MIRA 101
2017.2



I think you can tell by the title of this article how long I’ve been thinking about writing it…

You Cannot be Sirius
By Mike Frost


Twenty years ago, there was a newsagent shop called Martin’s in Rugby Town Centre – it closed a long time ago, Boots the Chemist now occupies the shop.  The newsagent used to have a table full of remaindered books on sale at a discount.  One day in 1993 copies of a title of interest to me appeared, “The Colours of the Galaxies” by David Malin and Paul Murdin, and of course I bought one.
It was a wise investment.  “The Colours of the Galaxies” was a lovely book - I remember recommending it to people at the time; such a pity (though not for me!) that it had to be sold off at a discount price; the book was a reprint of the 1984 first edition, perhaps the publishers over-estimated the demand for a reprint.  David Malin was the photographic scientist at the Anglo-Australian Observatory, Coonabarabran, NSW, and was widely regarded as the finest astro-photographer of his generation – he trained as a chemist and understood how to get the most out of film, yet pioneered a number of techniques which have continued to flourish in the digital era.  Paul Murdin was head of the Head of Optical Telescopes at the UK Observatory in the Canary Islands.
“The Colours of the Galaxies” was full of fascinating information.  There was a tremendous amount of astrophysics, explaining the reasons why stars and galaxies behaved the way they did.  David Malin spent a lot of time explaining his photographic techniques.  Both these strands were copiously illustrated with beautiful pictures, many taken by Malin at the Anglo-Australian Telescope.
Perhaps the book’s technical content was too ambitious for the general public, even if almost anyone would have appreciated the illustrations.  For me personally, Malin and Murdin’s book has had quite a profound influence on my astronomical activities, triggering several lines of enquiry that I have been pursuing doggedly ever since.  For example, the book’s discussion on atmospheric scattering came in useful when I was preparing my ever-popular lecture on the green flash, and the paragraphs on refraction within raindrops informed my talk on rainbows.
One particular section has stuck in my memory to such an extent that, twenty plus years after reading it, I am finally going to tell you about it.  Chapter five of “The Colours of Galaxies” is entitled “The changing colours of stars”.  There is a fascinating discussion of the evolution of stars, onto then on and then off the main sequence of the Hertzsprung Russell diagram, and how this can be used, for example, to determine the age of star clusters such as the Pleiades. 
Interesting though this chapter is, the portion of it that has stayed in my memory for so long is the closing section, “The Colour of Sirius”.  This presents evidence for an extraordinary claim – that the observed colour of Sirius, the brightest star in the night sky, has changed at some time in the last two thousand years.
What leads us to this conclusion?  In Ptolemy’s star catalogue, the Almagest, he described six stars as being “hypokirros”.  These six stars are Arcturus, Aldebaran, Pollux, Antares and Betelgeuse, which all have reddish, orangish or yellowish tints … and Sirius, which most observers would describe as being white.  Ptolemy’s exact meaning of “hypokirros” is not clear – Malin and Murdin suggest “yellowish”.
Ptolemy’s anomaly was first pointed out by Thomas Barker in 1780, and discussed by, among others, Sir John Herschel, Schiaparelli and Simon Newcomb.  But the person who really brought the issue to public attention was one of astronomy’s more colourful characters – TJJ See. 
Thomas Jefferson Jackson See was born and educated in Missouri, took a Ph.D. at Berlin, became an instructor at the University of Chicago, a member of the staff at Lowell Observatory, Flagstaff, and then took successive posts at the US Naval Observatories in Washington D.C., Annapolis and finally Mare Island, California.  His was a career that started with great promise; he was an outstanding visual observer, and may even have glimpsed craters on Mercury.  But illness blighted his career and he never achieved the heights that he thought himself capable of reaching.  Instead, bitter at being overlooked by the astronomical establishment, he took to arguing increasingly controversial theories in popular newspapers – for example, he never believed in relativity. 
On the subject of Sirius, See took up the baton in two articles; one in 1892, the other, not substantially different, in 1927.  See had certainly done research.  He produced 20 classical references which he claimed backed Ptolemy’s observation of a red Sirius.  Some of these were obscure, many were open to interpretation.
