Total Lunar Eclipse – 10 Dec 2011

•December 8, 2011 • 2 Comments

Imagine you are outside your house breathing some fresh air and enjoying the night view around you. You look up at the sky, hoping to catch a star or two, but what you saw instead was a reddish orangey full moon hanging in the sky. What happen to our Moon?!?

No fear, there is nothing wrong with our Moon – our Moon is as normal as it always is. It’s just passing through the Earth’s shadow, and we call that a lunar eclipse.

This is going to happen on Saturday (Dec 10), on a convenient time for us (in Malaysia). The Moon starts to enter the penumbral shadow at 7:33 pm and exit by 1:30 am the next day, with maximum eclipse at around 10:30 pm. This means that we no need to stay up late into the night to see it.

Credit: F. Espenak, NASA’s GSFC

For animation, click here. As you can see from the animation, as the Moon enters the Earth’s penumbra (P1 to U1), you may not observe any changes. The show really starts after 8:45 pm (U1), when the Moon starts to enter the umbra shadow. Look for the colour change (to reddish) as the Moon moves deeper and deeper into the shadow. Between 10:06 pm (U2) to 10:57 pm (U3) is what we called totality – this is the time when the whole Moon is in Earth’s umbra. After that, the Moon will slowly come out from the shadow, and by 1:30 am, you can pack and go to sleep.

Total Lunar Eclipse of 16 Jun 2011 taken outside my house. I did not managed to finish the whole sequence because the clouds rolled in. Credit: thChieh.

 

Why does the Moon turns reddish or orangey during totality? Shouldn’t it disappear as it enters the Earth’s shadow? The reason is our atmosphere. Take a look at the diagram below and it’ll explain everything.

If the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The presence of Earth’s atmosphere means that sunlight reaching the Moon must pass through a long and dense layer of air, where the light is scattered. Shorter wavelengths (blue) are more likely to be scattered, so by the time the light has passed through the atmosphere, the longer wavelengths (red) dominate. The scattering depends on the conditions/particles in our atmosphere, which in turns determine the colour of the totality Moon. Anything from bright orange to blood red is possible. If there has been a major volcanic eruption, for example, the atmosphere has so much dust that the shadow on the moon will appear dark throughout an eclipse.

What colour are we going to see this Saturday? It will be a surprise…

(When the eclipsed Moon is bright, the stratosphere is clear. On the other hand, a dark eclipse indicates a dusty stratosphere. There are atmospheric scientists out there who are studying lunar eclipses as a means of monitoring conditions in Earth’s upper atmosphere. How cool is that?)

 

But don’t just look at the red. A more not known colour is the turquoise blue. I’m not sure if it is visible to the naked eye, but I’m sure if you take a picture of it, you will see it (see picture below). This comes from light passing through the ozone layer, which absorbs red light and makes the passing light bluer. This can be seen as a soft blue fringe around the red core of Earth’s shadow. Start looking for the turquoise colour as the umbra eclipse begins (U1), it will be more obvious as the Moon moves into shadow, or when starts to come out of the shadow.

Total Lunar Eclipse of 16 Jun 2011 taken outside my house. The Moon had just fully entered the unbral shadow (U2). The top part is darker because it was deeper in the shadow; the bottom part is bluish due to the reason described above. Credit: thChieh.

If you are in an area without much light pollution, you can actually see the stars around the Moon during totality. Usually the bright full moon will drown all the stars around it, but during a totality, you can take picture of the full moon with the stars.

Go out and take a look at the Moon after dinner this Saturday, you won’t want to miss it, because this will be the last total eclipse until year 2014. Yes, you read it right. There will not be any total lunar eclipse for two whole years. So grab this last opportunity!

When to Launch Spacecrafts to Mars?

