This month in the news: The Rosetta mission and the funny shaped comet.
Comets, leaving icy trails across the sky, have doubtless been noticed by humans for millenia. They were usually taken as bad omens, ominous signs in the sky warning of impending deaths. In 1577, Tycho Brahe measured the parallax of the 'Great Comet' of that year, and in conjunction with observations by other astronomers, managed to determine that the comet was at least four times further away than the moon - and that it was not, as was widely believed at the time, an atmospheric phenomenon. In 1705, Edmund Halley used a method from Newton's Principia to study past comets, fitting their orbits to parabolas. He found that three of the previously observed comets had very similar orbits, and deduced that they were one and the same - predicting its return. This was of course the comet that came to be named after him, Halley's comet; without a doubt the most well-known of these objects. It was realized that comets contained some sort of volatile material which would vapourize due to solar radiation, forming the characteristic 'tail'. In 1950, the astronomer Fred Whipple proposed that comets were icy objects with some rock, as opposed to being rocky objects containing some ice - the so-called "Dirty Snowball" model. With the advent of the space age, a large number of probes and flyby missions were launched, and their observations appeared to support Whipple's model. A notable event was the study of Halley's comet in 1986, when it passed into the inner solar system. A number of spacecraft flew very close to the comet, including the ESA craft Giotto, which got as close as 596 km.
Today, the Rosetta mission is following in the footsteps of Giotto. Launched in 2004 after almost 10 years of planning, this remarkable spacecraft has been making its way across the solar system, slingshotting around the Earth three times and Mars once, to get to comet 67P/Churyumov-Gerasimenko, named after its discoverers. The spacecraft has just sent back the first images we have of the comet - before July 2014, the only picture the mission planners and scientists had was a single pixel. This makes it difficult to plan a landing. Thankfully, the Rosetta team now has access to a constant supply of images from the craft's onboard camera, with the first pictures being taken on July 14th, and released to the public on the 21st. The Rosetta twitter account is currently posting pictures of the comet each day, and as of the 25th of July the probe was roughly 3200 km away from its goal – with an anticipated rendezvous with the comet twelve days from then, on the 6th of August.
The shape of the comet came as a surprise to the observers, though. It seems to be composed of two differently-sized parts - reminiscent of nothing more than a slightly deformed rubber duck. Planetary scientists call this type of object a "contact binary". While contact binary asteroids have been seen before, this is the first contact binary comet that has been identified. The formation mechanism behind the binary is still unclear; it is likely to have resulted from a merger, but whether the two pieces came from different sources or from the breakup of a single, larger object is still a mystery - one that the Rosetta mission hopes to solve when the spacecraft gets closer.
However, this lopsided comet will make things slightly more difficult for the second half of the mission: once Rosetta reaches the comet and enters "orbit" around it (with a relative velocity of a few cm per second), the team will attempt to place a lander on the comet, in November 2014. This will be done after spending a couple of months a mere 25km from the nucleus of the comet, so that it will have mapped it in great detail. The shape makes it more difficult than expected, as the two different parts of the comet may have different densities, leading to an irregular gravitational field. It will also be harder to comprehensively image a comet of this shape, and th number of feasible landing sites may be quite small. Nevertheless, the team remain confident they will be able to pull of what has never been done before and land a probe (named Philae) onto the comet.
If everything goes well, the lander will take data using the plethora of instruments it has on board including cameras, an X-ray spectrometer, two evolved gas analysers, and a radar-like probe for sounding out the interior of the nucleus. Both Philae and Rosetta will stay with the comet as it goes around the sun and back away from it, studying what happens. The mission is due to end in December 2015, after which its fate remains uncertain!
More information about Rosetta can be found on the ESA website, and we also interviewed the retiring mission director, Gerhard Schwehm, at NAM 2014.
Interview with Prof. Elcio Abdalla
Professor Elcio Abdalla works on quantum field theory at the University of São Paulo, and applies this to cosmology. In this interview, he talks about the mathematical models that describe the electromagnetic radiation, visible matter, dark matter and dark energy that seem to make up our Universe, and about the universal effect of gravity that allows us to observe them. He discusses how such models are tested and refined or replaced using observations, and describes the upcoming BINGO telescope, which will trace cold hydrogen deep into the Universe's past.
The Night Sky
Ian Morison tells us what we can see in the northern hemisphere night sky during August 2014.
High in the south in the evening is the Summer Triangle of Deneb in Cygnus, Vega in Lyra and Altair in Aquila, with the four brightest stars in Cygnus forming the asterism of the Northern Cross. The small constellation of Delphinus the Dolphin lies below Cygnus. The Great Square of Pegasus is rising in the east, and its top-left star, Alpheratz, can be used to locate the Andromeda Galaxy. Starting there, move left to the next bright star, curve up and right to another, go sharp right one more star, then move the same distance again to find our nearest neighbouring large galaxy, once known as the Great Nebula in Andromeda. Cassiopeia is high in the north, above Perseus and its bright star Mirfak, with the Perseus Double Cluster between the two constellations.
