October 2023

Planets to look for

Saturn continues to make for good evening viewing. It is visible high in the northeast after sunset and up until about 2am if your nighttime proclivities favour a later viewing time. Have you heard about the latest simulations suggesting Saturn’s rings might have formed from colliding moons in the recent past? Saturn is joined by the Waxing Gibbous Moon on Oct 24. 

Image: Saturn and the moon within a couple of degrees of each other on Oct 24. 

Credit: Stellarium 

Jupiter makes its appearance about 9pm and dominates the eastern sky after that. Earth is catching up to Jupiter in its orbit, so the planet is getting brighter by the day. 

Venus is shining brightly in the east before sunrise. Bright and distinct, you can’t miss it. 

Mercury and Mars are both too close to the Sun in the sky this month. Both of them are in the process of moving behind the Sun so are drowned out by the glare of our star. 

Constellation of the month

Pegasus – The Winged Horse 

Pegasus is a large constellation visible low in the northern sky during spring. With its brightest star Epsilon Pegasi coming in at magnitude 2.4, this is a somewhat faint constellation. Observing the constellation begins by looking for the ‘Square of Pegasus’, a quadrilateral made up of lines between the stars Alpha, Beta and Gamma Pegasi along with Alpha Andromadae from the neighbouring constellation of Andromeda. 

Once the square is constructed, making out forelegs and a head isn’t too hard. Your imagination can then add the wings.  

Image: Pegasus, with the Square of Pegasus in green. Note that one of the stars forming the square, Alpha Andromedae is not part of the Pegasus constellation.  

Credit: Stellarium 

Pegasus makes its first appearance in Greek mythology during the tale of Perseus battling the snake haired Medusa, itself recorded in the nearby constellation Perseus. It eventually became the steed of Bellerophon, a slayer of mythological monsters.  

The constellation is home to the fascinating quasar Einstein’s Cross and the beautiful galaxy group Stephen’s Quintet, one of the first observational targets of the James Webb Space Telescope. 

Image: Stephan’s Quintet, as imaged by the James Webb Space Telescope. 

Credit: NASA, ESA, CSA, STScI 

Located in Pegasus is the Sun-like star 51 Pegasi. About 50 light years away, 51 Pegasi was the first main-sequence star around which an exoplanet was discovered. 51 Pegasi b, or ‘Dimidium’ as it was officially named in 2015, was discovered in 1995 by looking at the periodic doppler shifting of light coming from its parent star – a technique called the ‘radial method’ – and immediately challenged astrophysical models of planetary formation. 

Image: Periodic doppler shifting of the light from 51 Pegasi provided evidence of a large planet orbiting it, pulling it slightly off course and causing the observed doppler shifts. 

Credit: Mayor & Queloz 

The planet’s mass of 0.47 Jupiter Masses indicates that it is a gas giant, but its orbital period of 4.23 days shows it orbits its star at a distance of only about 8 million km (1/20th of the Earth-Sun distance), giving it a temperature of more than 1200 degrees. Gas giant planets are composed largely of hydrogen, helium and other volatile gases, which are much too unstable to exist so close to hot stars for the millions of years it takes to form a planet. 

Eventually the discovery of more ‘Hot Jupiters’ like 51 Pegasi b forced astronomers to reconsider models of planetary formation. Modern models of planet formation propose ‘planetary migration’ as the explanation for Hot Jupiters. The idea is that these larges, gaseous planets form further from their stars where it is cooler but eventually migrate inwards as they interact with other material in their solar system.  

Image: Artistic representation of the 51 Pegasi system. Note the extreme proximity between planet and star. 

Credit: NASA 

Going one step further presents the interesting question of ‘if gas giant migration seems to happen in a lot of solar systems, why didn’t Jupiter do the same in our Solar System?’ Fascinatingly, mathematical modelling shows that Jupiter likely did undergo inwards migration in the past – an event now called Jupiter’s Grand Tack –  but eventually Saturn’s gravitational pull was enough to drag it back out away from the Sun, saving the inner solar system, including Earth, from Jupiter’s interference.  

The discovery of 51 Pegasi b eventually earned it’s finders the Nobel Prize in physics in 2019. Planetary formation and solar system science remains a fascinating and active area of research.  

NGC 7331 

This galaxy, also called Caldwell 30, presents a challenging target for determined observers. Located in Pegasus, it is low on the northern horizon, but with a magnitude of 10.4 should be viewable in a modest scope. 

Image: NCG 7331. 

Credit: Optical image: SDSS 

NGC 7331 is a large spiral galaxy, somewhat similar in appearance to what the milky way is thought to look like. Fascinatingly, the central bulge of the galaxy rotates in the opposite direction to the rest of the stars in the outer disk.  

OSIRIS REx, Psyche and Asteroid Autumn 

The OSIRIS REx spacecraft (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer – say that 5 times quickly) was hot news recently after completing its sample return mission to the asteroid Bennu. 

Image: Artistic impression of OSIRIS REx at Bennu 

Launched in Sep 2016, OSIRIS REx reached Bennu in Dec 2018 and spent about 2 years studying it in detail before instantaneously touching down on the surface and collecting a sample of material from the asteroid, a “touch and go” (TAG) sample collection. The TAG was far more successful than anticipated, collecting about 250g of material and blowing more than 6 tonnes of debris off the surface of the asteroid.  

Video: OSIRIS REx sample collection of asteroid Bennu 

Credit: NASA  

On Sep 24 2023, after a two-year return flight to Earth, the spacecraft ejected this sample of asteroid material which re-entered Earth’s atmosphere and landed in the Utah desert. The case has been opened and scientists will spend many years carefully studying this material sample to discern information about Bennu. 