•December 4, 2011 • 1 Comment

Last month, Earthling had launched two spacecrafts to explore Mars – the Russian Phobos-Grunt with China Yinghuo-1 piggybacking and NASA Mars Science Laboratory Curiosity. Curiosity had a successful launch and now is on the way to Mars, but unfortunately for Phobos-Grunt, the engine of its Fregat upper stage failed to ignite and it is now stuck in low-Earth orbit. Currently, engineers are still trying to contact the silent probe, but without much success. You can get more updates from the web.

These mission, however, are not are not the main topic of today.

What I want to talk about is that do any of you ever wondered why these two different missions by two different nations* were launched at almost the same time? Is this a coincidence, or is there a reason? Take a look at the launch histories of Mars mission, and see if you can spot some trends…

Viking 1 (NASA): Aug 20, 1975
Viking 2 (NASA): Sep 9, 1975
Phobos 1 (USSR): Jul 7, 1988
Phobos 2 (USSR): Jul 12, 1988
Mars Observer (NASA): Sep 25, 1992
Mars Global Surveyor (NASA): Nov 7, 1996
Mars 96 (Russian Space Agency): Nov 16, 1996
Mars Pathfinder & Sojourner (NASA): Dec 4 1996
Nozomi (ISAS): Jul 3, 1998
Mars Climate Orbiter (NASA): Dec 11, 1998
Mars Polar Lander (NASA): Jan 3, 1999
Mars Odyssey (NASA): Apr 7, 2001
Mars Express & Beagle 2 (ESA): Jun 2, 2003
Mars Exploration Rover Spirit (NASA): Jun 10, 2003
Mars Exploration Rover Opportunity (NASA): Jul 7, 2003
Mars Reconnaissance Orbiter (NASA): Aug 12, 2005
Phoenix (NASA): Aug 4, 2007
Phobos-Grunt (Russia): Nov 8, 2011
Curiosity (NASA): Nov 26, 2011

We can see that the launches always happened roughly 2+ years apart. Why? Because we want to launch our spacecraft during Mars opposition, and Mars opposition happens every 2 years + 2 months. The reason to launch a mission to Mars during opposition is pretty obvious – this is the time when Earth is nearest to Mars. However, the best time to launch, in terms of how much energy is required for the trip, is a few months before that happens (compare the launch dates above with the Mars opposition dates below).

Mars Oppositions: Dec 1975, … , Sep 1988, Nov 1990, Jan 1993, Feb 1995, Mar 1997, Apr 1999, Jun 2001, Aug 2003, Nov 2005, Dec 2007, Jan 2010, Mar 2012 …

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“The launch window for Curiosity is from Nov 25 to Dec 18, 2011.”

“Although the launch window for a round-trip to Mars closed yesterday (November 21, 2011) for the Russia’s Phobos-Grunt probe, a one-way flight to the Red Planet will still be possible for another few weeks.”

“A delay that interferes with the launch window can cause the entire launch to be scrubbed.”

In spaceflight term, the time period in which a particular mission must is launched is called the “launch window”. If a spacecraft wants to meet with another spacecraft (for example a Soyuz wants go to the ISS), or go to a planet, an asteroid, or any other objects in space, the launch must be carefully timed so that the orbits overlap at some point in the future (meaning both objects will meet each other in the future). If the weather is bad or a malfunction occurs during a launch window, the mission must be postponed until the next launch window appropriate for the flight.

But why do we need a launch window? Why can’t we just find where Mars is now, point our rocket and launch it any time we like? To answer that, imagine you are standing in the middle of a field watching a 400 m running race in a stadium. Now, if you want to catch one of the runners, how would you do it? One way would be to simply chase the runner, and provided that you are fast enough, you might eventually catch him only after using up lots of energy and travelling a long way. Not efficient.

A better way to intercept your target is simply to walk to the other side of the running track. It uses a lot lesser energy and time to get there. But there is a catch – you have to time your walk carefully so that you reach there at the same time as the runner does. If too early, then you’ll have to wait; if too late, then you’ll miss him completely.

