Two days ago the space probe Lucy shot past the asteroid Dinkinesh... and found it has a satellite! Most of the apparent motion in this movie is caused by Lucy zipping by at 16,000 kilometers per hour.
Lucy is going on a grand tour of the asteroids:
On 3 December 2024 it will swing back past the Earth to get a gravity assist.
On 20 April 2025 it will visit Donaldjohanson, an asteroid in the inner belt that's part of a family created by a collision involving the asteroid Erigone 130 million years ago. Yes, some asteroids come in families!
On 12 August 2027 it will pass Eurybates, an asteroid near Jupiter's Lagrange point L4. Asteroids near L4 are called 'Greeks' and asteroids near the other Lagrange point, L5, are called 'Trojans'. Eurybates is the largest member of the only knoqn collisional family among the Greek or Trojan asteroids.
On 15 September 2027, Lucy will swing by Polymele, another Greek asteroid near Jupiter's L4 point. Its red color suggests it's covered with organic compounds called 'tholins', which also give the moon Titan its reddish haze.
On 18 April 2028 it will visit Leucus, another Greek. This is a D-type asteroid, very dark red.
On 11 November 2028 it will see Orus, another Greek. This may be a binary asteroid.
Then, after a while, on 26 December 2030, it will swing by Earth again for another gravity assist! It will be the the first spacecraft to return to Earth from Jupiter's orbit. The goal is to send it over to the Jupiter's other Lagrange point, L5, where the Trojan asteroids live.
On 2 March 2033, it will visit Patroclus–Menoetius, a binary Trojan.
Then Lucy will sail off into the night. For an animation of her trajectory, read on!
This is an animation of Lucy's trajectory in a coordinate system rotating along with Jupiter. In this animation the Sun is at the center, and Jupiter is the yellow dot on top, moving slowly up and down. Lucy's orbit is in purple. The Earth's orbit is in blue.
The asteroid Donaldjohanson is in green: this is the next asteroid Lucy will visit.
The asteroid Eurybates is in blue at left. It moves around Jupiter's Lagrange point L4, so it's called a 'Greek'. Lucy will visit it on 12 August 2027.
The double asteroid Patroclus–Menoetius is in red at right. It moves around Jupiter's Lagrange point L5, so it's called a 'Trojan'. Lucy will visit it on 26 December 2030 and then shoot off into space.
@johncarlosbaez I am always impressed with the precision that those calculations seem to have. Over millions of km, we hit a target zone that is just a few km. Amazing
@pjakobs - the calculations are great! But nothing is perfect, so like most of these space probes, Lucy does have engines that let it make course corrections. Here's a list of its engines:
@johncarlosbaez I thought so, bout still, with small thrusters like those, the possible corrections are not huge and dependent on the amount of available fuel on board - which I guess everybody wants to keep as low as possible.
and btw: TIL that catalytic monopropellant hadrazine thrusters exist - and have for many years.
@pjakobs - luckily, a tiny change in the direction a spacecraft is moving can affect its position by thousands of kilometers after it's traveled for months. So, the trick is to make the course corrections early enough.
@johncarlosbaez The orbit of Eurybates surprises me: it looks like it's too far ahead of Jupiter. I would have expected its trajectory in the rotating frame to librate (maybe with a fat oval) around a point 60 degrees ahead, relative to the Sun, but the angle seems bigger.
@mattmcirvin - Yeah, that orbit looks really weird now that you mention it! Everything I read suggests that Eurybates orbits Jupiter's L4 Lagrange point, 60° ahead of Jupiter's orbit in a 1:1 resonance. But that's not what the animation seems to show!
@johncarlosbaez@mattmcirvin Hm, nerd sniped and I tried my hand with my own n-body simulation. I figured it would help clear things up, but it just raises more questions for me! Although it should be noted, this is in the plane of the ecliptic! So Eurybates is moving into and out of the plane by about 0.7 AU, patroclus-menoetius is moving in and out by about 2.15 AU, and Jupiter by about 0.12 AU. The ploy extends from -5.7 to +5.7 AU. That could certainly account for the differences in shape.
@johncarlosbaez@mattmcirvin This is a comparison between NASA Horizons data ~20 years later, vs ~20 years of time evolution of my code. The cyan and red dots on the left are almost the same, so I'm taking this as evidence that my simulation is decently accurate.
@davidphys1 - that's really interesting, David Moore. Now I really wonder why Eurybates looks separate from Jupiter by a greater angle than Patroclus-Menoetius, even if you compare the "centers of their orbits". Is it because of how they're moving in and out of the plane of the ecliptic?
@johncarlosbaez@davidphys1 We're only seeing how Eurybates moves over a few cycles of Jupiter's orbit--maybe there is some longer-period motion that cycles around a different center on top of that? I know that the motions of co-orbital bodies in the rotating frame can look very complicated--there are "horseshoe orbits" that wander all over relative to the planet--I've seen these plotted out for some of Earth's co-orbital asteroids; I just thought the libration of Trojans tended to be simpler than that, but maybe it's not in this case.
@mattmcirvin@davidphys1 - it's time to do a bit of online research on the orbit of Eurybates. Since NASA is sending a space probe there, they must understand its orbit really well, and there could be some papers about it, especially if it's interesting.
@johncarlosbaez@davidphys1 Here's a good example of what I'm talking about--it's the orbit of one of the two known Trojan asteroids of Earth, and you can see that if you plotted a short bit of it in the co-rotating frame, it'd look as if it were centered in the wrong place:
@johncarlosbaez@RGrunblatt so that's not a bug, but a feature! And the up down variation represents the variation in the distance to sun (focus of the ellipse) hence the eccentricity of the orbit...
@antoinechambertloir@RGrunblatt - right! We work in this coordinate system where Jupiter is almost stationary so that we can easily see how two asteroids are orbiting its Lagrange points.
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