Most writers on the subject think that the resolution to the Sirius mystery comes in a misinterpretation of what Ptolemy said.  Perhaps “hypokirros” refers to the overall brightness of Sirius, or perhaps to the sometimes vivid flashes of colour due to twinkling when Sirius is at low altitude in the sky.  It’s even been suggested that “hypokirros” is a mis-transcription of “Sirius”, which makes sense in the context of Ptolemy’s text, where the name of the brightest star in the sky doesn’t otherwise appear.  However, the two words don’t look similar, and See was particularly dismissive of this possibility.  Unless further historical texts turn up, we’ll never know exactly what Ptolemy intended to say.
But can modern day astrophysics come to our assistance?  Is it physically possible that Sirius might have changed colour over the course of the last 2000 years?  Sirius is a main sequence star, of which we now know the physics well – such stars remain stable for billions of years at a time.  Besides which, we can see so many similar stars, that we’d stand a reasonable chance of catching a few such stars “in the act” of changing colour, and we haven’t seen any examples.
There is one other possibility. Sirius is part of a double star system.  In 1970 D. Lauterborn presented two papers on the evolution of such a double star system, starting as two stars of two solar masses and five solar masses.  The more massive star reaches its red giant state, so that the binary system appears red at great distances.  But the red giant rapidly loses mass to its companion, hastening its demise to a white dwarf, and boosting the previously less massive star to six solar masses, becoming the brighter star of the two, and intensely white in colour.  This fits the current observed state of the Sirius system, and gives a possible reason why Sirius may once have appeared red.  Unfortunately, it’s not currently possible to model a transition which settles down in mere millennia.  There are certainly binary star systems in which mass is transferred from one star to the other – but the transitions between stellar types still take millions of years to happen.  Inconveniently for See and others, modern day astrophysics doesn’t offer a solution.
What I like about this story are the layers of historical perspective.  The theories of Barker, See and others rely on the interpretation of the exact meaning of phrases written two millennia previously.  Malin and Murdin added their own perspective of a century; adding twentieth century astrophysics to the mix, and providing a historical perspective on See’s extraordinary career.  The fact that See was proven historically wrong on many other issues such as relativity doesn’t mean that he was wrong about Sirius, of course, but it certainly calls into question his judgment.
And now with another thirty years of perspective we can make further judgements.  First of all - David Malin and Paul Murdin, I’m pleased to say, are still going strong.  Malin retired to run his own website, www.davidmalin.com, which manages his superb library of astronomical images.  Paul Murdin is still very active at the Institute of Astronomy, Cambridge.
Does twenty-first century physics have anything further to say about the colour of Sirius? Unfortunately for See and company, there is still no realistic model for any way in which Sirius can have changed colour over the last two millennia.  Astrophysics may be wrong, of course, but the evolution of main sequence, “bog-standard” stars such as Sirius is no longer a subject of much argument – we know how such stars behave, and it doesn’t include changing colour.
But there is one person whose reputation might benefit from a historical re-evaluation, and that is TJJ See.  In See I see (no pun intended) shades of a figure from half a century later  Fred Hoyle.  There are similarities in their career trajectories and willingness to take their arguments directly to the public.  Hoyle was certainly the more important of the two astronomers – his work on nuclear fusion chains with Burbage, Burbage and Fowler was one of the central achievements of twentieth-century astrophysics, and goes a long way toward disproving the Sirius colour change thesis.  But in later years Hoyle espoused a number of very provocative theories, which he often pitched directly to the public by books and television programs rather than through standard academic routes.
So, See argued that Sirius had changed colour since antiquity – and was probably wrong. Hoyle argued that outer space is full of microbial life – and is also probably wrong. Probably. But it’s interesting that Hoyle’s panspermia conjecture doesn’t seem to be going away right now, rather mutating – I don’t think there are many scientists who now don’t accept that microbial life could travel, say, from Earth to Mars or vice versa, even though travel over interstellar distances is still beyond the pale.
My point, really, is that for science to advance, we need people like See and Hoyle, talented people who are not afraid of arguing contrary points of view even though doing so might hurt their careers.  Geniuses who are sometimes in error, but seldom in doubt. 

