Pluto and Charon Up Close

I was born in June 1989. Just 2 months after that, the Voyager 2 spacecraft flew past Neptune on its way out of our solar system and for the first time ever we saw real close up pictures of the solar system’s outermost giant planet.

Of course, I was too young to remember this, but it means that as I’ve been growing up I’ve been able to see amazing and beautiful photos of the 8 planets of our solar system. Pluto is no longer classified as a planet, but nonetheless it has been conspicuously missing from this set for my whole life.

That is, until this week. After travelling for 9 1/2 years, the New Horizons spacecraft finally flew past the only remaining “classic planet” that we had not yet explored. We are seeing real close up pictures of a new world, one that we’ve never seen like this before. And it’s glorious:

Pluto
Pluto in glorious colour | Image by NASA

This image is made by compositing the high resolution black and white image taken from LORRI (LOng Range Reconnaissance Imager) and colour imagery taken from Ralph (New Horizons’ colour imager). It was sent back to Earth by New Horizons prior to its exciting flyby of Pluto on Tuesday night (closest approach was 11:50 pm Tuesday 2015/07/14, New Zealand time), having been taken 16 hours ahead of time.

It sent back data ahead of time because during the 24 hour flyby New Horizons was busy collecting data. All of its instrumentation is built into the body of the spacecraft, so in order to point them the whole spacecraft needs to rotate. This means that in order to talk to Earth it has to look away from Pluto, so we had to wait in patient agony while it was collecting data.

After the flyby, New Horizons sent a packet of telemetry data back to Earth, containing information about how its systems were doing. The flyby was the most dangerous part of the mission; travelling at about 14 kilometres per second, an impact with even a tiny piece of debris could absolutely destroy the spacecraft and near Pluto is the most likely place to encounter such debris. Since New Horizons was travelling into the unknown, no one could guarantee this wouldn’t happen, although NASA was confident that the chance of such a collision was very low.

The “Phone Home” signal reached Earth at 12:52:37 pm on Wednesday 2015/07/15 (New Zealand time), telling us that everything went perfectly. New Horizons survived the flyby! The next stage of the mission is to send down all the data it collected, but getting data back from Pluto is hard. Even at the speed of light signals take over 4 hours to cross the distance, and the transfer rate varies from just 1 kb/s to a whopping 4 kb/s. It’ll take around 16 months to get all the data from the flyby down to Earth.

This morning (7am Thursday 2015/07/16 New Zealand time) NASA held a press conference in which they released some new images, including a high resolution image of Pluto’s largest moon Charon:

Charon
Pluto’s largest moon Charon | Photo by NASA

There’s a lot of interesting stuff going on here, for example that little notch you can see in the upper right is a canyon that’s 6-10 kilometres deep. So deep that you’re looking through it to the space behind Charon. The dark polar region at the top, which has been informally named “Mordor”, is also interesting. The fact that impact craters allow lighter material to peek through makes it seem as though the dark material on the surface is just thin layer. Apparently one possible cause of this could be some form of atmospheric transfer from Pluto.

At the press conference, NASA also released a much higher resolution of an area in the “heart” region of Pluto. The heart has been named Tombaugh Regio after Clyde Tombaugh, who discovered Pluto in 1930. Although he died in 1997, New Horizons carried some of his ashes on board to honour his request for his ashes to be taken to space. Here’s the image:

Zoomed in on Tombaugh Regio, Pluto (rotated 90 degrees counterclockwise) | Photo by NASA
Zoomed in on Tombaugh Regio, Pluto (rotated 90 degrees counterclockwise) | Photo by NASA

This is a very interesting image. The first thing you might notice is the mountains. These are about 3.5 kilometres tall, and almost certainly made of water ice. Also, there are no impact craters in this photo. That’s strange, really strange. It means that Pluto’s surface is new, probably less than 100 million years old.

Pluto is the first icy world we’ve seen that isn’t also the moon of a gas giant. Icy moons like Saturn’s moon Enceladus get stretched and heated by the tidal forces placed on them due to their close proximity to much more massive bodies. Tidal forces are caused by a gradient in the strength of gravity – parts of a body closer to a source of gravity experience a stronger force than those further away, and when this gradient is strong (which happens when you’re nearby a much larger body) the tidal forces are greater. The Moon’s tidal influence on Earth drives our oceanic tides, and the extreme tidal forces you’d experience crossing the event horizon of a black hole are what would turn you into a space noodle.

