Two trillion, and most of them out of sight

In 2016 a team led by Christopher Conselice at the University of Nottingham put the number of galaxies in the observable universe at about two trillion, roughly ten times the figure astronomers had been working with. The twist sits inside the result: the great majority of those galaxies are too faint for any telescope now operating to detect.

The paper states it plainly. There are “at least 2 × 10¹² (two trillion) galaxies in the currently visible universe, the vast majority of which cannot be observed with present day technology as they are too faint.” In other words, the count is less a tally of dots in our images than an estimate of how many dots the images are missing.

How you count galaxies you cannot see

The team did not add up galaxies one by one. Instead they used galaxy stellar mass functions, which describe how many galaxies of each mass exist in a given slice of space. They measured these at many points in cosmic history, reaching back to within roughly 650 million years of the Big Bang. Much of the deep data came from the Hubble Space Telescope, whose narrow, long-exposure images act like core samples drilled through the universe.

Fitting those measurements with a standard mathematical curve, the researchers worked out the density of galaxies at each epoch, then integrated across the whole observable volume and the whole age of the universe. The arithmetic ran down to galaxies as small as a million times the mass of the Sun, faint dwarfs that no survey can see at great distance.

Working with colleagues at Leiden Observatory in the Netherlands and the University of Edinburgh, the team turned narrow, deep images of the sky into three-dimensional maps. From those they measured the volume and the galaxy density of one small slice of space after another. Conselice likened the effort to an intergalactic archaeological dig, a way of estimating how many galaxies had been missed rather than counting the ones already on the photographic plates. The faintest galaxies the model implies would shine at around magnitude 29 on the astronomers’ brightness scale, far below the reach of the unaided eye and beyond all but the deepest exposures ever taken.

Two independent versions of the calculation gave 1.2 trillion and 2.8 trillion. The University of Nottingham reported the averaged figure of two trillion as the headline result, the product of what Conselice described as roughly fifteen years of work.

“We are missing the vast majority of galaxies because they are very faint and far away,” Conselice said when the study appeared. “It boggles the mind that over 90 percent of the galaxies in the universe have yet to be studied.”

The early universe was more crowded

The count is high partly because the young universe was busier than the present one. When the team tracked how galaxy numbers changed over time, the trend ran backward. A given volume of space held about ten times more galaxies when the universe was a few billion years old than it does today.

That sounds backward, since the universe has only grown older and larger. The explanation is that small galaxies have spent billions of years merging into bigger ones, so the population thins even as individual galaxies grow. Most of the missing multitude are low-mass systems, no larger than the dwarf galaxies that orbit the Milky Way, and they are exactly the ones too dim to register at great distance.

Why the number jumped tenfold

Before this work, the count was usually put in the hundreds of billions, with the often-quoted figure around 100 billion. Even careful counts from the deepest direct images, such as the Hubble Ultra Deep Field, reached only a couple of hundred billion galaxies. The 2016 model put the true total at almost ten times that direct-count number, roughly two trillion. That implies about ninety percent of galaxies are fainter than current telescopes can register, below an apparent brightness of about magnitude 29.

That gap is not just bookkeeping. The number of galaxies feeds into the total amount of starlight filling space, and into the centuries-old puzzle known as Olbers’ paradox, which asks why the night sky is dark if the universe is full of stars. A sky packed with two trillion galaxies might seem to make the paradox worse, not better.

The team’s answer is that the light of those distant galaxies never reaches us at full strength. Most of it is absorbed by gas in intergalactic space, and the expansion of the universe stretches the rest toward redder, fainter wavelengths. So the universe can be far more crowded than telescopes show while the night sky stays dark, with most of its contents waiting in the gloom for sharper eyes.

What this does and does not prove

The two-trillion figure is a model-dependent extrapolation, not a head count, and the authors are careful about its limits. The result hinges on two choices: the smallest galaxy mass they counted and the earliest epoch they reached. They note that pushing the mass limit lower would multiply the total by about seven, which is why they frame two trillion as a floor rather than a ceiling.

The bigger caveat arrived later. In 2021 a team used NASA’s New Horizons spacecraft, far enough from the Sun to escape the dust glow that brightens the sky near Earth, to measure the cosmic optical background, the summed light of all galaxies. From out there the sky was about ten times darker than the darkest view Hubble can manage. The researchers argued in public that the faintness of that glow points to a total closer to the hundreds of billions than to trillions. “We simply don’t see the light from two trillion galaxies,” one of them, Marc Postman, told reporters.

