A radargram image of the Moon's surface, captured by ESA's JUICE spacecraft as a test. Image credit: ESA/Juice/RIME, LOLA elevation map: LOLA Science Team
Have you ever wondered what happens when careful engineering meets pure cosmic luck? Welcome, dear readers, to FreeAstroScience. Today we’re following ESA’s JUICE spacecraft as it cruises toward Jupiter and, almost by chance, crosses paths with a visitor from another star: the interstellar comet 3I/ATLAS.
This article is written by FreeAstroScience only for you. We’ll see what that strange purple radar image shows, why astronomers are so excited about 3I/ATLAS, and why we must wait until 2026 to see JUICE’s photos. Stay with us to the end; the story ties together technology, patience, and a reminder of why curiosity matters so much.
What are we actually seeing in this purple “noise” image?
The image at the top looks like cosmic graffiti: purple speckles with bright orange ridges. It isn’t random art. It’s a radargram of the Moon, taken as a test by ESA’s JUICE spacecraft.
Here’s what that means:
- JUICE carries an instrument called RIME (Radar for Icy Moons Exploration).
- RIME sends radio pulses toward a surface and listens for echoes.
- The returning signal is plotted over time, building up something like an ultrasound scan of terrain.
Bright streaks in the image mark strong reflections, like rough or dense layers in the lunar surface. Darker areas show weaker echoes. When JUICE reaches Jupiter’s moons, the same technique will probe subsurface ice, hidden oceans, and buried structure.
So this fuzzy test image is our rehearsal: proof that the radar works before the spacecraft reaches its real targets.
What is ESA’s JUICE mission and why was it launched?
JUICE stands for Jupiter Icy Moons Explorer. Launched by ESA in April 2023, it’s designed to orbit Jupiter and then spend extended periods around Ganymede, while also studying Europa and Callisto.
These moons interest scientists because:
- They probably hide global subsurface oceans beneath thick ice shells.
- Liquid water, energy, and chemistry together might support some form of life.
- Their magnetic fields and radiation environments test our theories of planetary physics.
To give you a quick overview, here’s a compact timeline.
| Year | Mission Phase | Highlight |
|---|---|---|
| 2023 | Launch and early operations | Spacecraft checks, first instrument tests (including Moon radargram) |
| 2024–2030 | Cruise and gravity assists | Flybys of Earth, Moon, Venus, and Earth again to gain speed |
| 2031 | Arrival at Jupiter | Start of main science phase in Jovian system |
| 2034+ | Ganymede orbit | First ever spacecraft to orbit a moon other than our own |
During the long cruise, JUICE mainly focuses on navigation and spacecraft health. Most instruments stay quiet to preserve power and thermal margins. That’s why the decision to point them at comet 3I/ATLAS was so exciting: it wasn’t part of the original script at all.
What is interstellar comet 3I/ATLAS, and why does it matter?
Interstellar objects don’t belong to our Solar System. They formed around other stars and were kicked out long ago. When their paths cross the Sun, we get a rare chance to study material from another planetary system.
The naming scheme is simple:
- 1I/Ê»Oumuamua – the first known interstellar object (2017).
- 2I/Borisov – the first clearly comet-like interstellar visitor (2019).
- 3I/ATLAS – the third confirmed interstellar object, and the star of today’s story.
3I/ATLAS was spotted on July 1 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey in Hawaii.
Because it’s a comet, it carries:
- A solid nucleus, probably a dirty mix of ices and dust.
- A surrounding coma of gas and dust when it heats near the Sun.
- Long tails shaped by sunlight and the solar wind.
Now the key: those ices condensed around a different star, under different conditions. Measuring their composition is like dipping a thermometer into another planetary kitchen.
How does the brightness of a comet change with distance?
Before we follow JUICE, let’s put one simple physical idea on the table: the inverse-square law. Light from an object spreads out as it travels. Double the distance, and the same light covers four times the area, so it looks four times fainter.
In more formal language:
For an object of luminosity L and distance d, the observed flux F is:
Because JUICE was about 0.5 astronomical units (AU) from the comet during its observations, the flux it received was lower than that at closer spacecraft near Mars.
That makes the measurements challenging but far from impossible, especially when several instruments work together.
Which spacecraft observed 3I/ATLAS, and from where?
3I/ATLAS grabbed attention across the Solar System. Different missions saw it from different vantage points:
- NASA’s MAVEN orbiter captured ultraviolet images, revealing gases escaping from the comet’s coma.
- Mars Reconnaissance Orbiter (MRO) obtained some of the closest views as the comet passed about 19 million miles (roughly 30 million kilometers) from Mars.
