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The South African Astronomical Observatory Celebrates the life of an Astronomy Giant, Professor Michael William Feast

Professor Michael William Feast died peacefully early on Monday morning, 1 April 2019, aged 92. He is survived by his wife Connie, three children and eight grand-children. Prof. Feast was an Honorary Professor in the Astronomy Department at the University of Cape Town, a former director of the South African Astronomical Observatory (SAAO), a Founding […]

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Declaration of the South African Astronomical Observatory as a National Heritage Site

Image Credit: Illustrated London News, 21 March 1857/Ian Glass

On December 21, 2018, the South African Heritage Resources Agency(SAHRA) officially declared the South African Astronomical Observatory as a National Heritage Site.

This is a very exciting development for the SAAO, recognising the incredible achievements and their significance over the past two centuries, and will ensure this heritage is preserved. The declaration is made with the understanding that we are to remain a working site and that the Heritage status will not curtail our primary function as a world-class observatory.

SAHRA released the following statement along with a statement of significance, please find attached the entry in the Government Gazette:

“SAHRA identified the site as having qualities so exceptional that it is of special national significance and warrants declaration as a National Heritage Site.”

Statement of Significance

The South African Astronomical Observatory in Cape Town has played a highly significant scientific role over time as the oldest permanent observatory in the Southern Hemisphere. The site offers an overview of the history of astronomy both locally and internationally. It is a “living site” with almost 200 years of history while still retaining its prominence in the international astronomical community.

Contributions to astronomy from the site range from some of the first accurate measurements of the distance to a star(Alpha Centauri), first catalogues of the principal southern stars, the first photographic survey of the sky, accurate measurements of the distance to the Sun( a value that became the benchmark to measure all other cosmic distance and represented a paradigm shift in astronomy), development of spectroscopy, remeasurement of Lacaille’s Arc of Meridian, establishment of the true shape of the Earth in the Southern hemisphere and the first accurate geodetic surveys of southern Africa.

Architecturally, there are several buildings of historical value and not only reflect the changing architectural styles over the nineteenth century but also have a considerable scientific value due to their contributions to the field of astronomy. The Main Building(a Georgian Building) – designed by the British naval architect, John Rennie, and completed in 1828, the heliograph – the oldest dome on the site and which runs on cannon balls, and the McClean Telescope Building – designed by Herbert Baker.

The range of scientific object related to the observatory as a collection is integral to the scientific value of the site. Some of the instruments within structures have been used with varying degrees of continuity and consistency for over 180 years.

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SAAO contributes to MU69 exploration

On the 1st of January 2019, NASA’s New Horizon’s spacecraft performed a very exciting flyby of the most distant object ever explored, 2014 MU69, nicknamed Ultima Thule. MU69 is located in the Kuiper belt, the icy disk in the outer solar system that contains leftover material from the formation of the Solar System 4.5 billion years ago. In order to prepare for this flyby, and learn as much as possible about this object, astronomers at SAAO and around the world have been performing Earth-based observations from as early as 2017.

This image taken by the Long-Range Reconnaissance Imager (LORRI) is the most detailed of MU69 returned so far by the New Horizons spacecraft. It was taken at 5:01 Universal Time on January 1, 2019, just 30 minutes before closest approach from a range of 28,000 kilometres, with an original scale of 140 meters per pixel.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The New Horizons spacecraft, which initially visited Pluto in 2015, was then re-directed to explore an even more distant object. For this, scientists chose MU69, a body around 30km in size in the outer reaches of our solar system, and aimed to learn about its surface composition, its structure, and whether it hosts moonlets, a coma, or rings.MU69 offered a rare opportunity to learn more about objects in our outer solar system and how they may have formed.

However long before the 2019 flyby, observations of MU69 had already begun. Astronomers, using what are known as occultation observations, had already learned a fair amount about 2014 MU69. An occultation observation is performed as the object in question moved in front of a background star, briefly blocking the star’s light:

As 2014 MU69 passes in front of a background star, this occultation can be observed from Earth with carefully placed lines of ground telescopes or with SOFIA, an airborne observatory. [NASA]

In a recently published study led by Eliot Young (Southwest Research Institute), and including SAAO Astronomers, scientists searched for evidence of rings around MU69 using these occultation observations. Observations were performed on three dates in 2017: on 3 June, from SAAO and Argentina, on 10 June, from the airborne observatory SOFIA, and on 17 July, from Argentina. On these days, telescopes were set up with the goal of catching 2014 MU69 as it crossed in front of a background star.

