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Skywatching

Pulses from deep space

In the 1930s, we first started running high power radio transmitters at wavelengths that can penetrate the ionosphere and escape out into space. So today, any alien civilizations on planets orbiting stars at distances up to around 90 light years will be bathed in our radio signals. However, those signals will be very weak.

Today, with our highest power radio transmitters, we are probably able to send detectable signals out to distances of thousands of light years, our neighbourhood in the Milky Way. Therefore, imagine the power needed to transmit radio pulses that can be detected over billions of light years.

The fast radio bursts (FRBs) being detected by the CHIME radio telescope here at DRAO, and at other radio telescopes fit into that category. These bursts are a few milliseconds (thousandths of a second) long and cover much of the radio spectrum. To be detectable with a radio telescope on the Earth, one of these pulses from a billion light years away would require a transmitter with a power hundreds or even thousands of millions of times larger than the total power output of the sun.

Space is not quite empty; it has a few electrons per cubic centimetre and a very weak magnetic field. The result is that radio waves passing through it are slowed down very slightly, with longer wavelengths being slowed more than shorter ones. The result is the pulse is "dispersed," so we see it arrive at short wavelengths before it does at longer wavelengths. By measuring the degree of dispersion we can find how far the pulse has travelled. The pulses originate far outside our cosmic neighbourhood, in very distant galaxies.

Another important piece of information is the short duration of the pulses. Assuming the source of those radio pulses is radiating in all directions and not specifically towards us, it cannot be smaller than the distance radio waves travel over the duration of the pulse. Therefore a source of five millisecond radio bursts cannot be larger than about 15,000 kilometres, which is only a little larger than the Earth.

How can we make pulses with energies possibly billions of times larger than the sun's total energy output in such a small volume? Moreover, since FRBs can repeat, generating one does not destroy the source producing them.

Most of our radar systems produce radio pulses strong enough to produce detectable echoes off distant targets. They generate these strong pulses by accumulating energy in a storage device and then feeding it all to the transmitter in short pulse a millionth of a second or so long. In stars there is an excellent energy storage device, the magnetic field. We can store energy by twisting, stretching or compressing the magnetic field. This is how the sun accumulates over hours or days the energy driving solar flares or coronal mass ejections.

However, the magnetic fields in the sun are totally inadequate for storing the energies involved in making FRBs. Still, there is a way to overcome that problem. This leads to one of the many ideas as to how FRBs may be generated.

When large stars run out of fuel, they collapse and explode. The result can be a neutron star, where all the star's rotational and magnetic energy become concentrated in an object a few kilometres in diameter. The result is a rapidly rotating object with an extremely strong magnetic field. The magnetic field can then become wound up by the rotation, just like winding the spring of a clock, until the energy is released in a short, intense pulse.

Until recently there were more theories about what FRBs are than the number of FRBs that had been detected. Thanks largely to CHIME, this is no longer true. However we still don't really know what is driving those amazingly powerful pulses.

  • Venus shines brightly in the southwest after sunset and Mars, Jupiter and Saturn lie low in the southeast before dawn.
  • The moon will reach first quarter on March 2.


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How big is the universe?

This might sound like the ultimate unanswerable question, but thanks to astronomical knowledge accumulated over many centuries, and the power of modern astronomical instruments, we can make a guess. To do this we have three main tools.

The first is an effective means to measure how far away things are. There are stars such as Cepheid variables, and some classes of supernova explosions where we can deduce how bright they are, then measure how bright they look, and so calculate how far away they are. These techniques have enabled us to measure the distances of the most distant galaxies our telescopes can detect.  

The second tool is the expansion of the universe. As the universe expands it carries everything away with it. The further things are away, the faster objects are receding from us. Hubble and other astronomers established the relationship between distance and the rate things are moving away.  

With this information we can track the galaxies back in time and find that about 13.8 billion years ago, everything in the universe that we can see was all concentrated in one lump, which started to expand, a moment we call the Big Bang.

