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How big is the largest crater on Callisto?

Posted: 29 Mar 2018 10:00 PM PDT

Most of the United States could fit into Callisto's giant crater. Image Credit: NASA

Most of the United States could fit into Callisto's giant crater. Image Credit: NASA

Asked by Jon Gray

An impact basin known as Valhalla is Callisto's largest crater with a diameter of over 300 kilometres (190 miles), while concentric rings that surround it extend out to 3,000 kilometres (1,900 miles) – nearly large enough for the entire USA to fit within its boundaries. Made of rock and ice, this moon of Jupiter is one of the most cratered worlds in the Solar System. In fact, Callisto's surface is so battered that any new impacts on its surface will likely erase older craters.

Just how Callisto got its battered surface is a tale that's also true of other bodies in our Solar System, such as our Moon. It's suggested that Callisto got its distressed surface with the help of a period of heavy bombardment from asteroids and comets, around 4 to 3.8 billion years ago.

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April Night Sky: This month’s planets

Posted: 29 Mar 2018 08:56 AM PDT

VENUS (20:00 BST on 1 Apri)l

Constellation: Aries moving into Taurus

Magnitude: -3.9


Venus will return to the sky in April as a beautiful "Evening Star," bright enough to dominate the sky after sunset and draw the eye away from everything else. Not only that, but it will be in a part of the sky rich with star clusters, and will have a spectacular close encounter with the young Moon mid-month.

At the start of April Venus will be relatively low in the west after sunset, but with each day that passes it will climb a little further away from the Sun, improving its visibility until it is setting more than three hours after the Sun. To see Venus at its best you'll want to be somewhere with a clear view to the west, as your viewing won't be cut short by the planet disappearing behind trees, a hill or buildings. It will be immediately obvious to the naked eye, but if you have a telescope it will show you Venus as a bright, gibbous disc.

On 17 April, a beautiful, crescent Moon will be shining below and to the left of Venus. By the next evening the Moon will shine to the planet's upper left, and you should see the subtle lavender glow of Earthshine illuminating the dark part of the Moon's disc. After sunset on the 19th the Moon will have climbed further away to Venus' upper left, but they will still be a stunning sight together in the twilight.

In late April Venus will appear to drift up towards, and then pass, the famous Pleiades star cluster. On the evening of 24 April the planet and cluster will be just under three-and-a-half-degrees apart. This celestial fly-by will look particularly pretty through binoculars. Venus will then slide up between the Pleiades and the nearby V-shaped Hyades cluster. Look for Venus shining alongside the Hyades' brightest star, red-hued Aldebaran, on the 27 April.

Venus is often called "Earth's Twin" because it is roughly the same size, but the similarities end there. Earth is an oasis compared to the furnace-hot nightmare world of Venus. Venus is thought of by many planetary scientists as the forgotten planet; although a handful of space probes have been sent there, and other space agencies have studied it, other planets, notably Mars, tend to get more attention paid to them by NASA. Lots of missions to study Venus have been proposed over the years, but none have been approved. This is a great shame, because not only is Venus a fascinating planet in its own right, but studying its climate and weather in the same depth other missions have studied Mars and Saturn would tell us a lot about global warming and atmospheric science, which might help us combat climate change here on Earth.

MERCURY (22:00 BST on 24 April)

Constellation: Pisces

Magnitude: 1.2


The closest planet to the Sun will be so close to it in the morning sky this month that it will be almost impossible to see. If you are determined to try and find it you'll need to be scanning the eastern sky around half an hour before sunrise, preferably using a pair of binoculars. To prevent injuring your eyes, be sure to stop before sunrise.

MARS (04:00 BST on 07 April)

Constellation: Sagittarius

Magnitude: 0.3 brightening to -0.3


Mars will stay low in the sky this month. At the start of the month Mars will be very close to Saturn – just three Moon widths from it before dawn on April Fool's Day – but as the days pass they will move apart. Look for the waning gibbous Moon close to Mars and Saturn before dawn on 7 April, and to their left the next day.