But Pluto isn’t near any other large bodies that could exert this sort of tidal force on it. Charon is around half its size, but Pluto and Charon are both tidally locked to one another, meaning the same parts always face each other, so the tidal forces never change. There must be some other process driving geological activity on Pluto, and we don’t know what that is yet.

There’s a lot more interesting stuff coming out of the New Horizons mission, and of course much more to come now that we’re getting data back. I recommend you follow Emily Lakdawalla’s blog at the Planetary Society for up to date and accurate information. Her latest article is a great overview of what we learned at this morning’s NASA press conference.

This isn’t the end for New Horizons, even after the 16 month period of sending all its data down it’s going to continue into the outer region of the solar system known as the Kuiper belt, and hopefully visit one or two more icy worlds.

For now, though, we have finally completed our reconnaissance of the solar system. This is the end of the beginning.

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How Well Do You Know Your Moon?

Phil Plait, who writes the wonderful Bad Astronomy blog over on Slate, noticed something interesting in a recent episode of The Simpsons:

Photo by 20th Century Fox Film Corp.
Photo by 20th Century Fox Film Corp.

To the untrained eye, nothing about this cartoon image is likely to seem unusual. But if you spend a lot of time looking at the sky, and you have the additional context that this scene occurred in the evening, then the Moon is actually quite revealing.

(For those of you thinking “it’s just a cartoon, don’t expect it to be accurate”, I realise that. But if you decide to treat it as though it must be accurate then it can be interesting to think about so bear with me.)

To understand why that is and how we know, we have to have a think about how we look at the Moon.

The Moon orbits the Earth in a plane that’s pretty well aligned with the plane of the solar system. This means if you drawn a line in the sky tracing the path of the Moon, you’ll also find the Sun and the planets roughly on that line. This is why we experience solar and lunar eclipses, which happen when the Sun and Moon are lined up particularly well with the Earth. Because the line is where eclipses happen, it’s called the ecliptic by astronomers.

Earth’s axis is tilted by 23.5° relative to this plane, but if you’re not too close to the equator (for example, if you’re in New Zealand or the USA) then you can say that if you projected the equator into the sky it would be in roughly the same position as the ecliptic. If you’re in the southern hemisphere, that means it’s to the north, and if you’re in the northern hemisphere, it’s to your south.

So, if you’re looking at the Moon from New Zealand, you must be looking roughly to the north. If you’re looking at it from the USA, you must be looking roughly to the south. This also means the Sun and Moon, moving east to west across the sky as the Earth spins, appear to move right to left from the southern hemisphere and left to right from the northern hemisphere.

When we look at the Moon, we’re seeing the same thing no matter where we are on Earth except for one thing: which way is “down”. The “bottom” of the Moon in the part that’s closest to the horizon. If you travel to the other hemisphere, and you’re familiar enough with the Moon, you may notice that it appears upside down. That’s because the direction of “down” has swapped – from roughly north to roughly south (or vice versa if you’ve travelled from north to south). If you want to see what the Moon looks like from the other half of the world, you have to bend over backwards (or lie on the ground). This is also why the Moon will appear to have rotated if you compare it when it’s rising to when it’s setting.

One more thing: the Moon orbits us in the same direction as we’re spinning, which means it moves across the sky slightly slower than the Sun. Each day, the Moon rises roughly 50 minutes later than the day before, so that over its 28 day cycle of phases this sums to 24 hours.


Now, getting back to that image from the Simpsons episode. That scene was apparently in the evening, and the Moon is low on the horizon. That means the Moon must either be about to set or have just risen. If it had just risen after sunset, then it was recently full (because a full moon rises at sunset and the moon rises later each day), which means its phase would be a waning gibbous. Waning refers to the fact that it is on its way from being full to being new, and a gibbous is the shape made by a circle with a crescent cut out from it.

In the picture, the Moon is obviously a crescent, so it can’t have just risen. If it’s just about to set after sunset, then is must just have been a new moon (because a new moon sets at sunset and the moon rises later each day), which means its phase would be a waxing crescent. Waxing refers to the fact that it is on its way from being new to being full, and the crescent refers to its curved shape.

Another thing we know about the Moon is that its lit side always faces the Sun. For example, the lit side of a full moon points right back at us, because from its perspective the Sun shines on it from behind us. If the Sun has just set, and the Moon is just about to set as well, then the lit side of the Moon must be facing the Sun. As the Sun sets in the west, this means the lit side of the Moon should also be facing west if it is a waxing crescent.

