In just under a month, NASA will launch an ambitious mission through the asteroid belt and into Jupiter’s orbit. The spacecraft will not visit Jupiter, however: it will fly over several asteroids that share an orbit with the giant planet, seeking to investigate these fossils from the early days of the solar system.
This mission is pretty cool, but the flight it will take to get to where it’s going is just crazy.
This mission is called Lucy, named after the skeleton of Australopithecus afarensis, a hominid dating back over 3 million years, an ancestor of man. The skeleton itself was named after the Beatles song “Lucy in the sky with Diamonds”, which came full circle in that roundabout naming path.
It’s a recurring theme with this mission, as you’ll see in a moment.
Lucy’s main mission is to visit what is called Jupiter Trojan asteroids and study them with a suite of scientific instruments. Due to a quirk of orbital physics, when something like a planet orbits a star, there are several points along and near that orbit where there are gravitational stable points; if you put a small object in it, it will also orbit stably around the star. These are called Lagrange points, and are quite useful for space missions (James Webb Space Telescope, scheduled for launch in December, orbit around the Sun near one of the Earth’s Lagrange points).
Jupiter is a large planet, with powerful gravity, so its Lagrange points are very stable. Asteroids that roam there will tend to stay there. One of these points, called L4, is 60 ° ahead of Jupiter in its orbit, and another called L5 is 60 ° behind. Many asteroids cluster in these points – the first ones discovered were named after characters from the Trojan War myth, so they are nicknamed the Trojan asteroids, and these two stable points in particular the Trojan Points. By tradition, those at point L4 are named after Greek war figures and those at L5 after Trojans.*.
It is suspected that many of these asteroids sharing Jupiter’s orbit have been around for a very long time, even as long as the solar system formed. Sending a mission there is a really good idea – he can visit a lot of these asteroids more easily, as they come together. It’s like having many space missions combined into one.
The Lucy mission is expected to launch no earlier than October 16, 2021. It has a 23-day launch window; any launch within this time range will do. But it can’t launch before the window opens or after it closes because the planets literally have to be aligned for this to work.
And that brings me to the wonderful thing that Lucy’s trajectory is.
Getting to Jupiter’s orbit is difficult. It takes a lot of boost to lift something this far from the Sun, and Lucy is a pretty heavy spaceship. It runs on solar power (which is itself cool; it’s only recently that solar cells have become efficient enough to power a spacecraft this far from the Sun) and when deployed they will be 14 meters tall. wide and have a mass (including fuel on board) of 1,550 kilograms.
That’s a lot of Jupiter throwing machine. So the orbital engineers decided to get some help. And hey, once they’ve got some help, why not add a little bit to the mission and maybe get some more science for free?
The final path chosen for Lucy is, quite simply, incredibly cool. Strap on for that … and take a look at the ‘Where’s Lucy’ website for some fantastic animation that shows you where it will be and when.. It will really help you understand this next part.
It is launched on an Atlas V rocket from Florida. It will enter a high loop around the Earth, then a few months later will descend back down to our planet and use gravitational assistance to propel it into an even higher loop that will take it out a little beyond the orbit of Mars. . Then it falls back to Earth a second time to get another boost. This will project him through the main asteroid belt where he has his first scientific encounter with a 4 km asteroid called Donald johanson (named after one of the co-discoverers of Lucy’s skeleton!) on April 20, 2025.
Lucy (the spaceship) will then continue through the asteroid belt on a curved ellipse that culminates right at Jupiter’s orbit, perfectly timed to meet Jupiter’s L4 asteroids. It will pass very close to four of them: Eurybates (68 km wide and who has a tiny moon!), Polymel (21km), Leucus (~ 35 km), and Orus (50km).
Then it will descend back down through the asteroid belt and return to Earth’s orbit, turn around and return to Jupiter’s orbit again. Enough time will have passed that this time, when he does get there, he will encounter an asteroid at point L5 of Jupiter. And get this: it’s a binary asteroid! The two components are called Patroclus and Menoetius, each about 100 km in diameter. They revolve around each other at a distance of about 700 km every 4 days or so. I imagine the images we will get from this system as we approach will be incredible.
I think the coolest part of it all is that this pattern repeats every six years: Lucy will descend back down to Earth and then head into Jupiter’s orbit, cycling between asteroids L5 and L4 each time. As long as the spacecraft is healthy and NASA gives the green light to continue the mission, it could do so for a long time.
This is all brilliant orbital planning and I am very impressed with it. It uses minimal fuel and derives a tremendous amount of science from it. And we don’t have to wait very long!
But first: launch on October 16. This is the one I will be looking forward to.
*I learned something by researching this: So many Trojan asteroids have been discovered ten thousand – that the names quickly ran out, so the International Astronomical Union (the official custodian of names from space) authorized the names of Olympic athletes. It’s very cool.