Image: OSIRIS REx sample capsule after landing in the Utah desert. 

Credit: NASA 

Asteroid Bennu is a Near Earth object about 500 metres in diameter discovered in 1999. It will have 10 close encounters with Earth between the years of 2178 – 2185 with a 1-in-1800 chance of impacting our planet. 

Bennu is known to contain large amounts of carbonaceous material and other organic compounds, materials necessary for life on Earth. The asteroid has a very low density of 1190 kg/m³(compared to Earth’s 5510 kg/m³). This suggests it is a loosely bound agglomeration of material, a so called ‘rubble pile’ type of asteroid thought to have formed from material ejected from the breakup of a protoplanetary object billions of years ago. It is also thought to have been relatively unchanged since then, acting as a time capsule of sorts for a preview back to the early solar system. 

Image: Asteroid Bennu as imaged by OSIRIS REx 

Credit: NASA 

Now that its primary mission is complete, OSIRIS REx will live on as OSIRIS APEX, as it heads off to explore the asteroid Apophis (APEX = APophis EXplorer) 

Bennu’s carbonaceous composition leads us to the important point that asteroids come in different ‘types’, classified based on what they’re made of. Bennu is a C-type (carbonaceous) asteroid, and about 75% of asteroids fall into this category. There are also S-type (stony/silicon) asteroids that contain large amounts of rocky material. The final category is M-type (metallic) asteroids that are made largely of metals. 

This brings us to the asteroid 16 Psyche, a large M-type asteroid orbiting between Mars and Jupiter, so named because it was the 16th asteroid ever discovered, and soon to be the target of a dedicated spacecraft mission of the same name. If all goes well the Psyche spacecraft will launch on Oct 5 to study 16 Psyche, a potato shaped asteroid 220 km long known to contain large amounts of iron and nickel. 

Image: Artistic impression of 16 Psyche 

Credit: NASA/JPL-Caltech/ASU 

While rubble pile asteroids like Bennu seemed to have formed from the gentle accumulation of material blasted off the surface of protoplanetary objects billions of years ago, M-type asteroids are thought to have originated much deeper, coming from the metallic cores of protoplanets that were stripped of their outer layers. 

The general idea of planet formation is that ‘from little things big things grow’. In a protoplanetary disk of material that surrounds a newly formed star, grains of sand stick together to form pebbles, pebbles stick together to form rocks, rocks stick together to form bigger rocks and so on. At first, the accumulation is gentle, but as things get bigger their mergers become more violent. Eventually things get large enough such that the energy involved in their collisions is enough to completely melt the colliding objects. Then, just like in any liquid, the dense material sinks to the ‘bottom’, i.e. the core. This mechanism predicts/explains why rocky planets should have dense, metallic cores – the metal sank to the centre of the objects after they melted from violent collisions. 

Image: Metals that sunk down during formation now make Earth’s core.  

Credit: NASA 

There is speculation that 16 Psyche may once have been the metallic core of a protoplanetary object perhaps 1000 km across that had its outer layers blasted away. If this is the case, 16 Psyche could provide a fascinating insight into the core of the Earth and other rocky planets and the conditions under which they formed. Another school of thought posits that 16 Psyche could just be an asteroid that formed in a metal rich area of the protoplanetary solar system.  

Study of 16 Psyche’s origin is puzzled by its density of about 3970 kg/m³, much higher than Bennu, but not high enough to be entirely composed of iron/nickel, which would have a density of 7-9000 kg/m³. Earth based studies of 16 Psyche have found some evidence of large craters on the asteroid’s surface, suggesting a violent past from impacts and collisions, common when the solar system was young.  

 Image: Imagery of 16 Psyche suggests an irregular surface 

Credit: ESO 

The Psyche spacecraft will launch on a SpaceX Falcon Heavy rocket and perform a gravity slingshot at Mars before reaching 16 Psyche in 2029 for a 21-month observing mission. The spacecraft carries a number of instruments to elucidate the asteroids secrets.  

To determine 16 Psyche’s composition, the spacecraft has gamma ray and neutron spectrometers on board. These take advantage of the fact that high energy cosmic rays coming from deep space constantly impact the asteroid and excite the atomic nuclei in the surface material, causing them to emit gamma rays and neutrons that are detected by the spectrometers. Much like regular spectroscopy reveals information about electron energies that uniquely identify an element, the gamma rays and neutrons emitted also have a unique spectrum that identifies the elements as well, allowing the chemical composition of the asteroid to be determined. 

Image: Psyche undergoing testing before flight. Humans for scale.  

Credit: NASA 

There is also a magnetometer on board to map the asteroid’s magnetic field, which will provide clues to the distribution of material in its interior as well. Additionally, by carefully keeping track of the spacecraft’s movement as it orbits 16 Psyche, scientists can detect minute changes in its speed as small variations in the asteroid’s gravitational field pull it this way and that. The complete observations will allow accurate determination of the asteroids mass, as well as information on its distribution through the asteroid.  

Finally, the spacecraft carries a multispectral imager (read: fancy camera) to take those sweet, sweet pictures.  

If you’re wondering why it is Asteroid Autumn, NASA Administrator Bill Nelson had this to say: “With OSIRIS-REx, Psyche launch in a couple of weeks, DART’s one year anniversary, and Lucy’s first asteroid approach in November, Asteroid Autumn is in full swing.” 

Remember, NASA is in the northern hemisphere, it’s not Kambarang up there.