In the picture from the Simpsons, which we’ve established should be a waxing crescent, the lit side of the Moon is facing to the left. But remember, if you look at the Moon from the northern hemisphere you must be looking to your south, so west should be on your right. So if the waxing crescent moon is lit on its left, then you must be looking north to see it, which means you’re in the southern hemisphere.


Unfortunately, a lot of pop culture doesn’t get the Moon and its phases right. I know it’s such a tiny thing, and typically when they don’t get it quite right I can’t say I mind too much (although I often can’t help but notice), but I really love it when they put in that extra bit of effort to get it correct.

Almost all video games with day/night cycles where you can see the sky have the Moon orbit in 24 hours. Some of them include phases, although technically if your Moon always rises at sunset then it should always be full. I can forgive video games fairly easily though, I’m probably the only person who cares and I understand it could take significant development time to get proper lunar phases in. The only example I can think of that gets it right is Kerbal Space Program, where accurate celestial mechanics is an important part of the game.

Some books have issues with the Moon as well. Last year I was reading the book Ship of Theseus, and one scene describes the protagonist seeing the crescent moon rise as it gets dark. But crescent moons never rise as the Sun sets, light simply doesn’t work that way.

Movies often have trouble with it as well. The worst offender I’ve seen is the final scene from the movie Cloud Atlas:

Final scene from the movie Cloud Atlas
Final scene from the movie Cloud Atlas

Remembering that the lit side of a moon points towards its sun, and this applies even with multiple moons, that image implies some very strange things about the nature of light.

I’d love to see more media put more effort into getting this right. I know that one author who paid particular attention to this detail was J.R.R. Tolkien, who tried hard to get the phases of the Moon consistent with his dates when writing The Lord of the Rings and apparently did a pretty good job of it too.

My favourite example of well-documented in-depth world building doesn’t involve the Moon but I’d like to share it here anyway. My brother Jeremy (who’s currently working as a concept artist at Weta Workshop, a job he got soon after leaving Uni) worked on creating a deep and internally consistent fictional history to earn his masters degree in 2013. He ended up creating a fake National Geographic article from 1932 recounting the reporter’s visit to the settlement of Elkwood. The article only scratched the surface of all the thought he put into the work, if you want to see what he came up with you can read both the article and his exegesis explaining his research methods here: Creating Elkwood: building an alternate history

When fictional worlds are deep and internally consistent they become that much more enriched. If you know of any that have represented lunar cycles particularly well (or particularly poorly) let me know in the comments.

We Landed on a Comet

So, last night was exciting. The European Space Agency’s (ESA) robotic spaceship Rosetta arrived at the comet 67P/Churyumov-Gerasimenko in early August, after an amazing journey comprising of over 10 years and four gravity slingshots. Last night, it separated from its lander module, Philae, and sent it to touch down on the surface of the comet.

What I’ve been able to gather from watching the live stream last night and what I saw on Twitter when I woke up groggily for 2 minutes at 5:15 this morning is that not everything went to plan, but the landing seems to have been successful.

Philae (the lander) has several devices to make the landing easier. One of these is a “cold gas thruster”, a small engine to push it gently into the surface of the comet so it wouldn’t bounce off (remember the comet has extremely little gravity relative to something like the Earth or Moon). This engine failed to start working before the spacecraft separated, but the team decided to go ahead with the landing anyway.

Another device Philae has to help with the landing is a pair of harpoons to skewer the surface, but these also failed to fire. As far as I know they’re not sure yet why they failed, but Philae did make it to the surface, so the comet landing was a success.

The ESA be getting data back from Philae but I don’t think they know yet how it landed or where exactly it is relative to the landing site. There’s a danger it could be on its side, for example, which would prevent some of the experiments it’s carrying on board from going ahead. Time will tell, though.

A photo of the comet taken from Philae when it was only 3 km away has been posted to the official Twitter account:

Photo credit to European Space Agency, ROLIS camera on Philae

Photo credit to European Space Agency, ROLIS camera on Philae

UPDATE 2014/11/14

Since the landing a few other things have come to light. First, presumably because the harpoons failed to fire, Philae bounced of the surface twice. Although it bounced pretty much straight up, the comet was rotating beneath it so its final landing zone is a few hundred metres away.

