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Skywatching

How radar helps us see what's in space

Radar astronomy

For most of our history, astronomy has been a strictly spectator sport.

We observed and studied the light and other radiation that objects in the cosmos happened to be sending in our direction. These days, we are exploring the Solar System using spacecraft to fly-by, orbit or even land on other planets and moons. Since the Space Age started in late 1957, intriguingly, the first time we reached out and touched another body in space was in 1946, when a radar in the U.S. successfully bounced radio waves off the Moon. That marked the beginning of radar astronomy.

Radar—short for Radio Detection and Ranging—involves detecting, identifying and tracking distant objects by bouncing radio waves off them. Today it is an important facet of modern life, in peace and war.

Without radar, commercial air travel would be very difficult, and marine trade a lot more risky. It played a critical role in the Second World War, with the National Research Council of Canada becoming, along with the U.S. and U.K., a major development centre for radar systems. Canada's first radio telescope was made out of war surplus radar components.

Most radar systems work by transmitting high-power pulses of radio waves. The pulses are reflected by distant objects and are detected using a specially designed radio receiving system.

Light and radio waves travel at 300 metres per millionth of a second so by measuring the time interval between the transmission of the pulse and the reception of the echo, we can determine the range of the object reflecting the pulses. By noting the direction the antenna is looking, we can assign a position to the target. Moreover, we can do this at night and in bad weather.

However, there is one downside to radar. Because the signal has to reach the target and bounce back to us, increasing the range dramatically reduces the strength of the echo. If the target moves to ten times as far away, the echo becomes ten thousand times weaker. This meant that getting a radar echo off the Moon would be a challenge, and detecting objects even further away would be an even bigger one.

Thanks to improvements in electronics and signal processing techniques, and the availability of really big antennas, such as the 305-metre dish at Arecibo, radar observations have been made of Mercury, Venus and Mars, yielding information about their surfaces and rotation rates. Venus is covered by a permanent layer of thick cloud. Radar observations from the ground and then from the Magellan spacecraft as it orbited the planet revealed that hidden surface in high detail. It is not a place people are likely to visit any time soon.

Radar has also been used to study the rings of Saturn. In addition, it has been used to detect a number of comets and a good number of asteroids. It also enables us to track spacecraft, satellites and space junk in Earth orbit.

One very important application of radar in astronomy is to detect asteroids that could pose a risk of hitting Earth. Although radar systems cannot detect asteroids out to the distances achievable with optical telescopes, they have the huge advantage of producing almost immediate measurements of the speed and direction of any asteroids they detect. In addition, radar systems can be used during daylight, to detect anything coming from a sunward direction, and bad or cloudy weather does not affect them.

Because we now exploit our world very intensely, even an impact by a relatively small object, say 100 metres in diameter, would have very serious consequences. So astronomical radar systems are an important part of our planetary defences.

•••

• Venus and Mars lie very low in the dawn glow.

• Jupiter shines in the west after sunset, with Mercury hiding low in the sunset glow.

• The Moon will be full on March 25.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.





The tricky job of creating a calendar that reflects earth's actual rotation

Leap years and the calendar

This year is a leap year.

We have added a day to February in order to keep the date in step with the seasons and the annual motion of the Sun.

Here is how we determine if it is a leap year. If the year is divisible by four, it’s a leap year, unless the year is also divisible by 100, then it is not. If it is divisible by 400, it is.

How did we wind up with a procedure that sounds like something concocted by Revenue Canada?

If we didn't have seasons, setting up the calendar would have been easy. We could design it around any number of days or months. However, we do have seasons and we would like a calendar that stays locked to them. Achieving that has taken thousands of years of observation and calculation.

For many centuries, the Earth's axis of rotation has pointed towards the Pole or North Star. Since this star is not perpendicular to the plane in which the planets orbit the Sun, that means at one point in the year the Earth's northern hemisphere is leaning directly towards the Sun. Half a year later, we are at a point where we are leaning directly away.

When we are leaning towards the Sun, it is higher in the sky, the days are longer and we have summer. When we are leaning away from the Sun, it is lower in the sky. The days are shorter and we have winter. We call these points the summer and winter solstices respectively.