A bit like John McEnroe, actually. . .

Sources:

“Colours of the Galaxies”, David Malin and Paul Murdin (Promotional Reprint Company, 1993; original edition Cambridge University Press, 1984). The section on the colour of Sirius acknowledges correspondence with Robert Temple, author of “The Sirius Mystery”, which I haven’t read.

“A Career of Controversy: The Anomaly of T.J.J.See”, Sherill, T.J. (Journal of the History of Astronomy, 1999, p.25)

See’s papers on Sirius (which I haven’t read) were:
“History of the colour of Sirius”, See T.J.J., Astr. & Astrophys., 11, 269-274, 372, 457, 550 (1892)
“Historical researches indicating a change in the colour of Sirius, between the epochs of Ptolemy, 138, and of al Sufi, 980 AD”, See T.J.J., Astr. Nachrichten, 229, 245-72 (1927)

Lauterborn’s paper: “Evolution with mass exchange of Case C for a binary system with total mass 7 solar masses”, Lauterborn, D., Astr. & Astrophys.,7, 150-9 (1971)






Rendezvous with Comets and Asteroids
By Paritosh Maulik


Vesta and Ceres Geological Maps



Vesta Top.  Brown coloured areas are oldest and heavily cratered regions. In the Southern hemisphere there are two large impact craters, Rheasilvia, about 500km in diameter and formed about 1 billion years ago and Veneneia about 400km in diameter and formed about 2 billion years ago.  Purple colours in the north and light blue represent terrains modified by the Veneneia and Rheasilvia impacts, respectively.  Light purples and dark blue colours below the equator represent the interior of the Rheasilvia and Veneneia basins. Greens and yellows represent relatively young landslides or other downhill movement and crater impact materials, respectively.
Ceres Bottom, is a false colour image taken using infrared (920 nanometers), red (750 nanometers) and blue (440 nanometers) spectral filters were combined to create this false-color view.  Redder colours indicate places on Ceres' surface that reflect light strongly in the infrared, while bluish colours indicate enhanced reflectivity at short (bluer) wavelengths; green indicates places where albedo, or overall brightness, is strongly enhanced.
NASA images.


Human kind has been always fascinated by comets with their fleeting visits.  By the eighteenth century astronomers had learned to predict the appearance of planets.  In 1781 William Herschel discovered Uranus in the expected location.  Astronomy community were convinced that there must be planets between Mars and Jupiter and continue to look for them.  Then in 1801, Giuseppe Piazzi discovered a “missing planet”, it was named Ceres.  Subsequent observations suggested the size of Ceres to be 940 km (smaller than Pluto, which is 2300 km).  So Ceres was classified as a minor planet.  By 1845 more similar objects were discovered; astronomers classified these objects as asteroids and began to look for more such objects. For a brief history, see
http://www.esa.int/About_Us/Welcome_to_ESA/ESA_history/Asteroids_The_discovery_of_asteroids
Eventually it was realised that asteroids and comets were made from the same material which formed the Solar System and have remained almost unchanged during the last nearly 4.5 billion years.  By learning about these objects, we get a better understanding of the early Solar System.  As the technology improved, space scientists wanted direct encounters with comets and asteroids to analyse dust from these objects in-situ and/or bring back samples to the Earth for further analysis.  The European Gitto mission was the first mission to meet comet Haley and since then there has been one Japanese and one US mission to asteroids with the Europe's ESA Rosetta probe the latest.
 
Asteroids and Comets a brief Introduction Asteroids 
As the Solar system began to form, dust particles cooled down and formed planets.  Some of these did not quite make the grade planet, their mass was too small and weak gravity did not form a near spherical object.  A majority of the asteroids orbit between Mars and Jupiter.  From time to time, these get a knock from other astronomical bodies and leave their usual place of residence.

Comets
Like asteroids, comets are also made from dust and ice, but these are much smaller with sizes varying from hundreds of metres to a few kilometres.  These orbit the Sun in highly elliptical orbits.  As the comet approaches the Sun, the heat melts the ice and the Solar wind causes the evaporating materials to travel away from the Sun.  This appears as the tail of the comet.  We have discussed some these points in an earlier issue of MIRA Number 90, 2011.  
We only see the nucleus of the comet.  It is solid and is a mixture of ice, rock and dust.  Perhaps comets contain less of ice and more of rocky material and are very dark.  A typical size of a comet is about a few km across.  
As the comet moves nearer to the sun, the heat causes the ice to evaporate.  The water vapour and the dust are released from the surface and covers the surface of the comet which forms a cloud like feature around the nucleus of the comet. Light from the Sun makes it visible and this is the coma.
As the comet moves closer to the Sun, water vapour and dust particles form the tail behind the comet, making it visible to us.  The UV rays and the charged particles from the Sun causes ionisation of some particles; ionisation makes the particles electrically charged.  These electrically charged becomes magnetic.  Under the influence of solar wind these charge particles get defected and forms a separate stream called an ion tail.  Thus a comet has two tails, one an ion tail made of charged particles and the other a dust tail made up of silicate dust from the surface of the nucleus of the comet.  After observing comets for many years, the next obvious step was direct contact with a comet.