JUPITER (22:00 BST on 24 April)

Constellation: Libra

Magnitude: -2.4


The morning sky belongs to Jupiter this month. Strictly speaking the largest planet in the Solar System is an evening object, because at the start of the month it rises before midnight, and by month's end rises before 10pm, but it will be at its best in the early hours. Shining at magnitude -2.4, the planet will easily be the brightest thing in the sky until sunrise. To Jupiter's lower left you'll see the planets Saturn and Mars huddling close together, but neither will come close to Jupiter in terms of brightness or beauty. Look for the Moon shining to Jupiter's upper right on 3 April and to its upper left the next morning.

SATURN (04:00 BST on 07 April)

Constellation: Sagittarius

Magnitude: 0.5


Saturn is visible in the morning sky throughout the month, keeping brighter, redder Mars company low above the southern horizon until morning twilight. Make sure to look out for the Moon shining close to Saturn before dawn on the morning of 7 April, when they'll be just over four degrees apart.

URANUS (19:00 BST on 16 April)

Constellation: Pisces

Magnitude: 5.9


Although Uranus will be above the western horizon after sunset this month, it will not be visible because it will be too close to the Sun. Unlike bright planets such as Venus and Jupiter, Uranus is so faint that its weak light is overwhelmed by a bright background sky, and this month it will be setting barely an hour after the Sun.

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Deep Sky Challenge: Seek the Hunting Dog and the Great Bear’s night-sky jewels

Posted: 29 Mar 2018 08:55 AM PDT

Sunflower Galaxy (Messier 63). Image credit: NASA/ESA/HLA/STScl/ST-ECF/CADC

For those of us in the Northern Hemisphere, high overhead on spring nights can be found the constellations of Canes Venatici (the Hunting Dogs) and Ursa Major (the Great Bear). Both contain several bright distant galaxies along with many fainter ones, as well as an interesting nebula. Possibly the most famous of all the double stars, which is in fact a multiple star system, lies in this region too. There are some well-known galaxies that will be relatively easy target for even small telescopes but many will require a larger aperture and dark skies to see well.

Image credit: NASA/ESA

1 – The Whirlpool Galaxy (Messier 51)

Use the tip of the Great Bear's tail to find this interacting galaxy. You'll need at least a small telescope to pick out a diffuse patch of light with a bright central region at its heart.

2 – Pinwheel Galaxy (Messier 101)

Scopes with an aperture of about three inches will reveal a nebulous haze with a bright centre, while an eight-inch instrument will show a bright, condensed core surrounded by nebulosity.

3 – The Owl Nebula (Messier 97)

This is a planetary nebula – a star, which has shed its outer shell of gas. Larger telescopes will show two dark patches that give this deep-sky object its appearance.

4 – Messier 106

Spiral galaxy Messier 106 can be picked up with binoculars, while small telescopes show a diffuse patch with a bright centre. An eight-inch instrument will reveal details of the structure.

5 – Mizar and Alcor

The widest of the naked-eye double stars. Through the field of view, the stellar duo twinkle as a pair of white-blue jewels, where Alcor is the faintest of the pairing at a magnitude of 4.

6 – Sunflower Galaxy (Messier 63)

One of the prettiest spiral galaxies in the night sky. A large telescope with medium power shows it well. With the right aperture, usually ten inches or more, you'll can pick out the dust lanes.

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Top sights to see on the Moon this weekend

Posted: 29 Mar 2018 08:54 AM PDT


1. Plato

2. Archimedes

3. Apennine Mountains

4. Mare Crisium

5. Ptolemaeus, Alphonsus, Arzachel

6. Tycho

The different phases of the Moon offer different visual treats and delights. The crescent phase, whether it's a very young 'new' Moon hanging in the western sky after sunset, or an old waning Moon glowing above the eastern horizon before sunrise, is a beautiful sight. It can look particularly striking if it happens to be shining close to a bright planet. If the bright, sunlit crescent is quite thin you can often see the rest of the Earthfacing side of the Moon glowing with the subtle lavender light of Earthshine, too.

