Patrick M Shober, Curtin UniversityIf asked where meteorites come from, you might reply “from comets”. But according to our new research, which tracked hundreds of fireballs on their journey through the Australian skies, you would be wrong.
In fact, it is very likely that all meteorites — space rocks that make it all the way to Earth — come not from icy comets but from rocky asteroids. Our new study found that even those meteorites with trajectories that look like they arrived from much farther afield are in fact from asteroids that simply got knocked into strange orbits.
We searched through six years’ worth of records from the Desert Fireball Network, which scans the Australian outback for flaming meteors streaking through the sky. None of what we found came from comets.
That means that of the tens of thousands of meteorites in collections around the world, likely none are from comets, leaving a significant gap in our understanding of the Solar System.
When the Solar System formed, more than 4.5 billion years ago, a disc of dust and debris was swirling around the Sun.
Over time, this material clumped together, forming larger and larger bodies — some so large they swept up everything else in their orbit, and became planets.
Yet some debris avoided this fate and is still floating around today. Scientists traditionally classify these objects into two groups: comets and asteroids.
Asteroids are rockier and drier, because they were formed in the inner Solar System. Comets, meanwhile, formed further out, where ices such as frozen water, methane or carbon dioxide can remain stable — giving them a “dirty snowball” composition.
The best way to understand the origin and evolution of our Solar System is to study these objects. Many space missions have been sent to comets and asteroids over the past few decades. But these are expensive, and only two (Hayabusa and Hayabusa2) have successfully brought back samples.
Another way to study this material is to sit and wait for it to come to us. If a piece of debris happens to cross paths with Earth, and is large and robust enough to survive hitting our atmosphere, it will land as a meteorite.
Most of what we know about the Solar System’s history comes from these curious space rocks. However, unlike space mission samples, we don’t know exactly where they originated.
Meteorites have been curiosities for centuries, yet it was not until the early 19th century that they were identified as extraterrestrial. They were speculated to come from lunar volcanoes, or even from other star systems.
Today, we know all meteorites come from small bodies in our Solar System. But the big question that remains is: are they all from asteroids, or do some come from comets?
But might some of them have come not from asteroids, but from comets that originated in the outer reaches of the Solar System? What would such meteorites be like, and how would we find them?
Fortunately, we can actively look for meteorites, rather than hoping to stumble across one lying on the ground. When a space rock is falling through the atmosphere (at this stage, it’s known as a meteor), it begins to heat up and glow — hence why meteors are nicknamed “shooting stars”.
Larger meteors (at least tens of centimetres across) glow brightly enough to be termed “fireballs”. And by training cameras on the sky to spot them, we can track and recover any resulting meteorites.
The network’s data has resulted in the recovery of six meteorites in Australia, and two more internationally. What’s more, by tracking a fireball’s flight through the atmosphere, we can not only project its path forwards to find where it landed, but also backwards to find out what orbit it was on before it got here.
Our research, published in The Planetary Science Journal, scoured every fireball tracked by the DFN between 2014 and 2020, in search of possible cometary meteorites. In total, there were 50 fireballs that came from comet-like orbits not associated with a meteor shower.
Unexpectedly, despite the fact that just under 4% of the larger debris was from comet-like orbits, none of the material featured the hallmark “dirty snowball” chemical composition of true cometary material.
We concluded that debris from comets breaks up and disintegrates before it even gets close to becoming a meteorite. In turn, this means cometary meteorites are not represented among the tens of thousands of objects in the world’s meteorite collections.
The next question is: if all meteorites are asteroidal, how did some of them end up in such weird, comet-like orbits?
For this to be possible, debris from the main asteroid belt must have been knocked from its original orbit by a collision, close gravitational encounter, or some other mechanism.
Meteorites have given us our most profound insights into the formation and evolution of our solar system. However, it is now clear that these samples represent only part of the whole picture. It is definitely an argument for a sample-return mission to a comet. It’s also testament to the knowledge we can gain from tracking fireballs and the meteorites they sometimes leave behind.
But the rising Asian superpower is catching up fast: flying missions to the Moon and Mars; launching heavy-lift rockets; building a new space telescope set to fly in 2024; and, most recently, putting the first piece of the Tiangong space station (the name means Heavenly Palace) into orbit.
What is the Tiangong space station?
Tiangong is the successor to China’s Tiangong-1 and Tiangong-2 space laboratories, launched in 2011 and 2016, respectively. It will be built on a modular design, similar to the International Space Station operated by the United States, Russia, Japan, Canada and the European Space Agency. When complete, Tiangong will consist of a core module attached to two laboratories with a combined weight of nearly 70 tonnes.
