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January 2008

ASTRONOTES
Incorporating FRIENDS' NEWSLETTER Armagh's Visitor Attraction of the Year 2008's Space Odysseys Cosmic bully The sky this month A life for the stars?

ARMAGHPLANETARIUM


2 Astronotes January 2008

It's nice to be appreciated
By Colin Johnston, Science Communicator I am delighted to report that Armagh Planetarium has been awarded the title of Armagh Chamber of Commerce's Visitor Attraction of the Year. The award, which was sponsored by Armagh Down Tourism, was presented at the Armagh Chamber of Commerce Business & Tourism Awards 2007 evening on 11 December. The Planetarium was ranked first after being evaluated by both a panel of judges and a public vote. Winning this award is a tribute to the quality of our exhibits, the superb Digital Theatre shows and the hard work and excellent public service displayed by all of the team here. It's nice to have this effort acknowledged and thanks to all involved. Our trophy is currently proudly displayed near the Voyager CafИ, have a look at it next time you visit us.

Glittering Prize I accepted the award on behalf of the Planetarium. It was presented by Armagh Down Tourism.

2008's Space Odysseys
By Colin Johnston, Science Communicator Here are some interesting space and astronomy dates and anniversaries for 2008 to note in your diary. Remember spacecraft launch dates are subject to sudden change! 2008 will be the International Year of Planet Earth sponsored by UNESCO and the International Union of Geological Sciences. Their aim is "to ensure greater and more effective use by society of the knowledge accumulated by the world's 400,000 Earth scientists". The Year's ultimate goal will be to help to build safer, healthier and wealthier societies. 2 January: Earth is at perihelion (the closest point

Mercury, meet MESSENGER The probe's name is a cunning `bacronym' for MErcury Surface, Space ENvironment, GEochemistry and Ranging.

Image Credit: NASA

Image Credit: Armagh Down Tourism


January 2008 Astronotes 3 in its orbit to the Sun). 14 January: NASA's MESSENGER probe will make a flyby of Mercury on this date. It will be the first terrestrial visitor to the innermost planet since Mariner 10's last flyby in 1975 (see also October 6). January 24: Thirty years ago today, the nuclearpowered Soviet reconnaissance satellite Cosmos 954 made an embarrassing unplanned re-entry into Earth's atmosphere. Radioactive debris was scattered over Canada's Northwest Territories and a costly clean up was needed. February 1: Fifty years ago today the first American satellite, Explorer 1, was successfully launched. America's first satellite should have been launched as part of the US Naval Research Laboratory's Vanguard program but after an embarrassing launch failure, Explorer, a previously mothballed US Army Ballistic Missile Agency project, was hastily substituted. An onboard instrument revealed the hitherto unknown Van Allen radiation belt around the earth. February 7: An annular solar eclipse will be seen by observers in Antarctica, Australia and New Zealand.

Welcome to the Pleasuredome Armagh Planetarium in 1968. May 1: Forty years ago today Armagh Planetarium was officially opened. The fortieth birthday is a milestone, marking the point when youth begins to be left behind, and dignified maturity sets in. It will be the end of wild all night partying for the Planetarium; instead it will be evenings of dozing by the fire in slippers, cocoa, early nights and Phil Collins on the stereo. Forty is when the Planetarium grows up. Really? No chance! Forty is the new twenty! Armagh Planetarium will continue to be as exciting and innovative as it was back in the sixties! May 25: NASA's Phoenix spacecraft will land on Mars near the planet's water-ice rich north pole. June 22 : On this day in 1978, Charon, the largest satellite of Pluto was discovered. Now Pluto's mass could be accurately determined and it was found to be much smaller than previously estimated. In retrospect, this discovery marked the start of Pluto's decline from planethood. 30 June: This day marks the centenary of the Tunguska Event. This was the largest impact event known in historical times. Over 2000 square kilometres of Siberian forest were devastated by a falling comet or meteoroid. 4 July: Earth is at aphelion (the most distant point in its orbit to the Sun). July 20: This day is NASA's fiftieth birthday. President Eisenhower ordered the founding of the National Aeronautics and Space Administration on this day in 1958 to conduct America's civilian space activities. Since then the agency has been Earth's leading space agency, conduct-

"We will be hosting special events to celebrate Armagh Planetarium's 40th Anniversary "
February 21: A total lunar eclipse will be visible in North and South America, Europe, Africa and Western Asia. You can see it (weather permitting) between 0143 and 0509 GMT. February 22: ESA's first Automated Transfer Vehicle will hopefully be launched on its longdelayed voyage to the ISS (see the January 2007 Astronotes for a previous launch date). March 7-8: We will be hosting two days of special events to celebrate Armagh Planetarium's 40th Anniversary year.

