Astrophysicist

Category: science

The FCC issues first fine to a satellite operator

The US Federal Communications Commission (FCC) issued a press release on October 3rd 2023 describing the first time they have fined a satellite operator for failing to properly de-orbit one of their old satellites.  The fine might be small, but this is the first time they’ve used their regulatory powers to issue such a fine and so is a significant moment for the space sector.

We’re used to thinking of space as a big place, with distances dwarfing those we are used to on the Earth.  But Earth orbit is a finite resource.  Just as you can only fit so many cars on a motorway such as the M25, you can only put so many satellites into orbit before you risk collisions.

Since Sputnik 1 in 1957, humans have launched more than 15,000 objects into orbit.  Today, less than half that number are still operational.  Some defunct satellites have returned to Earth, many have burned up in the atmosphere, but a significant number are still in orbit many years, sometimes many decades, after they ceased operations.  

The debris problem

Active satellites are one thing, but decommissioned (or malfunctioning) satellites are quite another.  Once out of fuel, or uncontactable by ground stations, defunct satellites become nothing more than heavy, fast-moving, uncontrollable, dangerous projectiles.

Accidental collisions have happened, several times in fact, involving operational satellites, non-operational satellites, or mission-related debris (including parts of spent launch vehicles).  Debris has also been created accidentally through satellite break-up, and deliberately via anti-satellite weapons tests by various states, most recently in 2021 where the resulting debris threatened the safety of the International Space Station and the astronauts on board.

How much space junk is up there?  NASA’s Orbital Debris Program Office monitors and tracks space debris and estimates that there are approximately 500,000 marble-size fragments, and over 100,000,000 objects of 1mm or less in Earth orbit.  Even tiny fragments of material can do significant damage when moving at typical orbital speeds of several kilometres per second.  More debris makes space more hazardous for all operators.

Decommissioning satellites

With so much orbital debris, the question of how to deal with decommissioned satellites becomes vital for the sustainable use of low-Earth orbit (LEO) and the long-term viability of space-based activities.  Guidelines exist in many parts of the world (for example the European Space Agency has an office with a remit for space safety and requirements for their own satellites), but global standards (and enforcement of those standards) is not easy.

Satellites in LEO can be disposed of by giving them a nudge that sends them towards the atmosphere.  The drag provided by even the rarefied environment at the top of the atmosphere is enough to cause a satellite re-entry. 

Most satellites will burn up during their fiery descent, resulting in no debris hitting the ground, but creates a cloud of chemicals from disintegrating metal, electronic components, solar panels and batteries that slowly disperses in the air.  With many more satellites ending their operational lives this way, the long-term viability of dumping all this material in the atmosphere also needs to be considered.

Larger satellites do result in some debris making it through the atmosphere. Back in 1979 the Skylab space station broke up in the atmosphere, scattering debris across southern Western Australia centred on the small community of Balladonia. The local council issued NASA with a fine for littering. More recently, the launches of more than one segment of the Chinese space station in 2021 and 2022, resulted in 18-tonne rocket bodies re-entering the atmosphere in an uncontrolled manner, scattering debris over large areas.

Satellites in geostationary orbits are much higher above the surface of the Earth.  Bringing them down in the same way as satellites in LEO would require significant fuel for the necessary change in velocity (known as delta-v), as well as presenting the hazard of crossing the orbits of satellites operating at lower altitudes.  More often, defunct satellites in geostationary orbits are disposed of by sending them further out, requiring a much smaller delta-v and less fuel, into what is known as a disposal orbit, some 300km further up.

What happened with EchoStar-7

The subject of the FCC fine, Echostar-7 was a geostationary satellite operated by Dish Network, used to provide television services to the United States.  It had a mass of just under 2000 kg, excluding propellant, and spent its operational lifetime in geostationary orbit, some 36,000km above the surface of the Earth.  Launched in 2002, it was originally planned to operate for 12 years but was given a license extension in 2012 taking its operations through to May 2022 when it was expected to transfer to a disposal orbit.

However, in February 2022 the satellite operator, Dish Network, determined that the satellite has less propellant remaining than it should have.  Without enough propellant remaining on board, reaching the required altitude for the safe disposal orbit was no longer possible.  In the end, the satellite only reached an altitude 122km above the geostationary position, far short of the intended orbit required in their orbital debris mitigation plan.

So what?

Why does this matter?  With so many satellites in space, and the number increasing rapidly thanks to the advent of large constellations of satellites being launched by companies such as SpaceX, OneWeb, Kuiper (due to launch their first satellite this week) and others, the risks of a catastrophic collision increase all the time.  The risks are greater with defunct or uncontrollable satellites as they are unable to be moved to avoid a collision.  Failing to reach the 300km mark means that the satellite could become a significant hazard for operational satellites in geostationary orbit.

