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How We Found Jupiter’s 79 (At Least) Moons

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a tiny moon in front of jupiter

The moon Io is tiny compared to mighty Jupiter, but still among the easiest of Jupiter’s many moons to spot. (Credit: Cassini Imaging Team/SSI/JPL/ESA/NASA)

Jupiter is king of the planets. It’s huge, it’s bright in our night skies, and even four of its comparatively tiny moons are bright enough to see with the most basic of telescopes. We’ve sent nine probes either into orbit or on a close flyby of the planet. And yet, as recently as this past year, we discovered not one, but twelve new moons around Jupiter, bringing the total to 79. How haven’t we exhausted this particular moon mine yet?

The Easy Targets First

The answer is that most of Jupiter’s moons aren’t the grand companion that our own moon is to Earth, at nearly a quarter as wide as its host planet. The four moons first spotted by Galileo in 1610 — Io, Europa, Ganymede and Callisto — are big enough compared to our moon, but absolutely puny when compared to Jupiter, the planet they circle. And those are the easy targets. It makes discovering new moons against its bulk difficult.

It took the advent of photography before astronomers discovered any more moons around Jupiter, and the work over the next century or so was painstaking. By the time Voyager cruised by in 1979, the giant was up to 13 moons. Voyager added three to the count: Metis, Adrastea, and Thebe.

All three of these plus Amalthea (discovered in 1892 by famed astronomer E.E. Barnard) and the original Galilean moons comprise Jupiter’s regular moon group. This means they’re more or less spherical, orbit in the same direction that Jupiter spins and do so on well-behaved, near-circular orbits that don’t tip much out of the plane of Jupiter’s equator. In other words, what you probably imagine a moon to be.

The rest are the irregular moons, and these make up the vast majority of Jupiter’s satellites. These tend more toward potato shapes, and their orbits are often eccentric, tilted, or even retrograde, meaning they fly backwards to Jupiter’s spin. Most are probably captured asteroids or the results of long-ago collisions of larger bodies — perhaps past moons of Jupiter. They’re tiny and tend to orbit farther out from Jupiter than the regular moons. This makes them much harder to spot.

Astronomers found a few of these irregular moons. But after Voyager, the discoveries stopped for about two decades.

A Population Explosion

diagram showing prograde moons in blue, retrograde moons in red

One of Jupiter’s newest moons orbits prograde (normally), but since it’s among the retrograde (backwards) moon group, it’s probably marked for a deadly collision before too long. (Credit: Roberto Molar-Candanosa/Carnegie Institution for Science)

And then Scott Sheppard appeared on the scene. The Carnegie Institution for Science astronomer’s teams are responsible for 60 of the 79 known jovian moons — all irregular, but still an impressive feat. Sheppard’s team has been discovering moons around Jupiter since 2000. Just this past year, they added a round dozen to the list. The new moons add to our understanding of Jupiter’s neighborhood and help astronomers understand how the planet formed and its surroundings evolved over time.

It is true, though, that some of Jupiter’s moons have been “discovered” more than once. These glorified space boulders are sometimes spotted in images but their orbits are poorly understood. So when astronomers look for them again in a few months or years, sometimes they turn up missing and have to be found again.

These irregular moons are quite tiny — only a few miles to tens of miles across. They bear little resemblance to the complex worlds of Europa and Ganymede, or even our own moon. Instead, they are mostly misshapen hunks of rock, orbiting far out from Jupiter’s bulk. So the telescopes that find them have to be sensitive, and either look at a large swath of space or get very, very lucky.

Spotting Tiny Specks

The probes we sent to Jupiter, while far closer than Earth-bound telescopes, are mostly busy looking at the planet. They, too, would have to get quite lucky to catch one of these tiny irregular moons by accident while trying to image the planet. And frankly? The possibility of finding one more tiny space rock doesn’t tempt scientists who want to understand the deep mysteries of Jupiter’s storms or interior. They’re not wasting precious mission time looking very hard.

Sheppard’s most recent successes actually came while he was looking much farther out, trying to find a possible Planet Nine far past the giant planet’s orbit. But since Jupiter happened to be in the same region of the sky, Sheppard and his team checked to see if they could find any photo-bombing moons in their images. They got lucky, though hard work went into the find as well.

Astronomers have gotten much better at wide-field surveys where they scan large chunks of the sky at once. Our telescopes, of course, have also gotten better. But mostly, you have to be willing to spend a long time looking for very dim objects to discover any new satellites around Jupiter.

Most individual irregular moons aren’t considered groundbreaking discoveries on their own (though a few weird exceptions exist). But taken as a whole? Jupiter’s rowdy brood of moons, regular and irregular, tell a long and interesting story about what life is like around the solar system’s largest planet.

