February 2015: Rings

In the show this time, we talk to Prof. Chris Conselice about galaxy formation in the recent universe, Indy rounds up the latest news, and we find out what we can see in the February night sky from Ian Morison and Claire Bretherton.

The News

This month in the news: The tale of two robots.

On the 12th of November 2014, much of the scientific community and many interested members of the public all around the world held their breath as the Philae lander detached from the orbiting spacecraft Rosetta and began its slow descent down to the surface of comet 67P/Churyumov-Gersaimenko. This final step was the culmination of years of work by ESA, which started with the somewhat far-fetched idea of trying to land a robot on a comet. The resulting mission, Rosetta, was launched in 2004 from the Kourou Space center in French guyana and took 10 years to make it to the comet rendezvous location. The complex trajectoy flown by Rosetta was carefully calculated beforehand, involving several gravity assists around various planets. The craft finally reached 67P in August 2014, performing a series of burns to reduce its velocity relative to the comet from 775 m/s to 7.9 m/s.

Following this, the probe followed two successive triangular paths as it approached the comet's surface, before entering actual orbit around 67P at an altitude of around 30km on the 10th of September. the best image of the comet taken before Rosetta's arrival was but a few pixels across, and its shape and layout were completely unknown. The comet was mapped to a high degree of precision, revealing a sort of two-lobed shape, with one half bigger than the other. From some angles, the comet was somewhat reminiscent of a rubber duck! A landing site was chosen for Philae, and was dubbed Agilkia, in reference to the island on the Nile which housed the lander's namesake, the Philae obelisk. And so, on the 12th of November, after a 3-hour descent, Philae touched down on 67P. Or rather, bounced down. The harpoons meant to anchor the lander onto the comet failed to fire, and due to the extremely weak gravitational attraction exerted by the comet, Philae bounced back up a couple of times before coming to rest at the base of what appeard to be some sort of cliff. Unfortunately, this meant that part of it was in permanent shadow, and it could not replenish its batteries with solar power as planned.

Nevertheless, both Rosetta and Philae took (and in the case of Rosetta, are still taking) large amounts of important and interesting data while in the comet's vicinity using a plethora of different instruments. The first wave of scientific findings from these results has just been published in the journal Science, in a total of seven papers. According to these first results, the comet is very porous, with a density similar to that of wood - despite weighing roughly 10 billion tonnes! The surface of the comet nucleus seems to be rich in organic materials, and water ice is only rarely present. The probes also measured a lot of information about the coma of the comet, the cloud of gas produced by ices sublimating from the nucleus that surrounds the comet and trails behind it. It appears to change quite a lot on a day to day basis, both in terms of density and composition. The molecules present in the coma include H2O, CO, CO2, H217O H218O. Furthermore, the deuterium/hydrogen ratio in the water was measured and found to differ from the value found for Earth ocean-like water - going against the idea that this is the kind of water present in comets like 67P, known as Jupiter family comets.

Finally, Rosetta measured the size of the dust cloud surrounding the comet, finding roughly 10.000 grains larger than 5cm surrounding the nucleus in bound orbits - these are believed to have originated during the previous perihelion passage. Currently, the comet is only emitting grains up to 2 cm in size. Rosetta also observed the progressive ionisation of molecules surrounding the comet due to the growing influence of the solar wind as the comet gets nearer to the sun. Indeed, these initial measurements provide a reference point, both in terms of the global shape (the various surfaces of 67P have been given names taken from ancient Egypt) and the composition of the comet nucleus and coma. Rosetta will perform many more flybys before the closest approach to the Sun in August 2015, and will provide a wealth of information to track the evolution of the comet with time.in a related turn of events, scientists are debating whether or not to alter a planned flyby to look for Philae. The decision would mean sacrificing a so-called "zero-phase" flyby, taking the probe just six kilometers from the surface with the Sun behind it - thus providing shadow-free, extremely detailed images of the comet's surface. Instead, Rosetta would fly over a 20-by-200 metre strip of the comet where Philae is presumed to be located. This would provide a scientific benefit (knowing where the lander is exactly would enable the team on the CONSERT experiment, which sends radio waves between Philae and Rosetta to study the comet's interior, to process their data accurately) and also determine whether the lander would be able to re-awaken once the comet gets closer to the sun and the amount of light it recieves increases. Even if the scientists do decide to look for Philae, technical difficulties in altering the mission plan may still scupper the initiative. If the flyby does not go ahead, the Rosetta orbiter will only be able to get that close to the comet again in 2016. In the meantime, Philae is just going to have to wait things out, and hope it gets enough sun to wake up again.