Satellites orbit at high velocity, travelling several kilometres per second.  When they collide the resulting debris also travels incredibly fast in many directions.  Fast-moving debris has a lot of energy and any impact with another satellite, functional or not, will result in yet more fast-moving debris.  Because of the significant risks this poses, companies launching satellites must take steps to ensure the safe and responsible disposal of them when they reach the end of their operational lifetime.

While many countries and space programmes have their own voluntary guidelines and codes of conduct for satellite operators, this is the first case of such an operator being fined by a regulator for breaching licence conditions relating to disposal, a significant moment for the rapidly developing commercial space sector.

The risks are significant.  Debris can damage and destroy other satellites, taking out vital communications infrastructure.  Astronauts on board the International Space Station regularly have to perform manoeuvres to avoid either debris or active satellites that come within safely limits.  Satellite operators themselves are increasingly having to undertake collision avoidance manoeuvres because of other functional and non-functional satellites.  

The modern world relies heavily on satellite communications.  If enough debris accumulates in Earth orbit it will become difficult, if not impossible, to operate communications networks on which so much of modern life has come to rely, but also to send spacecraft (and potentially humans) out into the solar system. 

The fine is small in this case, just $150,000, but it is a clear signal that the FCC is willing to use its regulatory powers to fine operators who do not take seriously their responsibilities to the sustainable use of Earth orbit.  

It is worth remembering that the FCC only has jurisdiction over US-based operators.  To be effective as a deterrent and drive more responsible behaviour globally we need larger fines, and international cooperation between agencies and regulators so that our near-space environment isn’t trashed the same way we have destroyed so many habitats here on the Earth.  

The consequences of not acting would be disastrous for communications, banking, Earth observation, disaster relief, and many other sectors that rely on satellites and their applications.  Now is the time to act.

Exploding stars

If you’ve looked at any astronomy-related news over the last few days, you’ll have spotted that a supernova has exploded in a nearby galaxy, and astronomers are getting excited about it. Supernovae are not uncommon (we discovered more than 2000 of them just last year), but some get astronomers more excited than others. Why is this one interesting? Read on.

We think of stars like the Sun as lasting forever. On human timescales, that’s ok since most of them have lifetimes of millions to billions of years – our own Sun has been shining for some five billion years, and probably has enough fuel to last another five billion or so. But they are transient things, like everything else in the Universe, just on timescales it is difficult for us mere mortals to grasp. Even astronomers struggle!

Stellar lifecycles

Stars are giant fusion reactors, fusing hydrogen in the core to make helium, and releasing energy in the process. It’s this energy release that powers the heat and light we get from the Sun, and what makes all the other stars shine. Stars continue this fusion process for as long as the conditions are right in the core. You need a lot of hydrogen under very high pressure and at very high temperatures. Eventually, the hydrogen runs out and a star starts to die.

The way a star ends its life depends very much on one parameter: its mass. The amount of stuff that makes up a star. It’s a simple parameter, but it determines an awful lot about a star’s properties, lifespan, colour, luminosity, end point, and much else. Most stars are mostly composed of hydrogen (~75%) and helium (~25%) with small amounts of other elements. Stars that have formed recently (in cosmic terms) have a greater proportion of these “other” elements, for reasons I’ll come to, but they are still mostly hydrogen and helium.

When a star runs out of hydrogen in the core, the fusion process slows down, reducing the amount of energy released per second. That energy, in the form of photons, keeps the star from collapsing under gravity for most of the star’s life. When the number of photons decreases, the star can no longer counteract the relentless pull of gravity, and it starts to contract. In science terminology, the star is no longer in hydrostatic equilibrium.

As the star begins to contract under gravity, the pressure in the core begins to rise, the temperature also goes up, and we get to the point where conditions in the core are extreme enough that the fusion of helium can begin. This process also releases energy (although not at the same rate) and stops the collapse for a time. There isn’t so much helium though, so the helium-fusing period of a star’s lifespan is relatively short. The shell of mainly hydrogen around this hotter core is now also hotter, and can start undergoing hydrogen fusion too. Again, it doesn’t last anywhere near as long as the original hydrogen burning phase – a phase we call the main sequence lifetime, and which makes up the longest part of any star’s lifetime.

