Lights in the sky from Elon Musk’s new satellite network have stargazers worried



The panel of 60 Starlink satellites just before they were released to go into orbit around Earth.
Official SpaceX Photos

Michael J. I. Brown, Monash University

UFOs over Cairns. Lights over Leiden. Glints above Seattle. What’s going on?

Starlink satellites travel silently across the skies of Leiden.

The launch of 60 Starlink satellites by Elon Musk’s SpaceX has grabbed the attention of people around the globe. The satellites are part of a fleet that is intended to provide fast internet across the world.

Improved internet services sound great, and Musk is reported to be planning for up to 12,000 satellites in low Earth orbit. But this fleet of satellites could forever change our view of the heavens.

Will we lose the night sky to city lights and satellites?
Jeff Sullivan, CC BY-NC-ND

Starlink’s ambitious mission

Starlink is an ambitious plan to use satellites in low Earth orbit (about 500km up) to provide global internet services.




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What caused the fireballs that lit up the sky over Australia?


This is different from the approach previously used for most communication satellites, in which larger individual satellites were placed in high geosynchronous orbits – that stay in an apparently fixed position above the Equator (about 36,000km up).

Communications with satellites in geosynchronous orbits often require satellite dishes, which you can see on the sides of residential apartment buildings. Communication with satellites in low Earth orbit, which are much closer, won’t require such bulky equipment.

But the catch with satellites in low Earth orbit, which move quickly around the world, is they can only look down on a small fraction of the globe, so to get global coverage you need many satellites. The Iridium satellite network used this approach in the 1990s, using dozens of satellites to provide global phone and data services.

Starlink is far more ambitious, with 1,600 satellites in the first phase, increasing to 12,000 satellites during the mid-2020s. For comparison, there are roughly 18,000 objects in Earth orbit that are tracked, including about 2,000 functioning satellites.

Lights in the sky

It’s not unusual to see satellites travelling across the twilight sky. Indeed, there’s a certain thrill to seeing the International Space Station pass overhead, and to know there are people living on board that distant light. But Starlink is something else.

The first 60 satellites, launched by SpaceX last week, were seen travelling in procession across the night sky. Some people knew what they were seeing, but the silent procession of light also generated UFO reports. If you’re lucky, you may see them pass across your skies tonight.

If the full constellation of satellites is launched, hundreds of Starlink satellites will be above the horizon at any given time. If they are visible to the unaided eye, as suggested by initial reports, they could outnumber the brightest natural stars visible to the unaided eye.

Astronomers’ fears were not put to rest by Musk’s tweets:

Satellites are very definitely visible at night, particularly in the hours before dawn and after sunset, as they are high enough to be illuminated by the Sun. The Space Station’s artificial lighting is effectively irrelevant to its visibility.

In areas near the poles, including Canada and northern Europe, satellites in low Earth orbit can be illuminated throughout the night during the summer months.

Hundreds of satellites being visible to the unaided eye would be a disaster. They would completely ruin our view of the night sky. They would also contaminate astronomical images, leaving long trails across otherwise unblemished images.

The US$466 million Large Synoptic Survey Telescope, based in Chile, is an 8-metre aperture telescope with a 3,200-megapixel camera. It’s designed to rapidly survey the sky during the 2020s.

With the full constellation of Starlink satellites, many images taken with this telescope will contain a Starlink satellite. Longer exposures could contain dozens of satellite streaks.

Dark skies or darkened hopes?

Is there any cause for optimism? Yes and no.

Musk has produced some amazing feats of technology, such as the SpaceX Falcon and Tesla cars, but he’s also disappointed some on other projects, such as the Hyperloop tunnel transport plan.




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While Starlink certainly blew up on Twitter, for now at least, Musk is 11,940 satellites short of his 12,000.

Also, initial reports may have overestimated the brightness of the Starlink satellites, with the multiple satellites closely clustered together being confused with one satellite.

While some reports have indicate binoculars are needed to see the individual satellites, they also report that Starlink satellites flare, momentarily becoming brighter than any natural star.

If the individual satellites usually are too faint to be seen with the unaided eye, that would at least preserve the natural wonder of the sky. But professional astronomers like myself may need to prepare for streaky skies ahead. I can’t say I’m looking forward to that.The Conversation

Michael J. I. Brown, Associate professor in astronomy, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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What caused the fireballs that lit up the sky over Australia?



One of the fireballs (highlighted by the red circle) captured over the Northern Territory.
NT Emergency Services

Jonti Horner, University of Southern Queensland

Over the past few days a pair of spectacular fireballs have graced Australia’s skies.

The first, in the early hours of Monday, May 20, flashed across the Northern Territory, and was seen from both Tennant Creek and Alice Springs, more than 500km apart.

The second came two days later, streaking over South Australia and Victoria.

Such fireballs are not rare events, and serve as yet another reminder that Earth sits in a celestial shooting gallery. In addition to their spectacle, they hold the key to understanding the Solar system’s formation and history.

Crash, bang, boom!