- Perseverance rover on the Martian surface even recorded a faint glimpse in the sky, giving us the first image of an interstellar comet from the ground of another world.
- STEREO and SOHO, spacecraft near the Sun–Earth line, tracked the comet against the bright solar background.
- China’s Tianwen-1 orbiter, also around Mars, joined the observing campaign.
And then there was JUICE, far from Mars, quietly cruising toward Jupiter.
Here’s a simple comparison of the comet’s observers.
| Spacecraft | Location | Main data type |
|---|---|---|
| MRO | Orbiting Mars | High-resolution visible images |
| MAVEN | Orbiting Mars | Ultraviolet spectra and images |
| Perseverance rover | Surface of Mars | Wide-field sky images |
| SOHO & STEREO | Near Sun–Earth line | Solar and heliospheric imaging |
| Tianwen-1 | Orbiting Mars | Optical imaging |
| JUICE | En route to Jupiter | Remote sensing (from ~0.5 AU distance) |
When you look at that list, it hits you: a comet from another star is being watched by machines we placed around two planets, near the Sun, and on the path to Jupiter. That’s a science-fiction sentence that quietly became real.
How did JUICE end up observing 3I/ATLAS if it was “just cruising”?
Here comes the delightful part of the story.
During its cruise, JUICE was not expected to run many science observations. The spacecraft must manage:
- Thermal limits as it moves closer to and farther from the Sun.
- Power budgets for instruments and communication.
- Trajectory corrections, which always get priority.
But when astronomers realized that JUICE’s position would give it a view of 3I/ATLAS between November 2 and November 25, they went to work. ESA project scientist Olivier Witasse explained that the whole campaign was unexpected and required special planning during a supposedly quiet phase.
JUICE used five instruments:
- A visible-light camera.
- A near-infrared imaging spectrometer to study ices and minerals.
- A UV spectrometer for gas composition.
- A sub-millimeter instrument to probe dust and gases at longer wavelengths.
- A neutral atom sensor, which watches particles stripped from the comet by the solar wind.
Because the comet was relatively far away, the observations were purely remote sensing—no close-up nucleus images like Rosetta took at Comet 67P. Still, by combining all these data with results from Mars and near-Earth spacecraft, scientists can reconstruct a detailed physical picture.
Why don’t we get the JUICE comet photos until 2026?
This is the question that frustrates a lot of space fans: If JUICE already took the data, where are the pretty pictures?
The answer is surprisingly down-to-Earth: thermal protection and data rate.
When JUICE is closer to the Sun than Jupiter’s orbit, its instruments and electronics face higher levels of heating. To stay safe, the spacecraft points its large high-gain antenna toward the Sun and uses it as a kind of giant sunshade.
That solves one problem but creates another:
- The high-gain antenna is usually the main communication link with Earth.
- While it acts as a shield, JUICE must rely on a smaller medium-gain antenna.
- That antenna sends data much more slowly.
On top of that, during the 3I/ATLAS campaign JUICE was on the far side of the Sun from Earth. Radio signals had to skirt the solar corona, which complicates communication.
So, the data are stored onboard and trickle down at a low rate. ESA expects the full set of comet observations to reach Earth around February 2026.
This isn’t a secret plot or censorship; it’s simple space engineering: protect the spacecraft first, enjoy the science later.
What can JUICE learn from 3I/ATLAS, even from a distance?
Even though JUICE didn’t fly close to 3I/ATLAS, its instruments still carry a lot of scientific power.
Here are some key questions the data can address:
1. What is the comet made of?
By splitting light into spectra, JUICE’s instruments can measure:
- Ratios of water, carbon monoxide, carbon dioxide, and other gases.
- Presence of complex organic molecules.
- Dust grain properties, through how they scatter and emit light.
If certain gas ratios differ from known Solar System comets, that hints at different conditions around the comet’s birth star.
2. How does the coma react to sunlight and the solar wind?
The neutral atom sensor and UV spectrometer track how particles leave the comet and get ionized. That interaction shapes the comet’s tails and tells us:
- How active the nucleus is at a given distance from the Sun.
- How the solar wind strips material from interstellar ices.
3. How does 3I/ATLAS compare with comets born here?
To frame that comparison, here’s a simple HTML table.
| Property | Typical Solar System Comet | 3I/ATLAS (expected features) |
|---|---|---|
| Birthplace | Oort Cloud or Kuiper Belt | Another star's planetary system |
| Orbit shape | Elliptical, often returns | Hyperbolic, one-time visitor |
| Ice composition | Matches solar nebula chemistry | May show different element and isotope ratios |
| Scientific value | History of our Solar System | Direct sample of another stellar system |
We don’t have all the answers yet, but by the time the data are processed, astronomers should be able to place 3I/ATLAS on this comparison chart quite clearly.