The light curves produced by these occultations allowed the team to explore whether MU69 is encircled by additionally light-blocking rings. An example of candidate rings (red ellipses) ruled out by occultation observations on 17 July 2018 (yellow lines and map in the right panel). [Young et al. 2018]

The authors compared predicted light curves to the actual dimming of the background star captured during MU69’s occultations and concluded that MU69 was highly unlikely to host any rings. This lack of rings is consistent with the newly obtained low-resolution images from New Horizons’s flyby.

“Limits on a Ring System at 2014 MU69 from Recent Stellar Occultations,” Eliot F. Young et al 2018 Res. Notes AAS 2 224.


Occultations Suggest No Rings for Ultima Thule

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A mystery in the outer Solar System

Stellar occultations are a powerful technique that allows characterization of foreground objects by watching them block light from a distant star. This technique is particularly useful for studying small and/or distant objects in the Solar System, which are difficult to see from the ground. In March 2017, a team of astronomers from the South African Astronomical Observatory and the Massachusetts Institute of Technology observed a stellar occultation that was predicted for the large, Trans-Neptunian Object (TNO) Orcus (Fig. 1, left).

Prior to this observation, Orcus was known to be approximately 900 km in size and had one satellite, Vanth, with a size of roughly 280 km. The Orcus system is in a 3:2 orbital resonance with Neptune, similar to Pluto’s orbit. Although not expected to maintain an atmosphere, Orcus’ features could hint toward a possible resurfacing mechanism, such as cryovolcanism. Goals of the occultation observation included accurately constraining Orcus’ size, measuring any atmosphere around Orcus, and detecting rings or debris in the system.

Five sites in North America attempted the observation, with locations spanning from Chile to northern California. Two sites successfully detected the star disappearing: NASA’s 3-m Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai’i and a 1-m Las Cumbres Observatory (LCO) telescope at McDonald Observatory, Texas (Fig. 2). However, a quick analysis of the geometry and the timing showed that data did not align with the expectation of one star being occulted by one outer Solar System object. After months of careful study, the best guess was that one site observed an occultation by Vanth of the target star while the second site observed an occultation by Orcus of an unknown secondary star.

An external reviewer recommended obtaining high-resolution images in order to definitively solve the mystery. So, a proposal was submitted to the 8.1-m Gemini South telescope to observe the star using the Differential Speckle Survey Instrument (DSSI). The DSSI speckle images easily detected a companion star 250 milliarcsec away from the brighter primary (Fig. 3). Armed with confirmation of the location of the second star, the occultation data were reanalyzed to reveal that both detections were of Vanth, each blocking one of the two separate stars (Fig. 1, right).

The data allowed Vanth’s size to be measured at 443±10 km in diameter. They also placed a constraint of a few microbars on any global Vanth atmosphere and did not detect any material in the two star paths down to a limit of a few km in extent.

Click here for the Gemini Observatory press release, “Outer Solar System Object has Astronomers Seeing Double”.

Publication details:
“A Stellar Occultation by Vanth, a satellite of (90482) Orcus”
Amanda A. Sickafoose, Amanda S. Bosh Stephen E. Levine, Carlos A. Zuluaga, Anja Genade, Karsten Schindler, Tim Lister, and Michael J. Person
Icarus, volume 319, pages 657-668

Preprint available at

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SALT plays key role in the global hunt for Dark Energy

The Southern African Large Telescope (SALT), the largest optical telescopes in the Southern Hemisphere, has played an important role as part of the international Dark Energy Survey’s (DES, quest to pin down dark energy, the mysterious force accelerating the expansion of the universe.

As part of the hunt, SALT conducted follow-up spectroscopy of supernovae – stars that explode at the end of their lives – discovered by DES. Supernovae are so bright that they can be seen on the other side of the Universe and astronomers can accurately calculate the distances to a small subclass of them – the so-called Type Ia supernovae. Once their distances are known, Type Ia supernovae can be used to measure the acceleration of the expansion of the universe. Sorting through the chaff of variable objects to find and classify the Type Ia jewels was the important role undertaken by SALT and several other of the world’s biggest telescopes.

