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Skywatching

Spinning in space

Long sojourns by astronauts on the International Space Station have shown us that people can recover from extended periods of being weightless under free fall.

However, it would be nicer if those effects could be avoided. When astronauts arrive home, they are often lifted from the spacecraft.

It is very unlikely that when we arrive at a distant world after a long space journey there will be friendly aliens waiting to help us disembark.

The astronauts need to experience gravity during the trip, or something like it.

Weightlessness is what we experience when we are in free fall, moving only under the force of gravity. This applies not only when we are falling off something here on Earth, but also when we are in transfer orbits between worlds, with the engines shut down and purely moving in response to the gravity of nearby bodies.

The obvious thing would be to avoid being in free fall. For example, if we kept the spacecraft engines running so that we were adding 9.8 metres per second to our speed every second, we would feel just the same as we do on the surface of the Earth, with our usual weight.

We would run the engines for about half the journey, shut them down, turn the ship end over end and start them again. Our spacecraft would start decelerating, slowing the ship by 9.8 metres per second every second, so that by the time we arrive at our destination, we would be going slowly enough to go into orbit or land.

Unfortunately, as yet we have only two sorts of space propulsion. We have engines that can produce enormous thrusts for short periods, as needed to get from the surface of the Earth into orbit.

The other kinds are plasma engines that produce tiny thrusts, which can be run more or less indefinitely because they consume their fuel very economically.

However, they are not capable of giving the thrust required to keep our astronauts comfortable. We need engines giving a moderate thrust that can be kept going for long periods.

Decades ago, science fiction writers came up with another approach, namely to spin the spacecraft.

My family and I had a direct experience of this in an amusement park ride some years ago. The ride looked like a landed flying saucer, mounted so that it could spin around at high speed. Inside, attached to the outside wall of the "spaceship" were almost vertical benches.

We were instructed to lean against these benches. There were no windows, so we could not sense the moment we started spinning.

However, as the spin accelerated, we were pulled more and more strongly toward the wall, onto the benches, until the down direction was in the direction of the wall, which had now become the floor, and the original floor a vertical wall near our feet.

We can spin a real spacecraft in exactly the same way, so the astronauts would be pulled to the outside wall, experiencing something that feels like exactly like gravity. There have been science fiction stories of huge, hollow cylinders with large space colonies living in farms, villages and towns attached to the inside wall.

Arthur C. Clark's book Rendezvous with Rama features such a space ship. An alternative is a big, spinning wheel, like the space station in 2001, A Space Odyssey. In this case the passengers will be standing on the inside of the ring's outer wall.

Another concept is a vehicle consisting of two spheres connected by a long tube. By spinning this end over end, like a cheerleader's baton, passengers in the spheres would experience the same sort of pseudogravity.

There are technical challenges to building spinning spacecraft. One is the problem of stabilization. As people and other things move around onboard, the centre of gravity will shift slightly and the spin would wobble.

However, at the moment, spinning spacecraft seem the best option for arriving at distant planets and being able to walk off the ship.

  • Jupiter dominates the southern sky overnight.
  • Saturn rises around 10 p.m.
  • The moon will be full on the 16th and reaches last quarter on the 24th.




Jupiter rising

For the last couple of months Jupiter has been the most spectacular object in the sky after the sun and moon.

The giant planet lies in the south-east after sunset and dominates the southern sky for most of the night. Look for a very bright, starlike object, shining with yellowish-white colour.

Unlike stars, planets don't twinkle, so Jupiter shines steadily, like a plane's landing light.

The planets orbit the sun in almost circular concentric orbits.

Starting at the sun they are:

  • Mercury
  • Venus
  • Earth,
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

Pluto used to be included as the ninth and outermost planet.

In addition to having smaller orbits, the inner planets also move faster, so they take a lot less time to orbit the sun:

  • Mercury 0.24 years,
  • Venus 0.61 years,
  • Earth one year
  • Mars 1.9 years,
  • Jupiter 11.9 years
  • Saturn 29.4 years,
  • Uranus 84.1 years
  • Neptune 164.5 years.

The result is the inner planets regularly overtake those lying further out.

For Earth and Jupiter, this happened on June 10. At that time Earth was exactly between Jupiter and the sun.

That meant that as the Earth turned, Jupiter lay in the south exactly 12 hours after the sun lay in the south.

