on board to burn and impart energy to the probe. The question is, at what point in the orbit is it most advantageous to burn the fuel?
And the answer is at perigee, the lowest point, which can sound counter-productive, surely that’s when the Earth’s gravity is at its strongest? But think of it like this.
The probe has a mass of say 1000kgs, and you intend to burn 100kgs of fuel. At apogee, the highest point of the orbit, the entire mass has maximum gravitational potential energy and minimum kinetic energy, as it fall down to perigee, it trades potential energy for kinetic energy until at perigee it’s moving as fast as it will. Now imagine that it was possible to burn all 100kgs of fuel instantaneously (you can’t but run with it), suddenly the probe is 100kgs less massive, but the mass of the fuel had itself kinetic energy, and now it’s gone. Where? Over and above the impulse from burning the fuel, the probe now gains additional energy - momentum is conserved, the fuel and the momentum it had, has to go somewhere, and it goes to increasing the speed of the probe. This is the Oberth effect, the maximum ‘delta-V’, ie a change in velocity will happen if you burn fuel at the bottom of the gravitational potential well! *
Orbital mechanics is really not intuitive - I’ll leave you with a question. You have two spacecraft in identical orbits but one is 100km behind the other. You want to close the distance in advance of docking, so on the trailing spacecraft, you fire thrusters at the ‘back’ of the craft and increase speed. What happens?
* yes I know I’m interchanging energy and momentum, and yes I know they are not remotely the same, but for the purpose of explaination, give me a break eh?
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