What would it take to capture an interstellar visitor like ‘Oumuamua?

A proposal has been published for a mission to send a spacecraft to study a future interstellar object moving through our solar system. Although only two such objects have ever been confidently detected, advances in surveillance systems mean this number will almost certainly rise quickly. Recognizing that it will be a long time before we visit other galaxies, a mission to escort such an object as it leaves our environment could be our best chance to learn how our patch of space differs from everything outside it.

The gravitational forces of the larger planets occasionally eject comets and asteroids from the solar system, like spoiled children throwing away unwanted toys. Modeling suggests that there was a period nearly 4 billion years ago when the gravitational forces of Jupiter and Saturn did this much more often, sending millions of Kuiper Belt objects into the great beyond.

If so, it’s likely that the same thing is happening around other stars, and the galaxy is filled with icy wanderers, some of which randomly pass quite close to our sun. We know of two such objects, the comets ‘Oumuamua and Borisov, that were spotted passing through our solar system before they left.. Borisov seemed indistinguishable from any of our local comets, except for its orbit and high carbon monoxide concentration, but ‘Oumuamua is clearly something else. It is likely that ‘Oumuamua will be a subject of fascination and wonder for centuries to come.

Inevitably, astronomers will want to take a closer look at the next visitor – and probably several after that. A justification for the benefits of sending a spacecraft to overtake such an object, and the practical aspects of doing so, have now been published. Now it’s up to NASA to decide whether to make the idea a priority, and to Congress to fund it — unless another country decides to move first.

‘Oumuamua was discovered in 2017 and it’s been almost five years since Borisov was discovered, but it’s unlikely we’ll have to wait long to find other opportunities to encounter interstellar visitors. The Vera Rubin Observatory, expected to become operational in 2025, is expected to allow us to find millions of objects in the outer solar system, some of which will be visitors from elsewhere.

How many interstellar objects will be picked up in this way is unclear, but there will almost certainly be some. Dr. Alan Stern of the Southwest Research Institute and co-authors examined the likelihood that in the fairly near future someone will come close enough that a spacecraft could catch up to study them up close.

As principal investigator on the New Horizons mission, Stern has more experience than almost anyone when it comes to aiming spacecraft at distant ice balls.

Any interstellar object must travel at great speed or it would get stuck in the Sun’s gravity, making it easy to study. Nevertheless, Stern and colleagues show that a substantial pole can be expected to pass close enough and travel slowly enough to be captured by a spacecraft without technological advances over the spacecraft we have already launched.

The most important need will then be to have a mission ready, either on Earth with an accompanying launch vehicle or in space. By modeling a likely range of speeds, the authors predict that the likely time an interstellar object would spend within 10 AU (1.5 billion kilometers from the Sun, similar to Saturn’s orbit) is 770 days, so there is no time to lose.

Counterintuitively, a launch from Earth would allow us to intercept a larger portion of the modeled paths, but only if we could get a spacecraft ready to go into space within 30 days of detection. The authors describe this as an “unrealistically short period” because it would require “a planetary-class launch vehicle to be kept ready for several years until a suitable target is found.”

Instead, the best option is to ready and store the spacecraft at L1 so that even with a light push it can drop into low Earth orbit for gravity support once a suitable target is spotted. Unless we are very lucky with the target, the craft should be able to increase its speed by 3 km per second through its own systems. This is larger than, but comparable to, Cassini at 2.1 km/s, and is therefore considered feasible if not overloaded with too many instruments.

The idea of ​​overtaking an interstellar object has been explored before, but this work is much more detailed. It considers not only the practical feasibility of getting close enough to study such an object, but also the most suitable instruments for such a task. One strong candidate, a mass spectrometer, would slow the mission too much compared to the scientific value it would provide, the authors conclude. Other instruments that have proven useful during previous voyages in the outer solar system would not be suitable for such a small target.

All told, preparing such a spacecraft and putting it into orbit ready to take off would cost less than a billion dollars, the authors conclude. This puts it in a similar price range to projects such as the already launched Lucy and Psyche, and the delayed Venus missions Veritas and VenSAR.

The article is open access in Planetary and Space Science.

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