On any clear night, if you gaze skyward long enough, you will see meteors. These flashes of light are the result of objects impacting on our planet’s atmosphere.




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Specks of debris vaporise harmlessly in the atmosphere, 80-100km above our heads, all the time – about 100 tons of the stuff per day.

The larger the object, the more spectacular the flash. Where your typical meteor is caused by an object the size of a grain of dust (or, for a particularly bright one, a grain of rice), fireballs like those seen this week are caused by much larger bodies – the size of a grapefruit, a melon or even a car.

Such impacts are rarer than their tiny siblings because there are many more small objects in the Solar system than larger bodies.

Moving to still larger objects, you get truly spectacular but rare events like the incredible Chelyabinsk fireball in February 2013.

That was probably the largest impact on Earth for 100 years, and caused plenty of damage and injuries. It was the result of the explosion of an object 10,000 tonnes in mass, around 20 metres in diameter.

On longer timescales, the largest impacts are truly enormous. Some 66 million years ago, a comet or asteroid around 10km in diameter ploughed into what is now the Yucatan Peninsula, Mexico. The result? A crater some 200km across, and a mass extinction that included the dinosaurs.

Even that is not the largest impact Earth has experienced. Back in our planet’s youth, it was victim to a truly cataclysmic event, when it collided with an object the size of Mars.

When the dust and debris cleared, our once solitary planet was accompanied by the Moon.

The story behind the formation of the Moon.

Impacts that could threaten life on Earth are, thankfully, very rare. While scientists are actively searching to make sure no extinction-level impacts are coming in the near future, it really isn’t something we should lose too much sleep about.

Smaller impacts, like those seen earlier this week, come far more frequently – indeed, footage of another fireball was reported earlier this month over Illinois in the United States.

In other words, it is not that unusual to have two bright fireballs in the space of a couple of days over a country as vast as Australia.

Pristine relics of planet formation

These bright fireballs can be an incredible boon to our understanding of the formation and evolution of the Solar system. When an object is large enough, it is possible for fragments (or the whole thing) to penetrate the atmosphere intact, delivering a new meteorite to our planet’s surface.

Meteorites are incredibly valuable to scientists. They are celestial time capsules – relatively pristine fragments of asteroids and comets that formed when the Solar system was young.

Most meteorites we find have lain on Earth for long periods of time before their discovery. These are termed “finds” and while still valuable, are often degraded and weathered, chemically altered by our planet’s wet, warm environment.

By contrast, “falls” (meteorites whose fall has been observed and that are recovered within hours or days of the event) are far more precious. When we study their composition, we can be confident we are studying something ancient and pristine, rather than worrying that we’re seeing the effect of Earth’s influence.

Tracking the fireballs

For this reason, the Australian Desert Fireball Network has set up an enormous network of cameras across our vast continent. These cameras are designed to scour the skies, all night, every night, watching for fireballs like those seen earlier this week.

If we can observe such a fireball from multiple directions, we can triangulate its path, calculate its motion through the atmosphere, and work out whether it is likely to have dropped a meteorite. Using that data, we can even work out where to look.

A successful meteorite search by the Australian Desert Fireball Network.

In addition to these cameras, the project can make use of any data provided by people who saw the event. For that reason, the Fireballs team developed a free app, Fireballs in the Sky.




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It contains great information about fireballs and meteor showers, and has links to experiments tied into the national curriculum. More importantly, it also allows its users to submit their own fireball reports.

As for this week’s fireball over southern Australia, NASA says it was probably caused by an object the size of a small car. As for finding any remains, they are now likely lost in the waters of the Great Australian Bight.The Conversation

NASA’s record on the location marked in the Great Australian Bight of one of the fireballs over Australia this week.
NASA

Jonti Horner, Professor (Astrophysics), University of Southern Queensland

This article is republished from The Conversation under a Creative Commons license. Read the original article.

A ‘seiche’ wave can outpace a tsunami, and both can be triggered by meteorites and earthquakes



File 20190403 177184 r6mkdz.jpg?ixlib=rb 1.1
Waves can be generated in lakes and other bodies of water when seismic energy travels through land.
Leo Roomets / Unsplash, CC BY

Craig O’Neill, Macquarie University

A catastrophic event occurred on Earth 66 million years ago. A huge meteorite struck our planet in what is now Mexico, triggering mass extinctions of the dinosaurs and most other living creatures.

A new paper shows the first recorded victims of this impact were fish and other marine animals, stranded by a wave that left them high and dry in an ancient river in North Dakota, at a site called Tanis.

For scientists unpacking the evidence around the event, a full picture of the cataclysm has involved looking into the details of planetary surface physics during giant impacts.

But beyond the first layer of fascinating results – little glass impact beads stuck in the gills of fish, for example – one really interesting aspect of this work is around how water behaves when it’s exposed to extreme forces.

If you’ve never heard of a form of wave called a seiche, this is your chance to catch up.

This is a seiche – a standing wave – in a swimming pool, during a large earthquake in Nepal.