So the famous number is one side of an open argument, the high end of a range that newer data has pulled downward. What survives the dispute is the underlying lesson rather than the digit: most of the universe’s galaxies, on any of these estimates, are fainter than we can currently observe.

A number that keeps moving

The galaxy count has climbed and fallen as instruments changed, from a few hundred billion in the 1990s to two trillion in 2016 and back toward the hundreds of billions in the New Horizons reading. Each revision says as much about the reach of the telescope as about the universe itself.

That may be the honest takeaway. The number of galaxies is not a settled fact carved into the sky but a running estimate, one that shifts every time a new instrument sees a little deeper into the dark.

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A SpaceX Dragon cargo spacecraft is scheduled to leave the International Space Station on Tuesday, June 16, and head back to Earth carrying research samples and used hardware, with splashdown expected off the coast of California early on Wednesday, June 17.

According to NASA, the capsule will undock from the forward port of the station’s Harmony module at about 12:05 p.m. EDT, after a command from SpaceX ground controllers, then fire its thrusters to back away from the orbiting laboratory. Live undocking coverage begins at 11:45 a.m. EDT on NASA+ and the agency’s YouTube channel. The spacecraft is due to reenter the atmosphere the next day and splash down at about 5:08 a.m. PDT; NASA will not carry the splashdown live but plans to post updates on its space station blog. The flight closes out the 34th SpaceX commercial resupply mission, known as CRS-34.

Dragon is returning thousands of pounds of cargo. The science aboard includes bioprinted organ and cartilage tissue, data on storing cryogenic fuel for future missions, and DNA-inspired materials being developed into cancer treatments. The returning hardware includes an ocular imaging device used to track crew members’ eye health, an absorbent bed that filters trace contaminants from cabin air, and a separator pump from the station’s waste and hygiene compartment.

Several of the experiments target health problems on Earth. One NASA investigation grew blood-forming stem cells in microgravity, where researchers believe the cells better keep their ability to develop into the red and white blood cells used to treat blood diseases and cancers. Another, called MVP Cell-09, sent up lab-grown heart tissue deliberately infected with pneumonia-causing bacteria, which tend to grow more aggressive in orbit, to study a poorly understood link between pneumonia and heart disease. A third returns 3D-printed cartilage; NASA notes that more than 900,000 knee cartilage injuries occur in the United States each year, many requiring surgery, and that printing in microgravity spreads cells more evenly than is possible on the ground.

Partners flew experiments of their own. The European Space Agency’s Green Bone study tested a scaffold made from wood and designed to mimic real bone, work aimed at conditions such as osteoporosis, which weakens the bones of millions of people. Other returning samples include heart, liver, kidney, and brain tissue models dosed with experimental RNA-based drugs, and semiconductor crystals grown in orbit for use in sensors and lasers.

NASA frames these as early-stage investigations. The samples still have to be analyzed back on Earth, and the medical and engineering payoffs the agency describes remain prospective rather than proven.

Dragon arrived at the station on May 17, carrying nearly 6,500 pounds of crew supplies and experiments, two days after launching on a Falcon 9 rocket from Space Launch Complex 40 at Cape Canaveral Space Force Station in Florida. It docked to the Harmony module to deliver cargo to the Expedition 74 crew.

Once the capsule is recovered, its cargo heads to research teams across the United States and Europe, where the slower work of the mission begins: reading what weeks in microgravity did to the samples.

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For five years, the leading story about Kamoʻoalewa, a small asteroid that travels alongside Earth, was that it might be a stray piece of the Moon. A new peer-reviewed study argues it is probably something far more ordinary: a fragment of a distant asteroid family, weathered until its surface only looks lunar.

The timing is sharp. On or around June 7, after a 13-month cruise, China’s Tianwen-2 spacecraft slipped into orbit around Kamoʻoalewa, a maneuver tracked by independent radio astronomers even before China’s space agency formally confirmed it. Sample collection is expected within weeks. The asteroid that may decide a long-running debate about its own origin is now being circled by the machine built to bring a piece of it home.

Why the Moon theory took hold

Kamoʻoalewa was found in 2016 by the Pan-STARRS survey in Hawaiʻi, and it is one of a small set of asteroids called quasi-satellites: rocks that orbit the Sun but stay close enough to Earth to seem like loose companions. This one is the most stable of the bunch, expected to linger near us for a few hundred years. It is small, somewhere between roughly 40 and 100 metres across, and it spins fast, turning about once every 28 minutes.

Quasi-satellites are faint and awkward to observe, which is part of why this one drew such outsized attention once a clue to its makeup appeared. It sits close, it stays put for centuries, and it is bright enough to study with large telescopes during its annual passes. For an object that might preserve a record of the early Solar System, that combination is rare.