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Ecology

Today’s letters: ‘Visionary’ plans don’t always work in Ottawa

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The opinion piece written by Tobi Nussbaum, CEO of the NCC, declares that a “bold, visionary transit plan” would showcase the capital.

As a long-term resident of Ottawa, I’ve had it with visionary plans. In the 1950s, the streetcars serving Ottawa so well were sent to the scrapyards. In the early ’60s, Queensway construction bulldozed established neighbourhoods and ripped the city apart. Later in the decade, the downtown railway station, which could have formed the hub of a commuter network, was relocated to the suburbs. These actions, in the name of “progress,” were undertaken with the “vision” to make Ottawa a car-reliant city.

Now we have an LRT, built just in time for most people to realize that they do not have to go downtown as they can work from home.

Current thinking is pushing a new “link” between Ottawa and Gatineau, with yet more expensive and disruptive infrastructure projects being touted, including a tramway or another tunnel under the downtown core.

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Ecology

That was then: Biggest earthquake since 1653 rocked Ottawa in 1925

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A regular weekly look-back at some offbeat or interesting stories that have appeared in the Ottawa Citizen over its 175-year history. Today: The big one hits.

The Ottawa Senators were playing a Saturday night game against the Montreal Canadiens at the Auditorium, the score tied 0-0 halfway through the second period. Sens’ rookie Ed Gorman and the Habs’ Billy Boucher had just served penalties for a dustup when the building began to make “ominous creaking sounds.” A window crashed to the ground.

Nearby, at Lisgar Collegiate, all eyes were on teenager Roxie Carrier, in the role of Donna Cyrilla in the musical comedy El Bandido. She had the stage to herself and was singing “Sometime” when the building rocked, the spotlight went out, and someone in the audience yelled “Fire!”

At a home on Carey Avenue, one woman’s normally relaxed cat suddenly arched its back, rushed around the room two or three times, spitting angrily, and climbed up the front-window curtains.

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Ecology

Ottawa delays small nuclear reactor plan as critics decry push for new reactors

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TORONTO — Canadians will have to wait a little while longer to see the federal government’s plan for the development of small nuclear reactors, seen by proponents as critical to the country’s fight against global warming.

Speaking at the opening of a two-day virtual international conference on Wednesday, the parliamentary secretary to the minister of natural resources said the plan will lay out key actions regarding the reactors. Its launch, Paul Lefebvre said, would come in the next few weeks.

“We’re still putting the finishing touches on it,” Lefebvre said. “The action plan is too important to be rushed.”

Small modular reactors — SMRs — are smaller in size and energy output than traditional nuclear power units, and more flexible in their deployment. While conventional reactors produce around 800 megawatts of power, SMRs can deliver up to 300 megawatts.

Proponents consider them ideal as both part of the regular electricity grid as well as for use in remote locations, including industrial sites and isolated northern communities. They could also play a role in the production of hydrogen and local heating.

“SMRs will allow us to take a bold step of meeting our goal of net-zero (emissions) by 2050 while creating good, middle class jobs and strengthening our competitive advantage,” said Lefebvre.

Natural Resources Minister Seamus O’Regan had been scheduled to speak at the conference but did not due to a family emergency.

Industry critics were quick to pounce on the government’s expected SMR announcement. They called on Ottawa to halt its plans to fund the experimental technology.

While nuclear power generation produces no greenhouse gas emissions, a major problem facing the industry is its growing mound of radioactive waste. This week, the government embarked on a round of consultations about what do with the dangerous material.

Dozens of groups, including the NDP, Bloc Quebecois, Green Party and some Indigenous organizations, oppose the plan for developing small modular reactors. They want the government to fight climate change by investing more in renewable energy and energy efficiency.

“We have options that are cheaper and safer and will be available quicker,” Richard Cannings, the NDP natural resources critic, said in a statement.

Lefebvre, however, said the global market for SMRs is expected to be worth between $150 billion and $300 billion a year by 2040. As one of the world’s largest producers of uranium, Canada has to be part of the wave both for economic and environmental reasons, he said.

“There’s a growing demand for smaller, simpler and affordable nuclear technology energy,” Lefebvre said.

Joe McBrearty, head of Canadian Nuclear Laboratories, told the conference the company had signed a host agreement this week with Ottawa-based Global First Power for a demonstration SMR at its Chalk River campus in eastern Ontario. A demonstration reactor will allow for the assessment of the technology’s overall viability, he said.

“When talking about deploying a new technology like an SMR, building a demonstration unit is vital to the success of that process,” McBrearty said. “Most importantly, it allows the public to see the reactor, to kick the tires so to speak, and to have confidence in the safety of its operation.”

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