Space & Astronomy

Fleets of private satellites orbiting Earth will be visible to the naked eye in the next few years, sometimes all night long.

Companies like SpaceX and Amazon have launched hundreds of satellites into low orbits since 2019, with plans to launch thousands more in the works — a trend that’s alarming astronomers. The goal of these satellite “mega-constellations” is to bring high-speed internet around the globe, but these bright objects threaten to disrupt astronomers’ ability to observe the cosmos (SN: 3/12/20). “For astronomers, this is kind of a pants-on-fire situation,” says radio astronomer Harvey Liszt of the National Radio Astronomical Observatory in Charlottesville, Va.

Now, a new simulation of the potential positions and brightness of these satellites shows that, contrary to earlier predictions, casual sky watchers will have their view disrupted, too. And parts of the world will be affected more than others, astronomer Samantha Lawler of the University of Regina in Canada and her colleagues report in a paper posted September 9 at arXiv.org.

“How will this affect the way the sky looks to your eyeballs?” Lawler asks. “We humans have been looking up at the night sky and analyzing patterns there for as long as we’ve been human. It’s part of what makes us human.” These mega-constellations could mean “we’ll see a human-made pattern more than we can see the stars, for the first time in human history.”

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Flat, smooth surfaces on satellites can reflect sunlight depending on their position in the sky. Earlier research had suggested that most of the new satellites would not be visible with the naked eye.

Lawler, along with Aaron Boley of the University of British Columbia and Hanno Rein of the University of Toronto at Scarborough in Canada, started building their simulation with public data about the launch plans of four companies — SpaceX’s Starlink, Amazon’s Kuiper, OneWeb and StarNet/GW — that had been filed with the U.S. Federal Communications Commission and the International Telecommunications Union. The filings detailed the expected orbital heights and angles of 65,000 satellites that could be launched over the next few years.

“It’s impossible to predict the future, but this is realistic,” says astronomer Meredith Rawls of the University of Washington in Seattle, who was not involved in the new study. “A lot of times when people make these simulations, they pick a number out of a hat. This really justifies the numbers that they pick.”

There are currently about 7,890 objects in Earth orbit, about half of which are operational satellites, according to the U.N. Office for Outer Space Affairs. But that number is increasing fast as companies launch more and more satellites (SN: 12/28/20). In August 2020, there were only about 2,890 operational satellites.

Next, the researchers computed how many satellites will be in the sky at different times of year, at different hours of the night and from different positions on Earth’s surface. They also estimated how bright the satellites were likely to be at different hours of the day and times of the year.

That calculation required a lot of assumptions because companies aren’t required to publish details about their satellites like the materials they’re made of or their precise shapes, both of which can affect reflectivity. But there are enough satellites in orbit that Lawler and colleagues could compare their simulated satellites to the light reflected down to Earth by the real ones.

The simulations showed that “the way the night sky is going to change will not affect all places equally,” Lawler says. The places where naked-eye stargazing will be most affected are at latitudes 50° N and 50° S, regions that cross lower Canada, much of Europe, Kazakhstan and Mongolia, and the southern tips of Chile and Argentina, the researchers found.

A simulation shows the number and brightness of satellites visible from Canada at midnight on the June solstice if 65,000 satellites launch in the next few years. The center of the circle is straight overhead, and the edges mark the horizon. Yellow dots represent the brightest satellites and purple dots the dimmest. Curious about how the satellites might skew your view of the stars? Visit the researchers’ website to check simulations of the visibility near you.Samantha Lawler, Hanno Rein and Aaron Boley

“The geometry of sunlight in the summer means there will be hundreds of visible satellites all night long,” Lawler says. “It’s bad everywhere, but it’s worse there.” For her, this is personal: She lives at 50° N.

Closer to the equator, where many research observatories are located, there is a period of about three hours in the winter and near the time of the spring and fall equinoxes with few or no sunlit satellites visible. But there are still hundreds of sunlit satellites all night at these locations in the summer.

A few visible satellites can be a fun spectacle, Lawler concedes. “I think we really are at a transition point here where right now, seeing a satellite, or even a Starlink train, is cool and different and wow, that’s amazing,” she says. “I used to look up when the [International Space Station] was overhead.” But she compares the coming change to watching one car go down the road 100 years ago, versus living next to a busy freeway now.

“Every sixteenth star will actually be moving,” she says. “I hope I’m wrong. I’ve never wanted to be wrong about a simulation more than this. But without mitigation, this is what the sky will look like in a few years.”

