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Dawn reaches Ceres

by Tanya Hill
Publish date
5 March 2015
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When NASA’s Dawn spacecraft is captured into orbit around the dwarf planet Ceres on Friday, March 6, there will be no fanfare in mission control. In fact, the spacecraft won’t even be in radio contact. There’s no need, because Dawn’s path is set – this is a spacecraft unlike any other.

What makes Dawn unique is its ion propulsion system, which gives the spacecraft incredible manoeuvrability. Instead of using large bursts of thrust to get where it’s going, Dawn takes the slow and steady approach. Its ion engine delivers a tiny but continuous thrust that can last for days or weeks at a time.

Over the last two-and-a-half years, Dawn has been slowly reshaping its trajectory to bring it near Ceres and, most importantly, to match the dwarf planet’s speed – Ceres travels around the sun at nearly 64,000 kilometres per hour.

For other planetary missions, entering orbit is make or break. It’s an intense moment that hopefully ends in jubilant celebration when all goes as planned and the spacecraft momentously falls into orbit. But Dawn’s slow approach means that it is now right on course to guarantee capture by Ceres’ gravity.

Dawn is captured by Ceres' gravity The spacecraft’s approach trajectory with the white circles spaced at intervals of one day. This indicates the spacecraft’s speed – the closer the circles, the more slowly Dawn is moving.
Source: NASA/JPL

Come Friday, if the spacecraft’s propulsion were to be switched off it would remain under Ceres’ influence but would travel around the dwarf planet in a highly elliptical orbit. So over the next few weeks, Dawn will use its ion thrusters, together with Ceres’ gravity, to slowly draw it into a circular orbit – the first of four such orbital positions around the dwarf planet.

Not for the first time

Ceres is the second object that Dawn has orbited. Between July 2011 and September 2012, Dawn was in orbit around Vesta, which, like Ceres, resides in the asteroid belt located between Mars and Jupiter.

This marks the first time that one spacecraft has been able to orbit two different planetary objects. And it’s only possible because of Dawn’s ion engine.

A spacecraft powered in the usual way, using chemical propellant, would require ridiculous amounts of fuel to carry out such a mission. And even if it was possible for a spacecraft to carry that much fuel on-board, the cost of the mission would be astronomical.

Dawn's path to Ceres Dawn was launched in September 2007 and has taken the slow and steady approach to visit Vesta and now Ceres.
Source: NASA/JPL

At Ceres, Dawn will eventually travel in a polar orbit, travelling above the north and south poles. As it moves from north to south it will travel over the daytime side of the planet, and then during the second half of its orbit it will fly above Ceres' night side.

In its first orbital position, at a height of 13,500km, it will take 15 days for Dawn to complete one orbit. Since the planet takes only nine hours to rotate on its axis, this will allow Dawn to make a good map of the dwarf planet’s surface.

Throughout its 15-month mission, Dawn will vary its orbit three times, each one descending closer to the planet at heights of 4,400 km, 1,470 km and 375 km. To change orbits it will move through a complex series of spiral trajectories.

The descent to its lowest orbit will take two months and, during that time, Dawn will complete 160 revolutions as it constantly reorientates itself to ensure that one of its ion beams is thrusting in the right direction to continue its slow spiral descent.

Dawn's spiral descent Two months of downward spirals are needed to move Dawn into its lowest orbit - from the High Altitude Mapping Orbit (HAMO) to the Low Altitude Mapping Orbit (LAMO).
Source: NASA/JPL

Better than Star Wars

Ion propulsion systems, like the one that powers the Dawn spacecraft, have long been considered the next big thing for space exploration. In fact, they seemed so futuristic that they appeared in the Star Wars movies, powering Darth Vader’s TIE fighters or Twin Ion Engine fighters.

Science fiction to science fact The TIE fighters in Star Wars had twin ion engines, but Dawn does one better, with three ion engines.
Source: NASA/JPL

Ion engines were first used by NASA on Deep Space 1, which flew past the asteroid 9969 Braille in 1999 and comet Borrelly in 2001.

The Japanese Aerospace Exploration Agency (JAXA) has successfully used ion engines on its Hayabusa asteroid missions, the second of which was launched in December last year.