The core capsule, named Tianhe (Harmony of Heavens), is about the size of a bus. Containing life support and control systems, this core will be the station’s living quarters. At 22.5 tonnes, the Tianhe capsule is the biggest and heaviest spacecraft China has ever constructed.
The capsule will be central to the space station’s future operations. In 2022, two slightly smaller modules are expected to join Tianhe to extend the space station and make it possible to carry out various scientific and technological experiments. Ultimately, the station will include 14 internal experiment racks and 50 external ports for studies of the space environment.
Tianhe will be just one-fifth the size of the International Space Station, and will host up to three crew members at a time. The first three “taikonauts” (as Chinese astronauts are often known) are expected to take up residence in June.
Tianhe was launched from China’s Hainan island on April 29 aboard a Long March 5B rocket.
These rockets have one core stage and four boosters, each of which is nearly 28 metres tall － the height of a nine-storey building － and more than 3 metres wide. The Long March 5B weighs about 850 tonnes when fully fuelled, and can lift a 25-tonne payload into low Earth orbit.
During the Tianhe launch, the gigantic core stage of the rocket – weighing around 20 tonnes – spun out of control, eventually splashing down more than a week later in the Indian Ocean. The absence of a control system for the return of the rocket to Earth has raised criticism from the international community.
However, these rockets are a key element of China’s short-term ambitions in space. They are planned to be used to deliver modules and crew to Tiangong, as well as launching exploratory probes to the Moon and eventually Mars.
Despite leaving behind an enormous hunk of space junk, Tianhe made it safely to orbit. An hour and 13 minutes after launch, its solar panels started operating and the module powered up.
Completion and future
Tianhe is now sitting in low-Earth orbit (about 400km above the ground), waiting for the first of the ten scheduled supply flights over the next 18 months that it will take to complete the Tiangong station.
A pair of experiment modules named Wentian (Quest for Heavens) and Mengtian (Dreaming of Heavens) are planned for launch in 2022. Although the station is being built by China alone, nine other nations have already signed on to fly experiments aboard Tiangong.
How to see the Tiangong space station
Tianhe is already visible with the naked eye, if you know where and when to look.
To find out when the space station might be visible from where you are, you can check websites such as n2yo.com, which show you the station’s current location and its predicted path for the next 10 days. Note that these predictions are based on models that can change quite quickly, because the space station is slowly falling in its orbit and periodically boosts itself back up to higher altitudes.
The station orbits Earth every 91 minutes. Once you find the time of the station’s next pass over your location (at night – you won’t be able to see it in the daytime), check the direction it will be coming from, find yourself a dark spot away from bright lights, and look out for a tiny, fast-moving spark of light trailing across the heavens.
When the Sun ejects solar particles into space, how does this affect the Earth and climate? Are clouds affected by these particles?
When we consider the Sun’s influence on Earth and our climate, we tend to think about solar radiation. We are acutely aware of the skin-burning dangers of ultraviolet, or UV, radiation.
But the Sun is an active star. It also continuously releases what is known as “solar wind”, made up of charged particles, largely protons and electrons, that travel at speeds of hundreds of kilometres per hour.
Some of these particles that reach Earth are guided into the polar atmosphere by our magnetic field. As a result, we can see the southern lights, aurora australis, in the southern hemisphere, and the northern equivalent, aurora borealis.
This visible manifestation of solar particles entering Earth’s atmosphere is a constant reminder there is more to the Sun than sunlight. But the particles have other effects as well.
The impact of solar particles on atmospheric ozone was first observed in 1969. Since the early 2000s, thanks to new kinds of satellite observations, we have seen growing evidence that solar particles play an important part in influencing polar ozone. During particularly active times, when the Sun releases large amounts of particles into space, up to 60% of ozone at altitudes above 50km can be depleted. The effect can last for weeks.
Lower down in the atmosphere, below 50km, solar particles are important contributors to the year-to-year variability in polar ozone levels, often through indirect pathways. Here, solar particles again contribute to ozone loss, but a recent discovery showed they also help curb some of the depletion in the Antarctic ozone hole.
How ozone affects the climate
Most of the ozone in the atmosphere resides in a thin layer at altitudes of 20-25km — the “ozone layer”.
But ozone is everywhere in the atmosphere, from the Earth’s surface to altitudes above 100km. It is a greenhouse gas and plays a key role in heating and cooling the atmosphere, which makes it critical for climate.