The third tool is the speed of light. Light moves very quickly, just under 300,000 kilometres a second. That is, 300 metres in a millionth of a second. In our everyday lives this is far too fast for us to notice, but over cosmic distances it becomes very important. In fact, one of the standard units of cosmic distance is the "light year", the distance light travels in a year, which is a tiny bit less than 10,000,000,000,000 kilometres. When we look at Sirius, that bright star shining in the southern sky these evenings, we see it as it was about 8 years ago. So, as we look further and further away, we are looking further and further back in time. If we observe something nearly 13.8 billion light years away, we are seeing it as it was 13.8 billion years ago, as it was at the beginning of the universe. 

That means, from us to those objects at the beginning of the universe lies a distance of 13.8 billion light years, and that when that light set off on its journey to us, the edge of our observable universe lay 13.8 billion light years from us.

However, the universe is expanding. Just after the Big Bang, it expanded faster than light for a little while, and then it slowed down. Since then the expansion has been gradually accelerating. That means that today, what was 13.8 billion light years away 13.8 billion years ago is now closer to 46.5 billion light years from us. The expansion also tells us something about the shape of the universe. 

When we look at the expansion, we see exactly the same relationship between expansion rate and distance no matter what direction we are looking. There are only possible answers to this. One is that we are in the exalted position of sitting right in the middle of the universe, and that everything is moving away from us. A more reasonable possibility is that our situation is no different from any other astronomer out there, and that no matter what star they orbit or galaxy they live in, they are seeing exactly the same thing. This is what ants on an expanding balloon would see. 

Every ant would see the other ants being carried away at a rate related to how far away they are. In their case they are, as far as they can see, on a two-dimensional surface expanding in a third direction. In our case, we think our universe is a three-dimensional thing expanding in a fourth dimension. This would indicate that our universe is a sort of four-dimensional balloon with a diameter of around 93 billion light years, with us wandering around on its three-dimensional "surface." 

It also means we will never find the edge of the universe, but if we travel far enough we will wind up back where we started.

  • Venus shines brightly in the southwest after sunset and Mars rises in the early hours, Jupiter a bit later, with Saturn very low in the dawn glow. 
  • The moon will be full on the 23rd.  


Twisting space-time

Twenty or so years ago, the radio telescope at Parkes, Australia, found something unusual: a white dwarf star and a neutron star in close orbit around one another, taking only five hours to complete each orbit. 

In addition, the white dwarf is spinning very rapidly - once every 20 seconds. This has provided a unique opportunity to check out another strange aspect of Einstein's General Theory of Relativity: "frame-dragging."

Once upon a time there were two stars, one more massive than the other. As the more massive one aged, it expanded, and its neighbour made use of its gravity to capture some of its material. This meant the star losing material would not end its life by exploding, which would otherwise have happened, and it ended up as a white dwarf, which is what the sun will become when it runs out of fuel. 

However, increasing the mass of the second star meant it did end its life by exploding, with such an intensity that the atoms in its core were crushed into neutrons. The white dwarf is similar in size to the Earth, but the neutron star is only a few kilometres across. When the greedy star exploded, its material was thrown off into space and the white dwarf got some of it back, fortunately not enough to make it explode. Instead, just as when a ballerina spins faster when she pulls in her arms, the star spun faster, until it was spinning once every 20 seconds, almost fast enough to fly apart. This unusual "dynamic duo" of a neutron star and a rapidly-spinning white dwarf star provides a very good test bed for measuring frame-dragging.

All the objects in the universe are immersed in a multidimensional, stretchable and twistable medium called "space-time." When a planet or any other object moves through it or rotates, it drags space-time along with it. For small objects, like the Earth, this process is almost negligible. Precise measurements made using the Gravity Probe B reveal the Earth's frame-dragging. The effect is tiny; it would take about 36 million years to drag a full circle. However, it gets larger for more massive objects, like rapidly spinning white dwarf stars.

When a star shrinks to become a neutron star, it spins faster, and the magnetic flux in the original star is concentrated in this much smaller body. The result is powerful interactions in the magnetic fields linking the star to its surroundings. These drive a beam of strong radio emissions, which sweeps round like the light beam from a lighthouse.

Every time that beam sweeps in our direction, we receive a pulse of radio waves, which is why we often refer to neutron stars as pulsars. Since they are locked to the rotation of a very massive body, the spacing between the pulses is very stable, providing a very accurate clock. The neutron star in the dynamic duo is rotating 150 times a minute.

