Dr Megan Argo BEM

Astrophysicist

Category: night sky

Fireworks? In October?

19/10/2023 / / 0 Comments

No, not Bonfire Night. We’re talking celestial firework displays! It’s October, and once again we’re coming up on the time for the annual Orionid meteor shower.

There’s been a trend over recent years of various parts of the media getting a bit hysterical about various astronomical phenomena, and in some cases hyping them up waaaaaay beyond any sensible justification and raising expectations to totally unrealistic levels. So if you’ve arrived here having heard stories about how spectacular the Orionids will be on October 21st, should you believe the hype? TLDR: no, but…

What is a meteor, anyway?

A meteor is actually quite mundane: a small piece of rock, generally smaller than a grain of rice that disintegrates as it flies through our upper atmosphere while travelling at many kilometres per second.

You can see meteors on any clear night of the year, all you have to do is find somewhere dark, look up, and be patient. These are called sporadic meteors and are just the detritus left over from the formation of the solar system, or debris from collisions between rocky bodies in the solar system. In the absence of something like a planet to crash into, they just float gently around in space not doing very much.

Usually you have to wait a while before you see a sporadic meteor, although because they are distributed randomly in space they don’t turn up a regular intervals and you may see a handful close together if you’re lucky.

Meteor showers, on the other hand, can be much more spectacular, and come round predictably at the same time each year. In the case of the Orionids, that time is October 21st – or thereabouts.

Origins of the Orionids

All meteor showers are the result of the Earth passing through regions of space with a higher than average concentration of these dust and rock particles. This happens because stuff gets left behind when comets (and some asteroids) go about their normal business on elliptical orbits around the Sun.

Comets are a bit like giant, dirty snowballs, containing large quantities of both ice, frozen gases, dust and rocks, and other volatile substances. We currently know of more than 3800 comets in the solar system, some of which we have actually visited giving us a much better idea of their chemical composition. Samples collected by the Stardust mission even finding the presence of the amino acid glycine, one of the fundamental building blocks of all life as we know it.

Comets spend most of their lives in the far reaches of the solar system where conditions are very cold indeed. Since comet orbits are elliptical, and centred on the Sun, their orbits also take them into the inner solar system for some of the time.

As a comet approaches the inner solar system it get closer to the Sun and so absorbs more solar radiation, heating the nucleus and causing some of the ice to sublimate – that is it turns directly from a solid ice into a gaseous vapour. As the ice turns to gas, the dust and rock particles embedded in it are released and float away, leaving a trail of debris behind the comet as it travels around the Sun.

 When the path of the Earth happens to cross one of these debris trails, we see an increase in meteors coming through our atmosphere.  This is the origin of a meteor shower, and explains why they are regular with predictable dates of activity.

The Orionid meteor shower is the result of debris left behind by Halley’s comet, one of the most famous comets in our solar system. The comet only returns to our skies once every 76 years (and is not due back until 2061), but the Earth travels through the debris trail each year, giving us the regular Orionid shower in October, and also the less well-known Eta-Aquariid shower in April/May.

Variations

A given meteor shower may not have the same level of activity, year to year. Some years a shower might be fairly unimpressive, with peak rates of only a few per hour. Other years we might have a much larger spike in activity and see rates of several hundred an hour. Rarely we might see rates of more than 1000 per hour – rarely seen meteor storm.

Imagine an aeroplane passing through the sky on a sunny day, leaving a contrail behind it in the sky. If you sit and watch that trail, it slowly expands, becomes less dense, and eventually disappears. Comet debris trails are a little like that (although not as easy to see, being made up of tiny dark particles of rock!).

As that trail ages, it gets less well-confined, the particles move apart slowly. Each time the comet comes round on its orbit, it deposits a fresh trail of denser debris along its orbital path. Add in the fact that the gravitational influence of the likes of Jupiter (and any other planet the comet comes relatively close to) can alter the trajectory of the comet, and you start to get a sense of why the number of meteors we see varies from year to year as the Earth passes though denser or less dense parts of the debris trail.