Also, Philae has landed on its side. It’s still taking photographs and sending back data, so that’s good, but the fact that it’s on its side may mean that some of its experiments may not be able to go ahead. Phil Plait has a good write up explaining these updated on Bad Astronomy and Emily Lakdawalla has a more detailed one on her blog the Planetary Society.

Movement of the Moon

If you were up late last Wednesday, you’d have gotten the chance to watch the second total lunar eclipse this year. A lunar eclipse occurs when the Earth moves directly between the Moon and the Sun, therefore blocking the light to the Moon and making it turn dark.

This is very cool to watch, especially if you get a good look at the Moon during totality, when it is in the darkest part of the Earth’s shadow known as the umbra. At this point, the only sunlight reaching the Moon is that which is refracted through the Earth’s atmosphere. For the same reason as why sunrises and sunsets appear a lovely reddish colour, this light is also quite red, and as the Moon reflects some of this light back at us it appears a dim bronze colour.

This is one of my favourite space facts, and I find it quite poetic – you’re looking at the reflected light of all the sunrises and all the sunsets in the world, all at once. There are many spectacular photos of this effect online if you care to search for them too. But this is not what I want to write about in this article.

If you were watching Wednesday’s eclipse from Auckland, as I was, you’d probably have been disappointed to see that it was quite cloudy for the duration of totality. However, you were probably able to get a good view of the first part of the eclipse, when the Moon is moving into the outer part of the Earth’s shadow known as the penumbra.

Greg O’Beirne managed to put together a great compilation of this part of the eclipse:

Photo by Greg O'Bierne
Photo by Greg O’Beirne

As you can see, it looks rather like the Moon is having a bite taken out of it. In this compilation the position of the Moon is held roughly constant but in reality it’s the Moon that is moving here. During a lunar eclipse, we are given the rare opportunity to directly observe the Moon’s orbital motion.

Because the Earth is spinning, the Moon always appears to move across the sky from east to west (in the southern hemisphere, this effectively means it is moving right to left). The time it takes to move across the sky varies with the time of year, but it takes roughly 25 hours to do a full circuit.

Because of the Earth’s spinning, generally the only way we can usually observe the Moon’s orbital motion is by looking for it at the same time every day. If you do this, then instead of watching it migrate east to west over a day, you’ll see it move slowly from west to east over a couple of weeks.

In order to watch the Moon move directly, it would be possible to watch it move relative to a stationary background object. The stars could serve this purpose while the Moon is up at night, although generally it’s bright enough that it’s very difficult to see any nearby stars. The occultation of Saturn earlier this year, which I watched from home through my telescope, gave me a chance to observe its movement against the relatively stationary planet. Like with the stars though, you simply wouldn’t be able to observe this with your naked eye.

A solar eclipse is also an opportunity to directly watch the Moon’s orbital motion, as we can compare it to the Sun, as its movement is fairly negligible for everyday purposes when compared with that of the Moon. The problem there, of course, is that you can’t look directly at it without damaging your eyes. A lunar eclipse gives you the same opportunity except, unlike a solar eclipse, you can watch it directly.

If we ignore the Earth’s spin, then both the Sun and the Earth’s shadow (which, of course, must always be directly opposite one another) each take one full year to move the whole way across the sky. Because the 365 1/4 days in a year is very close to the 360 degrees in a circle, we can say that they move roughly 1 degree every 24 hours, or half a degree every 12 hours. This sounds pretty slow, but half a degree is roughly the size of the full Moon so it isn’t entirely negligible.

It takes about an hour for the Moon to move fully into the outer part of the Earth’s shadow, so in this time it moves roughly 1/12th the diamater of the Moon. For the sake of simplicity, let’s ignore this motion as well. Below I’ve put together a (rather clumsy and very imprecise) animated gif, using the images from Greg O’Beirne’s great compilation as its frames, to show this motion of the Moon holding the position of the Earth’s shadow roughly constant:

Lunar Eclipse Oct 2014 Animation

If you want to view this for yourself, the next lunar eclipse visible from New Zealand isn’t too far away. You may have read that there won’t be another total eclipse visible from New Zealand until 2018, but on the 4th of April 2015 there will be a partial solar eclipse in the evening where you’ll be able to see this. In the meantime, have a look up at the sky occasionally and notice where the Moon is (using landmarks as a guide to remember its position will help). If you make it habitual to do this at a specific time (I do it every day when I leave for work, for example) then you’ll be able to watch the Moon’s slow movement backwards across the sky.