Astronomers have been meeting with representatives from private companies, as well as space lawyers and government officials, to work out compromises and mitigation strategies. Companies have been testing ways to reduce reflectivity, like shading the satellites with a “visor.” Other proposed strategies include limiting the satellites to lower orbits, where they would appear brighter in telescope images but move faster across the sky. Counterintuitively, brighter, faster satellites would be better for astronomy research, Rawls says. “They move out of the way quick.”

But that lower altitude strategy will mean more visible satellites for other parts of the world, and more that are visible to the naked eye. “There’s not some magical orbital altitude that solves all our problems,” Rawls says. “There are some latitudes on Earth where no matter what altitude you put your satellites at, they’re going to be all over the darn place. The only way out of this is fewer satellites.”

There are currently no regulations concerning how bright a satellite can be or how many satellites a private company can launch. Scientists are grateful that companies are willing to work with them, but nervous that their cooperation is voluntary.

“A lot of the people who work on satellites care about space. They’re in this industry because they think space is awesome,” Rawls says. “We share that, which helps. But it doesn’t fix it. I think we need to get some kind of regulation as soon as possible.” (Representatives from Starlink, Kuiper and OneWeb did not respond to requests for comment.)

Efforts are under way to bring the issue to the attention of the United Nations and to try to use existing environmental regulations to place limits on satellite launches, says study coauthor Boley (who also lives near 50° N).

Analogies to other global pollution problems, like space junk, can provide inspiration and precedents, he says. “There are a number of ways forward. We shouldn’t just lose hope. We can do things about this.” More

A meandering trek taken by light from a remote supernova in the constellation Cetus may help researchers pin down how fast the universe expands — in another couple of decades.

About 10 billion years ago, a star exploded in a far-off galaxy named MRG-M0138. Some of the light from that explosion later encountered a gravitational lens, a cluster of galaxies whose gravity sent the light on multiple diverging paths. In 2016, the supernova appeared in Earth’s sky as three distinct points of light, each marking three different paths the light took to get here.

Now, researchers predict that the supernova will appear again in the late 2030s. The time delay — the longest ever seen from a gravitationally lensed supernova — could provide a more precise estimate for the distance to the supernova’s host galaxy, the team reports September 13 in Nature Astronomy. And that, in turn, may let astronomers refine estimates of the Hubble constant, the parameter that describes how fast the universe expands.

The original three points of light appeared in images from the Hubble Space Telescope. “It was purely an accident,” says astronomer Steve Rodney of the University of South Carolina in Columbia. Three years later, when Hubble reobserved the galaxy, astronomer Gabriel Brammer at the University of Copenhagen discovered that all three points of light had vanished, indicating a supernova.

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By calculating how the intervening cluster’s gravity alters the path the supernova’s light rays take, Rodney and his colleagues predict that the supernova will appear again in 2037, give or take a couple of years. Around that time, Hubble may burn up in the atmosphere, so Rodney’s team dubs the supernova “SN Requiem.”

“It’s a requiem for a dying star and a sort of elegy to the Hubble Space Telescope itself,” Rodney says. A fifth point of light, too faint to be seen, may also arrive around 2042, the team calculates.

In another Hubble image of the galaxy cluster MACS J0138.0-2155, the cluster split the light from a supernova into three points, SN1, SN2 and SN3. The other two points, SN4 and SN5, are predictions of where the light from the supernova will appear in future years.S. Rodney et al/Nature Astronomy 2021

The predicted 21-year time delay — from 2016 to 2037 — is a record for a supernova. In contrast, the first gravitational lens ever found — twin images of a quasar spotted in 1979  — has a time delay of only 1.1 years (SN: 11/10/1979).

Not everyone agrees with Rodney’s forecast. “It is very difficult to predict what the time delay will be,” says Rudolph Schild, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was the first to measure the double quasar’s time delay. The distribution of dark matter in the galaxy hosting the supernova and the cluster splitting the supernova’s light is so uncertain, Schild says, that the next image of SN Requiem could come outside the years Rodney’s team has specified.

In any case, when the supernova image does appear, “that would be a phenomenally precise measurement” of the time delay, says Patrick Kelly, an astronomer at the University of Minnesota in Minneapolis who was not involved with the new work. That’s because the uncertainty in the time delay will be tiny compared with the tremendous length of the time delay itself.

That delay, coupled with an accurate description of how light rays weave through the galaxy cluster, could affect the debate over the Hubble constant. Numerically, the Hubble constant is the speed a distant galaxy recedes from us divided by the distance to that galaxy. For a given galaxy with a known speed, a larger estimated distance therefore leads to a lower number for the Hubble constant.