Image Credit: Armagh Planetarium


4 Astronotes January 2008 final flyby in 2009 before entering orbit around Mercury in 2011. 13 October: On this day the British Interplanetary Society celebrates its 75th anniversary. It is the world's oldest space-advocacy organization. 28 October: The Lunar Reconnaissance Orbiter (LRO) and Lunar CRater Observation and Sensing Satellite (LCROSS) will be launched together on this day. Both probes will orbit the Moon. LRO is intended to locate sites for future human missions to the Moon. LCROSS is to observe the probes' booster impacting the Moon's surface. November 3: NASA launched Mariner 10 toward Mercury thirty five years ago today. November 15: Buran, the Soviet Union's space shuttle was launched by an Energia rocket on its unmanned maiden orbital spaceflight on this day in 1988. This was the first and last space flight for the Russian shuttle before the project fizzled out for lack of funding. The mothballed vehicle was destroyed when the roof of its hangar collapsed in 2002. 21-27 December 2008: Forty years ago this week Frank Borman, James Lovell and William Anders, the crew of Apollo 8, became the first humans to orbit the Moon. Their flight featured a memorable live broadcast including a reading from Genesis on Christmas Eve.

Image Credit: NASA

Discovery of Charon The satellite was found by astronomer James W. Christy who observed that images of Pluto were sometimes elongated (left) compared to the normal appearence (right). ing a hugely successful (although there have been occasional tragic failures) programme of manned exploration and an astonishing range of unmanned probes and science missions. 1 August: A total solar eclipse will be visible in the Canadian Arctic, Arctic Ocean, Siberia, Mongolia and China. Observers in the UK will see a partial solar eclipse. In Northern Ireland to see this spectacle at its best, you should watch the skies between about 0800 and 1000. 7 August: Space Shuttle Atlantis should take off for the final Hubble Space Telescope servicing mission. The HST will be fitted with new batteries and gyroscopes and have its instruments serviced. On its return Atlantis will be retired as part of the wind down of the Space Shuttle programme. 16 August: A partial lunar eclipse will be seen in Africa, the Middle East and Russia. October 2: This day is the `official' four hundredth anniversary of the telescope. On this day in 1608,Hans Lippershey, a Dutch lens maker, demonstrated the first publicly-displayed telescope in the Dutch parliament building. There is still considerable controversy over whether Lippershey personally invented the instrument or based it on the work of others. October 6: The MESSENGER probe will make another flyby of Mercury on this date. The spacecraft is scheduled to make a third and

Shuttle, meet Buran Some claim that the Soviet shuttle orbiter's design was heavily inspired by its American equivalent .

Image Credit: Edo Leitner


January 2008 Astronotes 5

Cosmic Bullying
Image Credit: X-ray: NASA/CXC/CfA/D. Evans et al.; Optical/UV: NASA/STScI; Radio: NSF/VLA/CfA/D. Evans et al., STFC/JBO/MERLIN

Fatal Beauty This composite image (about 180 000 light years across) shows the jet from the black hole at the centre of one galaxy striking another galaxy, the first time such an interaction has been found. In the image, data from several wavelengths have been combined. X-rays from Chandra (coloured purple), optical and ultraviolet data from the HST (red and orange), and radio emission from the VLA and MERLIN (blue) show how the jet from the main galaxy on the lower left is striking its companion galaxy to the right. By Wendy McCorry, Science Communicator It is sad but these days we hear a lot in the news about bullying of various types ­ physical, mental and the latest, of course, cyber-bullying. Cases of bullying usually take place between schoolchildren, work colleagues or even family members, but did you know that bullying also occurs in the cosmos? No, not between feuding astronauts, but between galaxies! Last month, the first ever observed case of inter-galactic bullying was reported by NASA scientists. The `bully', nicknamed the `Death Star Galaxy', was observed blasting a smaller neighbouring galaxy with a powerful jet of deadly radiation particles from its centre. Travelling close to the speed of light, the jet slammed into the lower section of the smaller galaxy, before dramatically twisting as it was deflected away. Neil Tyson, director of the Hayden Planetarium in New York dramatically comments, "It is like a black hole bully, punching the nose of a passing galaxy." Compared to the vast age of the Universe this event is relatively recent, having occurred around one million years ago. It is estimated that it may continue for another ten to one hundred million


