A massive collision billions of years ago may have dramatically reoriented Ganymede, Jupiter’s largest moon.

at Kobe University, Japan, and his colleagues studied Ganymede’s extensive furrow system, a series of concentric troughs believed to be remnants of the largest impact structure in the outer solar system.

The centre of the furrow system aligns closely with Ganymede’s tidal axis – the imaginary line running to Jupiter from the centre of the moon’s side that always faces its planet. This led the researchers to suggest that the impact that formed the furrows caused a significant redistribution of mass that reoriented the moon.

Through simulations, the researchers determined that the impactor responsible probably had a diameter of about 150 kilometres ­– significantly larger than the one that caused the extinction of the dinosaurs on Earth, which is estimated to have had a diameter of about 10 kilometres.

at the University of Illinois Chicago says that if an asteroid like that hit Earth, “it would be a global sterilising event, a bad day”.

Upon impact, this asteroid would have breached Ganymede’s icy crust into the liquid oceans below, creating a transient crater and hurling vast amounts of material across the moon’s surface.

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As this settled, it would have formed a thick blanket of ejecta around the impact site, creating a region where gravity is stronger due to the extra mass. Over time, this anomaly would cause Ganymede to reorient, aligning the impact site with its tidal axis, the simulation showed.

Hirata’s team compared this process with an event on Pluto, where a large impact created a basin called Sputnik Planitia, leading to a reorientation of the dwarf planet.

However, although it is likely that the Ganymede impact significantly affected the moon’s early history, estimating the size of the object that hit it is complicated because we lack good data on the gravity and topography of this frigid world, says Hirata.

Dombard says the model used in the paper doesn’t account for some of the complexities of Ganymede’s unique icy structure. “I think it is very good for establishing that this process could occur, but I don’t necessarily trust the numbers,” he says.

The Jupiter Icy Moons Explorer (JUICE) spacecraft is stopping by home in what the European Space Agency (ESA) is calling “a double world first”. It will fly past Earth and the moon on 19 and 20 August, marking the first of several complex manoeuvres along its looping path to Jupiter – and it may be possible to spot the spacecraft in the sky as it hurtles by.

JUICE launched on 14 April 2023, and since then it has been circling the sun roughly alongside Earth. But on 19 August, its journey will start to get interesting. It will pass within 700 kilometres of the surface of the moon, slowing it down slightly in what is called a gravity assist. Then, the next day, it will dip less than 7000 kilometres from Earth’s surface, bending its trajectory even more. That is when it may be possible for eagle-eyed skywatchers in South-East Asia to catch a glimpse of the spacecraft.

All of this is necessary because if JUICE were to fly in a straight line to Jupiter, it would need more than 60,000 kilograms of fuel to get there, and even more to slow down and enter orbit once it arrives. The mass of the entire spacecraft without fuel is 2420 kilograms, so that is not remotely feasible. Instead, the upcoming flyby of Earth and the moon will send the craft towards Venus, where it will start to speed up, and then it will make two more passes by Earth to gain just the right amount of speed to get to Jupiter in 2031.

The first manoeuvre may be the most complicated of the entire mission because JUICE must pass through precisely the right point in relation to both Earth and the moon to catch a ride to Venus. “It’s like passing through a very narrow corridor, very, very quickly: pushing the accelerator to the maximum when the margin at the side of the road is just millimetres,” said ESA’s in a . A double-flyby of Earth and the moon has never been attempted before, nor has any double gravity assist.

If all goes well, JUICE won’t just be on its way to Venus and then Jupiter – it will also perform the first real tests of its scientific instruments. For some of them, this is the only chance they will get to iron out any kinks while observing a planetary surface before the spacecraft arrives in orbit around Jupiter and begins its mission there: searching for hints that the giant world’s moons could host life.