The Giotto Mission
European Giotto mission was the first attempt to fly through the tail of a comet.  The comet was Halley's comet.  It was the first European Deep Space Mission.  It attempted to image the nucleus of two comets and to analyse the composition of comets.  Although Edmund Halley was not the first to observe the comet named after him, by using Newton's calculations, he predicted the periodic nature of the comet.  Halley also predicted the orbit of several other comets.  Italian painter Giotto included the comet in his painting The Adoration of the Magi in 1300.
The Giotto mission was originally planed as a joint ESA – NASA mission in 1980.  But due to financial constrains NASA pulled out of the mission and ESA decided to go alone.  The encounter with comet Halley consisted of a total of five probes.  Two Japanese probes to carry out the long distance measurements and two Soviet probes to located the nucleus of the comet.  Giotto used this information to attempt a flyby close to the nucleus of the comet. 
Giotto was launched on 2 July 1985 aboard an Arian Rocket.  Russian probes Vega 1 and Vega 2 took images of the nucleus of comet Halley in early March 1986.  The closest Vega 1 came to the nucleus was 8,889 km and on 14 March 1986, Giotto flew past the nucleus at a distance of 596 km. taking linages of the nucleus and carried out analysis of dust and gasses.  Giotto was hit by a particle, probably a fragment of the comet weighing between 0.1 – 1g.  This changed the controlled spinning of the probe, but the spinning was restored quickly.  It continued to gather data till 15 March 1986 and then it was turned off.  Another collision put the Halley Mulitcolour Camera out of action, however the data was sent back to the mission control.

Comet Halley's nucleus

The probe was switched off and then it flew by Earth to encounter a second comet, Grigg-Skjellerup in 1992.
What we learnt about comet Halley was the size of the nucleus, about 16x8x8km.  The nucleus surface is very irregular, with hills and depressions.  The density was estimated to be around 0.3kg/m3 and the ejected mass was 3 tonne/sec.  Average density of the Earth is around 5514kg/m3.
The mass of the dust was in the range of 1*10-18 to 4*10-1g.  There were predominantly two major classes of dust particles.  One was consisted of lighter elements like carbon, hydrogen, oxygen and nitrogen and the other was rich in mineral containing elements like sodium, magnesium, silicon, iron and calcium.  Water content about 80%, carbon monoxide about 10%, methane and ammonia about 2.5%, other hydrocarbons, iron and sodium were also detected.  The nucleus was very dark, this suggested a thick covering of dust.

Water on Earth
The temperature of the early Earth was too hot to retain water.  Now the question is how did the water get on the Earth?  It was suggested that water was carried to the Earth by comets.  But the Deuterium – Hydrogen (D/H) ratio of the terrestrial water does not match to those of comets Halley, Hyakutake and Hale–Bopp.  It may be that these three comets are odd ones out and there are other Kuiper belt objects with D/H ratio similar to terrestrial water.  Water associated with carbon-rich chondrites meteorites have similar D/H ratio as that of the terrestrial water.  These meteorites have their origin in the primordial Solar System dust cloud which formed the planetesimals and eventually the planets.  The D/H ratio of the Lunar rock is also similar to that of the Earth.  Hence water was already present on the Earth when the Moon was formed, so the water source of the terrestrial and Lunar is the same.  A theory based on these observations goes like this.  Jupiter temporarily moved into the inner Solar System or Jupiter formed much closer to the Sun and later on moved to the present position.  This destabilised the orbit of water rich meteorites (asteroids) and bought water to the Earth.  The ESA Herschel mission detected the presence of water in the asteroid Ceres.  NASA’s mission Dawn went into orbit around Ceres on 6 March 2015.  It has sent some images from a distance of 46,000 km (9,000 miles).
So better understanding of the comets and asteroids would help us to understand the origin of the Solar system.