6 Astronotes January 2008 years. Using the Chandra X-ray observatory, the Hubble and Spitzer Space Telescopes and the Very Large Array and the UK's Merlin (both are ground-based radio telescopes), astronomers have been able to build up a picture of the dramatic event, which occurred in a distant system called 3C321 - 1.4 billion light years (0.43 gigaparsecs) away from Earth. The two galaxies involved are in orbit around one another, and will eventually merge into one larger galaxy. Whilst merging galaxies are a common enough occurrence, such a violent outburst between two galaxies has never before been observed by astronomers. Earth is from the centre of the Milky Way) means that any planetary systems in the path of the deadly jet are likely to have been destroyed. It is thought that as many as tens of millions of stars, some of which may have had orbiting planets, were hit by the deadly emission. Should the Earth have been one of these planets, the effects would have been devastating. Over a matter of months, the jet would have stripped away the ozone layer, and compressed the protective magnetosphere, leaving the planet exposed to the harmful effects of both the Sun's rays and the jet itself. It is unlikely that any life forms on the surface would have been able to survive. Surprisingly, there may just be a happy ending to this particular episode of cosmic bullying. After all the death and destruction, Dr Martin Hardcastle, researcher at the University of Hertfordshire, believes that the effect of the jet smashing into the other galaxy could have a positive outcome. The emission may actually cause the galactic clouds of gas and dust within the smaller galaxy to contract, thus igniting star birth and resulting in the creation of a whole new generation of stars.

"...planetary systems in the path of the jet are likely to have been destroyed "
Most galaxies are believed to have supermassive black holes at their centre, and a small number of galaxies emit powerful jets from these black holes. How this occurs is still not fully understood. Such galaxies are called radio galaxies, as the jets are easily visible at radio wavelengths. Astronomers believe that the smaller of the two galaxies was unfortunate enough to enter the path of such a jet, emanating from the centre of its larger neighbour. The proximity of the two galaxies (around twenty thousand light years, about the same distance

Moon Phases, Jan 2008
Tue 8 Jan Tue 15 Jan Tue 22 Jan Wed 30 Jan NEW MOON First Quarter FULL MOON Last Quarter

January's Night Sky
By Paul O'Neill, Education Support Officer The Stars: The winter night sky is full of bright stars: Betelgeuse, Rigel, Procyon, Castor, Pollux, Aldebaran, Capella and of course the brightest star in the night sky ­ Sirius. The constellations of Orion, Taurus and Auriga are particularly prominent. As for the circumpolar constellations, the big bear (Ursa Major) is low down in the evenings, while Cassiopeia is high up (and looks like a large capital M). The Planets: Mercury is on the other side of the Sun from the Earth and will not reappear again until mid January as an evening object, visible low down in the western sky for a short time after sunset. It's always very difficult to find this little planet because of its proximity to the Sun and its small size (Mercury is only about one and a half times the size of our moon). Venus is still a spectacular object in the morning sky, about ten times brighter than Sirius. If you


January 2008 Astronotes 7 to look at Mars through a small telescope? Well most people are very disappointed with their first view of Mars. It often looks like a small orange featureless globe, any fine detail that may be visible is often obscured by atmospheric turbulence. Historical observations of Mars show how scientists can and do make mistakes and how these can lead to incorrect conclusions. In 1877 the Italian astronomer

Image Credit: www.wikimedia.org

Mars as it was 1 A map from a 19th Century German encyclopedia based on observations of Mars by Schiaparelli. Note the dark seas and lighter land masses. Giovanni Schiaparelli reported seeing canali on the surface of Mars. This was incorrectly transwere to look at Venus through a telescope you'd lated as canals, (it should have been translated see that it isn't a complete disk. The inner planas straight lines). An Irish astronomer Charles E. ets show phases (like the moon); during January Burton also reported straight lines visible on the Venus is about 75% full, the phase is waxing. planet surface.