Zoomed-in image of the supernova, highlighting it in one of the host galaxy’s spiral arms. For a brief time, the supernova can be as bright as the 100 billion stars of the host galaxy.
Image Credit: SALT/SAAO

“To measure the acceleration of the universe’s expansion by studying stars that died hundreds of millions of years ago takes the most powerful telescopes in the world, combined with meticulous analysis. SALT has provided a key contribution to the international Dark Energy Survey, the most sophisticated study of dark energy with supernovae yet” said Dr. Eli Kasai, former PhD student at SAAO, now lecturer at the University of Namibia, and Principal Investigator for the South African Astronomical Observatory DES program from 2014 to the end of the survey in 2018.

“We need to control systematic uncertainties to very high precision so that we have confidence in our conclusions. SALT, with its massive mirror and ability to rapidly target exciting new candidates, allows us to take a confirming spectrum when the supernova candidate is at its brightest. This translates into clean answers to exactly what kind of exploding star we are looking at.”, said Dr. Mathew Smith, who is based at Southampton University and was the PI of the SALT spectroscopic follow-up program of DES supernova candidates from 2013 to 2014.

DES began science observations in 2013, in Chile, South America, with an overall goal of measuring the expansion history of the Universe in order to place tight constraints on the quantity and properties of dark energy at an accuracy of about 1%. DES employs several methods of constraining dark energy, of which supernova observations, are a primary tool.

“20 years ago we discovered Dark Energy and the acceleration of the universe by carefully observing supernovae. Today, two decades later, dark energy is still one of the great mysteries of our time. These results, with the purest sample of supernovae to date, confirm yet again that dark energy is real, and will be a key target of investigation for the Square Kilometre Array (SKA), that will be built primarily in South Africa.” said Professor Bruce Bassett, a member of the SALT DES supernova follow-up program, astronomer at SAAO and head of the Data Science group at the South African Radio Astronomical Observatory (SARAO). SARAO has recently completed construction of the MeerKAT radio telescope that will form an important part of the Square Kilometre Array.

“Dark Energy is perceived to exist in the vast empty spaces between galaxies in the Universe known as voids and we believe that it is responsible for making galaxies move away from each other at ever-increasing speeds. In other words, it is responsible for the accelerated expansion of the Universe that we observe” said Prof. Roy Maartens, an SKA Chair in Astrophysics and Cosmology at the University of the Western Cape and a member of the SALT DES supernova follow-up program. He went on saying “observing more and more supernovae in many galaxies gives us a handle to quantify the properties of dark energy and also provides us insight into the true nature of supernovae”.

SALT consistently played a pivotal role of classifying into various types the discovered supernova candidates and successfully determining how far they were from Earth, two important parameters that were key to the success of the DES experiment, which came to an end at the end of February 2018. Spectral observations of the discovered SN candidates by SALT and other spectroscopic capable telescopes in DES played a crucial role in helping algorithms that could classify the discovered supernova candidates and determine their redshift using only the images in which such candidates were discovered. This type of classification and redshift determination is less accurate in comparison to that performed with spectroscopic data from SALT and other spectroscopic capable telescopes.

“The DES team has independently confirmed the existence of dark energy by combining four different cosmic probes: (1) supernova observations, (2) baryonic acoustic oscillations, (3) weak gravitational lensing and (4) galaxy clustering”, said Dr Eli Kasai. He continued by saying “The conclusions from DES from combining these four probes mean that for the first time we have been able to find strong evidence for cosmic acceleration and dark energy from a single experiment, instead of combining results from many different telescopes and different analyses.”

The past three weeks have seen the release of 8 DES papers to, reporting the findings of the analyses of DES supernova data observed over the first three years of the survey. The papers made use of the survey’s spectroscopic data including that taken with SALT.

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SALT contributes to the discovery of a bingeing white dwarf

A new binary star system has been discovered in which a small white dwarf star is cannibalising its larger Sun-like companion. Such objects are actually quite common, but for this new object, the white dwarf binged on its neighbour at a prodigious rate, heating part of it to nearly a million degrees. The object, named ASASSN-16oh, was found on 2 December 2016 by the All-Sky Automated Survey for Supernovae (ASASSN), a network of about 20 optical cameras distributed around the globe which automatically surveys the entire sky every night in search of transient events, objects which suddenly appear. ASASSN-16oh was found to be in the Milky Way’s satellite galaxy, the Small Magellanic Cloud, at a distance of ~200,000 light years.

