By the time she was noticed, she was already leaving.

“She,” in this case is asteroid 1I/2017 U1, affectionately named ‘Oumuamua,  which made headlines around the world last November when it was confirmed as the first interstellar object recorded to visit our solar system. ‘Oumuamua made its closest approach to Earth on October 14, but it wasn’t detected until October 19, when Robert Weryk (a post-doc researcher at the University of Hawaii’s Institute for Astronomy) noticed the unusual object recorded in data from the Pan-STARRS 1 telescope the previous night. By the time Weryk noticed it, the object was 20 million miles away from the Earth – that’s two thirds the distance from Earth to Mars –  and traveling at a cool speed of 27 miles per second towards the constellation Pegasus, never to return again.

In centuries of observing the sky and decades of expecting to find interstellar objects, astronomers have only found one.

The discovery of ‘Oumuamua is a boon for scientists who study planetoids and comets. Many have been captivated by its unique pencil-like shape and have noted the apparent  similarity of its composition to certain asteroids in our solar system. However, the detection of 1I (for the 1st Interstellar object) raises an intriguing question: Why was the first interstellar emissary elusive for so long? The existence of interstellar asteroids and comets has been widely assumed since at least the mid-20th century. Theories of how solar systems form indicate that there should be billions of interstellar objects left over from planet formation roaming the galaxy freely. It’s estimated that several of these interstellar visitors pass inside the orbit of the Earth every year – and yet, in centuries of observing the sky and decades of expecting to find interstellar objects, astronomers have only found one.

The easiest way to illustrate the difficulties that come with identifying interstellar objects is by examining the process through which asteroids and comets close to Earth (often referred to as Near Earth Objects, or NEOs) are tracked. Because NEOs have the possibility of hitting the Earth, significant effort has been made to identify them and determine their trajectories.  ‘Oumuamua was found by a telescope meant to track NEOs. Tracking NEOs after they are detected is relatively achievable – all you need are a few data points and some orbital physics equations. The detection is what’s difficult.

In an interview with Innovation, Robert Weryk (who, when he’s not detecting the first confirmed interstellar object in our solar system, is a NEO researcher) described the process of searching for NEOs using survey telescopes: “Every night when it’s clear enough and the weather is good enough, we observe as much of the sky as we can.” Scanning sections of the sky periodically throughout the night, survey telescopes such as Pan-STARRS and the Catalina Sky Survey feeds images into software, which attempts to detect objects – really just points of light – which move rapidly over the course of the measurements. Even using advanced algorithms, survey telescopes have a lot of noise and, says Weryk, “[The telescope] also includes a lot of false tracklets…things it thinks might be objects, but you need someone to look at and confirm.”

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B2CA764C-1A9F-4485-BDE29E6545E2CFC8_source
Trajectory of 1L/2017 (a.k.a. 'Oumuamua') through our solar system. The interstellar object followed a hyperbolic orbit which took it briefly by the sun, before flinging it back out of the solar system.

Graphic by: NASA JPL-Caltech

If an object is considered a likely NEO candidate, astronomers like Weryk try to corroborate the reading by matching it with data from previous nights. If they still think it’s a NEO, they’ll attempt to train telescopes on it in future nights, estimating its orbit based on previous measurements and scanning the sky where it’s expected to be, further confirming its status as an NEO. This process is effective – it is believed that over 90% of the largest and most dangerous NEOs  have been detected – but it still requires human oversight, and objects can slip through the cracks. For instance, the potentially disastrous asteroid 4581 Asclepius was detected over a week after it passed within 500,000 miles of Earth in 1989. Weryk also notes that it’s also possible to lose track of a likely object after first detection, adding “there have been times before where we’ve looked where we thought objects would be on a second night, and we never find them.”

The problems of detecting NEOs multiply when interstellar objects come into question. By definition, interstellar objects aren’t bound by the Sun’s gravity – they enter and depart the Solar System on a hyperbolic trajectory, not an elliptical one. The discovery of ‘Oumuamua was largely the result of Weryk and his colleagues noticing that the object that they had tracked didn’t seem to be on a typical orbital trajectory, and them being fortunate enough to have Pan-STARRS and an European Space Agency telescope collect enough data to triply confirm their findings. The quick thinking – and luckiness – of an alert group of astronomers allowed ‘Oumuamua to be detected against the odds. Indeed, without Fortune on their side, it’s likely that 1I/2017 U1 would have been a “one and done” object – one of those objects which Weryk mentioned that appears one night and is gone the next.

In that sense, the detection and continued tracking of ‘Oumuamua may improve interstellar object detection. Weryk notes that now researchers are aware of what interstellar trajectories look like – and can integrate the trajectories into the models which survey telescopes use.

Without Fortune on their side, it’s likely that 1I/2017 U1 would have been a “one and done” object.

“Now that we know [interstellar asteroids] exist, we can be more thoughtful in our follow-up strategy…if we find an object that we can’t follow up on the next night, that might flag the object for extra telescope time looking over other much broader follow-up regions.” Since the existence of interstellar objects is now confirmed, it’s worth astronomers’ time to look for them more purposefully during NEO surveys.

Still, even with an optimized follow-up strategy, Weryk describes the search for a second interstellar object as a matter of “pure waiting” – waiting for another visitor from afar to fly through a survey telescope’s field of view and give an opportunity for researchers to compare details between it and ‘Oumuamua. This wait would be understandably frustrating for many scientists, especially with the taste of the recent breakthrough still lingering in their mouths. However, even with the prospect of a lengthy pause in data, Weryk seems patient and clearly passionate about the future of the field (it helps that his advisor is as well). Declares Weryk, “moving forward, there’s so much work that this has opened up, in pure science, impact hazards…it’s going to keep us quite busy for quite some time.”

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Alden Hunt