Hayabusha
Mission to comet Itokawa, by the Japan Aerospace Exploration Agency was launched in 9 May 2003 from Japan to drop a lander on the surface of comet Itokawa.  It was also to collect dust from the comet and return to Earth.  The mission used ion propulsion and since the communication from the control centre was prohibitively long, it deployed an Autonomous Optical negation method.  Once the spacecraft arrived near the comet, it did not go into the orbit around the comet, but surveyed the comet from a heliocentric orbit.  After another survey from 20 km away from the surface, the craft was lowered further down.  The final aim was to land a probe weighing 591g (12cm diameter x 10cm high) with a collection device.  According to the original plan, the spacecraft was to fire pellets onto the surface of the comet and the lander would collect the dust; the lander would then be flown back to the space craft and eventually the dust would be retuned to the Earth.  The lander was also to image the surface of the comet.
But due to some some technical glitch the lander was released from a higher altitude; it did not touch the surface of the comet and flew into the space.  From the ground the mission appeared to be failure, however the orbiting  spacecraft did collect some dust and managed to return the dust to the Earth.  The landing site was in the outback of Australia on 13 June 2010.
What was the message from Hayabusa?
Asteroid Itokawa appears have two distinct types of surfaces.  A smooth zone with layer of sand and gravel (called regolith) and and an area with lots of rocks.  The combination of minerals in the Itokawa is typical of extraterrestrial chondrites.  Such mineral combination does not occur in terrestrial minerals.
The formation of the asteroid Itokawa from dust and gas from the original solar medium formed the asteroid.  Its original size was about 20 km.  The highest temperature of the original mass was around 800°C and then it cooled down slowly.  Eventually collisions with other astronomical objects occurred, and then gravity of some of the larger bodies pulled together to form the asteroid Itakawa.  Its present size is about 540m x 270m x 210m and the largest rock seen was about 50m. 
Some ground based observations suggest that the the density of the asteroid is not uniform.  The density of one side is 2.850 kg/m3 and the other side is 1.750 kg/m3.  These observations also found that the brightness of the asteroid changes as it rotates.  The rays from the Sun is reflected from the surface as heat.  If the surface is not uniform, the heat radiated from the surface is also not uniform.  This non-uniform heat radiation introduces a torque and affects the spin.  Currently Itokawa is loosing its spin by about 0.045 second/year. 
The presence of noble gasses in the minerals suggests that the asteroid is being bombarded with solar wind on the surface and and the asteroid is undergoing space weathering.  Evidence of bombardment by galactic cosmic rays is not evident.  The asteroid is loosing its surface by about 20 – 30cm per million years by space weathering.  The current size of the asteroid about 500m, and if this depletion rate continues, it is estimated that the asteroid may eased to exist in one billion years.

Chondrites
Chondrites are some of the most primitive rocks in the solar system.  These 4.5-billion-year-old meteorites have not changed much from the asteroid these came from.  Since these have never really got hot, these have not melted and as a result chondrites have a very distinctive appearance made from droplets of silicate minerals, mixed together with small grains of sulphides and iron-nickel metal.  This structure of millimetre-sized granules also gives chondrites their name, which comes from the Greek for sand grains 'chondres'.

NASA missions to Asteroids and Comets
STARDUST 
This was aimed to collect dust from comet Wild 2 and also interstellar dust and return to Earth.  The  mission was launched in February 1999, followed by a gravity Earth bypass, it encountered comet Wild2 in January 2002.  On it way, it images asteroid Annefrank.  The dust was returned to Earth in January 2006.
Comet Wild2 is less dusty than comet Halley.  So it was hoped that a better quality of image of the nucleus and the source of the dust could be obtained.  The dust was collected by an Aerogel plate. Aerogel is a very low density sponge like ceramic material.  The spacecraft travelled at a velocity of 6.1km/sec.  Laboratory experiments have shown that, at this the collision speed, the dust would not suffer any collision damage and would retain their identity.  After it has collected the cometary dust the collection plate was encapsulated in a container for return to the Earth (the conical clam shaped object at the lower left hand corner).

Interstellar Dust Collection
Another aim of the mission was to collect interstellar dust.  The velocity of the interstellar dust entering the heliosphere is about 30km/s.  Its direction of entry with respect to the ecliptic latitude is known.  Interstellar dust is affected by solar pressure, solar gravity and the interplanetary magnetic field.  The orientation of the spacecraft required for the optimum collection of interstellar dust is reasonably flexible.  The dust was collected for period of six months.  The reverse side of the Aerogel panel which collected the cometary dust, had another panel to collect the interstellar dust.
The spacecraft also had a spectrometer to analyse the dust in-situ.  Analysis of the retuned dust showed the presence of a protein. This protein is also found on Earth. The terrestrial protein is based on carbon 12 isotope, but the retuned dust sample contained carbon 13, which has extraterrestrial origin.  Presence of the carbon 13 isotope confirmed the extraterrestrial origin of the protein.  This is first time presence of protein, building block of life, has been found in comets. 
In the returned dust sample polycyclic aromatic hydrocarbons (PAH) were found.  These compound occurs in interstellar dust and also in soot from burning on Earth.  Some of the compounds were novel and are believed to be have been synthesised in the dust, that formed the solar system.  
Minerals found in the dust samples of comet Wild2 suggests that these minerals formed under different conditions. This indicates that different compounds formed in different locations in its life.  The chemistry of these minerals do not seem to have much altered by water, in other words these mineral had very little contact with water.  Some iron – nickel alloy and iron – nickel sulphides have also detected in the dust.