For stars like the Sun, that’s almost it. The helium also runs out, the core starts to collapse again. As the outer layers expand and cool, the star becomes a red giant. The core continues to shrink, eventually becoming a hot, dense object known as a white dwarf. The outer layers eventually drift off to form a (poorly-named) planetary nebula, the white dwarf core slowly cools and fades away over billions of years.

But for stars greater than a certain mass (somewhere around eight times the mass of our own Sun) the end is much more dramatic. These stars continue the fusion process, with the core developing like the layers of an onion, with each progressive layer fusing heavier and heavier elements as they are produced by the layers above them, all the way across the periodic table, up to iron. Iron is interesting, as its the first element in the periodic table where you cannot release energy through fusing it to create a heavier element. To fuse iron, you have to add energy. There’s nowhere inside a star for that injection of energy to come from, so the fusion process stops.

If there’s no fusion in the core, there is nothing stopping the layers above collapsing under gravity. And that’s what happens. Catastrophically.

The outer layers start to fall as the source of photon pressure to keep them up diminishes, until the iron core collapses under its own weight*, resulting in energetic shockwaves that literally rip the star apart. This is what we call a core collapse supernova. The physics of this are fascinating, and the theoretical modelling has come a long way in recent years as computers have become more powerful. (Other types of supernova happen too, astronomers recently detected radio observations from a thermonuclear supernova for the first time, but that’s for another day.)

Now, supernovae are not rare events. As I mentioned at the top, we discovered over 2000 supernovae just in 2022, with many more than that identified as candidate supernovae. Many of these are discovered by automated surveys that scan the sky each night looking for new objects, or astronomical objects changing in brightness. But some are discovered by dedicated amateurs with good equipment and an excellent knowledge of the sky.

SN2023ixf

This supernova, SN2023ixf, was discovered by one well-known supernova hunter, Koichi Itagaki from Japan. He’s discovered quite a few over the years! It is located in one of the spiral arms of the galaxy M101, also known as the Pinwheel galaxy due to its appearance, and it is pretty gorgeous:

M101 or the Pinwheel Galaxy, as seen by Hubble. Credit: ESA/NASA/Hubble.

The galaxy is some 21 million light years away, so the light reaching us now (including the light from this explosion) left the galaxy 21 million years ago and has only just reached us. Looking out into space is looking back in time, we only ever see the Universe as it was in the past.

So, if we discover so many of them each year, why are astronomers excited about this one in particular? Well, for one simple reason: its proximity. While we discover a lot of supernovae, most galaxies are a lot further away than M101, so most supernovae are also much further away than this. The closer a supernova is, the more easily we can study them, and the more detail we can achieve with our observations. A nearby event means that our telescope have better resolution (our images have more detail in them) and we collect more photons (an event of the same brightness twice as far away will appear four times fainter).

This galaxy has also been host to several supernovae over the years. Since we’ve been watching, we’ve spotted at least five in M101, the most recent being SN2011fe discovered in (no surprise!) 2011.

What we know so far

SN2023ixf was first discovered on May 19th and is being observed by astronomers around the northern hemisphere, so what do we know so far?

It’s a core collapse supernova, having gone through the evolutionary stages described above. Estimates from early modelling suggest that the progenitor star might have been about 15 times more massive than the Sun before it exploded. It has been getting brighter, heading for a peak magnitude of around mag 10 – far too faint to be seen with the naked eye, you’ll need at least a 4-inch/20-cm aperture telescope (and some patience!) to spot it.

There are lots of images over on David Bishop’s excellent Bright Supernova catalog page for this object, and plenty of updates if you do a quick search. It will likely stay bright form some weeks, so (as Catherine Heymans said on 5 Live at 8.50am this morning), if you have a telescope gathering dust in the attic, you might want to drag it out and have a look.

Why are they important?

So what’s all the fuss? It may be close, but it’s just one of many hundreds of thousands – so what?

Supernovae are cosmologically important. That nuclear fusion process I described makes elements up to iron in the periodic table. To get anything heavier than that, you need even more extreme conditions than in the core of a massive star, and that’s where supernovae come in.

In a supernova explosion, the extreme conditions created in the shockwave and subsequent blast result in a rapid burst of nucleosynthesis, creating elements much heavier than iron, including many that are important to the creation of things like the computer you are using to read this!

All of those heavy elements, as well as the stuff up to iron created before the explosion itself, get thrown out into the surrounding gas that make up what astronomers call the interstellar medium. This process is called enrichment, adding heavy elements to a gas that is mostly made of hydrogen and helium.

