'Oumuamua — first interstellar visitor
The first confirmed object from another star — cigar-shaped, oddly accelerating, endlessly debated.
The moment of 'Oumuamua — first interstellar visitor: https://nowsky.org/e/oumuamua — opens at the right place and time, replayable, no login.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 19 October 2017 the Pan-STARRS telescope on Haleakalā in Hawaii caught a faint, fast-moving point. Working out the orbit caused a stir: eccentricity 1.2, a hyperbola — an object not bound to the Sun. It was the first confirmed visitor from outside the solar system, and it had already rounded perihelion (0.25 AU) on 9 September and was on its way out.
The name is Hawaiian for "a messenger from afar arriving first". Its brightness swung tenfold every eight hours, implying an extreme shape — 100 to 400 m long and perhaps a tenth as wide, either cigar-like or pancake-like — a ratio no solar-system asteroid shows.
Stranger still was the acceleration. As it left the Sun a slight push beyond gravity was measured; in a comet that would be outgassing, yet no tail or coma appeared. Explanations poured in — a shard of hydrogen ice, a fragment of nitrogen ice, and, from Harvard's Avi Loeb, the hypothesis of an artificial light sail.
The window for observation lasted only weeks. By January 2018 it was beyond the largest telescopes, and it is now leaving forever toward Pegasus at 26 km/s. The orbit view replays the hyperbolic track slicing obliquely through the solar system.
Why it matters
'Oumuamua was the first eyewitness proof that material from other stars passes through our neighbourhood. Interstellar objects were a theoretical certainty, but this was the first ever caught, and with Borisov in 2019 and 3I/ATLAS in 2025 "interstellar visitors" became a field of their own.
The controversy itself is a legacy. The artificial-probe hypothesis never won majority support, but it put on the public stage the argument that when nature cannot explain something, artefacts must at least be considered. The practical result was "next time, chase it".
ESA's Comet Interceptor, planned for launch in 2029, is designed to wait at the Sun–Earth L2 point and intercept a new interstellar visitor or long-period comet. Once the Vera Rubin Observatory's all-sky survey is fully running, interstellar discoveries are expected to become an annual event.
How to watch
'Oumuamua is already beyond every telescope. It is now more than 50 AU from the Sun heading toward Pegasus, crosses Neptune's orbit around 2038, and approaches its next star in roughly 20,000 years.
Choose 'Oumuamua in the orbit view's event list to replay perihelion on 9 September 2017 and the hyperbolic track, inclined 122°, that pierces the solar system from above to below; slow the playback for closest approach to Earth on 14 October (0.16 AU).
In the 3-D interstellar view, with the solar system at a 100 AU half-width, the direction it came from (near Vega in Lyra) and the direction it leaves (Pegasus) can be judged against the sky.
See this for yourself — replay the moment in the orbit view
Background
At discovery it was designated A/2017 U1 as an asteroid; showing no cometary activity, it later received the first interstellar number, 1I. Its reddish surface resembled outer-solar-system bodies, and spectra found no trace of water or organics.
In 2023 researchers at Berkeley and Cornell proposed that the acceleration came from hydrogen trapped beneath the surface by cosmic rays and released by solar heating — an "ordinary comet" explanation that allows acceleration without a coma. The debate is not closed, but natural explanations prevail.
2I/Borisov, found in August 2019, was an ordinary comet with a clear tail, unlike 'Oumuamua, and 3I/ATLAS, found in July 2025, was the largest of the three at several kilometres across. The differences among the three visitors gave the first glimpse of the diversity of interstellar objects.
‘Oumuamua’s time inside the solar system was short. In early September 2017 it came within 0.25 AU of the Sun and rounded perihelion at 88 km/s, passed Earth’s orbit and was discovered a month later, and within two months of discovery was out near Jupiter’s orbit. Had it been found before perihelion, whether it had any atmosphere or outgassing could have been observed far more closely. That regret became an argument for strengthening surveys such as Pan-STARRS and ATLAS and for hurrying the Rubin Observatory’s all-sky survey.
FAQ
How was it known to be interstellar?
Its orbit was a hyperbola with eccentricity 1.2 — not bound to the Sun; solar-system bodies have eccentricities below 1.
Is it an alien probe?
Most scientists say no. Natural explanations such as trapped hydrogen release prevail, though direct confirmation is impossible.
How big was it?
Estimated 100–400 m long and perhaps a tenth as wide; it was never resolved in an image.
Can it be seen again?
No. It has been beyond every telescope since early 2018 and is leaving the solar system for good.
Related
- 2I/Borisov — the interstellar comet
- Halley's Comet perihelion (1986)
- Comet Hyakutake sweeps past Earth
- The Sun — object index
- Pegasus — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.