An artist’s impression of the supersoft X-ray binary system, ASASSN-16oh, with a small white dwarf star (left) accreting hot gas from its Sun-like companion (right), through an accretion disk. The stream of gas from the companion forms a flattened accretion disk and the gas gradually spirals down to the white dwarf, getting hotter as it does so. Eventually, the accreted gas impacts the equator of the white dwarf, heating it up to nearly a million degrees, emitting in soft X-rays.
Image credit: NASA/CXC/M.Weiss

Optical follow-up observations were conducted by the Southern African Large Telescope (SALT), the Polish OGLE telescope in Chile and the Las Cumbres Observatory (LCO) telescope network. It was also discovered to be a so-called “supersoft” X-ray source by the NASA Neil Gehrels Swift Observatory and the Chandra X-ray Observatory, produced by gas at temperatures of ~900,000 degrees. Such supersoft systems have previously always been associated with a thermonuclear runaway explosion on the surface of a white dwarf, as occurs in a hydrogen bomb, brought on by the accumulation of hot and dense accreted gas which eventually reaches a critical explosive limit.

“Supersoft sources are a really interesting class of transient events, and ASASSN-16oh is no exception”, says David Buckley, the Principal Investigator of the SALT Large Science Programme on transients, who is based at the South African Astronomical Observatory. “We were fortunate to be able to react quickly to its discovery and undertake crucial observations during the outburst phase”, he said. “Our SALT spectra showed all the hallmarks of a highly energetic system, with an intensely strong emission line from ionized helium which changed in velocity from night-to-night”, says Buckley. In addition, robotic observations were triggered with the LCO telescopes in South Africa, Chile and Australia, allowing for monitoring over a 34 hour period, beginning on Christmas Day 2016. “A nice Christmas present courtesy of the LCO Director who granted the time”, quipped Buckley. The SALT and LCO data were then quickly analysed by another member of the SALT transients collaboration, Andry Rajoelimanana, at the University of the Free State, in Bloemfontein, South Africa.

It became clear after the optical and X-ray observations were analyzed that ASASSN-16oh was no normal thermonuclear powered supersoft source. “In the past, the supersoft sources have all been associated with nuclear burning on the surface of white dwarfs,” said lead author Tom Maccarone, a professor in the Texas Tech Department of Physics & Astronomy, lead author of the ASASSN-16oh discovery paper that has just appeared in the December 3rd issue of Nature Astronomy.

If nuclear fusion is the cause of the supersoft X-rays from ASASSN-16oh then it should begin with an explosion and the emission should come from the entire surface of the white dwarf. However, the optical light does not increase quickly enough to be caused by an explosion and the Chandra X-ray data show that the emission is coming from a region smaller than the surface area of the white dwarf. The source is also a hundred times fainter in optical light than white dwarfs known to be undergoing fusion on their surface. These observations, plus the lack of evidence for gas expelled away from the white dwarf, provide strong arguments against fusion having taken place on the white dwarf.

Because no signs of nuclear fusion are present, the authors present a different scenario. As with the fusion explanation, the white dwarf pulls gas from its companion star, a red giant, in a process called disk accretion. The gas forms a large flattened rotating disk surrounding the white dwarf, becoming hotter as it spirals inwards, as shown in our illustration. The gas then falls onto the white dwarf, producing X-rays along an equatorial belt where the disk meets the star. The rate of inflow of matter through the disk varies by a large amount and when the rate of mass loss from the companion increases, the X-ray and optical brightness of the system becomes much higher.

“The transfer of mass is happening at a higher rate than in any system we’ve caught in the past,” added Maccarone. If the white dwarf keeps gaining mass it may reach a mass limit and destroy itself in a Type Ia supernova explosion, a type of event which was used to discover that the expansion of the universe is accelerating. The team’s analysis suggests that the white dwarf is already unusually massive, so ASASSN-16oh may be relatively close – in astronomical terms – to exploding as a supernova.

“Our result contradicts a decades-long consensus about how supersoft X-ray emission from white dwarfs is produced,” said co-author Thomas Nelson from the University of Pittsburgh. “We now know that the X-ray emission can be made in two different ways: by nuclear fusion or by the accretion of matter from a companion.”

Also involved in the study were scientists from Texas A&M University, NASA Goddard Space Flight Center, University of Southampton, University of the Free State in the Republic of South Africa, the South African Astronomical Observatory, Michigan State University, Rutgers State University of New Jersey, Warsaw University Observatory, Ohio State University and the University of Warwick.

NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.