"Most people are disappointed with their first view of Mars"
Mars is a prominent object in the evening sky. Last month Earth and Mars were at their closest point but during January the distance between the two planets increases rapidly. This means Mars will get dimmer during the month. If you were to record the apparent motion of Mars since last autumn you'd find that the planet seemed to be heading from the constellation of Taurus into Gemini but then started moving back into Taurus again. The Earth and Mars are both in orbit around the Sun. During last autumn Mars was ahead of the Earth but since the Earth is closer to the Sun it orbits faster and inevitably catches up then over takes Mars. From our point of view Mars then seems to be travelling backwards. This is called retrograde motion. What would you see if you were

"Percival Lowell reported that Mars was covered in a web of 585 canals "
A team led by Percival Lowell at the Flagstaff observatory in Arizona began making the most detailed study of Mars ever. They reported that the surface of Mars was covered in a web of about 585 permanent canals. These canals

Mars as it was 2 Maps drawn by Percival Lowell showing geometrically straight canals with `oasii' at their intersections.

Image Credit: www.wikimedia.org


8 Astronotes January 2008 disappear for a while just as they did for Galileo. The other major planets: Jupiter, Uranus and Neptune, are not well placed for observation this month.

"...in the early hours of January 4 you should see some meteors"
The only major meteor shower in January is called the Quadrantids. Meteor showers are named after the constellation from which they appear to radiate, so for example the Taurids appear to radiate from the constellation of Taurus, the Orionids from Orion and so on, but which constellation do the Quadrantids come from? The strange name comes from the disused constellation Quadrans Muralis (the Wall Quadrant ­ an old astronomical instrument used to measure star positions). This constellation appeared
Image Credit: NASA

Mars as it is A Hubble Space Telescope image taken in 2003 shows polar ice but there is not a canal to be seen.. seemed to run from the polar ice caps to the arid equatorial regions. Lowell believed the canals were artificial, implying that Mars was inhabited by intelligent life forms. It should be noted that Schiaparelli thought most of Lowell's observations were imagined. It was a January night almost 400 years ago that Galileo first turned his small telescope to the sky. He discovered the four largest moons of Jupiter and also found that Saturn was strange. In his words, "I have observed the highest planet (Saturn) to be triple-bodied. This is to say that to my very great amazement Saturn was seen to me to be not a single star, but

The Lost Rings of Saturn? The rings of Saturn all but disappear when viewed edge-on. Here we see the planet and several moons imaged by the Hubble Space Telescope. on some old star maps but never became fixed. The peak of the meteor shower lasts only a few hours, so can be easily missed. It is due around 06h40m UT on 2008 January 4 (nearly-new Moon). Meteors are best observed in a dark sky. This year, unlike 2007, the phase of the moon is favourable. If you go out in the early hours of January 4 you should see some meteors (look towards the northern part of the constellation of Bootes). Quadrantid meteors are usually quite fast and colourless. This is the last reasonably reliable meteor shower for a few months.

three together, which almost touch each other". Galileo never did understand that what he was looking at was a planet surrounded by a set of rings. Two years later he was puzzled to discover the triple star had become a single star. What Galileo did not know was that the Earth had passed through Saturn's ring plane and the rings were edge on and could not be resolved with his 20x telescope. In January 2008 the rings are inclined about 6є as seen from Earth ­ this angle is decreasing and in the near future the rings will

Image Credit: NASA


January 2008 Astronotes 9

A life for the stars?
By Colin Johnston, Science Communicator Last time we discussed the possibility that humans (or our robots) may one day travel to the stars. Using even the most advanced plausible rocket designs, such as the British Interplanetary Society's Daedalus concept, we seem resigned to one-way journey times of decades or centuries to reach even the nearest stars. Such vessels would be enormous, thanks to their colossal propellant requirements. Some of you may be thinking that this is hopelessly unimaginative. "What about travelling faster than light through warpdrive, wormholes or hyperspace?" you may be asking. Sadly we have no idea how these things could be made real. In fact they are almost certainly impossible. A century ago Albert Einstein proved that light travels at the fastest speed possible, some 300 000 km/s, and nothing can exceed this cosmic speed limit. Faster than light (FTL) travel is an essential element for much of science fiction but it is at heart a fantasy concept. I would really, really like to be proven wrong about this.