This number was once in dispute by a factor of two. Today the range is much tighter, from 67 to 73 kilometers per second per megaparsec. But that spread still leaves the universe’s age uncertain. The frequently quoted age of 13.8 billion years corresponds to a Hubble constant of 67.4. But if the Hubble constant is higher, then the universe could be about a billion years younger.

The longer it takes for SN Requiem to reappear, the farther from Earth the host galaxy is — which means a lower Hubble constant and an older universe. So if the debate over the Hubble constant persists into the 2030s, the exact date the supernova springs back to life could help resolve the dispute and nail down a fundamental cosmological parameter. More

One can only imagine what Grote Reber’s neighbors thought when, in 1937, the amateur radio enthusiast erected in his yard a nearly 10-meter-wide shallow bowl of sheet metal, perched atop an adjustable scaffold and topped by an open pyramid of gangly towers. Little could his neighbors have known that they were witnessing the birth of a new way of looking at the cosmos.

Reber was building the world’s first dedicated radio telescope. Unlike traditional telescopes, which use lenses or mirrors to focus visible light, this contraption used metal and circuitry to collect interstellar radio waves, low frequency ripples of electromagnetic radiation. With his homemade device, Reber made the first map of the sky as seen with radio-sensitive eyes and kicked off the field of radio astronomy.

“Radio astronomy is as fundamental to our understanding of the universe as … optical astronomy,” says Karen O’Neil, site director at Green Bank Observatory in West Virginia. “If we want to understand the universe, we really need to make sure we have as many different types of eyes on the universe as we possibly can.”

When astronomers talk about radio waves from space, they aren’t (necessarily) referring to alien broadcasts. More often, they are interested in low-energy light that can emerge when molecules change up their rotation, for example, or when electrons twirl within a magnetic field. Tuning in to interstellar radio waves for the first time was akin to Galileo pointing a modified spyglass at the stars centuries earlier — we could see things in the sky we’d never seen before.

Today, radio astronomy is a global enterprise. More than 100 radio telescopes — from spidery antennas hunkered low to the ground to supersized versions of Reber’s dish that span hundreds of meters — dot the globe. These eyes on the sky have been so game-changing that they’ve been at the center of no fewer than three Nobel Prizes.

Not bad for a field that got started by accident.

In the early 1930s, an engineer at Bell Telephone Laboratories named Karl Jansky was tracking down sources of radio waves that interfered with wireless communication. He stumbled upon a hiss coming from somewhere in the constellation Sagittarius, in the direction of the center of the galaxy.

Karl Jansky, shown here with his rotating radio antenna, stumbled on a radio hiss coming from the direction of the center of the galaxy, marking the beginnings of radio astronomy.NRAO, AUI, NSF, Jeff Hellerman

“The basic discovery that there was radio radiation coming from interstellar space confounded theory,” says astronomer Jay Lockman, also of Green Bank. “There was no known way of getting that.”

Bell Labs moved Jansky on to other, more Earthly pursuits. But Reber, a fan of all things radio, read about Jansky’s discovery and wanted to know more. No one had ever built a radio telescope before, so Reber figured it out himself, basing his design on principles used to focus visible light in optical scopes. He improved upon Jansky’s antenna — a bunch of metal tubes held up by a pivoting wooden trestle — and fashioned a parabolic metal dish for focusing incoming radio waves to a point, where an amplifier boosted the feeble signal. The whole contraption sat atop a tilting wooden base that let him scan the sky by swinging the telescope up and down. The same basic design is used today for radio telescopes around the world.

For nearly a decade — thanks partly to the Great Depression and World War II — Reber was largely alone. The field didn’t flourish until after the war, with a crop of scientists brimming with new radio expertise from designing radar systems. Surprises have been coming ever since.

Grote Reber erected the world’s first dedicated radio telescope – shown here – in his yard in Wheaton, Ill.GBO, NSF, AUI

“The discovery of interstellar molecules, that’s a big one,” says Lisa Young, an astronomer at New Mexico Tech in Socorro. Radio telescopes are well suited to peering into the dense, cold clouds where molecules reside and sensing radiation emitted when they lose rotational energy. Today, the list of identified interstellar molecules includes many complex organics, including some thought to be precursors for life.

Radio telescopes also turned up objects previously unimagined. Quasars, the blazing cores of remote galaxies powered by behemoth black holes, first showed up in detailed radio maps from the late 1950s. Pulsars, the ultradense spinning cores of dead stars, made themselves known in 1967 when Jocelyn Bell Burnell noticed that the radio antenna array she helped build was picking up a steady beep … beep … beep from deep space every 1.3 seconds. (She was passed over when the 1974 Nobel Prize in physics honored this discovery — her adviser got the recognition. But an accolade came in 2018, when she was awarded a Special Breakthrough Prize in Fundamental Physics.)