Publication Details:

“Unconventional origin of supersoft X-ray emission from a white dwarf binary”

Thomas J. Maccarone, Thomas J. Nelson, Peter J. Brown, Koji Mukai, Philip A. Charles, Andry Rajoelimanana, David Buckley, Jay Strader, Laura Chomiuk, Christopher T. Britt, Saurabh W. Jha, Przemek Mróz, Andrzej Udalski, Michal K. Szymański, Igor Soszyński, Radosław Poleski, Szymon Kozłowski10, Paweł Pietrukowicz, Jan Skowron, Krzysztof Ulaczyk

Nature Astronomy, Volume 2, No. 11, Article Number 2397-3366

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SALT chases hypervelocity stars

The Southern African Large Telescope(SALT) was recently involved in the identification of a hypervelocity star, flung across the galaxy by a supernova explosion that occurred 90 000 years ago. The discovery of this star and two others may help to solve a decades-old debate on how supernovae occur.

Video animation produced using Gaia Sky showing the path of the hypervelocity star candidate by Shen et al (2018), with its movement exaggerated by a factor of 300 billion times. The path of the candidate is first shown moving forward in time and subsequently going into the past, back to the supernova remnant. The circle indicates the location of the remnant of the supernova G70.0-21.5. Credits: S. Jordan, T. Sagrista

Credits: ESA/Gaia/DPAC, K. Shen, S. Jordan, T. Sagrista

Type Ia supernovae are the thermonuclear explosions of white dwarfs in binary star systems. They are one of the most common types of supernovae and have a fundamental importance as cosmological distance indicators. Despite this, the nature of the binary system and the details of the explosion has remained a mystery. Many theoretical models have arisen over the past few decades to explain how these stars explode, but there have been few pieces of direct evidence that any of these scenarios actually succeeds in nature.

One model, dubbed the “dynamically driven double-degenerate double-detonation” (D6) scenario, predicts the possibility that the other star in the binary system is another white dwarf that can survive the explosion of its companion. Such a surviving star would be flung away from the system when the gravitational pull of its companion disappeared and would continue zipping away at speeds between 1000 – 2500 km/s.

Shen et al. (2018) searched for such hypervelocity survivors in Gaia’s second data release in April 2018 and discovered three likely candidates. These stars were followed up with ground-based telescopes, including the Southern African Large Telescope (SALT), and found to possess many of the predicted features for survivors of D6 Type Ia supernovae: a lack of hydrogen and strong signatures of carbon, oxygen, and magnesium, as well as luminosities and temperatures unlike almost all other stars.  Furthermore, the past location of one of the stars is spatially coincident with a known supernova remnant, making it highly probable that it was ejected from a system that underwent a supernova.

The combination of Gaia, which precisely measured the high-speed motion of these hypervelocity stars in the plane of the sky, and the ground-based spectroscopic observations, which provided a measurement of the radial component of the stars’ motion, has shown that these are among the fastest freely moving stars in our Milky Way Galaxy.

Much follow-up work remains to be done to ascertain precise characteristics of these stars and the explosions that gave birth to their hypervelocity natures. However, it is very likely that these stars represent the first discoveries of surviving companions to Type Ia supernovae, and that they confirm the success of the D6 “dynamically driven double-degenerate double-detonation” model.

The team undertaking the observations and data analysis was led by Ken Shen, a researcher at the University of California at Berkeley (USA). The SALT observations were taken by Marissa Kotze at the Southern African Astronomical Observatory as part of a program led by Saurabh Jha from Rutgers, the State University of New Jersey (USA).


Figure 1: Orbital solution of the second white dwarf candidate for the D6-scenario, overlaid with H-alpha images from the Virginia Tech Spectral Line Survey (VTSS, Dennison et al. 1998). The blue trajectory extends 90,000 years into the past, the red trajectory extends the same amount of 90,000 years into the future. The green circle indicates the remnant of the supernova G70.0-21.5. Image credit: Shen et al. (2018)

Figure 2: The three hypervelocity candidates are shown here in the colour-magnitude diagram with the green, blue and orange circles. Some other white dwarfs are indicated in this diagram as well. The black circles and colored regions show the reliably measured stars from Gaia. Image credit: Shen et al. (2018)

The article by Shen et al. was published in the Astrophysical Journal on 20 September 2018.

