JUICE and 3I/ATLAS: An Unexpected Encounter in Space

Over the next few days and weeks, ESA's spacecraft JUICE will have a good view of the recently discovered interstellar object 3I/ATLAS.

To the point:

• Visitor from afar: 3I/ATLAS, the third known interstellar object to date, is currently traveling through the Solar System and reached its closest point to the Sun yesterday.
• Good view: In the next days, 3I/ATLAS is barely visible from Earth. ESA's spacecraft JUICE, however, is now launching a dedicated observation campaign.
• Cosmic water: With the help of the JUICE’s scientific instruments, researchers at the Max Planck Institute for Solar System Research (MPS) hope to learn more about the water on 3I/ATLAS.

On its eight-year journey to Jupiter, ESA’s space probe JUICE is meeting up with an unexpected diversion: In the coming weeks, the probe will have a good view of the interstellar object 3I/ATLAS. A few months ago, on July 1 of this year, the object first appeared in images taken by the Chilean ATLAS telescope. Its trajectory identified it as a visitor from outside the Solar System. Only two other interstellar objects are known to date. In recent months, researchers at the Max Planck Institute for Solar System Research (MPS) have helped prepare JUICE’s spontaneously convened observation campaign and are hoping for unique data.

For JUICE, observational conditions will be favorable in the coming weeks. At a distance of 210 million kilometers, 3I/ATLAS reached its closest point to the Sun yesterday, within the orbit of Mars. Under the influence of the Sun, frozen gases are likely to evaporate in increasing quantities from the surface of the well-traveled guest over the next few days and weeks, carrying dust particles with them and thus enveloping the cosmic visitor in a comparatively easily recognizable cloud of gas and dust. JUICE is currently only about 60 million kilometers away from this spectacle. Telescopes on Earth will have little chance of seeing the interstellar object in the coming days. Our planet now almost exactly on the other side of the Sun.

Water from the depths of space

The members of the JUICE team at MPS are particularly interested in the water that sublimates from 3I/ATLAS into space. The spacecraft’s Submillimetre Wave Instrument (SWI), which was developed and built under the leadership of MPS, captures radiation with wavelengths between approximately 0.25 and 0.5 millimeters. In this wavelength range, water molecules leave characteristic signatures. With its extremely high spectral resolution, SWI can provide information such as whether the water molecules originate from the surface of the interstellar object itself or additionally from ice grains in its coma. “We hope that SWI will enable us to take the first high-resolution measurements of water on 3I/ATLAS,” says MPS scientist Paul Hartogh, SWI Principal Investigator. “Among other things, this may help us understand the near surface water ice distribution of this active body”, he adds.

In the coming days, JUICE's Particle Environment Package (PEP) will also be switching on one of its sensors. The sensor captures energetic neutral atoms such as hydrogen, helium, or oxygen. These are created when charged solar wind particles collide with the gas cloud surrounding 3I/ATLAS – and the more gas the interstellar object spews into space, the more numerous they are. “The measurements can tell us which gases sublimate from the surface of 3I/ATLAS and in what quantities,” explains Elias Roussos, PEP team member from MPS. Since very few neutral particles are expected to be produced, PEP will measure for twelve days straight, hopefully capturing enough particles.

With a little luck, 3I/ATLAS will be particularly active in the next few days. While the object currently produces around two tons of water vapor per second, it could suddenly release even larger quantities shortly after passing the Sun. Such outbursts are known from comets and would facilitate the planned measurements.

Measurements under time pressure

The JUICE instruments that will be looking at 3I/ATLAS will be operating between November 2 and 25. From an operational point of view, the campaign is tricky. Probe and interstellar object are traveling in different directions, with 3I/ATLAS tearing through space at a speed of approximately 220,000 kilometers per hour. In addition, JUICE can only look directly at its target for no more than 30 minutes per day as to avoid exposing its sensitive side to too much solar radiation. Only PEP, with its significantly larger field of view, can measure for twelve days at a time despite the spacecraft slewing.

Whether JUICE’s efforts were successful will only become clear in February next year. Only then will the observational data be transmitted to Earth – a kind of delayed souvenir photo of a unique encounter.