Telescopes Are for Everyone

UPDATE 2014/17/01 1:54pm NZDT:

The content of the page has now been updated for the better, although its title still says “Telescopes for astronomy or land viewing; a great gift for him”.


Over the summer holidays, for the first time ever I took some binoculars outside on a clear night and looked up at the sky. I had some idea of what I could expect to see, but still the number of stars I could see surprised me. It was marvellous and elating, and settled my resolve to (finally) take up amateur astronomy as a hobby instead of just an idle interest. As a first step, I plan to buy a telescope. Earlier today, I searched online to see what was available and was very disappointed by something that I found.

The New Zealand website telescopes.net.nz sells telescopes, which is all well and good, but they have decided to market them as a “gift for him”. The title of the page is “Telescopes for astronomy or land viewing; a great gift for him.”, and the content advertises them like this:

A telescope makes the perfect Christmas gift for him and we have picked the best brands available in the market today. Whether you want a gift for your Dad, the man in your life or you just want to indulge yourself, you will find it here.

I was thoroughly disappointed and, horribly, not particularly shocked to see such sexism regarding this. As a white straight male, I have tremendous privilege; my male privilege would let me simply ignore this if I wanted to, as it’s not an affront to me. However, I follow various fantastic (I cannot stress that enough) female astronomers, astrophysicists, and astronauts through Twitter and blogs, and have long been aware of the existence of sexism in STEM fields. In fields such as astronomy, there is a large gender gap – these fields are dominated (in terms of numbers) by men – and this is only made worse by commonplace sexist attitudes. Even if some of this behaviour is entirely innocent, it still does active harm by excluding women and girls from astronomy. Although my privilege gives me the option to ignore this, I consciously choose to be a feminist and fight these attitudes. Telescopes are for everyone.

I shared this page on Twitter, and the great Katie Mack (@AstroKatie) sent them an email. Seeing this, I also sent an email to telescopes.net.nz, which I include in full below, urging them to change the content of their website and the attitude from which it sprang. If you would like to do the same, you can contact them at staff@telescopes.net.nz. Here is the email I sent them:

Dear Telescopes New Zealand,

I’m interested in amateur astronomy, having for the first time viewed the night sky through binoculars over the summer holidays and marvelled at how many stars I could see. I decided that in the new year, I would like to buy a telescope and take up amateur astronomy as a hobby. I found your website when searching for a telescope online.

However, when I came upon your telescopes page I was very disappointed to see it advertised telescopes as a “gift for him”, casually excluding all the women and girls that are interested in astronomy. Unfortunately, women already face significant sexism in STEM fields, including astronomy. Attitudes such as the one shown on your website only encourage this hostile environment, and harm astronomy as a whole.

I hope that you will revise the content of your website. I’m sure you can see that its current content, however innocent your intentions may have been, contributes to a harmful atmosphere of exclusion. Taking a more positive attitude toward women in astronomy will only benefit yourselves, amateur astronomers, and the field of astronomy itself.

Sincerely,
Mark Hanna

I’ve set up a change detection service monitoring the page, and hope to see it change for the better soon. If it doesn’t then, needless to say, I certainly won’t be buying my telescope from telescopes.net.nz.

Partial Solar Eclipse in Auckland

This morning there was a solar eclipse. From Auckland, I was able to see the sun about 87% covered by the moon just before 10:30 this morning. My girlfriend Eileen managed to get 4 pairs of solar viewing glasses from Stardome observatory last night (apparently the last 4 they had at the time), and I took 3 with me to work to share around.

I managed to take a few mediocre photos of it by putting the solar filter in front of my iPhone’s camera lens. Most of the time it was all glare, but if there was just the right amount of cloud coverage you could photograph the crescent shape. Here’s the best photo that I managed to take:

Partial solar eclipse
Partial solar eclipse

Meanwhile, Eileen was viewing it from Auckland Uni and sent me this great photo of a projection of the eclipse, which shows how you can safely view it if you lack the equipment to look directly at it:

Solar eclipse projection
A projection of the partial solar eclipse viewed from Auckland University

Like she said:

It is quite neat – holding the sun in the palm of your hand

On a related note, here’s a photo from earlier this year that she took of a similar projection showing the transit of Venus:

Projection of the transit of Venus
A projection of the transit of Venus