A rotating neutron star alone in space would give us a pulsar with a pulse timing that varies only slowly with time. When orbiting another object we will get changes in timing as it approaches and recedes, and when the rapidly-rotating white dwarf star gets anywhere near the line of sight, its frame-dragging will affect the timing too, in a measurable way, proving another test of Einstein's ideas.

For centuries Isaac Newton's idea of a uniform space and time ruled. Even today we can use Newton's ideas to navigate the solar system and understand much of what we see out there in space. Unfortunately, they break down in extreme circumstances, such as where there are very large masses, extreme speeds or temperatures, or very long periods of time. Einstein's ideas work better there. However, in what seems to be an increasingly bizarre universe, we cannot conclude they will continue to account for everything. Can they be improved? That's why the testing and exploration continue.

  • Venus shines brightly in the southwest after sunset and Mars rises in the early hours, Jupiter a bit later, with Saturn very low in the dawn glow. 
  • The moon will reach last quarter on the 15th. 


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The space junk hazard

Back in the late 1950s and early '60s, when our first artificial satellites were put into orbit, we didn't pay much attention to what went with them. 

The last stage of the launcher, various adapter rings, clamps, fairings and other bits were just allowed to float away. Actually in this case, “float” really meant moving at around 30,000 km/h. Then, eventually the satellite itself went dead, and became another contribution to what we now call “space junk.” 

Fortunately, objects in very low orbits experience a small amount of atmospheric drag. Their orbits shrink until they burn up in the atmosphere. However, space junk in high orbits will remain there for centuries or maybe even indefinitely. 

In those early days, nobody really thought of near-Earth space being choked with satellites, so space junk was not a problem. Now it is, especially with the launching of thousands of new satellites to provide global 5G Internet access and other communication services. 

In addition, we have orbiting observatories, and are looking forward to observing with the James Webb Space Telescope without worrying that something will hit it. There are tens of thousands of bits of space junk up there, ranging from dead satellites to nuts, bolts and flecks of paint, and there is a growing realization we really need to do something before it is too late; but what? Hypersonic collisions are not like low-speed ones. 

So what does happen when a piece of space junk hits a spacecraft?

Imagine a baseball approaching a bat. When it hits, the part of the ball in contact with the bat starts to move with the bat. However, the other side of the ball is still moving at the speed it was thrown. The ball starts to flatten against the bat and its kinetic energy is stored in the distortion of the ball. Then, when whole ball is moving with the speed of the bat, the distortion relaxes; the ball becomes spherical again and bounces off the bat. The result is the ball moves off with the speed of the bat plus a good fraction of speed with which it was thrown. 

The whole process takes place smoothly because the forces shaping the ball are transmitted through its fabric at the speed of sound, while the ball is moving much more slowly. Hypersonic impacts are very different. 

Here is the story of a typical impact experiment aimed at helping us better understand high speed impacts. An aluminum sphere 1 cm in diameter, fired from a special gun, moving at around 30,000 km/h, a typical speed for an object in orbit, hits a slab of aluminum around 10 cm thick. The speed of sound in aluminum is around 22,500 km/h, so the sphere is moving faster than sound and the material in the sphere never has a chance to distort, accommodating the impact. 

The result is that in about a few millionths of a second the kinetic energy is converted to heat, heating the sphere and part of the material it hit to tens of thousands of degrees. Solid aluminum becomes highly compressed aluminum vapour. As the force of the impact relaxes, the compression ceases, and that hot, dense ball of aluminum vapour explodes. The impact and explosion send shock waves through the slab, blowing off a layer of metal from the other side. The explosion blows a hole in the slab several centimetres wide and several centimetres deep. 

Since we cannot afford to give spacecraft walls several or more centimetres thick, we cannot live with an increasing volume of space junk; we have to come up with another way to deal with it.

Since there is no sort of catcher's mitt that will stop something moving at 30,000 km/h, the only way we know at the moment to deal with space junk is to rendezvous with each bit. We have then to match speeds with it, catch it, and then move on the next. This process is tedious and will require a lot of fuel. Innovative new solutions are needed.

Venus shines brightly in the southwest after sunset and Mars rises in the early hours, Jupiter a bit later, with Saturn very low in the dawn glow. 

The moon will be full on the 9th.



More Skywatching articles

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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