The Dawn spacecraft is fitted with three ion engines, although only one engine is used at any one time. And true to what we expect from science fiction, the spacecraft does emit a blue-green glow. This is a result of its xenon fuel.

The inner workings of an ion propulsion system. The inner workings of an ion propulsion system.
Source: NASA

Positively-charged xenon ions pass through two electrically charged grids. This accelerates the tiny ions and they shoot out of the engine at 144,000 kilometres per hour, providing the thrust to propel the spacecraft in the opposite direction.

Ion engines are around ten times more efficient than chemical rockets because the ions are ejected at roughly ten times the speed that a propellant is expelled by a rocket. The acceleration, however, is much slower.

It would take Dawn around four days to accelerate from 0 to 100 kilometres per hour but the trade off is that in doing so, it would only use 450grams (or just one pound) of fuel.

Why Vesta and Ceres?

Of course, the reason the technology is so marvellous is because it enables such fantastic science – the exploration of the two most massive objects in the asteroid belt, Ceres and Vesta.

The dwarf planet Ceres New images of Ceres, taken February 19 at a distance of 46,000km, show a mysterious double bright spot.

Don’t let their location fool you – these are not space rocks like typical asteroids. They are big worlds and, like Earth and the other terrestrial planets, Ceres and Vesta have a layered structure.

Vesta has an iron-rich core, a silicate mantle and a crust made of basalt. While Ceres is thought to have a rocky core, an ice mantle and a dusty surface.

The ice mantle is particularly interesting. It’s thought that around 30% of Ceres’ mass may come from water and potentially some fraction of that could be liquid water. Just last year, the Herschel Space Observatory made detections of what appear to be plumes of water vapour escaping from slightly warmer regions on Ceres.

The Dawn mission will continue until June 2016 and the latest images will be regularly posted here, while the Dawn mission blog is a great way to keep up-to-date on everything that happens.

Dawn of the Solar System

The space mission was called Dawn because if we think of Ceres and Vesta as protoplanets, then by better understanding these objects, we will gain insight into the early history of our solar system.

Vesta and Ceres size comparisons Ceres and Vesta more closely reflect half-formed planets than space rocks like asteroids.
Source: NASA

The planets of our solar system formed by a method of accretion. Starting out as specks of dust that collided and stuck together, they then grew bigger and formed rocks until eventually they were large enough to draw in enough material to form planets.

Vesta and Ceres seemed to have halted mid-way through this process. This is most likely due to the formation of Jupiter. Its gravity may have prevented objects in the asteroid belt from coming together to finish off the planet building.

As a result, Vesta and Ceres provide a unique opportunity for understanding the early formation of the planets, because they came so close to becoming planets themselves.

The early solar system The early solar system was born out of a dusty disc encircling the sun.
Source: William Hartmann. Courtesy of UCLA

The ConversationThis article was originally published on The Conversation. Read the original article.

Two eclipses for April

by Tanya
Publish date
11 April 2014
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Not one, but two eclipses will occur this month and both are partially visible from Melbourne.

Just before sunset on the 15th April, the Moon will rise already totally eclipsed. It should look quite eerie to see a red moon rising above the eastern horizon and it's always amazing how bright the Moon appears as it moves out of the Earth's shadow and returns to its usual splendour. While you are watching the eclipse, be sure to take a look at Mars, which will be just to the left of the Moon and the bright star Spica (in the constellation of Virgo) that will be found just above.

Lunar Eclipse The progression of a total lunar eclipse in August 2007.
Image: Phil Hart

Two weeks later on the 29th April, the Moon and Sun will come together in the sky and we'll see a partial solar eclipse. The eclipse will begin during the afternoon and reach its maximum point just before sunset. At the height of the eclipse 64% of the Sun's diameter will be covered by the Moon. The Sun will still be partially eclipsed as it sets below the western horizon.

Solar Eclipse The Moon takes a bite out of the Sun.
Image: Phil Hart

The timings for both the lunar and solar eclipse can be found from the Planetarium's monthly newsletter – Skynotes – which is a great guide for finding your way around the night sky.

Importantly, lunar eclipses are lovely to watch and you don't need any special equipment. Solar eclipses, on the other hand, require a bit of care and planning. Never look directly at the Sun.