Very clever folks (such as the IMCCE meteoroids and meteors group) take observational data on the number of meteors observed each year (collected by seasoned observers – and you can help!) and build models of the debris trails for each meteor shower, and use those to make predictions about the number of meteor showers we are likely to see the following year. They are usually pretty accurate, but sometimes we see unexpected spikes in rates that were not possible to predict from past data alone – so it’s always worth a look.

What to expect in 2023

This year we are expecting good observing conditions on the peak night of October 21st, with the Moon at 48% illumination but setting before midnight.  The best time to observe will be after midnight when the Moon sets and the constellation of Orion will have risen – this is the location of the radiant of this shower, the location on the sky the meteors appear to come from, and what gives the shower its name.

Some showers are particularly spectacular with more than one a minute on average.  We’re not expecting that this year for this shower, with predictions of around 20 or so per hour.  However, any shower has the potential to be spectacular, so it’s always worth a look!

The best way to observe is to find somewhere away from street lighting, wrap up warm, and look up!  Obviously, you also need clear skies, but don’t worry if October 21st is cloudy as the streams of debris that cause most meteor showers are wide enough to provide activity over more than one night.  Catching meteors takes patience, but can be worth the effort, and can contribute to citizen science projects.

Clouded out? I expect a lot of us (here in the UK, anyway) will be, thanks to Storm Babet. You might have more luck in other parts of the world. The nice thing about the Orionids is that they are equatorial, meaning you can see them from both the northern and southern hemispheres.

If you do miss them, don’t despair. There are plenty more meteor showers in the calendar!

In December we will see the return of the Geminid meteor shower with a predicted peak of ~150/hour! Much more spectacular! The Moon will also be favourable as it will be very close to the Sun and not up during most of the night, aiding dark sky conditions which help you see the fainter meteors.

So, if you miss out this weekend, make a note in your calendar of December 14th, and make sure you remember to take a look!

More asteroid near misses – and one hit!

15/02/2023 / / 0 Comments

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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Planetary spectacular

24/06/2022 / / 0 Comments

You may have heard Prof Lucie Green talking about the planetary conjunction on BBC Radio 4’s Today programme this morning – there were five planets lined up neatly in the early-morning sky this morning! Don’t worry if you didn’t see it today, or it you tried and had cloudy skies, you can still catch it over the next few days.  Here’s where to look, and what to look for.

Mercury, Venus, Mars, Jupiter and Saturn all appear in a line in the early morning sky.

Planetary alignment of late June 2022 – visible view at 4.15am BST – as seen from NW England.

First thing is, you’ll need to be up early!  The view above shows the sky at 4.15am.  The Sun rises at 4.43am from where I am, so you won’t see much after that as the sky will be too bright to see anything other than the Moon!  If you can drag yourself out of bed at that time, here’s what you will see.

Looking East, with a good horizon (ideally up a hill, but anywhere that you can avoid tress, hills or houses to your East) you should be able to see in order going up from the horizon: Mercury, Venus, the Moon, Mars, Jupiter and Saturn.  That’s quite a view!  The Moon is only 14% illuminated, so will appear as a nice crescent shape.  At magnitude -3.9, Venus will be the brightest of the set, less than 10 degrees above the horizon at 4.15am.  Mercury is the trickiest to spot, but will be between Venus and the glow of the pre-dawn Sun.  At magnitude -0.3, it will be a challenge to spot in the skyglow as by this time it is still only four degrees above the horizon.  If you have binoculars you will find it easier to catch, but be very careful NOT TO LOOK AT THE SUN!  Moving a little round towards the South, Mars is next.  At magnitude +0.5 it will still be easy to spot – you’re looking for something with a reddish/orange colour to it.  Moving up and further South again, you will find the next brightest of the set, Jupiter.  With a magnitude of -2.4, this planet is always hard to miss in the night sky.  If you have binoculars, have a look and see if you can spot the four largest Moons of Jupiter: Io, Europa, Ganymede and Callisto.  Further round, almost due South at this time, you will find the last of the set: Saturn.  At magnitude +0.6, Saturn is a little fainter than Mars, but yellow rather than red in colour.  If you have your binoculars handy, have a close look and see if you can spot the rings.  If you have good optics and a steady hand, you might just see them!