NASA DAWN  
Dawn mission left Earth in September 2007 and studied asteroid Vesta for 14 months from July 2011 to September 2012.  Then it headed for dwarf planet Ceres.  The spacecraft carries optical and infrared cameras and a spectrometer, gamma ray and neutron spectrometer.  There are other instruments as well to monitor other physical properties of both Vesta and Ceres.
Perhaps Ceres and Vesta are the last two remaining large proto-planets.  Vesta accreted material from the dust cloud for about only 5-15 million years, whereas Mars and the Earth continued to accreted for 30 and 50 million years respectively.  It is suggested that formation and relocation of Jupiter perhaps stopped the accretion process of bodies in the asteroid belt.  It is also likely that some of the asteroid belt objects collected materials from the comets.
Unlike Ceres, Vesta has seem many changes, basaltic lava flows similar to that of the Moon and the surface contains a lot of craters.  It is believed that we have meteorites on Earth originated from Vesta.  Minerals on Vesta have undergone changes due to the bombardment of cosmic rays.  These changes have occurred at least five times in the last 50 million years.  Vesta is about 573x560x450km in size and is nearly spherical.  It rotates once in 5hr 20m. Its mass is 2.6x1020kg (0.00002 of Earth’s mass).  There is a massive crater near the south pole.
Hydrogen was detected by gamma ray and neutron detectors on board the spacecraft.  This hydrogen is likely to be associated with minerals near the equator rather than as free water.  Free water is not expected to survive near the equator, but water ice can exist near the poles, where there is highest concentration of hydrogen.
These are some of the conclusions from the Dawn mission.  It is surprising that Ceres and Vesta are relative close to each other, yet Ceres remain dry and cool while Vesta retained sufficient heat from its origin to undergo melting.  These are some of the early findings of the Dawn mission.  Both of these asteroid belt objects are in ecliptic plane and their orbits are near circular.  Hence the one mission to cover both of these objects for the next phase of a future exploration.
After leaving Vesta, Dawn went into orbit around Ceres on 6 March 2015 and sent some images from a distance of 9,000 miles (14,500km).  When the very early images came in, there was an area with a white spot.  Now we have a 3D image of Ceres with the spot: the location is Occator crater, its diameter is about 90 km (60 miles).  The walls of the crater are generally smooth, but there are several surface features near the centre.  Images taken with different filters suggest that this bright feature is due to different mineral distribution, such as salts and highly reflective material.


By imaging with different filters, there appears to be evidence of water bearing minerals, a very weak atmosphere and frost on the surface of Ceres,  The surface has undergone very little modification since its formation.  Spectroscopic analysis indicate that the surface of Ceres is covered with clay like minerals and perhaps that is why we have not received any meteorites originated from Ceres.
Ceres measures 965x961x890km, (606x565 miles) with a mean radius of 470km, and rotates in 9 hs, 4.5 minutes.  Its mass is 9.4x1020kg (0.00015 of Earth’s mass).
The lower front cover image is a map-projected view created from images taken during its high-altitude mapping orbit, in August and September, 2015.  Images taken using infrared (920 nanometers), red (750 nanometers) and blue (440 nanometers) spectral filters were combined to create this false-colour view.  Redder colours indicate places on Ceres' surface that reflect light strongly in the infrared, while bluish colours indicate enhanced reflectivity at shorter (bluer) wavelengths; green indicates places where albedo, or overall brightness, is strongly enhanced.
Scientists use this technique in order to highlight subtle colour differences across Ceres, which would appear fairly uniform in natural colour.  This can provide valuable insights into the mineral composition of the surface, as well as the relative ages of surface features.
Since December 2015 until September 2016 Dawn orbited Ceres at 375km, now it is at 20,000km where it will stay.


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