The same thing applied in spaceflight. Now you are Earth and the runner is Mars. The calculations are actually more complicated then the example above since both Earth and Mars are moving in space. In order to reach Mars, we do not aim our spacecraft at where the planet is now, because by the time we reach the planet has already moved and is no longer there. Instead, spacecraft will travel in an elliptical orbit around the Sun that will eventually intersect Mars**. If the spacecraft reach the intersection point too early or too late, it’ll miss its target. (In the above example, if you are early you can wait, but in spaceflight a spacecraft cannot wait.) So timing the launch to allow a spacecraft and Mars to arrive at the same point and at the same time is important, and that’s how launch window comes in. If you miss your launch window, you miss your target.

The distance between Earth and Mars, the launch vehicle’s power, the spacecraft’s weight, and the desired geometry of approach to Mars are all factors in determining the range of possible launch dates.

A launch vehicle from Earth will take advantage of the Earth’s spin for an added boost, which translate to fuel saving. The point at which a launch vehicle uses the least amount of fuel to push a spacecraft onto the proper trajectory for Mars identifies an ideal launch date. The length of launch window for a mission is often determined by the type of launch vehicle, which has been designed to launch a certain amount of load at a certain velocity.

The farther the launch is from the optimal time for lift-off, the more energy needed to get to the target. If a rocket is just slightly more than enough to do its assigned job under the best launch conditions, the launch window becomes very narrow. The more energy a rocket delivers for a particular payload, the wider the range of conditions it can handle, thus the launch window widens. However, this consumes more fuels and money. Therefore, there has to be a balance between the mass of the payload and the energy capabilities of the launch vehicle.

You may think that a straight line is the best way to get to Mars, but that straight line translates into a huge, inefficient orbit around the sun. The launch vehicle required to put the spacecraft in such an extreme solar orbit would have to be very large, very powerful, very expensive and would waste an inordinate amount of fuel. Source: http://athena.cornell.edu/

For a mission to Mars, launch is usually scheduled prior to an ideal launch day, but within acceptable conditions for the launch vehicle. This allows for weather-related delays or delays in getting the vehicle ready. Other launch windows include a range of days after the ideal launch date. If these windows were missed, the trajectory to Mars becomes more difficult, the thrust of the launch vehicle becomes inadequate, and the window of opportunity for a mission to Mars closes for another two years.

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*three is you include China, but China Yinghuo-1 was launched by Russian rocket, so here I take it as two.

**In addition, the direction in which the spacecraft moves when it arrives should make it easy to match velocities with Mars, so that it is easier to enter Mars orbit. These trajectories and orbits are collectively referred to as Hohmann transfer orbits, first proposed in 1925 by the German engineer Wolfgang Hohmann. This will be a story for another post.

Moons and Rings

•November 6, 2011 • Leave a Comment

A quartet of Saturn’s moons posing for a picture with the planet’s rings taken by Cassini spacecraft. Credit: NASA/JPL-Caltech/SSI

The largest moon at the back is Titan with its hazy atmosphere. The white moon superimposed on Titan is Dione. The oblong-shaped moon to the right of the rings is Pandora while little Pan can be seen in the Encke Gap on the left of the picture.

Sidewalk Astronomy Night – 29 Oct 2011

•October 23, 2011 • Leave a Comment

Sidewalk Astronomy Night is back!

Sidewalk Astronomy Night is a programme where amateur astronomers get together and setup their telescopes at public areas to provide the public an opportunity to see, with their own eyes, celestial objects through telescopes. At the same time, the public will also be provided with information of what they are seeing, with the hope that it could raise public interest in astronomy and space science.

This year, our National Space Agency (ANGKASA) will be organising a Sidewalk Astronomy Night on Oct 29. Details of the programme are as follow:

Date: 29 Oct 2010 (Saturday)
Time: 8 – 10 pm
Venue: Taman Tasik Titiwangsa (previously site of Eye On Malaysia) Kuala Lumpur

All are invited to join this programme. Admission is free.