Its depletion above Antarctica had a cooling effect, which in turn pulled the westerly wind jet that circles the continent closer. As the Antarctic hole recovers, this wind belt can meander further north and affect rainfall patterns, sea-surface temperatures and ocean currents. The Southern Annular Mode describes this north-south movement of the wind belt that circles the southern polar region.
Ozone is important for future climate predictions, not only in the thin ozone layer, but throughout the atmosphere. It is crucial we understand the factors that influence ozone variability, be it man-made or natural like the Sun.
The Sun’s direct influence
The link between solar particles and ozone is reasonably well established, but what about any direct effects solar particles may have on the climate?
We have observational evidence that solar activity influences regional climate variability at both poles. Climate models also suggest such polar effects link to larger climate patterns (such as the Northern and Southern Annular Modes) and influence conditions in mid-latitudes.
The details are not yet well understood, but for the first time the influence of solar particles on the climate system will be included in climate simulations used for the upcoming Intergovernmental Panel on Climate Change (IPCC) assessment.
Through solar radiation and particles, the Sun provides a key energy input to our climate system. While these do vary with the Sun’s 11-year cycle of magnetic activity, they can not explain the recent rapid increase in global temperatures due to climate change.
We know rising levels of greenhouse gases in the atmosphere are pushing up Earth’s surface temperature (the physics have been known since the 1800s). We also know human activities have greatly increased greenhouse gases in the atmosphere. Together these two factors explain the observed rise in global temperatures.
What about clouds?
Clouds are much lower in the atmosphere than where most solar particles penetrate. Particles know as galactic cosmic rays (coming from the centre of our galaxy rather than the Sun) may be linked to cloud formation.
It has been suggested cosmic rays could influence the formation of condensation nuclei, which act as “seeds” for clouds. But recent research at the CERN nuclear research facility suggests the effects are insignificant.
This doesn’t rule out some other mechanisms for cosmic rays to affect cloud formation, but thus far there is little supporting evidence.
In other words, mangroves are some of our most precious ecosystems. Despite their importance, there is much we don’t know about these complex wetland forests. For example, when does their growing season start? And, how long does it last?
Usually, answering these types of questions requires frequent data collection in the field, but that can be costly and time-consuming. An alternative is to use satellite images. In the future, this will allow us to track the impacts of climate change on mangroves and other forests.
What is phenology?
Our research used satellite images to study the life cycles of mangrove forests in the Northern Territory, Queensland, and New South Wales. We compared the satellite images with field data collected in the 1980s, 1990s and 2000s, and found a surprising degree of variation in mangrove life cycles.
We’re using the phrase life cycle, but the scientific term is “phenology”. Phenology is the study of periodic events in the life cycles of plants and animals. For example, some plants flower and fruit during the spring and summer, and some lose their leaves in autumn and winter.
Phenology is important because when plants are growing, they absorb carbon from the atmosphere and store it in their leaves, trunks, roots, and in the soil. As phenology is often affected by environmental conditions, studying phenology helps us understand how climate change is affecting Australian ecosystems such as mangrove forests.
So how can we learn a lot in a short amount of time about mangrove phenology? That’s where satellite imagery comes in.
How we use satellites to study mangrove phenology
Satellites are an excellent tool to study changes in forest health, area, and phenology. Some satellites have been taking images of Earth for decades, giving us the chance to look back at the state of mangrove forests from 30 years ago or more.
You can think of satellite images much like the photo gallery in your smartphone: you can see many of your family members in a single image, and you can see how everyone grows and “blooms” over time. In the case of mangroves, we can see different regions and species in a single satellite image, and we can use past images to study the life cycles of mangrove forests.
For example, satellite images depicted below, which use data from the Australian government’s National Maps website, show how mangroves forests have changed in the Kimberley region of Western Australia between 1990 and 2019. You can see how the mangrove forest has reduced in some areas, but expanded in others. Overall, this mangrove forest seems to be doing pretty well thanks in large part to the fact this area has a reasonably small human population.
Our study of satellite images of mangrove forests in the Northern Territory, Queensland, and New South Wales – and how they compared with data collected on the ground – found not all mangroves have the same life cycles.
For instance, many mangrove species grow new leaves only once per year, while other species grow new leaves twice a year. These subtle, but important differences will allow us to track the impacts of climate change on mangroves and other forests.
Science cannot yet tell us exactly how mangrove phenology will be affected by climate change but the results could be catastrophic. If mangroves flower or fruit earlier than expected, pollinators such as bats, bees and birds may starve or move to a different forests. Without pollinators, mangroves may not reproduce and can die.
The next step in our research is to figure out how climate change is affecting the life cycles of mangroves. To do this, we will use satellite images of mangroves across Australia and factor in data on temperature and rainfall.