There are safe ways to watch a solar eclipse and the easiest is to purchase special eclipse glasses. They are available from the Scienceworks shop and will allow you to watch the event, while protecting your eyesight.

You can also create a simple "pinhole" projection. It's as easy as making a small pinhole in a piece of paper or cardboard. Do not look through the hole, but allow the Sun to shine through and project an image onto a second piece of cardboard. Even a blank wall or a clear patch of ground can make a good surface for projection.

And as I've mentioned previously on the Museum's blog, sometimes nature helps out too. If you can see sunlight travelling through the leaves of a tree, you’ve got yourself some ready-made pinhole projections. Check the ground and it might be covered with little crescent Sun images, just like this great example from the Astronomy magazine website.

Rings around an asteroid

by Tanya
Publish date
31 March 2014
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In a surprise discovery, two rings have been found around the asteroid Chariklo, making it the first small Solar System body known to have rings.

Saturn is known for its magnificent rings and the other gas giants - Jupiter, Uranus and Neptune - have ring systems too, though not quite as impressive. Careful searches had not found any other ring systems within the Solar System and many astronomers were beginning to think that rings might only exist around large objects, until now.

Rings from Chariklo An artist's impression of the newly discovered rings around Chariklo.
Source: ESO/L. Calçada/Nick Risinger (

Chariklo is just 250km across and lies beyond Saturn, at a billion kilometres away. It is much too small and far away for the rings themselves to be seen, but amazing detail is now known about them. The rings are dense but narrow, just three and seven kilometres wide, and are separated by a clear gap of nine kilometres. If you were standing on the surface of Chariklo, the rings would appear as wide as our Full Moon and stretch from horizon to horizon.

The discovery was made possible because last June, Chariklo passed in front of an obscure star (UCAC4 248-108672). Not only did Chariklo block the star's light for 5 seconds, but two tiny dips in the starlight were seen, just before and after Chariklo moved by. This video from the European Southern Observatory (ESO) shows faint dimming caused by the rings, just before and after Chariklo blocks the star completely.


This event, known as an occultation, could only be seen from South America and an observing campaign was coordinated across seven observatories, including two telescopes operated by the ESO at La Silla, Chile. Having observations from all seven observatories, ruled out other possible explanations, except for a ring system.

What I really love is the data from the new high-resolution camera on ESO's 1.54m Danish telescope. (Anyone who has been to my Discover the Night Sky series knows that I am particularly fond of beautiful graphs!) This new camera was developed to search for exoplanets and can take up to 40 images per second. It was actually able to see the gap between the two rings – now that's beautiful science!

Chariklo Data The data captured by ESO's 1.54m Danish telescope, showing Chariklo blocking out the light of the star (the main dip). On either side are two small, double dips, as the rings also passed in front of the star.
Image: F. Braga-Ribas et al.
Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (March, 2014)

The Planetarium's astronomer, Dr Tanya Hill, was recently appointed the Australian representative of the European Southern Observatory's Science Outreach Network.


Gravity waves, cosmic inflation and the multiverse

by Tanya
Publish date
20 March 2014
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Scientists have been whipping up a frenzy this week, following the announcement of a truly mind-boggling discovery last Monday. Astronomers from the Harvard-Smithsonian Centre for Astrophysics (USA) have captured the signature of gravity waves from the early Universe. Now, just like the Higgs Boson announcement from 2012, this one takes a bit of time to wrap your head around, but the consequences are out of this world.

Gravity waves from after the Big Bang The twisting pattern shown here represents the first direct image of gravity waves from cosmic inflation, in the moments just after the Big Bang.
Source: EPA/Harvard University

Firstly, this discovery provides the first evidence for gravity waves, a phenomenon that scientists have been searching for over many decades. Gravity waves are tiny ripples in the fabric of spacetime and they were predicted by Einstein's General Theory of Relativity. This theory has now survived every test that could be thrown at it. Even the bizarre notion of time ticking at different rates, depending on gravity and speed, is put into practical use every time we use a GPS to find where we are.

Finding gravity waves could open up a whole new window on the world around us. We are used to looking at the world using light, which are electromagnetic waves. But gravity waves hold just as great a potential. They could be used to see inside a neutron star, to watch a star collapse within a supernova or to examine two black holes colliding. They could even reveal new physics ideas and make it possible to pick up the vibrations of cosmic strings or determine the number of dimensions of the Universe.