How do gravity assists send JUICE to Jupiter—and to the comet?
You might wonder: how did JUICE end up in a position to see this comet at all?
The answer lies in gravity assists. Instead of firing huge rockets all the way to Jupiter, JUICE steals a little energy from planets by flying past them.
In simple terms, the spacecraft’s speed after a flyby depends on the vector sum of its incoming velocity and the planet’s orbital velocity. A basic expression for the speed needed to escape a body of mass (M) and radius (r) is:
<p>Escape velocity from a spherical body is given by:</p>
<code>v_escape = sqrt(2 * G * M / r)</code>
JUICE doesn’t quite reach escape speed from the Sun, but careful timing of Earth and Venus flybys reshapes its path so that:
- It spends less fuel.
- It arrives at Jupiter with the right speed to enter orbit.
- Its trajectory accidentally crosses a good line of sight to 3I/ATLAS.
That last point is pure serendipity. Mission planners design flybys years in advance; no one knew 3I/ATLAS even existed back then.
Why didn’t JUICE photograph Venus during its flyby?
If JUICE can take images of a distant comet, why not snap high-resolution pictures of Venus up close?
The answer is the same heat-shield trick we saw earlier. During Venus flybys, JUICE once again points its high-gain antenna at the Sun to protect its instruments from intense heating. Because of this orientation, the remote-sensing instruments can’t safely operate, so no Venus images are taken.
It feels like a missed opportunity, but it keeps the mission alive for the main goal: Jupiter’s icy moons and long-term observations in the outer Solar System.
What comes after JUICE and 3I/ATLAS? Are we getting better at catching comets?
Yes. Each mission teaches engineers and scientists how to do more with the next one.
ESA’s Rosetta mission orbited Comet 67P/Churyumov–Gerasimenko and, in 2014, dropped the Philae lander onto its surface. That gave us detailed maps of a comet built inside our own Solar System.
Building on that experience, ESA is now developing Comet Interceptor, scheduled for launch later this decade. The idea is clever:
- Place a spacecraft in a stable waiting position near the Sun–Earth line.
- When astronomers spot a promising new comet heading inward, send the craft to intercept it.
- Study a pristine object from the outer Solar System—or, in a very lucky case, another interstellar visitor.
While the odds of Comet Interceptor meeting another interstellar object are small, they’re not zero. That’s astronomy’s version of buying a ticket for the cosmic lottery.
What does all this tell us about science, patience, and curiosity?
Let’s pause for a small reflection.
Right now, somewhere between the orbits of Mars and Jupiter, a spacecraft launched from Earth is carrying on-board images and spectra of a comet born around another star. We can’t see those data yet. They’re stored in memory chips, wrapped in metal panels, protected from the Sun by a dish that doubles as both shield and antenna.
We wait.
This delay isn’t just a technical detail; it’s part of the story. Space exploration often asks us to accept long gaps between effort and reward:
- Launch now, arrive in eight years.
- Take data today, download it next year.
- Discover a phenomenon now, understand it decades later.
In a world that loves instant results, missions like JUICE remind us that patient curiosity still works. They also remind us why we must keep reason awake. When we stop asking questions, myths and fears rush in to fill the gap—the sleep of reason breeds monsters.
FreeAstroScience exists to push back against that sleep, one clear explanation at a time.
So, what should we watch for next?
When the JUICE data from 3I/ATLAS arrive around early 2026, expect:
- First processed images of the comet from the spacecraft’s camera.
- Spectral analyses comparing its ices with Solar System comets.
- Joint studies combining Mars-based observations, Sun-orbiting spacecraft data, and JUICE’s unique viewpoint.
Together, they’ll show whether 3I/ATLAS is “just another comet” or something chemically distinct, shaped by a different star’s history.
Until then, we can already appreciate the feat: a mission built to study Jupiter’s icy moons has given us a bonus encounter with material from another planetary system, on the way to its main target. That’s the kind of quiet cosmic magic that keeps many of us in love with space science.
This post was written for you by FreeAstroScience.com, which specializes in explaining complex science in simple, honest language. Our goal is to nurture curiosity and remind everyone that when reason falls asleep, monsters—of ignorance, fear, and misinformation—take its place. Stay curious, stay awake, and come back soon for more stories from our shared universe.
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