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The General Assembly of the International Astronomical Union to be hosted on African soil for the first time in 2024

Thursday 30 August 2018. Vienna, Austria

The International Astronomical Union (IAU) today announced the 32nd General Assembly of the IAU in 2024 will be hosted by Cape Town, South Africa. This will be the first time in the 105 year history of the IAU that the General Assembly will be held on the African continent. The award recognises the incredible strides that African astronomy has taken in recent years.

The South African astronomical community in collaboration with the Academy of Science of South Africa (ASSAf), and with strong support from the South African Government and astronomy stakeholders across the African continent, last week formally invited the IAU to Africa at the 30th IAU GA currently being held in Vienna, Austria.

Africa has a long and rich relationship with astronomy, dating back millennia. The world recognized the unique geographical importance of Africa in global astronomy almost two centuries ago with the establishment of the Royal Observatory, Cape of Good Hope in 1820. Since then Africa’s contributions to global human knowledge have both independently and collaboratively grown from strength to strength.

The beginning of the 21st century has seen a renewal of Africa’s strong heritage of astronomical excellence. The IAU has held Middle East and Africa Regional Meetings since 2008. The Entoto Observatory in Ethiopia, has been operating as an independent research centre since 2013.

Since the establishment of the IAU’s global Office of Astronomy for Development (OAD) in 2011, Africa has become the home of three such regional offices coordinating activities across East Africa from Ethiopia, West Africa from Nigeria, and Southern Africa from Zambia. The mandate of the regional offices is to ensure that the region benefits maximally from the practice of astronomy. In 2017, the 1-metre Marly telescope was installed in Burkina Faso as a research telescope as part of the University of Ouagadougou.

Africa is also host to the world-renowned HESS telescope in Namibia. The continent is developing the very exciting African Very Long Baseline Interferometry Network (AVN), and a number of countries are rapidly developing their own astronomy programmes and instruments. At the General Assembly currently underway in Vienna, Algeria, Ghana, Madagascar, Morocco and Mozambique all became new national members of the IAU.

Today, Africa is home to the largest optical telescope in the southern hemisphere (SALT), the largest and most powerful radio telescope in the southern hemisphere (MeerKAT) and will play host to a large part of the international Square Kilometre Array (SKA) Project, whose African partnership includes Botswana, Ghana, Kenya, Madagascar, Mauritius, Mozambique, Namibia, South Africa and Zambia.

The winning bid is particularly timeous as the SKA telescope is expected to start conducting science observations in the mid-2020s.

“The support for the bid from not only astronomers but also industry, academic institutions and government have been phenomenal, and its success is a testament to what we can accomplish through our united efforts. For astronomers, this is like winning the bid to host a Football World Cup or the Olympics. It’s time for Africa! We are excited and look forward to welcoming our colleagues from around the world to the first of hopefully many IAU General Assemblies on African soil.” says Dr Shazrene Mohamed, member of the bid committee, an astronomer at the South African Astronomical Observatory and the University of Cape Town.

The General Assembly in Cape Town in 2024 is an occasion to give voice to Africa in the global astronomical endeavour and will bring attention to the excellent science and education conducted on the continent. It is expected that the opportunity for many African astronomers to take part in one of the world’s biggest astronomy meetings will contribute to an enduring legacy of astronomy on the continent.


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Pluto: small, distant, and fascinating

Pluto is an interesting object located in the outer region of our Solar System.  Although only 2376 km in diameter, it hosts five moons and a thin atmosphere.  As shown in Figure 1, Pluto has a high orbital inclination (17 deg) and eccentricity (0.25), and it requires 248 years to travel once around the Sun.  At 120 deg., Pluto has unusually high obliquity, which is the angle between the rotational pole and the orbital plane.  Thus Pluto’s north pole (as defined by the right-hand rule) lies 40 deg. below the orbital plane. This combination of high orbital eccentricity and obliquity results in extreme seasons: Pluto’s most distant location from the Sun is nearly twice as far away as the closest, and each pole is exposed to the Sun for more than a century at a time.  This geometry is expected to strongly affect the atmosphere.

Figure 1: Schematic plot of Pluto’s orbit in our Solar System. In 2018, Pluto crossed the ecliptic plane and is moving away from the Sun. Pluto was at perihelion in 1990 and won’t be there again for more than two hundred years. Credit: A. Verbiscer;

Pluto’s micro-bar atmosphere was first definitively detected in 1988.  In 2002, measurements showed that the atmosphere had expanded, even though Pluto was moving away from the Sun.  By 2015, when NASA’s New Horizons spacecraft [] flew through the Pluto system, the atmosphere was roughly the same size it had been in recent years.  However, the story gets more intriguing: models of the mass and distribution of surface ice, which include thermal inertia, predict that Pluto’s atmosphere could collapse out completely.  For these reasons, continued observations of Pluto are important.