Despite what many observers will tell you, the full Moon is not the worst lunar phase to observe. True, with the Sun beating down mercilessly from high above there is no surface relief to see, no shadows are cast behind the Moon's jagged mountains or into the bowls of its deep craters, but the full Moon is when it is easiest to see the contrast between the dark lunar seas and its rugged highlands, and to identify its major features too. Full Moon is also the best time to see the bright 'rays' streaking across the Moon's face – trails of dusty debris sprayed out across the Moon by the impacts which blasted the youngest craters out of the surface. Also, few sights in astronomy can compare with seeing a bloated full Moon rising up from behind the trees like an enormous silvery hot air balloon.

However, I have always thought that one special day of the lunar month offers the best of both worlds, and provides stunning views through binoculars and small telescopes. When the Moon is just slightly gibbous, a day past first quarter – what many people call a 'half Moon' – it offers the observer fantastic views of every type of lunar feature. With the terminator – the line between lunar night and day – running almost straight down the middle of the Moon's face the light is just perfect for seeing its craters, mountain ranges, sprawling seas and long debris rays, too.

Binocular views of the Moon the day after first quarter are fascinating, with the seas on the eastern side of the Moon's face clearly visible as dark, bluegrey splodges, and the largest craters along the terminator looking like pock marks. Through a small telescope with a low-power eyepiece, with the Moon almost filling the eyepiece, you can easily imagine you're a space tourist, flying towards the Moon in a spaceship.

Increase the magnification so you're looking straight down into the craters along the terminator and you'll feel like you're standing behind the astronauts of the future as they descend towards the surface, looking for a safe landing site, just as Armstrong and Aldrin did in 1969 when they guided the Eagle lunar module towards its historic landing on the Sea of Tranquility.

When the Moon has just passed first quarter you will be able to see the sweeping curve of the jagged Appenine mountain range, right on the terminator towards the north. Just above those mountains the crater Archimedes will stand out from the surface in stark relief, looking as fresh as if it had been made the day before. To the east, next to the curving limb, the oval Mare Crisium will look like a dark thumbprint on the Moon, and between it and the terminator other dark seas will form the shape of a crab's claw. In the centre, just to the right of the terminator, a chain of three craters, Ptolemaeus, Alphonsus and Arzachel will look very impressive. At the top of the disc, on the terminator, the dark-floored crater Plato will stand out clearly, while back towards the bottom of the terminator, the young crater Tycho will be starting to emerge from the shadows. As you stare down into it, Tycho might remind you of a bullet hole in a wall, or a pit left on the surface of a frozen lake after the impact of a heavy stone.

This month the Moon is at first quarter on the evening of 22 April, and if you observe it on that night you will still have fantastic views, but the following night those views will be just a little better. If you don't believe us, take a look yourself!

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Space Rocks: An interview with former Ash guitarist Charlotte Hatherley

Posted: 29 Mar 2018 06:48 AM PDT

AAS: Are you excited for Space Rocks? What can we expect from you at the event? 

Yes, I'm really excited! I mean it's the first actual solo gig I've done in years, maybe four or even five years. So that's exciting. But then the whole event is just so cool, with this mix of science fiction, culture, film and music. This is so up my street as a science fiction fan.

I'm going to be there the whole day, because there's like three different events, a daytime thing and a panel discussion. I'm just going to be there geeking out and trying to introduce myself to Tim Peake.

AAS: So what's inspiration behind this combination of science fiction and music?

Well I've always been interested in science fiction, particularly Phillip K. Dicks when I was growing up. Then I discovered the world of 1980's sci-fi with films such as Bladerunner, Aliens, The Terminator, Robocop and The Thing. All of those films made a huge impression on me.

For some reason, I missed out on Star Wars. I don't know why. I was born in 1979, and I just didn't go to the cinema to see it, or see it on TV. So I missed that more mainstream sci-fi, and went straight into the dark stuff, like horror. That's the stuff that stayed with me, really. So when I came to making my new records, I just indulged myself in all those influences.

