Lucy in the sky a diamond
MANY space-related events have captured attention this year, but coming up is one of the most exciting: the launch of NASA’s Lucy mission to investigate two collections of asteroids known as the Trojans, which share Jupiter’s orbit around the sun.
We’ve never seen one of those up close, but scientists expect them to offer important information about the history of our solar system.
Lucy’s launch is scheduled for Saturday at 7.34pm (Tasmanian time).
Although the asteroids in each of the two clusters are quite spread out, the groups are centred 60 degrees ahead and 60 degrees behind Jupiter — that is, one-sixth of Jupiter’s orbital path on each side of the giant planet.
The possibility that objects could be in these locations was known well before any were discovered. An understanding of this came from the brilliant minds of the mathematician-astronomers Leonhard Euler (1707-83) and his student, Joseph-Louis Lagrange (1736-1813).
Euler discovered that if we consider two masses — one much greater than the other, such as the sun and Earth — there were three points along a straight line at which the gravitational and centrifugal forces on an object are in balance. One point is on the opposite side of Earth’s orbit, hidden behind the sun and about 300 million kilometres away, and the others are 1.5 million kilometres towards and away from the sun along the line joining the sun and Earth.
Lagrange, working in Paris, made a significant discovery: there are two other such points, ahead and behind a planet along its orbit. Even though Euler found the first three, the five points are known collectively as the Lagrangian points.
An object at one of the first three points, called L1, L2 and L3, is in unstable equilibrium. That is, it’s a bit like a ball being placed atop a smooth rounded hill: any slight change in its position will cause it to roll down the hill. However, L4 and L5 are different. They are rather like the bottom of a curved bowl, in which objects moving away from the bottom will tend to return to it.
If you are ever in Heidelberg in Germany, take a stroll along the Philosopher’s Way and take in the view looking over the river to the castle and to the hill beyond called Konigstuhl. On the night of February 22, 1906, at the Konigstuhl Observatory, astronomer Max Wolf made the first discovery — photographically — of an object trapped in the vicinity of the L4 point around Jupiter’s orbit. This object, and subsequent discoveries, confirmed Lagrange’s mathematics.
We now know of about 5000 Jupiter Trojans. The first three were named Achilles, Patroclus and Hektor, characters from the Iliad, a poem by Homer about the mythological Trojan War. The reference was continued. Jupiter’s L4 asteroids are named after Greek characters, and those related to L5 are named after Trojans. However, the L4 and L5 asteroids are collectively called, simply, the “Trojans”.
While I love the connections between science and history — including mythology — let’s get back to the science.
Close observation of the Trojan asteroids will be important because they are objects that have been preserved in those locations for a very long time. They are thought to be leftover material from the building blocks of material that formed the outer planets, and distinct from the main “asteroid belt” between Mars and Jupiter. Indeed, because of the Trojans’ relationship to Jupiter, study of them may offer important clues as to the interior composition of Jupiter itself.
Lucy will investigate four asteroids in the L4 location in 2027-28, then swing back through the inner solar system and examine one of the L5 bodies in 2033.
Through experience in ventures such as this, we know that what we shall see and learn is likely to include some surprises. To me, apart from giving us plenty of scientific information to digest, that is what can make missions like Lucy’s so thrilling.
You may have been wondering why this mission is named Lucy. It is not one of NASA’s clever acronyms: it is named after the hominin Lucy, whose partial skeleton was discovered in Ethiopia in 1974. Just as that Lucy helps us to understand our origins, so, hopefully, will the spacecraft Lucy about our planetary system.