Pulsars are “not only interesting for being a discovery in themselves,” Lockman says. They “are being used now to make tests of general relativity and detect gravitational waves.” That’s because anything that nudges a pulsar — say, a passing ripple in spacetime — alters when its ultraprecise radio beats arrive at Earth. In the early 1990s, such timing variations from one pulsar led to the first confirmed discovery of planets outside the solar system.

More recently, brief blasts of radio energy primarily from other galaxies have captured astronomers’ attention. Discovered in 2007, the causes of these “fast radio bursts” are still unknown. But they are already useful probes of the stuff between galaxies. The light from these eruptions encodes signatures of the atoms encountered while en route to Earth, allowing astronomers to track down lots of matter they thought should be out in the cosmos but hadn’t found yet. “That was the thing that allowed us to weigh the universe and understand where the missing matter is,” says Dan Werthimer, an astronomer at the University of California, Berkeley.

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And it was a radio antenna that, in 1964, gave the biggest boost to the then-fledgling Big Bang theory. Arno Penzias and Robert Wilson, engineers at Bell Labs, were stymied by a persistent hiss in the house-sized, horn-like antenna they were repurposing for radio astronomy. The culprit was radiation that permeates all of space, left behind from a time when the universe was much hotter and denser than it is today. This “cosmic microwave background,” named for the relatively high frequencies at which it is strongest, is still the clearest window that astronomers have into the very early universe.

Radio telescopes have another superpower. Multiple radio dishes linked together across continents can act as one enormous observatory, with the ability to see details much finer than any of those dishes acting alone. Building a radio eye as wide as the planet — the Event Horizon Telescope — led to the first picture of a black hole.

The Event Horizon Telescope, an international network of radio observatories, took this first-ever image of a black hole, at the center of galaxy M87.Event Horizon Telescope collaboration et al

“Not that anybody needed proof of the existence [of black holes],” Young says, “but there’s something so marvelous about actually being able to see it.”

The list of discoveries goes on: Galaxies from the early universe that are completely shrouded in dust and so emit no starlight still glow bright in radio images. Rings of gas and dust encircling young stars are providing details about planet formation. Intel on asteroids and planets in our solar system can be gleaned by bouncing radio waves off their surfaces.

And, of course, there’s the search for extraterrestrial intelligence, or SETI. “Radio is probably the most likely place where we will answer the question: ‘Are we alone?’” Werthimer says.

The ALMA radio telescope network in the Atacama Desert in Chile captured this image of what appears to be a planet-forming disk around the young star HL Tauri.ESO, NAOJ, NRAO

That sentiment goes back more than a century. In 1899, inventor Nikola Tesla picked up radio signals that he thought were coming from folks on another planet. And for 36 hours in August 1924, the United States ordered all radio transmitters silent for five minutes every hour to listen for transmissions from Mars as Earth lapped the Red Planet at a relatively close distance. The field got a more official kickoff in 1960 when astronomer Frank Drake pointed Green Bank’s original radio telescope at the stars Tau Ceti and Epsilon Eridani, just in case anyone there was broadcasting.

While SETI has had its ups and downs, “there’s kind of a renaissance,” Werthimer says. “There’s a lot of new, young people going into SETI … and there’s new money.” In 2015, entrepreneur Yuri Milner pledged $100 million over 10 years to the search for other residents of our universe.

Though the collapse of the giant Arecibo Observatory in 2020 — at 305 meters across, it was the largest single dish radio telescope for most of its lifetime — was tragic and unexpected, radio astronomers have new facilities in the works. The Square Kilometer Array, which will link up small radio dishes and antennas across Australia and South Africa when complete in the late 2020s, will probe the acceleration of the universe’s expansion, seek out signs of life and explore conditions from cosmic dawn. “We’ll see the signatures of the first structures in the universe forming the first galaxies and stars,” Werthimer says.

The Square Kilometer Array will connect radio dishes and antennas across South Africa and Australia, offering an unprecedented look at the universe.SKA Observatory

But if the history of radio astronomy is any guide, the most remarkable discoveries yet to come will be the things no one has thought to look for. So much about the field is marked by serendipity, Werthimer notes. Even radio astronomy as a field started serendipitously. “If you just build something to look at some place that nobody’s looked before,” he says, “you’ll make interesting discoveries.” More

When considering where to look for extraterrestrial life, astronomers have mostly stuck with what’s familiar. The best candidates for habitable planets are considered the ones most like Earth: small, rocky, with breathable atmospheres and a clement amount of warmth from their stars.