Apart from ANGKASA, other astronomical bodies around Klang Valley, i.e. Starhunter Astronomy Society, Starfield Instruments Supply, Starfinder Astronomy Society and Jabatan Mufti W.P, KL will also be joining in the programme.

The Moon and planet Jupiter will be easily visible. If weather and sky conditions permit, we can also see some bright constellations such as Cygnus and Pegasus.

Conjunction of The Moon and Jupiter

•October 15, 2011 • 1 Comment

Two days ago, the almost full moon had a meeting with Jupiter. And I was there to take a group photo of them.

The top left is (of course) our Moon. Bottom right is Jupiter. Click photo to enlarge. Credit: thChieh.

This picture is a composite of two pictures – one exposed for the Moon and the other exposed for Jupiter. It is impossible to get this picture in one shot, because the Moon is too bright and Jupiter is too dim (relatively speaking; Jupiter by itself is consider very bright). If the picture is exposed correctly for the Moon (meaning with a fast shuttle speed), then Jupiter will not show up. If exposed correct for Jupiter (meaning with a slow shuttle speed), then the Moon will be over-exposed. So we need a composite to get the best of both worlds.

After I took the picture and looked at Jupiter in the camera screen, I felt very unsatisfied. Jupiter was not a nice round dot. It was oval. Camera shake? Maybe. I took it again. And again. Still the same oval shape. Maybe the tripod is OK but Jupiter had drifted when the picture is taken? I don’t think so – the exposure time is just 1 second or less and the zoom is not that big (125 mm focal length). I give up on trying to understand why and go back into the house.

I loaded the images into my computer and when I opened the files, voila! Mystery solved! THE MOONS! Jupiter’s moons blended together with the planet and make the “dot” oval. I can’t believe it. I never realised that Jupiter’s moons can be so easily captured. Just a camera plus a 18-135 mm lens* plus a tripod were all that you need! I fired up Stellarium to check on the positions of the moons, and yes, they were where they were supposed to be.

Look carefully at Jupiter at the bigger version picture; you may be able to see its moons – two above (Callisto & Ganymede) and two below (Io & Europa). I cropped it out as below, just in case you are having trouble seeing it. The quality may not be spectacular, but still, to be able to capture the moons is something that I’ve never think of just by using an off-the-shelf camera.

Comparison of Jupiter taken with Nikon D70, 18-135 mm lens with Stellarium.

Later, I went out again…

A day later… click here…

The Moon had moved to the east a day later, so it was now below and farther away from Jupiter.

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*If you are wondering why I’m using 125 mm focal length instead of 135 mm that the lens can offer, the answer is I don’t know. I actually set it to 135 mm, but I think maybe the lens slid back a bit due to gravity when I pointed the camera to the sky. A lesson learned here is that I may need to secure the lens to prevent any movement. What I can think of now is using a (or a few) rubber band.

Aurora – What It Is and Some Amazing Vidoes

•October 13, 2011 • 3 Comments

Recently, I kept seeing pictures of aurora in the web. It’s not a surprise since the Sun is quite active lately. What a coincidence is that I also kept on hearing the word “aurora” from the people around me. We live at the equator, most of us only heard of but never really know what an aurora is, thus aurora is not something you’ll talk about in your daily conversation with others.

Is that a sign that I’ve to finish my article on aurora which I’ve plan to write long long time ago? I’ll take that as a Yes.

So, what is an aurora?

Wikipedia answer: “An aurora (plural: auroras or aurorae) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere.” In the northern hemisphere, it is called aurora borealis (northern lights) whereas in the southern hemisphere, it is called aurora australis (southern lights).

We always see aurora as static, because most of us only see aurora through pictures, which only capture it as a moment in time. If we can look at an aurora over time, as in the videos below, we can see that they are in fact changing and “moving”. This is why some time they are also called “the dancing lights”.

You must watch all the videos below because they are just beautiful! I guarantee you will not regret.


Timelapse of Aurora Borealis over Tromsø, Norway by Tor Even Mathisen. The greenish “dancing light” is aurora. Another cool thing about aurora is that you can see stars through its “curtains”.