That is why astronomers refer to the planet being in "opposition". Around opposition is one of the best times to get out the telescope to observe planets in orbits outside the Earth's.

They are at their closest to us, and we have the longest time to see them without the Sun being in the sky.

When Galileo first pointed his telescope at Jupiter back in 1610, what he saw revolutionized his picture of the universe.

Today, we have far better telescopes and around now is the best time to turn them on Jupiter. Even if you have just a pair of binoculars, there will still be something to see, such as what got Galileo so excited.

You will see a tan-coloured disc crossed by darker lines or belts. Near the planet, you will see up to four starlike objects, all in a row, like beads on a wire. These are Jupiter's four largest moons, moving out from the planet:

  • Io
  • Europa
  • Ganymede
  • Callisto

Over successive nights, you will see them changing position. They vanish behind the planet and pass in front, and on occasion you can see the shadow of one of the moons cast on the planet.

Galileo observed this, carefully noting how the positions of the moons changed with time and concluded the moons were orbiting the planet.

Since this was a time when it was taught that absolutely everything circled the Earth, finding an exception was a wedge opening the doorway to new ideas.

This led to a growing realization that a lot of things we observe in the sky become a lot easier to interpret if we assume the Earth and other planets orbit the sun.

If you have a telescope, you can have a closer look at the planet itself. That tan disc is not the body of the planet; it is just the top of a very deep atmosphere.

Jupiter has a diameter more than 10 times that of the Earth, but rotates on its axis in around 10 hours. Whereas a point on Earth's equator is moving eastward at around 1670 km/h, a point on Jupiter's equator is moving at around 45,000 km/h.

This furious rotation pulls the clouds into belts and drives huge and long-lived storms.

On Earth, our weather happens in a thin skin over the planet, a few kilometres thick. The rough, rocky ball rotating underneath has a major effect on how our weather patterns form and evolve.

Jupiter is different. The planet is a gas giant, and is mostly atmosphere, so what we are seeing is a weather machine without any ground beneath.

This raises a very interesting question. With no ground anchor underneath, why has the great red spot stayed in the same place for hundreds of years?

This is something to think about while looking at that fascinating planet.

  • Jupiter dominates the eastern sky after dark.
  • Saturn rises around 10 p.m.
  • The moon will reach first quarter on the 9th and be full on the 16th.


Free falling in space

We often hear people explaining that "Things float around in space because there is no gravity.”

This is not true.

Gravity extends across the whole universe and more locally, it keeps the moon in orbit around the Earth and the Earth and other planets orbiting the sun.

What is really going on?

A good way to start is to imagine a hugely scaled-up version of the CN Tower.

The rotating restaurant and observation decks are 410 km above the ground. We chose this height because as we eat, we can occasionally see the International Space Station whiz past.

Since this is an imaginary tower, we can ignore the very obvious safety issues. As you enjoy your meal, you can also enjoy the amazing panorama of the Earth and ever-changing cloudscapes far below.

The only obvious difference will be that the restaurant and viewing galleries will be airtight and pressurized. As we get higher above the ground, the air pressure drops. Above about five kilometres, the air pressure is low enough for each breath not to bring in enough oxygen, so we have trouble breathing.

At the top of our tower, there is virtually no air. One thing you might not notice, because it is so familiar, is that the restaurant tables are laid out exactly as they are at restaurants closer to the ground.

  • Knives
  • forks
  • plates
  • wineglasses

None shows any tendency to float off the tables, and you can walk around more or less normally. In fact, you weigh almost 90% of what you weigh at sea level.

Imagine you are wearing a space suit and are on the special outside viewing platform. Would you step off it and go for a space-walk?

If you did, you would find yourself falling toward the Earth with plenty of time to wonder what you had got wrong. One thing you will notice is that you are now weightless.

This is what we experience if we are falling freely under gravity. We get to experience this for a short time when bungee jumping or on one of the more thrilling amusement park rides.

Astronauts on space walks or doing acrobatic things inside the International Space Station experience weightlessness because they too are falling freely under gravity.

This condition is often described as free fall. To make the situation clearer let’s imagine that on top of the tower there is a very big cannon, along with a good supply of gunpowder and cannonballs.

Load the cannon and fire it. As you expect, the cannonball follows a downward curving path under gravity until it hits the ground.

Load it again, using more powder. The cannonball is moving faster and follows a more gradual downward curve until it hits the ground further away.