But as more planets outside the solar system have been discovered, astronomers have debated the usefulness of this definition (SN: 10/4/19). Some planets in the so-called habitable zone, where temperatures are right for liquid water, are probably not good for life at all. Others outside that designated area might be perfectly comfortable.

Now, two studies propose revising the concept of “habitable zone” to account for more of the planets that astronomers may encounter in the cosmos. One new definition brings more planets into the habitable fold; the other nudges some out.

“Both papers focus on questioning the classical idea of the habitable zone,” says astronomer Noah Tuchow of Penn State University. “We should extend the range of places that we look, so we don’t miss habitable planets.”

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Some overlooked planets could be much bigger than Earth, and potentially receive no starlight at all. Astrophysicist Nikku Madhusudhan of the University of Cambridge and colleagues propose a new category of possibly habitable planet that could be found at almost any distance from any kind of star.

These hypothetical planets have a global liquid-water ocean nestled under a thick hydrogen-rich atmosphere (SN: 5/4/20). Madhusudhan calls them “Hycean” planets, for “hydrogen” and “ocean.” They could be up to 2.6 times the size of Earth and up to 10 times as massive, Madhusudhan and colleagues report August 25 in the Astrophysical Journal. That thick atmosphere could keep temperatures right for liquid water even with minimal input from a star, while the ocean could protect anything living from crushing atmospheric pressure.

“We want to expand beyond our fixated paradigm so far on Earthlike planets,” Madhusudhan says. “Everything we’ve learned about exoplanets so far is extremely diverse. Why restrict ourselves when it comes to life?”

In the search for alien life, Hycean planets would have several advantages over rocky planets in the habitable zone, the team says. Though it’s difficult to tell which worlds definitely have oceans and hydrogen atmospheres, there are many more known exoplanets in the mass and temperature ranges of Hycean planets than there are Earthlike planets. So the odds are good, Madhusudhan says.

And because they are generally larger and have more extended atmospheres than rocky planets, Hycean planets are easier to probe for biosignatures, molecular signs of life. Detectable biosignatures on Hycean planets could include rare molecules associated with life on Earth like dimethyl sulfide and carbonyl sulfide. These tend to be too low in concentration to detect in thin Earthlike atmospheres, but thicker Hycean atmospheres would show them more readily.

Best of all, existing or planned telescopes could detect those molecules within a few years, if they’re there. Madhusudhan already has plans to use NASA’s James Webb Space Telescope, due to launch later this year, to observe the water-rich planet K2 18b (SN: 9/11/19).

That planet’s habitability was debated when it was reported in 2019. Madhusudhan says 20 hours of observations with JWST should solve the debate.

“Best-case scenario, we’ll detect life on K2 18b,” he says, though “I’m not holding my breath over it.”

Astronomer Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany, thinks it probably won’t be that easy. Planets in the Hycean size range tend to have cloudy or hazy atmospheres, making biosignatures more difficult to pick up.

It’s also not clear if Hycean planets actually exist in nature. “It is a really fun idea,” she says. “But is it just a fun idea, or does it match up with reality? I think we absolutely don’t know yet.”

Rather than inventing a new way to bring exoplanets into the habitable family, Tuchow and fellow Penn State astronomer Jason Wright are kicking some apparently habitable planets out. The pair realized that the region of clement temperatures around a star changes as the star evolves and changes brightness.

Some planets are born in the habitable zone and stay there their entire lives. But some, possibly most, are born outside their star’s habitable zone and enter it later, as the star ages. In the August Research Notes of the American Astronomical Society, Tuchow and Wright suggest calling those worlds “belatedly habitable planets.”

When astronomers point their telescopes at a given star, the scientists are seeing only a snapshot of the star’s habitable zone, the pair say. “If you just look at a planet in the habitable zone in the present day, you have no idea how long it’s been there,” Tuchow says. It’s an open question whether planets that enter the habitable zone later in life can ever become habitable, he explains. If the planet started out too close to the star, it could have lost all its water to a greenhouse effect, like Venus did. Moving Venus to the position of Earth won’t give it its water back.

On the other end, a planet that was born farther from its star could be entirely covered in glaciers, which reflect sunlight. They may never melt, even when their stars brighten. Worse, their water could go straight from frozen to evaporated, a process known as sublimation. That scenario would leave the planet no time with even a cozy wet puddle for life to get started in.

These planets are “still in the habitable zone,” Tuchow says. “But it adds questions about whether or not being in the habitable zone actually means habitable.” More