There is NO observing tonight (21/11)
There is NO observing tonight (20/11).
The BepiColombo mission to Mercury is a joint ESA/JAXA (Japan Aerospace Exploration Agency). The craft is named in honour of Giuseppe (Bepi) Colombo (1920-1984), an Italian scientist who first used the gravity assist (‘slingshot’) method of sending spacecraft across the solar system by bringing them close to other planets, thereby using their gravity to increase speed.
Mercury is odd in that it has a very dense large core with a total size not much different from that of Earth’s. This almost certainly came about when Mercury hit something massive while the solar system was still being formed. (What Prof Rothery calls ‘planetary embryo collisions’.) But where was Mercury’s embryo when it got bashed? He is intrigued why there is so much sulphur (2-5%) on the surface but the collision theory would also explain that away in that Mercury probably formed further out in the cooler parts of the solar system and was therefore mostly the hit and run impactor itself, and most of the lighter stuff from the two bodies was dumped into space, leaving the dense core.
Mercury’s surface has a rather dark grey, flat albedo, even lower than that of the Moon. There are areas of slight stepping, indicating tectonic thrusting as the surface cooled.
The Messenger spacecraft was busy orbiting and mapping Mercury from around 2011-15. Joint UK/Italian scientists are using the data to create 15 geological maps in total. As regards BepiColombo, the UK contribution is an x-ray spectrometer (see below) which sees the fluorescence on the surface. The Sun shines x-rays onto Mercury and the resulting fluorescence and reflected x-ray photons tells us what elements there are and the abundance thereof. The more active the Sun is the more fluorescence there is so you get more information.
Unfortunately the Messenger data did not cover the south pole. The north pole had much better coverage. The elements detected were silicon, sulphur, iron, magnesium, potassium, calcium, among others.
BepiColombo consists of two craft which will not be able to separate until they arrive. MMO (Mercury Magnetospheric Orbiter) is Japan’s half, MPO (Mercury Planetary Orbiter) is the ESA half. The UK and Finland have created MIXS (Mercury X-ray Imaging Spectrometer mentioned above).
Launch may go ahead next year. There will be two Venus flybys and five Mercury flybys to get it into orbit, and this is going to take 8-9 years. Once in position, MMO will fly very close in ellipses, so that its orbits will take 2 to 3 hours (400-1500km) and MPO will have a far more elliptical orbit, with its closest being 400km also.
It was amusing to hear Prof Rothery’s defence of the cost of the mission in the face of nasty comments from people on public media. The mission is likely to cost a total of 3.3 billion euros. Not that much when you consider about 8 billion dollars is likely to be spent on lipstick in the next year….
Patrick Irwin is Professor of Planetary Physics at RAL, and has a particular interest in gas like and exoplanets and the atmospheres thereof. He was involved with some of the Venus Express, Rosetta and Cassini equipment. He started off as a member of Bath AS and now asks how we can detect life on exoplanets.
Before 1995 the only planets we knew were those in our solar system, i.e., the terrestrial ones and the gas giants. Then along came the discovery of 51 Pegasi b, which I found out is a hot Jupiter with a two day orbit. We discovered it by the radial velocity of the spectral lines on the star; the planet is making its star wobble so the spectral lines keep shifting as the pair orbit around the system’s centre of gravity. (Hot Jupiters are large gas giants orbiting very close to their parent stars. If you’ve attended some of our previous talks on exoplanets you will know that these planets’ orbits are decreasing and sending the planets into certain doom.)
Transit spotting is a stock method for detecting exoplanets because they are so much fainter than their parent star. Unfortunately the orbital orientation can’t be ascertained. That can only be done if we can actually see the planet transiting the star. If you get a dip in the star’s light then you can assume a planet is transiting, and if the orbit is just off horizontal to us then you get a mini dip when the planet going behind its star. I loved his demonstration with a golf ball and a football stuck on either end of a stick hanging from a string. He also showed us the best picture I’ve ever seen of Venus in transit.