Colliding black holes In this artist's impression, two black holes are entwined in a gravitational tango that will eventually collapse the pair into a single black hole.
Source: NASA

Secondly, the discovery provides the best proof of cosmic inflation – the idea that within a fraction of a second after the Big Bang, the Universe went through a period of rapid expansion. Picture a child blowing bubbles, but in this case, the Universe grew from absolutely miniscule to larger than the observable Universe in the blink of an eye.

Only cosmic inflation predicts the rise of gravity waves in the early Universe. And now that the theory has passed this test, other predictions of inflation may well hold true; such as spacetime being infinite and that the multiverse concept is real. We are one step closer to the possibility that there exists an infinite number of Universes, separate to ours, that are continually erupting, just like ours did, out of some sort of ever inflating "sea".


How the detection was made: First hints of gravitational waves in the Big Bang's afterglow by Krzysztof Bolejko (University of Sydney) and Alan Duffy (University of Melbourne), in The Conversation

Solar eclipse from space

by Tanya
Publish date
31 January 2014
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During the early hours of this morning, from 12:30am to 3am, NASA's Solar Dynamics Observatory captured a stunning solar eclipse from space.  The Observatory sees a number of eclipses each year, but this was the longest one that's been recorded so far.


The eclipse was visible from the Observatory's vantage point, orbiting 36,000km above Earth. Since it could only be seen from space, the event is technically called a lunar transit. At its peak, the Moon covered up to 90% of the Sun.

Just as the Moon moves away, you can see a solar flare erupting from the left hand side of the Sun. This is just the kind of activity that the Observatory is helping scientists to better understand.

Solar Flare from the Solar Dynamics Observatory Perfect timing as the Sun releases a solar flare.
Source: NASA

Launched into space on 11 February 2010, the Observatory is on a 5 year mission to study the Sun as part of NASA's Living with a Star program. Our Sun is very active releasing flares and eruptions that can send energetic particles hurtling towards Earth. This can play havoc with our technological systems, bringing down power grids and causing blackouts. The ultimate aim is to better understand the cause of the Sun's activity so that one day we may be able to predict when such flares will occur to give us some prior warning.

The Observatory takes an image of the Sun every 0.75 seconds, and you can see all the beautiful images at the Observatory's Gallery. We have been loving the Gallery here at the Planetarium, and some of the footage will be featured in a new planetarium show to be released later this year.

A Christmas star for 2013

by Tanya
Publish date
13 December 2013
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Just in time for Christmas, a new star has appeared in our southern sky!

Nova Centauri 2013 is the brightest nova to be seen since 1999. It is about as bright as the fifth star in the Southern Cross and is very easy to spot from Melbourne, and across Australia in general. It sits right next to Beta Centauri, one of the famous two pointer stars that leads the way to the Southern Cross.

Nova Cen 2013 from ESO This photograph was taken from La Silla Observatory in the Chilean Atacama Desert on the morning of 9 December 2013.
Image: Yuri Beletsky
Source: ESO

The nova appears just to the left of Beta Centauri, the bluer and higher of the two bright stars in the lower-right part of the image. The Southern Cross and the dark Coal Sack Nebula are also captured near the top of the image.

The nova was discovered on 2 December by John Seach from New South Wales. It is a 'classical nova' and is caused by a dead white dwarf star having a brief, but intense new-lease on life. White dwarfs are stellar embers, where nuclear fusion (the fire that keeps a star shining) has ended. However, this white dwarf has a close companion star. If enough gas from the companion falls onto the white dwarf it triggers a brief explosion on the star's surface.

The star undergoes an extreme burst of brightness. But unlike a supernova, the white dwarf remains intact and lives to tell the tale.

What better reason is there to slip away from the Christmas madness and spend a quiet moment or two, under the stars.


Ideas about the 'real' Christmas star from Phil Plait's Bad Astronomy blog

ESO Science Outreach Network - Australia

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Updates on what's happening at Melbourne Museum, the Immigration Museum, Scienceworks, the Royal Exhibition Building, and beyond.