‘Comet hunter’ to intercept and observe visitors
Instead of launching a probe once a new interstellar object has been spotted, the European Space Agency, ESA, will station the Comet Interceptor probe permanently in space. When telescopes spot an alien guest or a new comet from the outskirts of the Solar System, the probe can immediately give chase.
As he was the first to spot the new body, it was named 2I/Borisov. Just like Oumuamua, Borisov follows a hyperbolic path, meaning that it only sweeps by the Sun, then leaving the Solar System again.
Planet was torn by a star
While Borisov looks like an ordinary comet that is far away from home, Oumuamua is a more unusual object. Its shape is unlike anything else scientists have observed in the Solar System. It seems to be elongated, at least six times longer than it is wide, whereas ordinary asteroids are more commonly shaped like balls or potatoes. For a while, this curious shape led to conjecture that Oumuamua could be a space probe or a part of a spacecraft from an alien civilisation. But in July 2019, an international team of 14 astronomers concluded that the object, about 400 metres long, is probably perfectly natural. None of the measurements possible from a distance indicated otherwise. The odd shape remains difficult to explain, but perhaps Oumuamua is a small fragment of a world that was torn apart on its way through space when it came a little too close to a star. The matter could only be settled by getting in closer to Oumuamua.
So far, Oumuamua and Borisov have been the only two visitors yet observed from ‘alien’ worlds, but far more of them probably pass through the Solar System – we just need to take a better look, using bigger and improved telescopes. Astronomers have their hopes pinned on the new wide-angle telescope known as the Vera C. Rubin Observatory (formerly LSST), which has been under construction in Chile since 2015. The telescope will be equipped with the world’s biggest digital camera, and when it starts to photograph the sky in a few years from now, interstellar objects will have a much harder time hiding in the dark.
However, it is not enough for astronomers to watch these rare heavenly bodies through telescopes; the observed small bright spots do not offer much information. Scientists would like to get much closer, and this might be possible once the European Space Agency, ESA, launches the planned Comet Interceptor probe in 2028.
The aim is to park the probe in orbit 1.5 million kilometres from Earth for up to three years. When the Vera C. Rubin Observatory or another telescope spots an alien object in the Solar System, this will bypass the time required to construct and launch a probe from scratch. Instead the Comet Interceptor can set out in pursuit immediately.
“Pristine or first-time visitor comets are completely new to us... fascinating objects to study closely by means of a space probe,” says the ESA’s Günther Hasinger.
However, the whole project gambles on the arrival of an object within the available time window of the Comet Interceptor. Only time will tell if a suitable interstellar object will be spotted within the three years during which the ‘parked’ Interceptor retains sufficient fuel to wait and then intercept.
Probe to catch up with Oumuamua
Engineers from the British Initiative for Interstellar Studies, on the other hand, aren’t planning to wait around. They aim to try to catch up with Oumuamua, despite the fact that by the time this issue of Science Illustrated publishes on 11 February 2021, Oumuamua will already be at a distance of 3347 million kilometres from Earth, heading out of the Solar System at a harrowing speed of some 100,000km/h.
The scientists behind the mission, which is called Project Lyra, have calculated that a probe needs to reach a speed of some 200,000km/h to catch up with Oumuamua over a space journey lasting a couple of decades. The launch of such an unmanned probe would require one of the most powerful rockets yet built, such as NASA’s future Space Launch System or Space X’s Falcon Heavy. The engineers envisage a launch from Earth in May 2033, intercepting
Oumuamua in 2052. En route, the space probe would travel close by Jupiter and around the Sun to gain extra speed.
So far, the enthusiastic engineers of the Initiative for Interstellar Studies have simply made the calculations to prove that the project is theoretically possible. It remains uncertain, probably unlikely, whether the project can be realised: it would be so expensive that a major space organisation (or very wealthy benefactor) will be required to make the Project Lyra dream come true.
Huge planet ejects siblings
Astronomers’ current knowledge of asteroids and comets is based entirely on objects that we know from our own Solar System. Not until we manage to come very close to a truly interstellar object will astronomers find out whether the asteroids and comets we know from our own Solar System are similar to those from other regions of the galaxy.
If a probe registers dust particles, or discovers that the object has a magnetic field, astronomers will be able to offer a qualified guess of how it formed, and perhaps answer the question of why the alien heavenly body left its own solar system.
At this point, astronomers try to answer these questions using computer models that simulate how stars and their planets form and develop. These models are, of course, based on our own Solar System, now a well-organised system of planets and small heavenly bodies that orbit the Sun in predictable paths. However, calculations show that it was not always so: the Solar System was much more chaotic during the initial period after its formation.
In the disc of dust and gas that formed around the newborn Sun, larger lumps of material, known as ‘planetesimals’ quickly accumulated, then developed into larger planets over time. But there were still some planetesimals left over after the planets had formed, and when these lumps came close to a large planet, one of three things could happen. They could be absorbed and end up as a part of the planet. Or the influence of gravity from a large planet could either force them into the Sun, or fling them outwards into the Solar System.
Only very large planets would have the gravity to eject smaller objects entirely. In our own Solar System it would have been Jupiter that was probably responsible for the vast majority of ejections. According to astronomers, material corresponding to far
100 observable interstellar objects may pass by our Solar System annually.
more than Earth’s mass must have ended up as interstellar objects which are now travelling through space and might be visiting alien planetary systems.
In the same way, ejected planetesimals from remote stars could visit our Solar System. We know that a great many stars in the sky are orbited by planets, but so far scientists have had little idea how many ejected objects might end up reaching us. Two astronomers from Yale University in the US, PhD student Malena Rice and Professor Gregory Laughlin, have now calculated how many objects might be observed by the new Vera C. Rubin telescope. They estimate that every year the Solar System should be visited by 100+ observable objects with a radius over a metre, including a few that will be larger than Oumuamua. The numbers are encouraging, though subject to uncertainty.
Previous visitors
Two Harvard scientists, astronomers Amir Siraj and Abraham Loeb, also believe that interstellar objects might have hit Earth without us noticing it. In a database of meteor strikes on Earth over three decades, Siraj and Loeb stumbled across a meteor some 45cm across that burned up in the sky above Papua New Guinea on 8 January 2014. The meteor entered Earth’s atmosphere at a speed above 160,000km/h, so fast that scientists believe that it must have come from a different solar system.
So it is not only telescopes on Earth that can identify interstellar objects. Siraj and Loeb calculate that a telescope the size of Hubble, if aimed at the Moon’s surface, should be able to identify previous meteor strikes caused by interstellar objects. The Moon has no atmosphere in which meteors burn up, and the resulting dust cloud when an object hits the Moon’s surface might reveal its make-up. Scientists could establish the speed of impact, and whether the object consisted mainly of ice or of rock.
Scientists are sure that our Solar System receives visitors from other regions of the galaxy which could tell exciting stories about our universe, especially if it were possible for us to get a closer look via a probe. Perhaps interstellar objects from the remote corners of the galaxy even brought us some of the complex molecules that were necessary for life on Earth to originate.
So far, we have only been able to watch these visitors at a great distance. But in the future, we may get that opportunity to get in closer. The Comet Interceptor is the first space mission that could explore an interstellar object, but it will not be the last. The obvious next step is a space probe that can land on the surface of an alien object and perhaps, like Hayabusa2, even take samples that could be returned to Earth for analysis. Landing on such an interstellar object would represent our first indirect visit to an alien solar system.