What It Would Take To Catch An Interstellar Visitor Like ‘Oumuamua?

A proposal has been published for a mission to send a spacecraft to study a future interstellar object passing through our Solar System. Although only two such objects have ever been detected with confidence, advances in surveillance systems mean that number is almost certain to shoot up very soon. Acknowledging that it will be a long time before we visit other star systems, a mission to accompany such an object as it leaves our vicinity could be our best chance to learn how our patch of space differs from everything beyond.

The gravitational forces of the larger planets occasionally throw comets and asteroids out of the Solar System, like spoilt children disposing of unwanted toys.  Modeling suggests there was a period almost 4 billion years ago when Jupiter and Saturn’s gravity combined to do this much more frequently, sending millions of Kuiper Belt objects into the great beyond.

If so, it is likely the same thing happens 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, comets ‘Oumuamua and Borisov, that have been spotted passing through our Solar System before leavingBorisov appeared to be indistinguishable from one of our local comets, other than its orbit and high carbon monoxide concentration, but ‘Oumuamua is clearly something else. It is likely ‘Oumuamua will be a subject of fascination and wonder for centuries to come.

Inevitably then, astronomers want a closer look at the next visitor – and probably several after that. A justification for the merits of sending a spacecraft to catch up with such an object, and the practicalities of doing so, has 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 nation decides to get in first.

‘Oumuamua was discovered in 2017 and it’s almost five years since Borisov was detected, but it’s unlikely we will have to wait long to find other opportunities to encounter interstellar visitors. The Vera Rubin Observatory, scheduled to begin operations 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.

Just how many interstellar objects will be picked up this way is unclear, but there will almost certainly be some. Dr Alan Stern of the Southwest Research Institute and co-authors explored the likelihood that in the reasonably near future one will come close enough that a spacecraft could catch up with it to study at close range.

As the Principal Investigator for the New Horizons mission, Stern has more experience than almost anyone else when it comes to pointing spacecraft at distant balls of ice.

Any interstellar object must be traveling at great speed, otherwise it would get trapped in the Sun’s gravity well, making it easy to study. Nevertheless, Stern and colleagues show that a substantial pool can be expected to come near enough and be traveling slowly enough that they could be caught by a spacecraft with no advances in technology over those we have already launched.

The key need then will be to have a mission ready to go, either on Earth with an accompanying launch rocket, or in space. Modeling a likely range of speeds, the authors predict the likely time an interstellar object would spend within a radius of 10 AU (1.5 billion kilometers of the Sun, similar to Saturn’s orbit) is 770 days so there’d be no time to waste. 

Counter-intuitively, an Earth-based launch would allow us to intercept a larger proportion of the modeled paths, but only if we could get a craft ready to go into space within 30 days of detection. The authors describe this as an “unrealistically short period” since it would require “holding a planetary class launch vehicle ready for multiple years until a suitable target is found.”

Instead, the best option is to prepare the spacecraft and store it at L1, so even a gentle push would see it fall into Low Earth Orbit for a gravity assist, once a suitable target has been spotted. Unless we get quite lucky with the target, the craft would need to be able to boost its velocity through its own systems by 3 km per second. This is greater than, but comparable to, Cassini at 2.1 km/s, and therefore considered achievable if not overburdened with too many instruments.

The idea of catching up with an interstellar object has been explored before, but this work is far more detailed. Not only does it consider the practicality of getting close enough to study such an object, it also considers the most suitable instruments for such a job. One strong candidate, a mass spectrometer, would slow the mission down too much for the scientific value it would provide, the authors conclude. Other instruments that have proven useful on previous outer Solar System journeys would be unsuited to a target this small.

All told, the preparation of such a spacecraft and placing it in orbit, ready to go, 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 paper is open access in Planetary and Space Science.

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