The Europa Clipper spacecraft may soon be pummeled by charged particles — particles ripped away from the ion tail streaming from the interstellar comet 3I/ATLAS.
That’s the new prediction from two European researchers whose computer code allows them to identify when spacecraft can align with a comet‘s tail and the sun. Completely harmless to the spacecraft, the event provides a rare and fortuitous alignment — and a unique opportunity to directly sample material from a comet from beyond our region of the cosmos.
“We have virtually no data on the interior of interstellar comets and the star systems that formed them,” Samuel Grant of the Finnish Meteorological Institute, who led the research, told Space.com. “Sampling the tail in this way is the closest we can currently get to a direct sample of such an object, and thus a different part of the galaxy.”
Despite this huge opportunity, however, there are a number of things that could work against Europa Clipper detecting charged particles from 3I/ATLAS. For instance, the U.S. government shutdown persists. This is an issue because Europa Clipper is currently in cruise mode as it continues its journey to Jupiter and not all of its instruments are activated. The alignment between Europa Clipper, the comet and the sun falls into place between Oct. 30 and Nov. 6, and if the shutdown is not resolved by then, it’s unclear whether scientists can awaken Europa Clipper to make the measurements even if they want to.
If the measurements can be made, though, they will provide insights into the composition of the interstellar comet, allowing comparisons with comets from our own solar system.
Comet anatomy
Comets possess two tails formed from particles and dust lifted off a comet’s surface, or outgassed due to rising temperatures on the comet as it nears the sun. The heat causes pockets of gas just beneath the surface to expand and burst out.
A comet’s dust tail is the most prominent and follows the trajectory of the comet. No spacecraft is currently in position to fly through 3I/ATLAS’ dust tail and sample it.
The ion tail, however, is another matter.
“We study cometary bodies because they act as time capsules, sealing in material from their formation billions of years ago,” said Grant. “This material is ejected on approach to the sun, a portion of which is transported away from the sun by the solar wind to form the ion tail.”
Because it is driven by the solar wind, which is radiating out from the sun, the ion tail always points away from the sun.
Between Oct. 30 and Nov. 6, Europa Clipper will be in position to potentially receive some of those ions transported by the solar wind towards the spacecraft at several hundred miles per second.
Along with Grant, Geraint Jones — of the European Space Agency and principal investigator of the Comet Interceptor mission — was able to predict the ion tail crossing thanks to their Tailcatcher program, which is computer code that can track the movements of packets of solar wind material.
“We use the velocity measured at [a packet’s] arrival to trace back the path it took to travel from the sun to the spacecraft, and we can compare this path to the position of the comet,” said Grant.
Their calculations indicate that Europa Clipper could intercept some of the solar wind packets that came close enough to the comet to steal away some of the ions in its ion tail.
The solar wind carries ions of its own from the sun, so how can scientists differentiate between solar-wind ions and ions stolen from the comet?
“Cometary ions can be distinguished in a number of ways, most simply by chemical abundances — cometary ions include significant amounts of heavier species, particularly water-group ions, compared to the proton and helium-dominated solar wind,” said Grant. “Additionally, the act of loading additional mass into the solar wind causes a general slowing and deflection of the ambient solar wind flow.”
Probability of a particle shower
A successful detection also relies on properties intrinsic to the solar wind. First, the solar wind has to be flowing in the right direction; although it always flows from the sun, it is not always perfectly radial and sometimes it can flow at an angle, in which case it might miss Europa Clipper.
The wind also has to be strong enough at the right time to be able to carry the heavier ions all the way out to Europa Clipper. The comet is at perihelion — its closest point to the sun — on Oct. 29, when it will be at a distance of 126 million miles 200 million kilometers), inside the orbit of Mars. Meanwhile, Europa Clipper is currently over 186 million miles (300 million kilometers) from the sun having conducted a flyby of Mars earlier this year.
On the plus side, as 3I/ATLAS edges closer to the sun, activity on the comet will continue to ramp up. If this trend continues, then, at perihelion, the ion tail will be at its broadest, increasing the chances of Europa Clipper being able to detect some of its ions.
The European Space Agency’s Hera spacecraft, on its way to the double asteroid system of Didymos and Dimorphos, will also be in a position to intercept solar wind packets carrying cometary ions between Oct. 25 and Nov. 1. However, Hera has no instruments for measuring the solar wind’s charged particles and magnetic field. Europa Clipper does, since part of its mission is to study Jupiter’s magnetic and radiation environment and its impact on Europa.
Yet, neither Grant nor Jones are members of the Europa Clipper team, and therefore have no say on how or whether the team makes the measurements.
But even if Europa Clipper misses out this time, Tailcatcher has been used to successfully predict ion tail crossings before, such as when the European Space Agency’s Solar Orbiter spacecraft detected ions from the comet C/2019 Y4 in 2020. Its history of successful predictions means that it will undoubtedly be able to identify tail crossings in the future — perhaps even for the next interstellar comet, whenever that might turn up.
Indeed, the possibility of tail crossings adds an extra dimension to studying comets. In 2029, the European Space Agency will launch the Comet Interceptor mission, which will hold station in space until scientists identify a comet worthy of being visited by the spacecraft. Interstellar comets are high on the list.
“Comet Interceptor’s primary objective is to fly close enough to an interstellar or long-period comet as to directly sample the coma and head of the tail,” said Grant, who is not a member of the Interceptor team. “It would be an amazing opportunity if another spacecraft was able to cross upstream of the tail at a similar time.”
Grant and Jones’ prediction of the ion tail crossing is reported in a pre-print posted to the arxiv pre-print paper repository.