There are also two ground based observatories that can do direct imaging of exoplanets by using a coronagraph, which is a way of blocking out the parent star’s light so the exoplanet becomes easier to see. Gravitational lensing can also reveal a double image of a star passing behind a darker star. When the two stars are in a line of sight the gravity of the front star bends the light coming from the star behind it. If the star has a planet you get a double spike in the two light curves of the star. Of course this is just a one off, line of sight, event. Mind you, 25 exoplanets have been observed using this microlensing effect, as it is called. But, really that’s a tiny proportion of the 2950 exoplanets that have already been confirmed, and the unconfirmed 2505 possibilities from the Kepler data. Prof Irwin concludes that almost any population II stars (metal rich, second generation stars) have planets, including hot Jupiters and Earth sized.
As regards finding planets that may harbour life, we need to look at exoplanets in the ‘Goldilocks zone’, as it is called, where liquid water can exist on the surface. This depends on the planet’s distance from its star and the temperature of the star. Mind you in theory it seems that Venus and Mars are supposed to be suitable in our Solar system.
Transit observations give us the most data. Prof Irwin did not go into detail about the equipment involved, but then we have had previous speakers talk about exoplanet searches. Kepler is still in space in an Earth-trailing orbit, although its steering has been compromised. It had been observing a patch of sky in Cygnus and in spite of only managing three years of its intended ten years of data gathering it is still providing information. It is a Schmidt plate camera with a 55 inch primary.
Pictures can be taken at different wavelengths. Red gives shallow readings, green and blue go in deeper, so you can get better absorption readings of the planet’s atmosphere. Different gases have different absorption signatures, such as methane, ozone, carbon dioxide.
An exoplanet can also exhibit a change in light as its phase changes, so it is higher as the star moves behind it. Even thermal emissions can be detected.
Our Earth’s atmosphere does not have a uniform depth; the ozone holes and oxygen are not permanent and need to be constantly replenished. They are clear signs of life. Intriguingly, the temperature of the carbon dioxide in our upper atmosphere is warmer over Antarctica than elsewhere.
Prof Irwin gave us a review of the Cassini-Huyghens mission and the equipment he was involved with. It is still the largest ever interplanetary craft that we have launched. He was involved with CIRS (Composite Infra-Red Spectrometer), which operated at long IR wavelengths and had to be cooled to 80 kelvin. It was pointed at Saturn and had a black shield which radiated heat into space. He brought in a model of the instrument, with its tiny germanium lens and hollow titanium legs, also tiny and quite fragile. It had to be flight tested so that it could survive two years of travel with its protective cover on before it could be deployed. He said the shake test involved standing it on a large woofer.
In July of 2004 Cassini-Huyghens was sent on lots of elliptical orbits round Saturn and observed lots of swirly bits along belt zone boundaries. He got quite excited about them and still does not know why these swirly bits exist. He had also noted how the rings appear bent as they go behind Saturn but concludes Saturn has an atmosphere which bends their light.
He was also involved with VIMS (Visible and Infra-red Mapping Spectrometer), which looked deep into Titan’s atmosphere, wavelength around five microns. It also revealed lots of structure in Saturn’s atmosphere, even more than in Jupiter. The rings’ reflections on the surface and backlighting from the rings is exquisite. Internal heating is detected.
Titan’s clouds have a visible orange smog but near IR shows deeper methane clouds and a methane rain (hydrological) cycle.
CIRS took spectrum readings of Titan, and took temperature readings at different heights and latitudes.
The hexagon shaped vortex on the north pole of Saturn was actually detected by Voyager two, but the image was confusing at the time. It is very clear at five microns and backlit. A big storm in June 2011 recorded the hottest temperature at the uppermost part of Saturn, visible in a colourful swirl.
Prof Irwin’s final thoughts were on the future of ground based telescopes as well as the space based scopes such as the JWST.
We WILL be observing tonight at Frillford Heath from 8pm onwards.
See here for details.
There is NO observing tonight (18/9).
Professor Martin Hardcastle of the University of Hertfordshire is one of those people we need to invite back in the future. His talk was about surveying the sky for radio galaxies. In 1933, Karl Jansky’s first radio survey showed the Milky Way and the Sun and that’s all. Since then, we know that galaxies have a wide range of radio profiles.
The first major radio surveys were done at the Cambridge Lord’s Bridge site. The old radio array (do they still call them Yagi?) is where the 3rd and 4th Cambridge Catalogue of radio sources was compiled (the 1st and 2nd were full of spurious readings). You will have heard of the Parkes radio source catalogue, from the radio observatory in Australia. There is also the B2 catalogue of around 10,000 sources from 1960 to 1975. Please google for further information.