Stars form from the interstellar medium. Over dense regions in the gas start to clump together, gravity increases and more material gets pulled in. Slowly, over hundreds of thousands to millions of years, the gas accumulates, the temperature goes up, the pressure in the centre increases, until you have a hot ball of gas where nuclear fusion can begin in the core – et voila, a star is born!

The gas around the core then goes on to form planets in many cases. If that gas cloud only contains hydrogen and helium, you only get gas giants forming. If the gas also contains heavy elements in sufficient quantities, produced in previous supernova explosions, then you get rocky planets forming too.

If you’ve ever heard Carl Sagan talk about us being made of star stuff, this is what he means. Much of the chemicals that make life possible, that make up the crust the Earth, that make up you and me, and our computers, were made inside massive stars and released into space in supernova explosions.

Don’t panic?

Our own galaxy, the Milky Way, is also forming stars, some of which are massive enough to go supernovae in this way. We actually haven’t had one in the Milky Way since 1604 so we’re rather overdue for one.

Some stars we know of are big enough to explode this way. Betelgeuse is a red giant that has got astronomers very excited in the last couple of years by behaving in a way that could signal its impending doom. It hasn’t exploded yet, but keep an eye on it.

If a supernova did explode in the Milky Way it would likely appear extremely bright due to its closeness. Astronomers would go bananas.

Supernovae are energetic explosions, generating blasts of neutrinos, as well as strong radiation across the electromagnetic spectrum – radio, infrared, visible, ultraviolet, X-rays, gamma rays. Any planets in the neighbourhood would get a good dose of radiation.

Don’t worry though, there are no stars in the vicinity of the Sun that are big enough to be a threat to life on Earth by going supernova. The supernova candidates are all far enough away to not be a threat. They would be pretty cool to watch though!

Edit: I just spoke to BBC Radio 5 Live about this supernova! Listen back (5.50pm): https://www.bbc.co.uk/sounds/schedules/bbc_radio_five_live

Edit: thanks to Jost Migenda for spotting my mistake!

More asteroid near misses – and one hit!

The early hours of January 27th 2023 saw the closest approach to Earth of asteroid 2023 BU.  The fact that this particular space rock was only discovered on January 21st, just a week earlier, combined with it passing just 3,600 km from the surface of the Earth (0.03x the distance between the Earth and the Moon) got the media rather excited.  It’s trajectory brought it closer to the Earth than orbit of our geostationary satellites, but still well above the 200-300 km of things like the International Space Station located in low Earth orbit.  Given how far apart geostationary satellites are, our communications infrastructure was not in any significant danger (this time).

This particular asteroid was estimated to have a diameter of 4-8 metres and was travelling at a speed of around 9.3 kilometres per second as it passed by.  This might sound big, but it’s tiny by asteroid standards.  If it had hit the atmosphere, it would have most likely burnt up entirely, leaving only tiny fragments reaching the ground, if at all.  For comparison, the rock that disintegrated over Chelyabinsk in 2013 was estimated to be 20 metres in diameter – that one exploded in the atmosphere, showering small chunks of debris over the town.  Assuming a similar density to the Chelyabinsk rock, asteroid 2023 BU likely had a mass of less than 1,000 tonnes.

The asteroid moves rapidly past the Earth at closest approach before moving away again and slowing down.

Animation showing the close approach of asteroid 2023 BU on January 27th 2023. Image credit: ESA.

The thing is with an asteroid passing this close to a much larger object, the encounter will change its future orbital trajectory.  Prior to this encounter, observations show that this asteroid orbited the Sun every 359 days.  Observations made after the encounter allowed experts to model its new orbit, finding that it now orbits the Sun every 425 days.  It won’t be back at the Earth now until December 24th 2029 when it will be some 14 million km at closest approach.  Nothing to worry about.  In fact, they’ve modelled its position all the way to 2139.  The closest it will pass to us in that time is 528 thousand km in January 26th 2066.

The thing is, this happens all the time.  As of today, according to the IAU’s Minor Planet Center, there are 31,207 known near-Earth asteroids, 850 of which are larger than 1 kilometre in size, and 2,328 potentially hazardous asteroids.  And we’re finding new ones all the time.  Just this year (we’re still only in February) we’ve had at least eleven objects pass closer than the Moon, at least five of which were not discovered until after closest approach!  Again, don’t panic, they’re all pretty small and would be highly unlikely to do any damage.

2023 CX1 entering the atmosphere.  By Wokege.

2023 CX1 entering the atmosphere on Feb 13th 2023. By Wokege.

One of these actually impacted the atmosphere.  Asteroid 2023 CX1 was discovered less than seven hours before impact!  Again, don’t panic, it was tiny, about 1 metre in diameter, and burned up as an impressive fireball somewhere over the English Channel / Northern France (above).  You can see reports of sightings on the IMO fireball report catalogue.  This was only the seventh impacting asteroid to be discovered before it actually hit the atmosphere.  It’s still pretty difficult to find these things in advance.

