When an interstellar comet goes behind the Sun from the Earth’s perspective, science does not hit pause it relocates. In the case of 3I/ATLAS, the best observatory for a critical phase of the orbit was not on Earth, but already in operation around and on Mars.
In early October, NASA orchestrated the observation of 3I/ATLAS, the third known interstellar object that passed through the solar system, by three Mars-orbiting assets: Mars Reconnaissance Orbiter (MRO), MAVEN, and the Perseverance rover. When it was closest, the comet was 30 million kilometers away from Mars, a position that enabled spacecraft to take advantage of the same solar glare that made viewing difficult from Earth during conjunction.
The mission required the use of Mars orbiters as deep space trackers. MRO’s HiRISE camera, intended to image surface detail, was turned and pointed to track a target that is much fainter than usual. This maneuver has its roots in the past, as HiRISE did the same thing during the 2014 flyby of comet Siding Spring, but it is still a challenging task in terms of attitude control and exposure. The images obtained did not contain a nucleus but instead mapped the coma, which was 1,500 kilometers wide at about 30 kilometers per pixel. HiRISE principal investigator Shane Byrne explained the significance of these opportunities: “Observations of interstellar objects are still rare enough that we learn something new on every occasion.”
Such images represented just one part of a multi-instrument observation. The Imaging Ultraviolet Spectrograph on MAVEN extended these observations into the ultraviolet, distinguishing lines associated with hydrogen, hydroxyl, carbon, and oxygen, species that can be used to interpret a patch of light as a chemical signature. By monitoring hydrogen emissions, the researchers placed an upper limit on the ratio of deuterium to hydrogen, a marker that can be used to link volatile chemistry to the formation environment. It’s just “incredible,” said MAVEN principal investigator Shannon Curry of the first findings, we’ve “only scratched the surface” of what the data can provide.
Perseverance added to this from the surface with Mastcam-Z, which has different constraints. Since there is no gimbaled telescope mount, the rover had to use long exposure times that create star trails; the comet was just visible. This is also the strength of the mission: the rover showed that even without a gimbaled platform, it can still add to an observatory network.
Independent observatories added to the physical model around the same time. The James Webb Space Telescope showed that the comet is unusually rich in carbon dioxide, while Hubble Space Telescope images narrowed the nucleus size to below 1 kilometer. Coupled with Mars observations, these provide a complete set of boundary conditions from a passing event: dust dynamics in visible, volatile species in ultraviolet, and composition in infrared.
Interstellar comets are believed to originate from planetesimals that were ejected from their own systems. The results of modeling close stellar encounters indicate that flybys can eject material with typical speeds of 0.5–2 kilometers per second relative to the parent star, whereas planet scattering can lead to faster ejection. In the case of 3I/ATLAS, the hyperbolic trajectory and high speed of approach impose strict conditions on possible ejection histories, and isotopic ratios such as D/H form another independent axis of comparison. The take-home engineering message is subtle but significant: a planetary mission design is more valuable if it can be repurposed, retimed, and interconnected. For an object that could not be tracked continuously from Earth, Mars filled the missing node in the observation network to harvest photons that would otherwise be lost due to geometry.”