Interview with Prof. Chris Conselice

Prof. Chris Conselice is a Professor of Astrophysics at the University of Nottingham where he specialises in the formation and evolution of galaxies. Chris discusses the possible formation mechanisms of the first galaxies, including the idea of smaller galaxies in the early universe merging to form the larger galaxies that we see today. Chris talks about his focus on observational astronomy, using some of the largest space and ground based telescopes to study large populations of bright, easy to find galaxies. Using the idea that galaxies don't tend to lose mass, he tells us how observing them at different epochs can act as tracers for their evolution. Finally he explains how studies of galaxy populations can probe larger scale cosmological questions and highlights some of the problems that remain in extragalactic astrophysics.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during February 2015.

The constellation of Orion the Hunter is in the south in the evening, now a little towards the west. The three stars of his Belt point up towards Taurus the Bull, with its Hyades and Pleiades Clusters, and down towards Canis Major and the brightest of night stars, Sirius. Orion's top-leftmost star is the red giant Betelgeuse, and to its left are Canis Minor and the bright star Procyon. Above these stand the Gemini Twins, with the higher bright star of Castor and the lower of Pollux. Up and right of Gemini is Capella, a bright star in the constellation of Auriga, wherein binoculars also reveal the open clusters M36, M37 and M38. Leo the Lion rises in the east, and to the right of its bright star Regulus is the even brighter planet Jupiter, which outshines all the stars and other planets during the night.

The Planets

• Jupiter reaches opposition (opposite the Sun in the sky) on the 6th, and is visible almost from dusk to dawn. At a maximum brightness of magnitude -2.6, it is highest in the sky around midnight. Jupiter moves westwards in retrograde motion from Cancer into Leo on the 4th, and it shrinks slightly in angular size from 45.3 to 44.6" during the month. A telescope reveals the planet's larger moons, its equatorial bands and, at the right times, its Great Red Spot.
• Saturn rises before dawn: around 03:00 UT (Universal Time) at the beginning of the month and 01:40 by its end. It is situated close to the leftmost star of the fan marking Scorpius' head and claws. It grows in apparent size from 16.2 to 16.9" during February, and shines at magnitude +0.5. Saturn's low elevation in the south-east - no more than 22° - makes surface details fuzzy, but the ring system is visible as it has now opened out to 25° from the line of sight.
• Mercury reached inferior conjunction (between the Earth and the Sun) on the 30th of January, so it appears low in the south-eastern pre-dawn sky only towards the end of February. It then achieves greatest elongation (largest angular separation from the Sun in our sky) on the 24th, but binoculars (to be used only before sunrise) may still be needed to see it.
• Mars is still resident in the evening sky, moving eastwards from Aquarius into Pisces on the 11th. It dims slightly from magnitude +1.2 to +1.3 over the month, while its angular size, decreasing from 4.4 to 4.3", prevents observation of surface details. It is best observed low in the southern-west as darkness falls, but sets some two hours after the Sun at month's end, and it is near to the much brighter planet Venus in the sky.
• Venus sets around 90 minutes after the Sun at the start of February and has a magnitude of -3.9. It is easily visible above the south-western horizon an hour after sunset if you have a low horizon, and, although it will appear fuzzy, refraction by our atmosphere may split its light into a spectrum when viewed through binoculars or a telescope. During the month, its angular size increases from 11 to 11.6".

Highlights

• Jupiter is spectacular this month, reaching 55° elevation around midnight each night.
Venus and Neptune appear close together around 18:00 UT in the first few days of the month. Binoculars or a small telescope are required to spot Neptune at magnitude +8. Venus is 46' to Neptune's lower left on the 1st.
• Looking east before dawn on the 13th, a waning crescent Moon can be seen just 3.5° to the left of Saturn.
• A thin, waning crescent Moon is 2.5° to the left of Mercury in the south-east before dawn on the 17th.
• Mars and Venus lie within 2° of each other from the 17th to the 26th. Seen about 1 hour after sunset, Mars is 8° up and to the left of Venus on the 7th. Venus then rises rapidly, and lies to Mars' left on the 26th. Their closest approach is 23' on the 22nd, when binoculars or a telescope may be needed to perceive Mars against the glare of Venus.
• A thin, waxing crescent Moon is just 6° to the right of Mars and Venus on the evening of the 20th.
• A first-quarter Moon passes in front of the Hyades Cluster on the 25th, moving within a few arcminutes of the red giant star Aldebaran just after midnight UT as the 26th begins. The star, at magnitude +0.8, is not near to the Hyades in space, but lines up with it in the sky.