If you want to look at the population characteristics, JPL’s Center for Near Earth Object Studies has some data and charts you can play with – I’ve included a couple below showing the discovery rate of NEOs, colour-coded by survey, and the size distribution.

Bar chart showing the increase in discoveries in recent years.

Discovery rate of NEOs, colour-coded by survey, dated Jan 31st 2023. Credit: CNEOS.

The above plot shows the increase in discovery of near-Earth objects.  The surveys that have discovered the most objects are the Catalina Sky Survey and Pan-STARRS, although many are still discovered by amateur astronomers – including 2023 BU and 2023CX1!  This is one of the science goals of the Vera C. Rubin telescope‘s Legacy Survey of Space and Time (LSST), to make an inventory of the solar system.

There are far more small NEOs known than large ones.

Size distribution of NEOs discovered to date, dated Jan 31st 2023. Credit: CNEOS

This one shows the size distribution of NEOs discovered so far.  As you can see, there are not many in the 1000+ metres category – luckily!  Those are the ones most likely to cause us damage, but they are also the easiest to spot.  The thing with space rocks is that they are rocks.  Rocks are usually pretty dull looking, they are often dark colours and don’t reflect much light.  That is a problem when your trying to find them with an optical telescope – they don’t reflect much light, so are pretty faint and therefore difficult to detect.

If you’re a keen astronomical observer and are looking for a project, here’s the Minor Planet Center’s list of NEOs needing confirmation.  More observations are always welcome, helping to pin down asteroid orbits, and you don’t need sophisticated equipment to contribute.

Keep watching the skies – there will be more of these!  But they will get harder to spot as we launch more and more satellites, and install more and more lights.

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Space Juice

It’s been a busy week or so for news stories about the solar system!  Last Friday was the list time we would get to see Juice, Jupiter Icy Moons Explorer, before it was shipped off to Kourou in French Guiana for launch on an Ariane 5 rocket in April.  Juice is heading off to the Jupiter system to explore the planet, its magnetic fields, and some of its largest moons: Europa, Ganymede and Callisto.  This mission has been in development for years, having been selected in 2012 as the first “large-class” mission in ESA’s Cosmic Vision 2015-2025 programme, and has contributions from both NASA and the Israeli Space Agency.

Jupiter Icy Moons Explorer

As BBC local radio stations amusingly described it last Friday, Space Juice will achieve several firsts.  It will be the first spacecraft to orbit a moon in the outer solar system – we’ve orbited our own Moon, but never the moon of another world.  It will also be the first flyby of the Earth-Moon system (called a lunar-earth gravity assist), which is a flyby of the Moon first and then another flyby of the Earth just 1.5 days later – by doing this manoeuvre, Juice will save a significant amount of propellant.

Flybys are always important for getting to the outer solar system, they can save you a lot of propellant which gives you more mass to use for funky, exciting science instruments!  In this case, although Jupiter is only about 600 million km from Earth, there is no rocket powerful enough to go directly there.  By making flybys of Earth (August 2024), Venus (August 2025), and Earth again (September 2026, January 2029), Juice will travel more like 6.6 billion kilometres!  It’s worth it though, for the extra science payload.

What’s it going to do?  Juice will give us the most detailed view of Jupiter and its icy/water world moons (Ganymede, Callisto and Europa).  Jupiter is the archetype gas giant planet.  We keep finding Jupiter-like planets around other stars, but they are very difficult to study due to their distance – it’s very hard to image them.  Jupiter is much easier to study, and learning more about this solar system giant can help us understand those Jupiter-like exoplanets in more detail.  All three moons thought to have subsurface oceans of liquid water, so are important places to go searching for evidence of life.

Exploring moons

Jupiter and its moons are like a mini solar system.  Jupiter sits in the middle of a dancing melee of smaller rocky objects in (almost) circular orbits around it.  Why will Juice visit these three moons in particular?  Well, we think they all have some liquid water below the surface.  And they are all quite different, so comparing them will be really interesting.

Europa has an obvious icy crust, so the surface features are actively changing.  Imagine watching the ice creak and move slowly in Antarctica – but on a planetary scale!  Juice will make a couple of flybys of Europa to search for biosignatures, to see how much water there might be under the surface, and explore the moon’s geology and activity.  Europa is very close to Jupiter, it’s a very harsh environment so Juice will only make two flybys of this moon.