Figure 2: Image from NASA’s New Horizons spacecraft of Pluto and Charon. This image was taken in July 2015, while the spacecraft was 5.4 million km from the bodies. The colours are approximately those that would be seen by the human eye: it is obvious that Pluto and Charon have very differently-coloured surfaces. Credit: NASA/JHU APL/SwRI.

In August of 2017, researchers from the South African Astronomical Observatory (SAAO) and the Massachusetts Institute of Technology (MIT) observed a relatively rare event, a stellar occultation by the dwarf planet Pluto.   The stellar occultation technique requires accurate measurements of the positions of a distant star and a foreground body, in order to predict exactly when and where on Earth a shadow will fall.  In this case, Pluto was predicted to occult a star of approximately 15th visible magnitude, with the moderately-sized shadow path (less than 1/3 the diameter of Earth) falling over the northern Pacific Ocean.  The event was observed remotely from Cape Town, using NASA’s 3-m IRTF (Infrared Telescope Facility []) with the high-speed, accurately-timed, visible-wavelength instrument MORIS (MIT Optical Rapid Imaging System). Figure 3 contains Images taken before and after the occultation, which show Pluto and it’s largest satellite, Charon, as they approach the star and then after they pass by.

Figure 3: Animated images of the Pluto system moving up to, and then past, the occultation star on 07 August 2017. Charon is clearly discernible from Pluto, with a separation of approximately 0.8 arcsec. These 60-sec images are subframes of an unfiltered dataset taken on NASA’s 3-m IRTF with MORIS. Discrete jumps occur between separate data cubes as well as during the occultation, when separate data were taken at significantly faster cadence. Credit: A. Sickafoose; N. Erasmus; SAAO

These data show that Pluto’s atmosphere still existed in late 2017.  They are currently being analyzed, along with additional occultation measurements from 2018, to determine the most recent measurements of Pluto’s atmospheric characteristics. Results will be presented at the 2018 American Astronomical Society’s Division of Planetary Sciences meeting [] in October
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SAAO to contribute to the global effort to detect Near Earth Objects

The South African Astronomical Observatory(SAAO) will play host to the next generation of asteroid-hunting telescopes as part of the NASA funded Asteroid Terrestrial-Impact Last Alert System (ATLAS). On 13 August NASA confirmed that it will fund two asteroid-hunting observatories in the Southern Hemisphere at a cost of US$3.8 million, SAAO will host the first with the location of the second still to be decided.

In 2013, NASA announced the Asteroid Grand Challenge (AGC) “to find all asteroid threats to human populations and know what to do about them.”  The 2013 meteor strike in Chelyabinsk, Russia in which a 20m rock exploded mid-air injuring numerous people was a clear reminder of the busy neighbourhood in which we live and of the destructive potential of asteroids.

The ATLAS project was designed to address these concerns and two telescopes are currently operational on the islands of Maui and Hawaii, run by the University of Hawaii. Since it began operations in 2015 ATLAS has discovered over 300 asteroids which pass near the Earth’s orbit. However, since these telescopes are located in the Northern Hemisphere they are blind to roughly 30% of the southern sky and therefore to any asteroids in that region.

The new telescope in Sutherland will aid in the detection, tracking and characterization of near-Earth objects (NEOs)  and address the current gaps in sky coverage by imaging the entire sky twice per night. This will be performed using a fully robotic 50-cm diameter telescope with a 110 MP CCD Camera.

The ATLAS system also has software which is optimized to detect fast-moving objects. In early June, the system assisted in providing data on the trajectory of a 1.8-metre asteroid called 2018 LA that entered the atmosphere over Southern Africa. Fragments of this asteroid were found in Botswana using this information.

The telescope will be able to detect a 100-meter diameter asteroid at a distance of 40 million kilometres (~ 3 weeks warning) and a 10-meter diameter asteroid at a distance of 4 million kilometres (~2 days warning). Newly discovered NEOs can then be followed up using SALT and other SAAO telescopes to determine the type, rotation rate, and other important information.

SAAO’s involvement in the ATLAS project offers an excellent opportunity for South African staff, scientists and students to collaborate with NASA and share valuable technology and expertise.

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