In 2021, astronomers reported that its reflected light was a closer match to lunar-like silicates than to any common asteroid type, with an unusual extra redness. That resemblance was the spark for the lunar-fragment idea. A 2024 study went further, proposing a specific birthplace: the young Giordano Bruno crater on the Moon’s far side, from which an impact could have flung debris into an Earth-like orbit.

It was a tidy and romantic picture. A souvenir of the Moon, looping around its parent planet, waiting to be collected.

What the new study actually did

Writing in Nature Communications, a large international team led from the Chinese Academy of Sciences reopened the case from three directions. First, they reanalyzed the asteroid’s spectrum and pinned down a key absorption feature at a wavelength of 1.001 micrometres, a value they report as consistent not with lunar material but with LL chondrites, a common type of stony meteorite.

Then they tested whether ordinary meteorite could be made to look like Kamoʻoalewa. In the lab, the team blasted LL chondrite samples with laser pulses to mimic space weathering, the slow darkening and reddening that solar wind and tiny impacts inflict on any airless surface over millions of years. Heavily weathered chondrite powder, they found, reproduced the asteroid’s spectrum. Solid chips of the same meteorite did not.

Finally, they traced where such an object could have come from. Their modeling points to the ν6 secular resonance, a gravitational escape route in the inner asteroid belt, and more specifically to the Flora family, a cluster of asteroids born from an ancient collision. Kamoʻoalewa’s makeup, they write, resembles the well-studied asteroid Itokawa and seven Flora-family members, only more weathered.

The resonance matters because it offers a plausible delivery route. Rocks that drift into it can have their orbits stretched until they cross Earth’s path, which is one way an inner-belt fragment could end up as a quasi-satellite rather than a lunar one. The argument, in short, is that you do not need the Moon to explain Kamoʻoalewa. An ordinary asteroid nudged out of the Flora family and aged in sunlight can account for the same colour, the same orbit, and the same odd companionship with Earth.

Why a powder, not a slab, is the key detail

That powder-versus-chip result is the quiet heart of the paper. A fresh asteroid surface and a deeply weathered one can be the same rock and still reflect light differently, and grain size matters as much as chemistry. The team’s reading is that Kamoʻoalewa is coated in fine, long-exposed dust, which is exactly what would redden an ordinary asteroid until its colour drifts toward the lunar range.

If that holds, the redness that first suggested the Moon is not a fingerprint of lunar rock at all. It is what time does to a small, slow-tumbling asteroid that has been left out in the solar wind for ages. The comparison with Itokawa carries weight here, because Japan’s Hayabusa mission already returned grains from Itokawa and confirmed it as an ordinary chondrite body. The new study places Kamoʻoalewa in that same broad company.

What this does and does not prove

This is a competing explanation, not a verdict. The authors are careful to say Kamoʻoalewa probably came from the Flora family, and the hedge is doing real work. Reflectance spectra are notoriously slippery; weathering, grain size, viewing angle and composition can all nudge an asteroid’s colour in similar directions, which is why two capable teams can look at the same faint object and reach different conclusions.

The lunar-origin case has not been withdrawn, and the dynamical argument for a recent lunar escape still has its defenders. It is also worth noting that the new paper is an early-access version of an accepted manuscript, released ahead of final editing, so small details may still change. What the study does is weaken the single strongest pillar of the Moon theory, the matching red spectrum, by showing that a plain meteorite can be pushed to look the same.

In other words, the resemblance to the Moon is real. The new work simply argues it is a coincidence of weathering rather than a sign of shared birth.

The sample that will end the argument

What makes this debate unusual is that it has an expiration date. Most arguments about an asteroid’s origin are settled, if at all, by ever-finer remote measurements. This one will be settled by holding the rock.

The stakes are not trivial. If the grains turn out to be lunar, Kamoʻoalewa becomes a free sample of the Moon’s far side, delivered to Earth’s neighbourhood without a crewed landing. If they are ordinary chondrite, the asteroid becomes a well-preserved relic of the early Solar System and a test case for how badly space weathering can disguise a familiar rock. Those are two very different prizes, and the same capsule settles which one we get.

Tianwen-2 is expected to collect material from Kamoʻoalewa within weeks and send a capsule back toward Earth in 2027, after which the spacecraft heads onward to a comet. A returned sample can be measured for the isotopes and minerals that distinguish lunar rock from a chondrite asteroid, the kind of test no telescope can perform from afar. For once, a question about where a space rock came from will be answered by the rock itself, and the answer is already on its way.

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