 


The Aurora by Galip Koral

 


Astronauts Capture Aurora Australis from the International Space Station. If you are an astronaut, you also can observe aurora, but from above looking down, rather than from below looking up! Click here for more pictures of auroras from space.

 

For an aurora to happen, you need three ingredients – charged particles, magnetic field and atmosphere. The charged particles come from the Sun, and the magnetic field and atmosphere is from Earth*. Our Sun is an active ball of gas; it constantly gives out streams of charged particles known as solar winds. When these charged particles reach Earth, our Earth’s magnetic field guides these particles to the north or south pole, where they collide with air molecules in the upper atmosphere (at altitudes above 80 km). The collisions excite/ionise the air. When the electron returns to its original energy level or recombines with the air molecule, energy is release in the form of light creating the visible aurora. See the video below for a nice animation on how aurora is forms.


Click here for a more detail (and longer) video on how aurora forms.

Since the charged particles will follow the magnetic field lines to the north and south pole, aurora is usually visible in the far northern and far southern parts of the planet, in a band known as the auroral zone. The auroral zone is typically 10 to 20 degrees from the magnetic pole. However, during a geomagnetic storm (in simple words, when the Sun is more active), the auroral zone can expand to lower latitudes making aurora visible to temperate latitudes. Large magnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after that peak. The current solar cycle predicted to peak around 2013/2014, so we can expect more light shows for the years to come.


Click here for animation. The IMAGE satellite captured this view of the aurora australis on September 11, 2005. Shown in the image is the ring of light that the solar storm generated over Antarctica, glowing green in the ultraviolet part of the spectrum. Because the Earth’s magnetic field is three-dimensional, the aurora appears as an oval ring around each of Earth’s pole. It isn’t a perfect circle because the Earth’s magnetic field is distorted by the solar winds. From the Earth’s surface, the ring would appear as a curtain of light shimmering across the night sky.

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Colours of the Aurora

Variation of colours in the aurora with altitude. Credit: Causes of Color

If we look at pictures of aurora, we can see that aurora comes in different colours, with green as the most common. As we all know (do you?), our atmosphere consists of different compounds such as nitrogen and oxygen. The colour of the aurora depends on which gas in our atmosphere is being excited, and on how excited it becomes. Oxygen is responsible for the green and red colour; nitrogen causes blue and deep red hues.

Oxygen is unusual in terms of its return to ground state after excitation: it can take three quarters of a second to emit green light and up to two minutes to emit red. However, collisions with other atoms or molecules will absorb the excitation energy and prevent emission. At the top of the atmosphere, there has a higher percentage of oxygen and it is sparsely distributed such that collisions are rare. This allows time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen thus only green is visible, and eventually even green light emissions are prevented at even lower altitude.

This is why there is a colour differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras, followed by pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly pure blue.

Watch the video above The Aurora again and see if you can spot the layers of different colour with altitude.

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*Earth is not the only planet to have auroras. Auroras occur on other planets too – Jupiter, Saturn, Uranus and Neptune also have their own light shows since they also have magnetic fields and atmosphere. Similar to the Earth’s aurora, they are visible close to the planet’s magnetic poles.

Jupiter Aurora. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

 

Saturn Aurora. Credit: NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI)

Crescent Moons and the Ring

•October 11, 2011 • Leave a Comment

Crescent moon is always a lovely sight to behold, just imagine you can have two at one go plus rings as bonus.

Credit: NASA/JPL-Caltech/SSI

These are Saturn’s rings and moons taken by the Cassini Spacecraft. The smaller one (just below the rings) is Enceladus and the bigger one at the bottom is Tethys. I don’t have anything to comment, the picture is posted here purely for its beauty, and I don’t think anyone will disagree with me.

Quick facts: Enceladus is 504 km across and Tethys is 1062 km across. This view was obtained at a distance of approximately 272,000 km from Enceladus and 208,000 km from Tethys.

 
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