- Jupiter reached superior conjunction (behind the Sun in the sky) on the 24th of July, so it only becomes visible in the middle of August, low in the east-north-east before dawn and shining at magnitude -1.8. By month's end, it is 20° above the horizon at 05:30 BST (British Summer Time, 1 hour ahead of Universal Time). With a disc 32" across, a small telescope can be used to observe the equatorial belts and the accompanying Galilean moons. Jupiter lies within 4° of Venus from the 14th to the 21st.
- Saturn is in Libra, near the double star Alpha Librae. Its brightness declines from magnitude +0.5 to +0.6 during the month. It is 20° above the horizon at sunset, and a telescope reveals its 17" disc, ring system and largest moon, Titan.
- Mercury is so low above the horizon at nightfall that it is very difficult to spot this month.
- Mars begins August between the stars Spica and Alpha Librae, and moves from Virgo into Libra on the 10th. Its apparent size shrinks from 7.7 to 7" during the month, while it dims from magnitude +0.4 to +0.6. Its low elevation as darkness falls makes it difficult to observe surface details through a telescope, due to atmospheric scintillation.
- Venus rises in the east-north-east before dawn, reaching 20° above the horizon by sunrise on the 1st. Its angular size drops from 10.7 to 10.2" over the course of the month, but its percentage illumination increases from 82 to 97% as it moves towards the far side of the Sun from our perspective, so its brightness remains at magnitude -3.8 all month. Venus moves from Gemini into Cancer on the 11th, and then into Leo on the 27th, by which time it is only 14° above the horizon at sunrise.
- You can use binoculars or a telescope to observe the globular cluster M13 and the Double Double this month. The former is a lovely group of stars in Hercules, the latter is a pair of double stars in Lyra.
- Neptune can be found using binoculars or a small telescope this month. It is at opposition (opposite the Sun in the sky and near its closest approach to Earth) on the 29th and has a magnitude of +7.9, lying in Aquarius and reaching a maximum elevation of 27° around 01:00 BST. A telescope of 8 inches or more in aperture may reveal Neptune's largest moon, Triton.
- The Moon, Mars and Saturn are close together in the sky an hour after sunset on the 3rd, low in the south-south-west after sunset.
- The Moon makes a close approach to the Earth just as it becomes full at around 19:00 BST on the 10th. It rises at 20:15 BST, and is about as large in angular size as it ever appears on Earth, leading to the name of 'megamoon'.
- The annual Perseid meteor shower reaches its maximum this month, although the nearly-full Moon hampers viewing. Rising in the north-east, its radiant in Perseus is highest in the east and most active before dawn on the 12th and 13th as the Earth passes through the debris of Comet Tempel-Tuttle.
- Venus and Jupiter are just 12' apart in the east-north-east just before dawn on the 18th, and the open cluster M44 - also known as the Beehive Cluster or Praesepe - is just a couple of degrees away from them. You need a low horizon to see them, as they are just 5° above the horizon at 05:00 BST.
- Venus, Jupiter and a waning crescent Moon appear together an hour before sunrise on the 23rd, but require a low eastern horizon to be visible.
- A lunar feature known as the Straight Wall can be seen well using a telescope on the 4th and 17th, as the terminator is close to it, casting a long shadow. It is actually more of a gentle escarpment than a straight wall.
Ian Morison's new book, An Amateur's Guide to Observing and Imaging the Heavens, is out now.
Claire Bretherton from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during August 2014.
This month sees Scorpius and Sagittarius high overhead in the evening sky. Scorpius, a winter constellation, is easy to spot by its orange star Antares, which lies just east of the zenith. A curve of bright stars stretches out towards the right, forming his tail. Antares is a red supergiant star with a radius more than 800 times that of the Sun. To Māori, this group of stars is known as Te Matau a Māui: the fish-hook of Māui. Māui used this hook to pull a great fish out of the ocean which became the north island of New Zealand: Te Ika-a-Māui. The red star is known as Rehua, and represents a drop of blood that Māui took from his nose to use as bait. Below Scorpius is an upside-down teapot shape formed from the brightest stars in Sagittarius. The broadest and brightest part of the Milky Way lies towards Scorpius and Sagittarius, high in our eastern evening sky.
The centre of the Milky Way provides a whole assortment of stunning nebulae and star clusters to observe. Lying along the tail of the Scorpion is NGC 6231, a bright cluster of stars which looks like a small comet. Estimated to be only 3.2 million years old and nearly 6,000 light-years away, if the cluster were placed at the same distance as the Pleiades then some of its stars would be amongst the brightest in the night time sky. About halfway between the Scorpion's sting and the spout of the Teapot is M7. This is an open cluster of stars easily visible to the naked eye, and a lovely sight through a good pair of binoculars. Nearby and somewhat fainter, M6, the Butterfly cluster, is also well worth a look in binoculars. To the left of the Teapot's spout, and just about visible to the naked eye, is the Lagoon Nebula, or M8. This is a huge cloud of interstellar gas and dust where new stars are being formed, and where their ultraviolet radiation causes leftover hydrogen gas to glow. Along with the nearby Trifid Nebula (M20), the Lagoon Nebula is a good target for binoculars or a small telescope. The Trifid nebula combines emission and reflection nebulae with an open cluster of stars. This part of the sky also contains a number of globular clusters, each hosting hundreds of thousands of ancient stars that date back more than 12 billion years. Over 150 globular clusters are found in the halo of the Milky Way, and their distribution provided early evidence of the scale of the Milky Way and our position within it. The brightest globular cluster is M4, and this is also one of the easiest to find as it lies just 1.3° west of Antares. Appearing as a small fuzzy ball in binoculars and small telescope, a slightly larger telescope will begin to pick out individual stars. Also in this region, near the top of the Teapot, is M22, one of the closest globular clusters to us at distance of around 10,000 light-years.