If you keep firing the cannon over and over again, using more powder each time, the ball will hit the ground further and further away.

However, you will reach a point where something weird happens. As expected, the Earth's gravity pulls the ball into a downward curving path, but underneath, because the Earth is a sphere, the ground is curving away underneath it. The result is that ball does not hit the ground, it just keeps curving around the Earth; it is in orbit.

To make this happen, the cannonball needs to leave the muzzle of the gun at 27,600 km/h. That is the speed the International Space Station is whizzing past the restaurant windows, because like that cannonball, the station, along with its passengers, are freely falling around the Earth.

In practice we don't only choose to avoid having our spacecraft hit the ground, we need to keep them above the atmosphere too, which will slow them and bring them down.

With no convenient tower or cannon, putting something into orbit involves lifting it above the atmosphere and then accelerating it horizontally so that it is moving fast enough so that as it falls, it keeps missing the Earth.

Remember, those astronauts performing their acrobatics are just as subject to gravity as we are on the ground.

  • Jupiter dominates the eastern sky after dark. Saturn rises around 10 p.m.
  • The moon will be new on Tuesday, and will reach First Quarter on the 9th.


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Exploding stars tell a story

Understanding the expansion of the universe depends on our ability to measure the distances of very far-off galaxies.

Most of these are so remote even our biggest telescopes cannot see them other than as faint blurs.

Fortunately for us, occasionally we see a brilliant, starlike thing appear, rapidly brightening until it is shining brighter than the entire galaxy hosting it.

The object can be seen for maybe several weeks, before it fades back into invisibility. What we saw was a supernova, an exploding star.

There are various types of these. One marks the demise of a giant star that has finally run out of fuel. With energy production declining in its core, the temperature falls and so does the pressure holding up the star's outer layers.

Because there is no way the core material can support the load, the star collapses and explodes. They are scientifically very interesting.

However there is another, rather odd type that is useful in another way, because we can use it as a ruler to measure cosmic distances.

The unique thing about this class of supernova, known as Type 1a, is that these explosions involve what are possibly the most stable objects in the universe — white dwarf stars.

At the ends of their lives, with their fuel exhausted, massive stars blow themselves up. Lower-mass stars like the sun do not.

As the energy supply runs down and the temperature in the core drops, the star shrinks until the material in the middle is compressed enough to support the overlying layers.

Since this is a less massive star, the material can support the load and no catastrophic collapse happens. The star has shrunk to about the size of the Earth and is glowing white hot.

It has become a member of the class of stars known as white dwarfs. Held together by strong gravity, but not compressing their cores intolerably, white dwarf stars are possibly the most stable objects in the universe.

In addition, they are so miserly in the amount of energy they radiate; they take many billions of years to cool off.

However, if a white dwarf has a partner, things can be very different.

Many stars in the universe are double or multiple, closely orbiting one another. They were born together and happened to stay together. Imagine a double star.

It is likely that one will get old before the other, and run out of fuel. It swells into a red giant, sneezes off it outer layers, and the remnant becomes a white dwarf. 

Some time later its sibling starts getting old, and also swells into a red giant.

Red giant stars have a very weak gravitational hold over their outer layers, and the white dwarf starts to pull the other star's material down onto its surface.

This can be a very big mistake. As the material accumulates, the additional weight has to be supported by further compressing the core. There is a limit to how much it can do this. Eventually that limit is crossed and the star collapses and explodes.

What is convenient for us is that we know stars with masses more than 1.4 times the mass of the sun blow up and those with lower masses don't. This is known as Chandrasekhar's Limit, named after the astrophysicist who worked this out.

This means we have a good estimate of the mass of that white dwarf when it blew up, which means we can calculate how much energy was released.

By measuring how bright it looks to us, we can calculate how far away it is, and of course for remote galaxies, the distance of that galaxy. This is why so much time is spent by amateur and professional astronomers in searching for supernovas in distant galaxies. 

Without exploding dwarf stars, establishing cosmic distances would be much harder, maybe impossible, and we would know much less about the history of the universe.

  • Jupiter lies in the east after dark. It shines very brightly and steadily, like an aircraft landing light. Get out the binoculars or telescope for a look.
  • Saturn rises around 10 p.m.
  • The moon will reach Last Quarter on the 25th, and be new on the 2nd.


More Skywatching articles

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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