All sorts of crazy things have been suggested about 3I/ATLAS, the third known interstellar object we’ve discovered. Some are simply conspiracy theories that it is an alien spacecraft, while others are well-thought-out suggestions, such as using Martian probes to observe the comet as it passed in front of the Red Planet.
A new paper pre-published on arXiv and accepted for publication by the American Astronomical Society Research Notes by Samuel Grand and Geraint Jones, of the Finnish Meteorological Institute and ESA, respectively, falls into the latter category and suggests using two spacecraft already en route to their separate destinations to potentially detect ions from the object’s spectacular tail that formed on approach of the Sun.
A few weeks isn’t a lot of time to set up a quick experiment to perform a test that neither spacecraft was designed for. But sometimes science means doing the best with what you have, and in this case, these two spacecraft are our best bet for studying the tail of an interstellar comet.
This tail has continued to grow since the discovery of the comet in early June. Recent reports of its water “gushing” indicate how massive the tail has become, leaving behind a trail of water particles, but potentially more importantly, ions. The comet has also recently disappeared from view of Earth systems, although it is assumed that its tail will continue to grow until it reaches perihelion on October 29.
As the paper explains, getting into part of its tail isn’t as simple as passing directly behind it as it moves through the solar system: the solar wind pushes particles further from the Sun, following a curved path away from the comet. The speed at which the wind hits these particles plays a major role in their location, and therefore exactly where the spacecraft should pass to collect data directly on the tail.
To make these estimates, the authors used a model called “Tailcatcher” which estimates where the path of cometary ions will go based on different wind speeds. He then calculated the “minimum miss distance” for a given spacecraft for the central axis of the comet’s tail. Unfortunately, the model is only as accurate as solar wind data, which is usually only definitively collected ex post facto – and certainly not in enough time to help achieve this potential mission goal.
Even with the program’s best estimates, the two spacecraft would be millions of kilometers from the central axis – about 8.2 million for Hera and 8 million for Europa Clipper. However, it is still within reach to be able to directly collect data on tail ions, as they can propagate millions of kilometers from very active comets like 3I/ATLAS.
The downside of this plan is that at least one of the spacecraft – Hera – has no instrument capable of detecting either the expected ions in the tail, or the magnetic “drape structure” that characterizes the effect of the comet’s atmosphere on the magnetic field carried by the solar wind. However, Europa Clipper does: its plasma instrument and magnetometer are exactly what would be needed to directly detect these ions and magnetic field changes.
Acting on this small coincidence is difficult to say the least, but it is also very limited in terms of time. It’s unclear whether the mission controllers of Hera, or perhaps more importantly, Europa Clipper, will see the message in time to do anything about their possible trip through the coma. But if they do, they could be the first in human history to directly sample the tail of an interstellar comet – and wouldn’t that be something to brag about that has nothing to do with their original planned mission?
THE original version of this article was published on The universe today.