Obviously there must be two other points during the year when we are neither leaning towards nor away from the Sun, when the same number of hours of daylight and darkness. These are the spring and autumn equinoxes.

It is easy to see the march of the seasons by watching the movement of the sunrise or sunset points along the horizon. At the summer solstice, the Sun rises and sets at its northernmost point on the horizon, and at noon it is at its highest elevation. At noon on the winter solstice, it rises and sets at its southernmost point on the horizon and it is at its lowest elevation.

At the equinoxes, the Sun rises in the east and sets in the west. For much of our history this was enough. However, when we wanted to measure the passage of days more precisely, for example setting the dates for religious festivals, we needed something much more precise—a calendar.

That posed a problem. The Earth does not take a whole number of days to complete its orbit around the Sun. It takes 365.2425 days, which means that is the actual length of a year. The result is a 365-day calendar would slip by almost a quarter of a day a year. Maybe that would not be a problem for a year or two, but after a few years it would.

The first and biggest step towards fixing this was made by Julius Caesar. He introduced a 365-day calendar with an extra day being added every four years. He made an average calendar year 365.25 days long. This was a big step, but not exactly right. Over four years the slippage is not one day, it is actually 0.97 days, so as the years passed the errors gradually built up, just more slowly.

In the 16th Century, the error had reached a point where setting religious festivals was becoming a problem, so Pope Gregory produced a refinement. If the year was divisible by 100, there would be no leap year. This was better. The average length of Pope Gregory's calendar year was 365.24 days. However, this was still not 365.2425 days. So it was decided that every 400 years there would be a leap year and a day added whether or not the year was divisible by 100.

We still use the Gregorian calendar today. Corrections are still needed. Occasionally, at midnight on Dec. 31, a leap second is added. This happens for two reasons—to handle the residual errors in the calendar and to correct for the Moon slowing down our planet's rotation, by about 1.7 milliseconds a century. Calendar correction is going to be an ongoing job.

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• Venus and Mars lie very low in the dawn glow.

• Jupiter shines high in the south after sunset.

• The Moon will reach first quarter on March 16.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



Avoiding the fate of the dinosaurs

Nudging asteroids in space

Around 65 million years ago, an asteroid 10 kilometres to 15 kilometres in diameter hit the Earth close to what is now the Yucatan Peninsula.

Already stressed ecosystems were devastated, leading to the extinction of 75% of the species living at the time, including the dinosaurs, ammonites, belemnites and others.

We now live in a world filled with people, in highly stressed ecosystems. We would be very unlikely to survive a similar impact were it to happen now. Therefore, we are working hard to avoid it. There are telescopes dedicated to detecting potential Earth-threatening asteroids, and programs to estimate whether they are an immediate or long-term threat and the probability of an impact.

Of course, detection is only part of the solution. If we find a potentially threatening asteroid, what can we do to remove that threat? Proposed solutions include sending a missile to the object and blowing it up or finding a way to nudge it onto a different and safer path.

A lot depends upon what asteroids are like. Are they rigid lumps of rock or are they rubble piles held together by their weak gravity. That has led to space missions to asteroids to see what they are like and, recently, an experiment to see if we could change its orbit enough to avert a hit.

A few close encounters with asteroids by spacecraft suggest most of them are basically loosely consolidated rubble piles. Whether one of these could be diverted into a new orbit using available space technology led to a NASA mission to Dimorphos, an asteroid with a diameter of about 180 metres (small, but large enough to do a lot of damage). This asteroid happened to be orbiting a somewhat larger asteroid, Didymos, with a diameter of around 780 metres.

The spacecraft, named DART, was intended to smash into Dimorphos and see whether its orbital path was changed by a useful amount. A double asteroid was chosen because the change in their orbits around one another would be far easier to detect, over less time.

The spacecraft was launched in November 2021 and hit it head-on in September 2022. The mission was a success in that the orbit was changed by more than expected.

However, something else happened. The shape of the asteroid was changed. Dimorphos was a rubble pile and it is likely that, if it were hit harder, it would have come apart. If it was an Earth-threatening asteroid, that could be a disaster.

Being shot with a rifle is bad. Being shot by a shotgun is far worse. If a larger rubble pile were heading in our direction, how hard could we hit it in order to change its path without smashing it? Another important consideration is the spacecraft took close to a year to get to the asteroid. We have to know at least a year in advance in order to act usefully.