We think rising temperatures are causing longer periods of leaf growth, a theory we plan to test by studying data from now with satellite images from the 80s and 90s.
But satellite monitoring is not enough on its own and cannot capture the detail you can get on the ground. For example, satellites cannot capture the flowering or fruiting of mangroves because flowers are often too small and fruits are often camouflaged. Also, satellites cannot capture what happens under the canopy.
It is also important to recognise the work of researchers on the ground. Ground data allows us to validate or confirm the information we see in satellite images. When we noted some mangrove forests were growing leaves twice per year, we validated this observation with field data, and confirmed with experts in mangrove ecosystems. Field data is crucial to understand the life cycles of ecosystems worldwide and how forests are responding to changes in the climate.
In addition to the year’s other reliable performers we’ve included one wild card: the Aurigids, in late August. Most years, the Aurigids are a very, very minor shower, but they just might put on a show this year.
So here is our pick of the meteoric highlights for 2021.
For each meteor shower, we give you a finder chart showing the radiant (where the meteors appear to come from in the sky) and where best to look in the sky, the full period of activity and the forecast peak. Most meteor showers typically only yield their best rates for about a day around maximum, so the peak night is definitely the best to observe.
The Zenithal Hourly Rate ZHR is the maximum number of meteors you would expect to see under perfect observing conditions. The actual number you will see will likely be lower.
Most meteor showers can only really be observed from either the northern [N] or southern [S] hemisphere, but a few are visible from both [N/S].
Lyrids [N/S; N favoured]
Active: April 14–30
Maximum: April 22, 1pm UTC = 11pm AEST (Qld) = 7am CST = 3am Hawaii time
The Lyrids are one of the meteor showers with the longest and most storied histories, with recorded observations spanning millenia. In the past, they were one of the year’s most active showers, with a history of producing spectacular meteor storms.
Nowadays, the Lyrids are more sedate, putting on a reliable show without matching the year’s stronger showers. They still throw up occasional surprises such as an outburst in excess of 90 meteors per hour in 1982.
This year’s peak Lyrid rates coincide with the first quarter Moon, which will set around midnight, local time, for most locations. The best time to observe will come in the early hours of the morning, after moonset.
For observers in the northern hemisphere, the Lyrid radiant will already be at a useful altitude by the time the Moon is low in the sky, so some brighter meteors might be visible despite the moonlight in the late evening (after around 10:30pm, local time).
Once the Moon sets the sky will darken and make the shower much easier to observe, yielding markedly higher rates.
For observers in the southern hemisphere, the Lyrid radiant reaches a useful altitude in the early hours of the morning, when the Moon will have set. If you’re a keen meteor observer, it could be worth setting your alarm early to get out and watch the show for a few hours before dawn.
Lyrid meteors are fast and often quite bright so can be rewarding to observe, despite the relatively low rates (one every five or ten minutes, or so). Remember, this shower always has the potential to throw up an unexpected surprise.
Eta Aquariids [S]
Active: April 19–May 28
Maximum: May 6, 3am UTC = 1pm AEST (Qld/NSW/ACT/Vic/Tas) = 11am AWST (WA)
The Eta Aquariids are an autumn treat for southern hemisphere observers. While not one of the big three, they stand clear as the best of the rest of the annual showers, yielding a fine display in the two or three hours before dawn.
The Eta Aquariids are fast meteors and are often bright, with smoky trains. They are fragments of the most famous comet, 1P/Halley, which has been laying down debris around its current orbit of the Sun for tens of thousands of years.
Earth passes through that debris twice a year, with the Eta Aquariids the best of the two meteor showers that result. The other is the Orionids, in October.
Where most meteor showers have a relatively short, sharp peak, the Eta Aquariids remain close to their best for a whole week, centred on the maximum. Good rates (ZHR > 30 per hour) should be visible before sunrise on each morning between May 3–10.
The Moon will be a waning crescent when the Eta Aquariids are at their best. Its glare should not interfere badly with the shower, washing out only the faintest members.
Observers who brave the pre-dawn hours to observe the Eta Aquariids will have the chance to lie beneath a spectacular sky. The Milky Way will be high overhead, with Jupiter, Saturn and the Moon high to the east and bright, fast meteors streaking across the sky from an origin near the eastern horizon.
Active: July 17–August 24
Maximum: August 12, 7pm–10pm UTC = 8pm–11pm BST = August 13, 4am–7am JST
The Perseids are the meteoric highlight of the northern summer and the most observed shower of the year. December’s Geminids offer better rates but the timing of the Perseid peak makes them an ideal holiday treat.