Some of the sources were very bright, but visually very dim, so the 3rd Catalogue’s sources took 30 years to record at visual wavelengths; he mentioned one 19.5 magnitude galaxy seen between two bright radio lobes. These lobes had emitted from the nucleus of the galaxy. Now Prof Hardcastle started warming to his task!
It seems all reasonably large galaxies have a black hole in the centre, caused when orbiting stuff gravitates towards the centre over time. And when stuff starts building up gravitationally around it you also get a build up of magnetic fields. And, as it’s rotating, so the particles in the stuff get twisted around. He showed us a computer simulation on auto-replay showing how there is a massive flash then two blobs appear in the polar regions, which then become the lobes over 100million years. Mesmerising stuff. Depending on circumstances there may already be a halo of debris around the galaxy or the jets just spread out into the intergalactic medium. The strong magnetic fields are what makes some of these jets reach relativistic speeds near to the central black hole, i.e., half of the speed of light or even faster. The big elliptical galaxies are most exciting. My notes on M87 say it is an active old mess in the visual but far more active in radio wavelengths and has a huge radio envelope.
As a result of such activity you can imagine there is a wide frequency range needed to monitor all this radio output.
The Chandra (an orbiting x-ray telescope) website shows lots of x-ray and radio sources, and reveals that the envelope is hot gas interacting with the ‘intercluster medium’. Google Cygnus A.
The VLA (Very Large Array = 27 radio dishes which can be laid out in four different layouts for different resolutions) has a range of 75MHz (so a bit shorter wavelength than Radio Oxford at 95.2 MHz) down to 43 GHz. When working to GHz wavelengths its resolution is sub arc seconds. It does have a small field of view, however. It has recorded 100,000 sources and counting, from 1995 onwards. Its radio survey is called FIRST (Faint Images of the Radio Sky at Twenty cm) is a shorter wavelength study (1 to 4GHz). It has 5 arcsec resolution but again only has a small field of view.
More recently and increasing in size and resolution is the LOFAR (LOw Frequency Array) with a range of 30 to 300MHz (Anything less than 30Mhz is absorbed by the ionosphere). This area is mostly unexplored and FM radio sits in there too. These long wavelengths need very long base lines, so they are observed by lots of little antennae, like poles with strings attached. They are camped in fields in large units called Superterps. The HQ is in Groningen, Netherlands, but there are five stations in Germany, with others in France, Sweden, Poland and Ireland. We have one in Chilbolton, Hampshire. The data is coordinated via VLBI (Very Long Base-line Interferometry).
LOFAR is dishing out masses of data, and this is accessible to you (yes, you) via the galaxy zoo website for citizen science.
Another new kid on the block is the SKA, (Square Kilometre Array), which also has pole antennae but also dishes for shorter wavelengths to 14GHz. There are stations in South Africa and Western Australia. I googled SKA and on April 28th the Global HQ is now being built in the grounds of Jodrell Bank.
This data, along with data from Space telescope Herschel, is now creating quite a map of what is in the sky, and is able to separate the active galactic nuclei from those that are star forming, as a result of core activity in the past. Prof Hardcastle hopes we will end up with a Hertsprung-Russell diagram in the radio, with size versus temperature, just as there is with stars.
Between now and April 20th Mercury should be visible to the naked eye in the evening twilight. Wait about an hour after sunset and you might catch a glimpse of this small world shinning in the evening sky. At the moment it is very low, only about 7 degrees above the horizon. The picture below shows Mercury at 20:15 BST for the 28/3/2017:
Tomorrow you can use the Moon to guide you as Mercury will be 8 degrees to the right of a very thin crescent Moon (NB: the Moon is much larger in this picture than it will be in the sky):
On April 1st the planet will be at its greatest elongation from the Sun, but it will still be low down in the sky. After that its orbital motion will bring it rapidly closer to the Sun and by April 20th it will be at inferior conjunction and no longer visible to us.
Don’t forget to watch Stargazing Live 2017 8pm (BST) tonight, tomorrow and Friday. All the way from Siding Spring, New South Wale, Australia.