Waves of damage

The Chicxulub meteorite crater in coastal Mexico is strongly associated with the mass extinction of the dinosaurs (and 75% of all species), 66 million years ago.

The first victims were right at the site. Any marine creatures close to the point of impact would have been instantly vaporised (sadly leaving no fossil record), along with much of the surrounding rock.

Around the periphery, the energy of the impact melted and ejected tonnes of molten rock, which together with condensing rock vapour, formed little glass beads (“impact spherules”) that can be found in a layer around the world at this time.

The shock wave itself pulverised the adjacent rock enough to metamorphise it, forming features like “shocked quartz” – fractured quartz indicative of enormous pressures. It carried the energy equivalent of a magnitude 11 earthquake – 1,000 times more energy than the 2004 Boxing Day quake which killed almost 230,000 people.

Vast inland sea now gone

North Dakota is more than 3,000km away from the Chicxulub crater, and was a similar distance at the time of the meteorite impact event.

Separating them back then, however, was a vast inland sea that covered much of midwest USA, from Texas up to the Dakotas. Feeding into that inland sea was a river system upon which the Tanis site in North Dakota was formed. This site has preserved the earliest recorded deaths of the Chicxulub impact.

Different views of the Tanis site. A: Tanis (starred) within a regional context (large map) and on a national map (inset). B: Photo and interpretive overlay of an oblique cross-section through Tanis. C: Simplified schematic depicting the general deposits at the site (not to scale). Most fish carcasses were found at point 3.
Robert A DePalma and colleagues

The site itself is unusual. The deposition of sediments can tell us about the flow of water in the river.

Most ripples (or flame structures) indicate a southerly flow of the river before and after the Tanis deposit. However, these flow indicators point the wrong way during the time the Tanis unit formed. Water was flowing upstream, fast.

At the site are also found the fossilised remains of species, like sharks and rays, that occupied brackish water, rather than the freshwater of the stream. These had to be brought inland from the sea by something, and left to die, smothered in sediment, on a riverbank.

Stranded in Dakota

The obvious candidate is an impact tsunami. Perhaps the impact of the meteorite hitting the ocean generated a huge wave that carried fish from the inland sea, and against the flow of fresh water, to leave the creatures stranded in Dakota?

But there are problems with this hypothesis. The tiny impact spherules that formed in Chicxulub can be found throughout the deposit (many clogging the gills of fish), and pockmarks in the sedimentary layers means rocks were still raining down. This means the surge of water occurred within around 15 minutes to two hours of the impact itself.

For a tsunami to travel the 3,000km from the point of impact, to the Tanis site across the inland sea, would have taken almost 18 hours. Something else killed these creatures.

The seismic waves from the impact would have travelled through the Earth much faster than a tsunami travelled across water – and arrived near Tanis between 6-13 minutes later. The authors of the Tanis study suggest these seismic waves may have triggered an unusual type of wave in the inland sea, called a seiche.

Standing waves

Seiches are standing waves in bodies of water, and are often found in large lake systems during strong winds. The winds themselves cause waves and water displacement, which can have a harmonic effect, causing the water to slosh side to side like an overfull bathtub.

However, earthquakes are also known to cause seiches. Particularly dramatic seiches are often seen in swimming pools during large quakes. The interaction of the seismic wave’s period (the time between two waves) with the timescale of waves sloshing in a pool can amplify their effect.

But seiches can affect larger bodies of water too.

During the 2011 Tohuku earthquake in Japan, seiches over 1m high were observed in Norwegian fjords more than 8,000km away. With an energy more than 1,000 times greater, the Chicxulub event could quite conceivably have generated bigger than 10 metre swells in the North American inland sea – the scale implied by the deposition of the Tanis site.

These waves in Norwegian fjords were created by seismic waves from the 2011 Tohoku earthquake in Japan.

Given a seiche can be driven by seismic waves, it’s conceivable that one drove the surge that stranded marine creatures at Tanis, resulting in the short time between the impact debris and the surge deposit.

Still lots of questions

But a lot remains unclear regarding exactly what did happen 66 million years ago.

Could the fish stranding have been driven by the first seismic activity to appear at Tanis (the P and S waves in science parlance, which travel through the interior of the Earth, arriving at Tanis 6 and 10 minutes after impact, respectively), or the more destructive but slower surface waves at the top of the Earth’s crust, which arrived 13 minutes after impact?

How might seiche waves have interacted with global hurricane-strength wind storms caused by the impact?

Would the period of sloshing of a seiche be consistent with the scale of the inland sea? (The inland sea was much larger than most lakes seiches are traditionally observed in – and may or may not have been open to the ocean). Given so little is really known about the dimensions of the inland sea, this is hard to constrain.

The Tanis site has given us an incredible window into the first few hours of a mass-extinction. But it has also highlighted how little we have probed into the fatal surface physics of these extreme events.The Conversation

Craig O’Neill, Director of the Macquarie Planetary Research Centre/Associate Professor in Geodynamics, Macquarie University

This article is republished from The Conversation under a Creative Commons license. Read the original article.