Ganymede is older and has a less active crust.  It has an older surface which is rocky, rather than icy, and gives us a window on a geological record that spans billions of years of solar system history.  It’s also the only known moon with a magnetic field, implying that it has a molten core like the Earth.  Juice will explore Ganymede’s magnetic field, look for subsurface pockets of water or evidence of a sub-surface ocean, measure its complex core, its interaction with Jupiter, and help us determine the potential for habitability – now, or in the past.

Callisto has the oldest known surface in the solar system.  It appears heavily cratered, an indicator of its age, and is inactive (no volcanos on Callisto!).   Given its age it will help us explore the history of our own solar system.  It may also contain a salty subsurface ocean, something else the sensors on board Juice will be looking for.

After launch (hopefully!) in April, Juice will then set off on its eight-year cruise out to Jupiter.  On the way it will have to brave harsh radiation and temperature environments (+250C at Venus flyby, -230C at Jupiter!), but it has been designed to cope with all this.  For its science operation phase it will be a long way from Earth, so it will need a powerful antenna to send back data, and largely autonomous systems due to the time delay.  Sunlight is 25 times weaker at Jupiter than on Earth, so it also has very large solar panel arrays (an area of 85 square metres!) producing 700-900 Watts (plus batteries for use during eclipses).

Jupiter will be a busy place over the next decade or so.  Juno is already in orbit, mapping Jupiter’s gravity and magnetic fields, and NASA are also sending Europa Clipper which will arrive in 2030and will work in collaboration with ESA’s Juice.  Keep an eye out for results from these missions!

For much more on Juice, its instruments and mission goals, see ESA’s Juice Launch Kit.

Nothing to see here – how satellites are ruining our shared skies

Have you ever camped somewhere quiet? Pitched your tent, carefully lit a small fire, watched the stars appear as the sky goes dark, picked up your phone to ask the internet if that’s Venus you can see – and then realised there’s no signal? With several private companies racing to develop the first operational space-based WiFi network, this could soon become a thing of the past – but so might your view of the stars.

Since the 1950s, humans have been launching satellites. From the early days of the space race the technology has developed dramatically, with satellites today carrying out tasks from communication to earth observation, measuring everything from ice coverage and wildfires, ocean currents and weather, to enabling the tracking of land use changes, predicting droughtsand spotting leaks from gas pipelines.

Arguably, the next logical step is a network of satellites providing global, universal internet access, and many companies are working on developing exactly this. These networks of satellites, known as constellations, will provide relatively cheap, low-latency and accessible internet to anyone on Earth. This will be particularly beneficial for remote locations where the potential benefits of internet access would be significant for human and economic development but cost of installing fibre is prohibitive.

Affordable, universal internet coverage enables fast, reliable communication, transcending socio-political boundaries, bridging the digital divide and helping people everywhere access education and reducing global inequality.

Is there space in space?

There is clearly huge potential, and lots to be gained. But is there enough room in low Earth orbit? According to data maintained by the UN Office for Outer Space Affairs, there are currently 8,221 satellites in Earth orbit, though less than half are operational. In total since the 1950s, humanity has launched more than 12,000 satellites into space. The number of objects launched in a year doubled between 2016 and 2017, and in 2021 alone we launched 1,800 objects. Including debris fragments, there are over 23,500 objects larger than 10 centimetres in size currently orbiting the Earth.  The accidental collision in December 2021 of the Chinese meteorological satellite, YunHai 1-02, with debris from a rocket launched in 1996 really hasn’t helped matters.  Neither did the Russian anti satellite test back in November that deliberately created a cloud of over 1,500 pieces of debris that threatened the International Space Station.

Space may be big, but the available space in Earth orbit is finite. Satellites have to avoid each other, they must communicate with other satellites in their networks and with those on the ground without interfering with each other, and we need to know accurately where they all are to avoid collisions. These are significant challenges; the safe use of Earth orbit requires cooperation.

Each of the companies developing these networks is planning and launching its own constellation, consisting of several hundred (and up to tens of thousands of) individual satellites. If we want to provide cheap, accessible internet to every community, then we will need to safely launch and operate many tens of thousands of satellites. How do we ensure this is done in a sustainable way?

The UN Committee on the Peaceful Uses of Outer Space produced guidelines on the sustainability of outer space activities, covering the avoidance of contamination (both of other planets and of the Earth), the safe re-entry of defunct satellites, sustainable use of the (finite) radio spectrum, the sharing of space weather and forecasting, among other things. However, these are guidelines, not laws, and are voluntary. Going forward, and in order to protect both the space around the Earth, the safety of the people on it and their view of the universe, we need international cooperation and agreement.

