The GRACE mission was designed to map the Earth’s gravitational field in exquisite detail (and it has just celebrated a decade in orbit). To achieve this objective, the mission consists of a pair of twin satellites in orbit and the instruments which can measure changes in their separation to incredible accuracy. Over long periods of time the GRACE satellites have built an incredible map of how gravitational field strength varies across the Earth.
Early evening March 17th 2002 I had my telescope set up in the garden just outside the house. It was still twilight but it was dark enough to see the constellations: the brightest stars of Gemini, Taurus and Orion were easily visible. Jupiter and Saturn were also shining and very high in the sky. Then I noticed a bright new star in the sky not far from Jupiter. For a moment I thought it was a satellite but then realised that it didn’t look like it was moving. My next thought was that it might be a new supernova! I got the telescope – a 10 inch Dobsonian – on to the new star very quickly.
The view through the eyepiece was unusual; it was disk shaped – a bit like Jupiter – but bigger and with a brighter core at the centre. Through the eyepiece it did actually appear to be slowly moving. At this point I realised that I was probably watching a rocket from beneath and looking directly into the exhaust.
I watched for a few minutes and then went to get the digital camera. I’d taken a few shots of the moon before, by holding it to the eyepiece. No control over the exposure or flash but I was hopeful I’d get something. Returning to the telescope I watched in amazement as the disk seemed to shatter into hundreds of pieces and disperse. I took this picture:
Two of the pieces seemed more substantial and I watched them for a few more minutes as they slowly drifted apart. I was always curious about what rocket I actually saw there and why it ended up in pieces like that.
So having found the picture again recently I saw that the date and time was stored in the image properties! I googled the date and found that the GRACE mission was launched that day.
GRACE was launched atop a Rokot launch vehicle from Russia earlier that day so I initially thought I’d seen the separation of the individual satellites. I tried to find a way of confirming whether GRACE was visible from the UK on the day of launch. After a bit of digging I found a paper which seemed to confirm that various manoeuvres had to take place in the within visibility of several ground stations. The path of the satellite (shown in the paper) would have made it visible from the UK.
So….a minor mystery solved. I’m not sure I saw the actual separation of the satellites from the rocket – that was supposed to happen 90 minutes or so after launch (I was watching many hours later). It was probably a burn to separate or correct the separation of the satellites. I’m happy to have what may be one of the few pictures of the start of this incredible mission!
The dark evenings of November provide a great opportunity to view the Milky Way as it flows through the northern constellations of Cassiopeia and Perseus. There are so many star clusters to see! As a young astronomer 30 years ago, I remember looking with binoculars from my bedroom window and discovering a nice semi-circle of stars below a brighter star. The view is shown in the large red circle below.
This scattered cluster of stars was not listed in any of the deepsky object catalogues that I knew about (Messier, New General Catalogue) and it was many years until I discovered that this object had a name. The bright star at the focus of the semicircle is called Mirfak (or alpha Perseii) and the cluster of stars is called the “alpha Persei Moving Cluster“. It is also known as Cr39; it is the 39th entry in a catalogue compiled by the Swedish astronomer Per Collinder in 1931. It also goes by the name Melotte 20.
So what is it? Well “cluster” is too strong a word. Astronomers call this an “association” instead. The stars are more loosely gravitationally bound than a cluster and the stars may eventually go their separate ways. For now, they’re all moving in roughly the same direction through space.
The cluster is approximately 600 light-years away from us and the 50 or so stars are mostly massive, very hot blue-white stars. The exception is the brightest member, Mirfak, which is slightly cooler and more yellowish. The age of the stars is estimated to be about 70 million years. Binoculars show the cluster very well; telescopes have a field of view just a bit too big to do this group of stars justice.
There are many other clusters of stars to be found in this part of the sky. Just go outside with your binoculars and take a look for yourself!
Week 5 on the course was devoted to the mid-term exams in all the academic subjects; so no lectures or seminars.
In Week 6 we got stuck into more algebraic tools and methods; namely polynomial division, factor theorem and remainder theorem. Read on for more details.
The Taurid meteor shower reaches peaks on November 3rd and again on the 12th. For many amateur astronomers the month of November is associated with the Leonids - a much more famous shower which has occasional stormy outbursts. But I have to say, I prefer the Taurids because they have a more interesting backstory!
About 20,000 to 30,000 years ago a huge comet - perhaps 50 km in diameter or more - became embroiled in a series of close approaches to the planet Jupiter. Nothing unusual about that - Jupiter has a huge family of comets even today and we;ve seen first hand how Jupiter can change comet orbits and even tear comets apart.
These days, all we have left of the original giant progenitor comet is a small, faint comet with an orbital period of 3.3 years (called Encke's Comet) and a complex series of dust streams which the Earth encounters in November each year. The dust released by the fragmented nuclei over periods of thousands of years have gradually been spread out into a broad swathe of the inner solar system. Actually, the Earth also encounters one of the streams during June but during the daylight hours. It is speculated that the devastating Tunguska event of 1908 was due to a larger fragment disintegrating in the Earth's atmosphere. That's another story.
On Earth, each November, we see the remains of a giant comet, streaking into our atmosphere as a shower of shooting stars and appearing to come from the constellation Taurus. It takes the Earth weeks to cross these lanes of dust and in doing so we encounter two distinct peaks - evidence of the complex evolution of the meteor orbits. So in November we see the Northern Taurids near the start of the month and and the Southern Taurids around two weeks later. Activity is fairly low - typically about seven or eight per hour. Compare this to, say, the short sharp spike in activity of the Perseids in August or the Geminids in December.
Will we see another giant comet? Interestingly, there is evidence to suggest a big comet, seen in our skies in 1106, broke apart and the sun-grazing fragments have produced several of the best comets of past millennium.
There's also the possibility that a swarm of Kreutz sungrazing comets (probable fragments of the Great Comet of 1106) are en-route to the inner solar system and will arrive in the coming decades. If true then there are good prospects of seeing another Great Comet (or Comets).
So, no major showers this month. But chances are good this month that if you spend enough time outside watching the sky you will see the remnants of an ancient, giant comet ending their existence in a brief flash of light.
For a more in depth technical article about the Taurids --- see here.
* Had to fight an urge to add the word Batman! to this title.
Here, in one place, are some photos of our home world taken by humans and robots from low Earth orbit and beyond.
That final picture remains the most distant picture of the Earth ever taken. Taken at the request of Carl Sagan, the Voyager 1 spacecraft captured a series of images which were stitched into Family Portrait of the major planets of the solar system:
The Voyager Portrait was from taken by a camera leaving the solar system and looking in towards home. A more recent portrait was assembled by the MESSENGER mission team, this time looking out from a vantage point the near the centre of the solar system:
Clearly, these images put our place in the solar system (and universe) into perspective! You don't have to travel too far before the Earth is rapidly scaled down from an entire planet with oceans, cities and people to less than a single bluish pixel before spacecraft have left the outer solar system,
I'll leave the final word to Carl Sagan, who said this about The Pale Blue Dot:
“Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there-on a mote of dust suspended in a sunbeam."
Dr Adrian Jannetta
Guitar strummin' explorer of the universe. Mild mannered maths teacher by day and astronomer by night.