Interstellar Objects and Unsolved Signals: The Space Mysteries Science Still Cannot Close

Three visitors from other star systems have crossed our Solar System. One radio burst from 1977 has never repeated. Millisecond flashes arrive from deep space with no settled cause, and a distant star dims in a pattern no planet explains. These are active research programs — and interstellar objects are the fastest-moving part, because new surveys should soon multiply today’s sample of three.

This guide separates what is measured from what is merely proposed — a distinction this field too often blurs.

1I/’Oumuamua: An Acceleration We Can Measure but Not Explain

On 19 October 2017, Canadian physicist Robert Weryk, using Pan-STARRS in Hawaii, found the first confirmed interstellar object, 1I/2017 U1 — ‘Oumuamua. It was already leaving, on a hyperbolic trajectory no solar orbit can capture; its home system and age remain unknown.

It showed no coma despite passing well inside Mercury’s orbit. Its light curve implied tumbling motion and an unusually elongated body, perhaps 100 to 1,000 metres long. Then, in 2018, Micheli and colleagues reported non-gravitational acceleration: it sped up slightly more than gravity alone allowed, away from the Sun.

Outgassing would explain the push, except none was detected, and the required gas-to-dust ratio is at least a hundred times higher than in known comets. Alternatives include sublimation of molecular hydrogen ice, nearly invisible to the instruments used. Radiation pressure works only for an extremely thin or low-density body — the starting point for lightsail speculation, a hypothesis rather than a finding, and not the consensus. The visitor is now too distant to test further.

2I/Borisov: Familiar Machinery, Foreign Chemistry

Comet 2I/Borisov, the second interstellar object, discovered in August 2019 by amateur astronomer Gennadiy Borisov, arrived with a bright coma and tail — recognisably a comet, and the first chance to directly sample material formed around another star.

In December 2019, ALMA measured its released gases. Hydrogen cyanide, relative to water, looked ordinary. Carbon monoxide did not: nine to twenty-six times the average Solar System comet concentration, among the highest ever recorded inside 2 astronomical units. It stays frozen only in extreme cold, so Borisov almost certainly formed beyond its ice line, in a distant disk’s frigid outer region.

3I/ATLAS in 2026: The Third Visitor, Caught in Time

On 1 July 2025, the ATLAS survey in Chile discovered 3I/ATLAS, the third confirmed interstellar object. Its eccentricity, about 6.14, is the highest yet recorded for such a visitor; its excess velocity is roughly 58 kilometres per second. Perihelion came on 29 October 2025 at 1.36 astronomical units; closest Earth approach, a safe 1.8 units, followed on 19 December. Through 2026 it has been outbound and fading, never to return.

Caught months before perihelion, it was observed by Hubble, Webb and ESA’s JUICE spacecraft. Webb delivered the headline: a coma extraordinarily rich in carbon dioxide, at a ratio near 8 to 1 against water, far above the Solar System comet trend.

Two cautions: estimates that it may exceed seven billion years in age are model-based inferences, not measurements, and proposals such as subsurface “ice volcanoes” — like the artificial-probe claims that circulated during its passage — remain unconfirmed. What is confirmed is enough: a third data point, chemically distinct from both predecessors.

The Wow! Signal: Stronger in 2025, Still Unexplained

On 15 August 1977, Ohio State’s Big Ear radio telescope recorded a powerful narrowband signal lasting 72 seconds, near the 1420 megahertz hydrogen line — long considered a logical channel for a deliberate transmission. Astronomer Jerry Ehman wrote “Wow!” on the printout. It has never been confirmed to repeat, despite decades of follow-up.

In 2025, the Arecibo WOW! project, led by Abel Méndez, reanalysed over 75,000 digitised pages of the Big Ear archive. The signal became stranger: a revised frequency of 1420.726 megahertz, peak strength above 250 janskys — four times the commonly quoted figure — and a narrowed source region. Their tests ruled out terrestrial interference, known satellites and lunar reflections.

Their leading proposal is natural and unproven: a magnetar flare striking a cold hydrogen cloud, briefly driving a maser-like “superradiance” burst never yet observed at this frequency. Two weaker, similar archive signals make it plausible, not proven. Like the cycles behind gold vs silver, it returns because each analysis adds information — and still stops short of an answer.