AAS: What can listeners enjoy from your exclusive sound?

It's an emotional record and it's very honest, which I don't think any of my previous solo records have been. I've been quite guarded with my lyrics [in the past], but this time I was going through a difficult breakup and reflecting on previous relationships, trying to fix what I thought was going wrong in my life. I suppose I used this album as therapy, really.

I think that people can really relate to the lyrics and the sentiment of the songs, but at the same time, I didn't want to do a boring indie-rock record like I've done many times before. So now I have a completely different sound. It is kind-of melancholy electro-pop [genre]. With the videos and artwork, I made the decision to be quite out there visually. So the whole aesthetic is like an inspired story about a heartbroken alien travelling the universe.

So hopefully it's going to a mixture of very down-to-earth, grounded, emotional music mixed with a quite visually interesting presentation of me on stage with the artwork and the video. I really put a lot of love into that.

AAS: What made you go from rock band Ash to this new futuristic sci-fi sound?

Age! I mean, when I was in Ash I was 18, and I primarily wanted to just be a guitar player in a band, I suppose. I spent about eight years touring and travelling with them. So when I left, I was 26/27 [years old] and I knew that I wanted to write music, but I felt a bit lost when I left Ash. It took a few years to find my feet, and I think the one thing that has been constant in my life is that I've written songs.

As I've become more of a session musician, I've played with other musicians and I've travelled with other musicians. But I've also had the solo projects on the side. I've also not been successfully able to promote it, because I'm usually so busy with other people and other projects. So I suppose it's something I always turn to, and after every record I put out I say, "Well that's the last one I'll do, I'll never do that again." And then a couple of years down the line, I feel like doing it again.

So it's quite an unusual trajectory my career, I suppose, with it going from Ash to where I am now. But it has been in 20 years. It's crazy! I can't believe it! The passage of time has somehow led me to be dressed up as a full-blown alien.

AAS: Could you have ever imagined your career playing out like it has?

I don't know, but I think I would have been quite proud of myself, if I'd of known. I've always been into quite prog-rock [progressive rock] music, like I love Kate Bush, Peter Gabriel and David Bowie. People who are quite flamboyant and constantly changing their style. I don't think it's anything that wild, but I guess it is a little bit coming from 4 piece rock band to where I'm at now. I'm just glad I'm not doing the same thing over and over again. I think that's important.

AAS: We hear that you've previously worked with the European Space Agency (ESA) on music videos…

Yes. I did a video for my first single called A Sign from the new record, directed by Gavin Rothery, who worked on the film Moon and is a big sci-fi fan. We did this music video together and I put it up online, and Mark McCaughrean from ESA got in touch. He sent me a direct message basically saying, "I love this video and I love the music." And he's like the senior science adviser at the ESA, which kind of blew me away.

He [Mark] was involved in the Rosetta mission, and a big part of his job is mostly outreach, to promote ESA. So we got talking about how we could work together, and Space Rocks was sort of born from that conversation. Then I met Alexander Milas, the promoter, and we talked about how we could make it happen, and that was just over a year ago. It's amazing how quickly it has all come together.

AAS: What aspects of space excite you in particular?

I mean I could have dipped my toes into reading about new science, as there are so many amazing books out there, but I have limited brain capacity for this stuff.

I just really love reading about it. Then when you just sit and talk to someone like Mark [McCaughrean], it just blows you away. All the stuff he knows and talks about, that's his life. But I think I'm more of a happy passenger in a project like this [Space Rocks]. I love reading about science, and I'm really interested in futurism and modernity. Also the actual parallels between books, like the Phillip K. Dicks books that were written in the 1950s, which you can see happening now, as if they were living in a really unique time. So I try to digest as much as I can, but I'm in no means a particularly clever person.

Space Rocks takes place at the indigo at The O2 in London on Sunday 22 April. Book your tickets now! For updates follow Space Rocks on Twitter and Facebook