The Perseids are debris shed behind by comet 109P/Swift-Tuttle, which is the largest known object (diameter around 26km) whose orbit currently intersects that of Earth.
Perseid meteors are fast, crashing into Earth at a speed of about 216,000km/h, and often bright. While the shower is active, at low levels, for more than a month, the best rates are typically visible for at the three nights centred on the peak.
For observers at European latitudes, the Perseid radiant rises by mid-evening, so the shower can be easily observed from 10pm local time, and remains high all through the night. The later in the night you look, the higher the radiant will be and the more meteors you’re likely to see.
Aurigids [N favoured]
Active: August 28–September 5
Maximum: Potential Outburst on August 31, peaking between 9:15pm–9:40pm UTC = 10:15pm–10:40pm BST = 11:15pm–11:40pm CEST = September 1, 1:15am–1:40am Gulf Standard Time = September 1, 5:15am–5:40am AWST (WA)
Where the other showers are reliable and relatively predictable, offering good rates every year, the Aurigids are an entirely different beast.
In most years, the shower is barely visible. Even at its peak, rates rarely exceed just a couple of meteors seen per hour. But occasionally the Aurigids bring a surprise with short and unexpected outbursts of 30-50 meteors an hour seen in 1935, 1986, 1994 and 2019.
The parent comet of the Aurigids, C/1911 N1 Kiess, moves on an orbit with a period far longer than the parent of any other shower on our list.
It is thought the orbit takes between 1,800 and 2,000 years to complete, although our knowledge of it is very limited as it was only observed for a short period of time.
In late August every year, Earth passes through debris shed by the comet at a previous passage thousands of years into the past. In most years, the dust we encounter is very sparse.
But occasionally we intersect a denser, narrow stream of debris, material laid down at the comet’s previous passage. That dust has not yet had time to disperse so is more densely packed and hence gives enhanced rates: a meteor outburst.
Several independent research teams studying the past behaviour of the shower have all come to the same conclusion. On August 31, 2021, the Earth will once again intersect that narrow band of debris and an outburst may occur, with predictions it will peak around 21:17 UTC or 21:35 UTC.
Such an outburst would be short-lived. The dense core of the debris stream is so narrow it will take the Earth just ten or 20 minutes to traverse. So you’ll have to be lucky to see it.
The forecast outburst this year is timed such that observers in Eastern Europe and Asia will be the fortunate ones, with the radiant above the horizon. The waning Moon will light the sky when the radiant is above the horizon, washing out the fainter meteors from the shower.
The Aurigids tend to be fast and are often quite bright. Previous outbursts of the shower have featured large numbers of bright meteors. It may just be worth getting up and heading outside at the time of the predicted outburst, just in case the Aurigids give us a show to remember.
Active: December 4–17
Maximum: December 14, 7am UTC = 6pm AEDT (NSW/ACT/Vic/Tas) = 3pm AWST (WA) = 2am EST
The Geminid meteor shower is truly a case of saving the best until last. By far the best of the annual meteor showers, it graces our skies every December, yielding good numbers of spectacular, bright meteors.
The shower is so good it is always worth observing, even in 2021, when the Moon will be almost full.
Over the decades, the Geminids have gradually become stronger and stronger. They took the crown of the year’s best shower from the Perseids in the 1990s, and have continued to improve ever since.
For observers in the northern hemisphere, the Geminids are visible from relatively early in the evening, with their radiant rising shortly after sunset, and remaining above the horizon for all of the hours of darkness.
As the night progresses, the radiant gets very high in the sky and the shower can put on a truly spectacular show.
For those in the southern hemisphere, the situation is not quite as ideal. The further south you live, the later the radiant will rise, and so the later the show will begin.
When the radiant reaches its highest point in the sky (around 2am–3am local time), it sits closer to the horizon the further south you are, so the best meteor rates you observe will be reduced compared to those seen from more northerly locations.
Despite these apparent drawbacks, the Geminids are still by far the best meteor shower of the year for observers in Australia, and are well worth a look, even on the moonlit nights of 2021.
Peak Geminid rates last for around 24 hours, centred on the official peak time, before falling away relatively rapidly thereafter. This means that observers around the globe can enjoy the display.
The best rates come when the radiant is highest in the sky (around 2–3am) but it is well worth looking up at any time after the radiant has risen above the horizon.
So wherever you are on the planet, if skies are clear for the peak of the Geminids, it is well worth going outside and looking up, to revel in the beauty of the greatest of the annual meteor showers.