The need to apply a push that changes the path enough without smashing the asteroid, together with having enough time to reach it, means we need to identify the threatening asteroids well in advance. That way we can apply a gentler push, or over a longer time.

We certainly know how to determine orbits with precision. However, predicting where an asteroid will be in a few years' time is made harder by the constantly changing gravitational influences of the other planets, especially Jupiter, the largest planet in the Solar System. Small perturbations build up rapidly over time, changing asteroid orbits significantly.

On the other hand, we are getting better at it, and spacecraft can be made that make small navigational changes en route. It is a challenge spotting small, dark objects against a black sky early enough to identify threats and act on them. However the incentive is certainly there.

•••

• Venus and Mars are getting lost in the dawn glow.

• Jupiter shines high in the southwest after sunset.

• The Moon will be new on March 10.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.





Fast radio bursts raise questions about highly magnetized star

Magnetism in space

Fast radio bursts (FRBs) are intense bursts of radio emissions, just milliseconds duration. They were discovered by accident.

A large radio telescope, which could "see" a patch of sky far smaller than the full Moon, just happened to be pointing in the right direction at the right time. Now we have radio telescopes like CHIME, a Canadian instrument located at our observatory, which have a field of view large enough to capture a good fraction of the sky and thousands of these FRBs have been captured, turning up all over the sky.

Most of them come from galaxies millions, or billions, of light years away, showing how much energy must go into producing each of them—more energy than the Sun produces in a year. A few months ago, one of them was detected from an object in our own galaxy. It was a highly-magnetized neutron star, or “magnetar,” named, poetically, SGR 1935+2154.

Since then, astronomers watched, and waited for it to do it again, which it did in October 2022. And CHIME spotted it.

NASA was monitoring the object with two advanced X-ray satellites—NICER (Neutron Star Interior Composition Explorer) is on the International Space Station, and NuSTAR (Nuclear Spectroscopic Telescope Array) is in low-Earth orbit. They observed over several hours, bracketing the fast radio burst so it was possible to put together a picture of what happened.

First some background. Giant stars can end their lives as neutron stars, where the explosion in the outer part of the star compresses the atoms in the core to the point where they collapse completely. Atoms are mostly empty space, so collapsing them completely can take an object around 1.5 million kilometres in diameter down to around 15 km.

The electrons and protons in the atoms have been squeezed together, becoming neutrons. The gravitational attraction on the surface of a neutron star is tens of billions of times the attraction at the surface of the Earth. The magnetic field is around 1e12 (1 followed by 12 zeroes) times stronger than the magnetic field at the surface of the Earth. The highest mountains on a neutron star would be just a few centimetres high.

However, there's one factor that makes the geology of a neutron star very different from the geology of the Earth or other rocky planets. Rock is really good for handling compression, which is why we make buildings out of it. However, it is much worse at handling shear or stretching. On a neutron star the magnetic field makes things different. The surface is pervaded by the intense magnetic fields, which link the neutron star to material orbiting around it. The result is the stresses stored in the stressed surface material before a “starquake” can be enormously stronger. The neutron star discussed here has particularly intense magnetic fields, and is thus is known as a magnetar".

The NASA telescopes detected a "glitch", a small increase in the rotation rate of the object. That happens when a starquake occurs and where material cracks and falls inward, like a skater spinning faster when she pulls in her arms. Large earthquakes here on Earth cause the same thing. There was a glitch - a starquake, and soon after the fast radio burst was detected. About four hours later the rotation rate had fallen to what it was before the glitch, then there was another. The magnetic fields could easily have stored enough energy to drive the fast radio burst.

One suggestion as to why the rotation rate slowed again is that the first starquake caused the surface layers to rotate a bit faster than the deeper layers. Over the next few hours, the surface layers were dragged back into step with the rest of the star.

There is still a lot of vigorous discussion going on here, but on the other hand, it is stunning how much we have learned so far. There is more to do.

•••

• Venus and Mars lie close together low in the dawn glow.

• Jupiter shines high in the south after sunset.

• The Moon will reach it last quarter on the March 3.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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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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