Southern Hemisphere

Claire Bretherton from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during February 2015.

Venus is stunningly bright for an hour after sunset, low in the west. Mars, though fainter, is higher up to its right at the beginning of February, but it gets lower each night until it slips into the evening twilight at the end of the month. Venus and Mars are only 0.4° apart on the 22nd. Jupiter is low in the north-east in the evening, and crosses the sky just in time to set at dawn. This is because it is at opposition on the 7th, and so is on the opposite side of our sky from the Sun. As well as being due north around midnight, it means that the planet is at its closest to us and appears as big and bright as it ever can.

Jupiter lies in the constellation of Cancer, which contains just five stars that are visible to the naked eye. However, Cancer does host M44 - the Beehive Cluster - at its heart, which is to Jupiter's north. At magnitude 3.7, it is visible to the naked eye and is one of the closest open clusters to the Earth. Galileo studied it with his early telescope and found about 40 stars, and around 1000 are now known. The bright star Regulus, in Leo, is on the other side of Jupiter and marks the Lion's head and mane. Shining at magnitude 1.35, it actually comprises four stars arranged in two pairs. The double star Algieba (the Mane), below Regulus, was first split by William Herschel in 1782, and its separation of 4" is a nice challenge for modern observers with telescopes of 8 centimetres or more in aperture. Leo rises higher in the north as the night wears on, and it also contains the Leo Triplet, consisting of the interacting spiral galaxies M65, M66 and NGC 3628. At 35 million light-years away, these provide the opportunity to witness the gravitational dance of galaxies in the local Universe. All three show tidal disturbance, with NGC 3628 exhibiting a tidal tail 300,000 light-years long. The Leo Triplet appears near the bright star Denebola and about halfway between the stars Theta Leonis (Chertan) and Iota Leonis. While M66 is visible in large binoculars, the other two galaxies can be found using a small telescope. A group of at least eight galaxies is nearby, including M95, M96 and M105.

The constellation of Gemini is on the other side of Cancer to Leo, and its bright stars Castor and Pollux, representing the Heavenly Twins, can be seen in the north after sunset. Although Pollux appears brighter in our sky, Castor is a system of no fewer than six individual stars. Eta Geminorum, at the foot of the figure of Castor, is near to the open cluster M35. While just visible to the naked eye, this group of stars makes a lovely sight in binoculars or a wide-field telescope.

Odds and Ends

A team of Dutch-US astronomers have uncovered a seriously cool exoplanet in data from the SuperWASP observatory. By observing the change in brightness of the 16 million year old star J1407 they noticed a series of complex eclipses, lasting 56 days. To uncover the source of these eclipses they modeled a planet with a ring system passing in front of its parent star, which you can check out here. Named J1407b, this is the first confirmed glimpse of a ringed planet outside of our own solar system, and what's really awesome about this ringworld is its scale. The team predicts it has 37 rings, which extend to almost 2/3 of the distance from the Earth to the Sun, and have a total mass of 100 of our own moons. If you replaced Saturn's rings with these and observed it from Earth it would appear brighter than the Moon and dwarf it on the sky. You can check out the full story and some artists impressions of J1407b here. And for any budding astronomers out there, the team need you to help them learn more about the system by monitoring J1407 and reporting to AAVSO. You can find its location by searching for J140747.93-394542.6 in the NASA Exoplanet Archives.

An ancient solar system has been discovered using Kepler data. The host star, Kepler-444, is 11.2 billion years old and has five terrestrial planets orbiting it. They range in sizes from that of Mercury to Venus and have orbital periods of approximately 10 Earth days. This short orbital period and hence close proximity to the Sun-like star prohibits the possibility of life. However, this is the oldest known terrestrial solar system to be found to date. There could be another in the galactic disk with planets in a more habitable zone. The full story can be found here.

On Monday 26th January asteroid 2004 BL86, passed the Earth at a distance of 1.2 million kilometres. As it passed astronomers on Earth were observing the 325m wide object using radar imaging and discovered that 2004 BL86 was not alone but in fact had its own moon, a 70m diameter companion to the main asteroid. Radar images of the pair can be found here.

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