Hitching a Ride on an Interstellar Comet

It is hard to believe that 3I/ATLAS, the third confirmed interstellar object to enter our solar system, was only discovered two months ago. It appears that every observational instrument has studied it in some way, with the latest being the James Webb Space Telescope. The data returned so far is quite intriguing.

The discovery of 3I/ATLAS further advances our understanding of interstellar objects, which started nearly seven years ago with the detection of the first such object, 1I/'Oumuamua, marking the beginning of a new era in planetary science. Though long theorised, these interstellar objects were beyond our technological reach until recently and are primarily believed to be comets—dirty snowballs—that were ejected from their original solar systems billions of years ago and are now drifting through space. In other words, they act as natural probes since they are untouched samples from far-off star systems we will never be able to reach. Planetary science is a comparative discipline where each object is examined and understood in relation to the group of objects. These interstellar objects are no exception. In addition to examining 3I/ATLAS by leveraging our knowledge of comets within our Solar System, we are also comparing it to the two prior objects: 1I/'Oumuamua (2017) and 2I/Borisov (2019). Here is what we've learned so far.

Given its trajectory, 3I/ATLAS seems to have originated from the Milky Way's thick disk, where older stars reside, implying that it could be at least 7 billion years old; in comparison, our Solar System is 4.6 billion years old. Like a typical comet nearing the sun, the volatiles in 3I/ATLAS' nucleus start to outgas in response to the increased exposure to the Sun's radiation. This allows us to perform a spectral analysis of the venting gas and accompanying dust forming the cloud surrounding the comet, known as the coma. As such, various instruments have characterised the coma, revealing the presence of large amounts of carbon monoxide (CO), carbonyl sulphide, and water ice, with carbon ratios far exceeding those that are typically found in our Solar System, supporting the idea that 3I/ATLAS is exotic.

The interstellar comet is on a hyperbolic escape trajectory and will make its closest approach to the Sun (perihelion) on October 29, 2025, at a distance of 1.36 AU—approximately 203 million kilometers or 126 million miles. Alas, by then, it will be behind the Sun, which will prevent us from making direct observations at the peak of the outgassing. Nevertheless, as more volatiles are released from the comet in the coming months, we should expect more compounds to be identified. Based on what we know so far, it is probable that 3I/ATLAS was formed near the carbon dioxide frost line of its original solar system or has been exposed to significant solar radiation from its parent star.

Estimating its size is challenging because the coma envelops the nucleus, making precise measurements difficult. However, 3I/ATLAS appears to be the largest interstellar object observed in our Solar System so far, with a diameter of a few kilometers. As a reminder, 1I/ʻOumuamua measures between 500 and 1,000 meters in length, while 2I/Borisov is believed to be approximately 500 meters in diameter.

Considering the possibility of gaining further insights from this cosmic messenger, which holds secrets from a star system we will never visit, there have been suggestions to redirect one of the Martian probes to more closely observe 3I/ATLAS as it approaches its perihelion. In fact, with interstellar objects now expected to be detected regularly (see my July post), the European Space Agency has a mission planned to deal with future similar objects: the Comet Interceptor Mission, scheduled for launch in 2029. It will orbit the Sun-Earth L2 point and wait to intercept a "dynamically new" long-period comet or, ideally, an interstellar object where it will study it in detail, making it one of the most exciting missions in the coming decades.

I would propose going one step further and using such interstellar objects as a hitchhiker catches a ride. Theoretically, we could 'land' a probe on a comet, much like the Rosetta and Philae mission did in 2014, and use it to travel across the immense distances of interstellar space. We would have to match the velocity of the interstellar comet if we are to rendezvous with it. Currently, 3I/ATLAS has a velocity of 58 km/s (36 mi/s; 208,800 km/h; 129,740 mph) while the fastest probe leaving the Solar System is New Horizons, travelling at 16.26 km/s (10.10 mi/s; 58,500 km/h; 36,400 mph). We could achieve greater speeds. The Parker Solar Probe holds the record as the fastest probe, reaching 191 km/s (118.7 mi/s; 690,000 km/h; 430,000 mph). Although it's designed to orbit the Sun rather than follow the escape velocity path of interstellar comets, it shows that with appropriate gravity assists, we can propel probes to the necessary high speeds.

Space is immense, and even at these incredible speeds, reaching the nearest star system, Proxima Centauri, would still take tens of thousands of years. But once a probe has safely landed on an interstellar comet, it could utilise the comet's resources (volatiles, rocky materials, etc.) to sustain itself for a while. Interstellar comet probes could thus act as time capsules, akin to the Golden Records on the Voyager spacecraft or the Pioneer Plaques, and carry messages to convey the essence of humanity throughout our galaxy to any entities that might encounter them. Perhaps, a probe could reshape a comet or recreate famous monuments or artworks on its surface; I'm thinking Stonehenge, the Great Sphinx of Giza, or even the statue of David by Michelangelo. I can't imagine a better way to show others we were present than by sharing what we cherish the most. On this highly speculative note, as always, onwards and upwards.

Image credit: Image of interstellar comet 3I/ATLAS captured by the Hubble Space Telescope’s Wide Field Camera on 21 July 2025. Credit: NASA, ESA, D. Jewitt (UCLA); Image Processing: J. DePasquale (STScI).