From its bright centre in Sagittarius, the Milky Way stretches out from east to west in the early evening. Along its path are found the majority of the bright stars in our night-time sky. In the north, just to the left of the Milky Way, is the bright star Vega, which forms part of the constellation of Lyra the Lyre. Opposite, in the southern sky, the second brightest night-time star, Canopus, can be found in the constellation of Carina. To Māori in Aotearoa (New Zealand), this star is Atutahi or Ao-tahi, which means 'to stand alone'. Running back along the Milky Way towards Scorpius, we pass the False and Diamond Crosses before arriving at Crux, the Southern Cross. The smallest of the 88 official constellations, it has the appearance of a diamond shape of four bright stars along with a fifth fainter star. It is known to Māori as Te Punga, the anchor of Tama-reriti's Waka. Alpha Crucis appears to the unaided eye as a single star of magnitude 0.9, but small telescopes reveal it to be a double star with blue-white components of magnitudes 1.4 and 1.9. Nearby is NGC 4755, an open cluster of stars also known the Jewel Box. It is rich and bright with stars, showing delicate colours accentuated by an orange-red supergiant. It can easily be seen with the naked eye, but binoculars and telescopes reveal much more detail. Just to one side is a dark patch known as the Coalsack Nebula. This is a cloud of interstellar dust and gas that obscures the light from more distant stars, appearing as a darkened area against the bright backdrop of the Milky Way. To Māori it is known as Te Pātiki or the Flounder. East of Crux are the two bright pointer stars, Alpha and Beta Centauri, marking the front hooves of Centaurus the Centaur. The brightness and number of stars rapidly drops off when we look away from the path of the Milky Way, and after sunset the constellations of Virgo and Corvus can be seen to its west.
- Mars and Saturn sit near to Spica, the brightest star in Virgo, with the orange star Arcturus below. Mars and Arcturus have red hues, while Spica is a brilliant blue-white and Saturn is yellow. Mars and Saturn are at their closest on the 25th. Small telescopes reveal the Saturn's rings and its largest moon, Titan, looking like a small star around four ring diameters away from the planet. More powerful telescopes should reveal faint banding in the Saturn's atmosphere, along with gaps and colour variation in the rings, as well a number of smaller moons. Mars appears as a small, red disc, and is getting fainter as we move away from it on our faster inner orbit.
- Also in the western sky is the planet Mercury, climbing higher as the month progresses and setting 1.5 hours after the Sun by month's end. It is close to the crescent Moon the 27th.
- Brilliant Venus and Jupiter appear in the morning sky and are at their closest on the 18th, when there will be less than the diameter of the full Moon between them.
Odds and Ends
NASA has announced that astronomers using data from the Cassini Space craft have identified 101 distinct geysers erupting on Saturn's sixth largest moon Enceladus. Astronomers also now believe that the water in these geysers is erupting water vapour from the liquid water ocean underneath Enceladus's icy crust rather than ice vapourised by frictional heating on the moons surface. The NASA article can be found here.
An unusual art project has seen a bonsai tree and a bouquet of flowers photographed at the edge of space, or at least very high above the Earth. Azuma Makoto worked with "DIY space program" JP Aerospace to launch helium balloons to a height of 28 kilometres, with the fauna and a camera attached. The result is a strange and beautiful contrast between the isolated life forms, the curve of the Earth, the light of the Sun and the blackness of space.
Russian space agency Roscosmos has lost ground control of its satellite Foton M4. Foton M4 is home to five geckos; one male and four females. Roscosmos sent them to space to observe their reproductive habits in zero-gravity conditions. While they are still able to take data from the experiment, they will not be able to give the satellite the relevant commands to bring it back to Earth, so it looks like the geckos are stuck up there. More information is available from here or here.
|Interview:||Prof. Elcio Abdalla and Indy Leclercq|
|Night sky:||Ian Morison and Claire Bretherton|
|Presenters:||Adam Avison, Fiona Healy and Mark Purver|
|Editors:||Adam Avison, Claire Bretherton, John Field, Indy Leclercq and Mark Purver|
|Segment Voice:||Iain McDonald|
|Website:||George Bendo and Stuart Lowe|
|Cover art:||A meteor in the Perseid Meteor Shower photograhed by Ron Garan from the ISS in August 2011 CREDIT: Ron Garan, ISS Expedition 28 Crew, NASA|