Fast Radio Bursts: Canada’s CHIME Is Rewriting the Census

Fast radio bursts are millisecond flashes from other galaxies. Their emission mechanism is unsettled: magnetars — highly magnetised neutron stars — are the leading progenitor candidate, but whether one population explains every burst remains open.

The decisive instrument sits in Canada. CHIME, near Penticton, British Columbia, has found the overwhelming majority of repeating sources. Its latest catalogue adds 30 new repeaters, bringing the total to 80 — seventy-nine from CHIME. Only about 2.4 percent have been seen to repeat, yet the statistics fit most or all repeating eventually. CHIME’s Outriggers now pinpoint even one-off bursts: FRB 20250316A, among the brightest ever recorded, was localised in March 2025 to galaxy NGC 4141, 130 million light-years away, within roughly 42 light-years.

Tabby’s Star: The Megastructure That Turned Into Dust

KIC 8462852 — Tabby’s star, or Boyajian’s star — dimmed by up to 22 percent for days at a time in Kepler data announced in 2015: too deep, irregular and aperiodic for an ordinary transiting planet. Speculation about an alien megastructure followed. Could observers catch a dip in real time?

A dedicated observing campaign did so in 2017: over 200 researchers found the dimming is chromatic — some wavelengths are blocked far more than others. A solid, opaque structure cannot do that; fine dust can. The megastructure explanation is effectively ruled out. Only the dust’s origin stays open: exocomet swarms, colliding planetesimals and a disintegrating exomoon are published candidates.

What the Vera Rubin Observatory Changes

Every conclusion above rests on a sample of three. The NSF-DOE Vera C. Rubin Observatory in Chile, with the world’s largest digital camera, will spend ten years on the Legacy Survey of Space and Time, imaging the whole visible sky repeatedly — the depth, area and cadence that finding a faint, fast visitor requires.

Estimates suggest Rubin should detect between five and fifty interstellar objects over that decade. Even the low end transforms the subject: a population to compare, instead of single anomalies to debate.

Rubin’s unprecedented data firehose also makes data stewardship a scientific issue — the same reason protecting your own data trail matters, as our guide to AI scraping and your data explains. Discovery depends on fragile infrastructure too, much like the energy flows through the Strait of Hormuz chokepoint.

The Next Decade Will Sharpen These Mysteries, Not Simply Close Them

Better instruments rarely deliver instant answers; they deliver better questions with error bars attached. ‘Oumuamua’s acceleration survived scrutiny without yielding a unique cause. The Wow! signal survived digitisation by becoming stronger. Fast radio bursts survived localisation by multiplying into populations.

Within ten years, interstellar objects should be a statistical sample and bursts a mapped population, with refined Wow! coordinates guiding searches into the signal’s 50th anniversary in 2027. Some mysteries will close; at least one will be replaced by something stranger.

Frequently Asked Questions

How many interstellar objects have been discovered?

Three: 1I/’Oumuamua (2017), 2I/Borisov (2019) and 3I/ATLAS (2025), each on a hyperbolic trajectory proving an extrasolar origin. Rubin should raise that total substantially.

Was ‘Oumuamua an alien spacecraft?

No. Its acceleration has several proposed natural explanations, including invisible outgassing of volatile ices, none proven. The lightsail idea is a minority hypothesis without direct evidence.

Is 3I/ATLAS dangerous to Earth?

No. It passed no closer than about 1.8 astronomical units in December 2025 and is leaving the Solar System permanently. NASA and ESA describe its trajectory as no threat.

Has the Wow! signal ever repeated?

None has been confirmed. The 2025 reanalysis refined its frequency, strength and source region, so some earlier searches may have aimed slightly off target.

What causes fast radio bursts?

It is unsettled. Magnetars are the leading candidate, especially for repeaters, but whether all bursts share one origin remains open.

Can I see such a visitor with a backyard telescope?

Usually not: all three were faint and fast, found by survey telescopes. Smart telescopes still show comets, nebulae and galaxies — and amateurs, as Borisov proved, make real discoveries.