Blinded by the light

What about that view of the stars from our campsite? In good conditions we can see a total of around 10,000 stars with the naked eye, spread across both hemispheres. The darker your location, the more stars you can see. Over time our night skies have become brighter and brighter, with every new streetlight or security light making the stars harder to see. As a consequence, astronomical telescopes are confined to remote places, often on the tops of isolated mountain peaks.

However, this dramatic increase in satellite numbers now threatens these facilities and the science they enable. New constellations are launching at a time when several large, multi-country, expensive telescopes are under construction or becoming operational. The problem with the proliferation of satellites is that it becomes impossible to avoid them when trying to observe the universe. At least one of these companies is working with astronomers to try to minimise the effects on observations, but there is no regulatory compulsion to do so.  The big science questions these telescopes were built to address will become impossible to answer if we can no longer see the sky properly.

One telescope at risk is the Vera C. Rubin Observatory, a large optical telescope with a 3.2 gigapixel camera under construction in Chile, which aims to rapidly scan the sky to search for transient astronomical events and asteroids, among other things. This telescope should see first light this year, but will be particularly adversely affected by the extreme photobombing caused by large numbers of satellites passing through its large field of view.

It isn’t just optical telescopes that will suffer, either. For current and future radio telescopes, the proliferation of transmitters in orbit will make trying to observe the universe like trying to hear a violin over the sound of a jet engine. The cosmos will become as invisible to radio telescopes as the stars are to the human eye during the day.

Now, no astronomer is arguing against the human and economic benefits of universal internet (astronomy itself is being used as a basis for development projects around the world), but there is concern that this will ruin the view for everyone, and render much of modern astrophysics impossible in the long term. This ultimately leads to a waste of taxpayers’ money, a loss of training opportunities in the high-level technical and analytical skills that astronomy provides (most astronomy graduates do not become astronomers, but work in IT, business and the civil service, where they put those analytical skills to good use), and a loss of part of our shared cultural heritage.

One solution is obvious: we could just send all our telescopes into space. That’s fine if you have the money to do it, and it does happen (the Hubble Space Telescope and the James Webb Space Telescope being the best-known examples). In practice, this is always a more expensive option, it is a lot more risky, if something goes wrong it is difficult to fix, and you can’t build your telescopes as big for the same amount of money. This may be the long-term future of astronomy, but we are a long way from that being normal. It also does nothing for the view from our campsite.

Don’t look up?

For the moment, these constellations are being launched only with the go-ahead from country-based agencies (mainly in the US), but they impact the whole planet’s view of the universe. The night sky and the space around the Earth can (and, perhaps, should) be thought of as a natural wilderness and protected as such for everyone to enjoy and use, but that requires serious international cooperation and agreement – which, at present, does not exist.

Having internet access can drive economies forward and provide an excellent gateway to education. Is protecting the night sky for the minority of those who want to look at it (including amateur astronomers and children) more important than providing affordable internet in remote areas? Definitely not, but if we want to preserve the night sky for future generations, we need to act now. After all, if we can’t see the sky, we might not find the next incoming comet or asteroid until it’s too late.

[This article was originally published in Green World on July 7th 2020.  The numbers have been updated.]

OMG an ASTEROID the size of the EIFFEL TOWER!!!

You might have spotted the story in the media yesterday about an asteroid called Nereus which makes a close approach to the Earth today.  It’s described as the size of the Eiffel Tower and will zip past us pretty close, coming within 3.9 million kilometres.  Are we all doomed?  Of course not.  Here’s why.

Asteroid Nereus (or to give it its full designation, asteroid 4660 Nereus) is a lump of rock about 330 metres in diameter.  It was discovered in 1982, about a month after it passed within 4.1 million kilometres of the Earth.  It’s not spherical, more egg shaped, according to radar observations.  In late 2001/early 2002, astronomers used a radio telescope at Goldstone to send radio waves at the asteroid during one of its previous close approaches, using the reflected signals to measure its size and shape.  They found that it has dimensions of  510 by 330  by 241 metres, and it rotates on its own axis every 15 hours or so.

There are actually many thousands of lumps of rock this size in the inner solar system.  None of them are known to be on a direct collision course with the Earth, but we haven’t found all of them yet – not by a long way.  The current count of known asteroids is 1,113,527 (according to this NASA page), and this number is increasing all the time as we find more of them.  Some of these space rocks are large and easy to spot from Earth, but most are much smaller and are quite hard to find.