[Aside: If you look carefully, you will also note that Uranus makes an appearance in the lineup.  You are unlikely to spot this without a telescope though, as it has a magnitude of +6.  In good conditions and with good eyesight, you might spot this with the naked eye during darkness, but not in the early hours with the Sun brightening the sky.  Not far from Venus is the Pleiades cluster of stars – now that is worth a look with the binoculars as it’s always an impressive sight.]

“Why are the planets in a line?” I hear you ask.  That’s a good question, and it comes down to perspective.  The planets are actually always in a line, it’s just that it only becomes obvious when you have a close alignment such as this.  The reason for this is because all of the planets orbit the Sun is a very similar plane – you can imagine the solar system sitting on a dinner plate with the Sun at the centre and all the planets moving in (almost) circular orbits around the surface of the plate.  If you imagine yourself as an ant sitting on the dinner plate, you would see the planets sitting on a circle around you.  How does this look to us?  Here’s the same view as above, but now with this plane drawn on:

The planets all lie close to the plane of the ecliptic on the sky.

Planetary alignment of late June 2022 – visible view but with the plane of the ecliptic added.

This plane is actually the projection of the path of the Sun around the sky as seen from Earth.  We’re orbiting the Sun of course, not the other way around, but from our perspective we see the Sun move across the sky relative to the background stars over one calendar year.  The path the Sun takes across the sky is called the ecliptic by astronomers.  We do like our jargon.

The orbits of the planets in the Solar System lie close to the ecliptic line, which is the apparent path of the Sun as seen from Earth projected out into the sky.

Planetary alignment of late June 2022 – visible view but with the ecliptic and orbits of the planets added.

The above view is the same, but now I’ve added the paths of the planets as well.  You can see that, as the planets orbit the Sun, their orbits never take them very far from the ecliptic.  That’s because of that dinner plate effect I talked about earlier.  The planets are all moving about close to the plane of the solar system, and so are we, so they appear to closely follow the path of the Sun on the sky.  It’s not exact because the planets all have slightly non-circular orbits, and their orbits are all very slightly tilted compared to that of the Earth, but the planets are essentially always in a rough line from our perspective.  Pretty cool, huh?

Finally, if you’re finding it annoying that the Sun makes Mercury so hard to spot, you’re not alone.  Many astronomers have rarely caught a glimpse of it!  Since Mercury never moves very far from the Sun, and it’s quite small and rocky so doesn’t reflect a lot of light, it can be challenging to observe.  The best solution to this problem?  Visit the Moon where you don’t have an atmosphere to contend with!  If you viewed the sky at the same date and time from the (far side) of the Moon, here’s what you would see:

With no atmosphere, the sky does not appear bright blue as it does on Earth, hiding the stars during the day. Instead, the stars are visible all the time, whether the Sun is in the sky or not.

The same planetary alignment, but viewed from the Moon where there is no atmosphere to hide Mercury!

This is the view at the same date and time, but from a location of 25°43’N 157°19’E on the Moon’s surface.  The Sun is in the sky, but because the Moon has no atmosphere to speak of, there is no scattering of the Sun’s light, and the sky does not appear bright blue.  Instead, all the stars are still visible, just as if it were night time.  The Earth is below the horizon from here, so it’s not in the sky right now from this location.

As visiting the Moon is (sadly) not an option for most of us any time soon, my advice is to choose a nice hill, pack yourself some sandwiches and a flask of your favourite beverage, and go for an early morning hike.  Or camp up there with an alarm clock.  Good luck!

All images made with Stellarium.

© 2023 Dr Megan Argo BEM

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