The problem with hunting for asteroids is that they are mostly (a) small, and (b) made of rock.  Small things reflect less sunlight, so they are fainter and need a bigger telescope to spot, and things made of rock tend not to be terribly reflective.  You’ve probably seen snow on mountains – the snow is much brighter than the rocky areas because white things reflect more sunlight than black/brown things.

Nereus is quite reflective for a rocky asteroid, but still tricky to spot because of its small size.  One estimate puts its maximum apparent magnitude (how bright it will appear to an observer on the Earth) at 12.6 which is pretty faint.  If you are lucky enough to live somewhere with dark skies, you can probably see stars an faint as about magnitude 6.  [This is one of those annoying astronomical measurements that doesn’t make intuitive sense – bigger numbers relate to fainter objects, so the Sun has an apparent magnitude of -26.74 while faint distant galaxies can have magnitudes of +20 or more.  The scale is also logarithmic as well, which  means that a difference of one magnitude is actually a factor of 2.5 in brightness.]  At its brightest, at closest approach, Nereus will be magnitude 12.6 which is more than 400 times fainter than the faintest stars you can see unaided.

Now, most of these floating space rocks are never likely to cause us a problem.  They orbit the Sun in the asteroid belt, a region between Mars and Jupiter where there is a concentration of asteroids.  Not all of them are in the asteroid belt though, many have elliptical orbits around the Sun, rather than the almost circular orbit of the Earth, and sometimes those orbits can bring an asteroid close to the Earth.   (The trick is to spot them coming!)

Nereus has an elliptical orbit within the inner solar system, taking 1.8 years to complete each orbit.  This orbit causes it to pass close to the Earth from time to time, but it also comes pretty close to Mars as well.  This actually makes it an excellent candidate for a sample return mission, and it was originally one of the candidates for the Hayabusa mission – but a delay meant that probe ended up visiting asteroid Itokawa instead.

OK, that’s the science background.  Should we worry about it coming so close?

Well, first some perspective.  At its closest on this occasion, Nereus will be 3.9 million kilometres from the Earth – that’s about ten times the distance between the Earth and the Moon.  Just on that count, we don’t really have anything to worry about.  This distance may be small when you compare it to the size of the entire solar system, but it’s still a long way.  Nereus will actually come even closer in the future; in 2060 its orbital path will bring it within 1.2 million kilometres of Earth.  That’s three times the Earth-Moon distance, so still not a threat.  If you are interested, here are the predictions for future encounters (including a close approach to Mars in 2089.

So no, we don’t need to worry.

I’ve talked about other asteroids though, and how the inner solar system contains an awful lot of them.  If we didn’t spot this one until it was already a month past a close approach, how easy might we miss another one that might come closer?  Well this is a risk.

In planetary terms, an asteroid impact could potentially do a lot of damage.  How much damage depends on the size of the impactor, the relative velocity of the asteroid and the Earth, and what the asteroid is made of.  There are lots of impact simulators around, but this one suggests that a collision with a 330-m object on land would result in a crater more than 4 km in diameter, and an earthquake of magnitude 7.8.  Their estimate is that an impact on this scale occurs on Earth (statistically) every 18,000 years.

There are plenty of efforts ongoing to find these asteroids and determine their orbits.  Many telescopes do this sort of work, and it’s something the Vera C. Rubin Observatory will be able to do really well (if the light from all the satellites being sent into low Earth orbit doesn’t cause too much of a problem… but that’s a topic for another day).

If we do find an asteroid that could impact the Earth, we need to have some way of dealing with it.  We can’t rely on Bruce Willis, so just last month NASA launched the DART mission to the binary asteroid system Didymos and its smaller companion Dimorphos.  The idea with DART is to test planetary defence techniques by, well, literally smashing a 500 kg projectile into Dimorphos at ~6.6 km/s (~15,000 mph) and watching how its orbital path changes.

The physics is simple (its just conservation of momentum) and, if all goes well, following the impact in October 2022 we should be able to observe a small change in the orbital period of Dimorphos.  It’s not the first time humans have visited as asteroid, and it’s not the first time we’ve deliberately bashed into one, but it is the first time we’ve actively tried to alter the trajectory of a solar system object.  If we do spot something dangerous headed our way, it makes sense to know how to make it less of a threat!

So, should we worry about Nereus?  No.  Categorically no.  It’s not a threat.  Should we worry about something we haven’t spotted yet hitting us in the future?  Well, it’s unlikely, but we really ought to be expending some resources to look.  It’s in our own interest as a species, after all.

Don’t have nightmares.

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