Bussard Ramjet Starship 1 An artist's impression of the concept. It is hard to depict the craft's scale: almost certainly it would be kilometres across. It will be millennia before we could hope to construct such a vehicle. between the stars. Imagine a cube of interstellar space 1000 km across, it initially appears entirely empty but if you carefully examine its contents you will find about a gramme of material. About 99% of this thin stuff is gas of which about 90% is hydrogen and nearly 10% helium with traces of other elements. Bussard Ramjets would scoop up this material as they flew (hence their alternative name of ramscoop), use the hydrogen as nuclear fusion fuel and accelerate continuously through their flight. If the acceleration could be maintained at roughly 9.8 m/sІ (1g or the gravitational acceleration near the Earth's surface), the crew would be able to comfortably walk on their vessel's decks as though it were sitting on the Earth's surface. As it accelerated, a strange thing would happen to time on board the ramjet starship. This is no place to explain Einsteinian Relativity, it is sufficient to say that century old mathematics show that time runs more slowly on a fast moving starship than it does on the worlds it left behind. The closer it approaches the speed of light, the slower time will run on board. For example, at 0.95c (ninety-five percent of the speed of light), shipboard time is running about a third of

"Space is not quite a vacuum "
Robert W. Bussard, who passed away in October 2007, was a US nuclear physicist who is best known for a speculative space propulsion concept he proposed in 1960, back in the heady pioneering days of astronautics. The Bussard Interstellar Ramjet offered the intriguing possibility that one day it could be possible for astronauts to get on board a spaceship to travel anywhere in the galaxy or even beyond in their (and note, only their) lifetimes. A Bussard ramjet ( in aviation a ramjet is a rarely used form of jet engine, not to be confused with Bussard's concept) avoids the problem of excessive mass by not carrying its fuel with it. Instead it would gather fuel as it flew between the stars. Space is not quite a vacuum. An incredibly tenuous interstellar medium pervades the space

Image Credit: NASA


10 Astronotes January 2008 1g for half the trip then decelerating a 1g for the other half, would do it in about 7.5 years as observed by people on Earth- but about four years to its crew! Perhaps human beings could travel to the stars in person after all! This effect proves more and more useful for longer distances. A ramjet could travel the fifty light years to the sun-like star 51 Bussard Ramjet Starship 2 Another artist's impression by the very talented Adrian Pegasi (home of at Mann. If you are interested in more thought-provoking artwork of futuristic space least one planet) craft see his website http://www.bisbos.com/rocketscience/. in about 52 years the rate it does on Earth. To the ramjet's crew nothing unusual is happening but a stationary observer would see them moving in slow motion, (apologies to any relativistic physicists reading, I am simplifying an awful lot here). At the end of their 15 minute tea break, an hour and four minutes have passed on Earth. This effect gets more extreme as the ramjet accelerates. At 0.999c, an hour on board the starship passes in a day of Earth time. measured on Earth, but less than eight years to the crew. On their arrival back on Earth, less than two decades will have passed for the intrepid explorers, but they will return more than a century after they departed. The Earth they come back to may be as alien to them as any exoplanet. Even more fantastic trips would be possible; a really determined young crew could set off on a round trip to the Galactic Core, 26 000 light years away. They would experience a 40 year journey, but the by now middle-aged crew would return to Earth 52 000 years after they left. Even intergalactic missions would be possible. A Bussard ramjet offers return trips anywhere in the Universe and one-way trips to the future. The concept has been a wonderful gift for science fiction authors. Could we build such a craft one day? Even on a superficial examination there are difficulties with the idea. The Bussard ramjet must be boosted to a certain minimum speed before the scoop action functions. This speed may be as high as 6% of light speed (0.06c), necessitating a Daedulus-style rocket as an auxiliary propulsion system. The ramjet would need an enormous collection area to capture its fuel. Bussard calculated that a 1000 tonne ramjet vessel would need a scoop

Image Credit: © Adrian Mann, reproduced with permission

"Einstein's time dilation sounds crazy but will really happen"
This, Einstein's celebrated time dilation, sounds crazy, but there is excellent experimental evidence that this will really happen. No spacecraft yet built has travelled fast enough to experience this effect, but accelerating short-lived subatomic particles to near-light speeds physicists have lengthened their time of existence. Imagine if we could really build a Bussard Ramjet. The robotic Daedalus probe discussed in the last issue would cross the six light years to Barnard's Star in about fifty years, a ramjet, accelerating at


January 2008 Astronotes 11 more than 100 km across. Doubtless this collector would not be a material object but cleverly shaped magnetic and electric fields generated by the vessel. This could be useful: as the ramjet approaches its destination, the polarity of the fields could be reversed, creating a drag force, slowing the ramjet to an eventual halt. Possibly the toughest of all the technical issues with the concept is that trying to use hydrogen (as opposed to a hydrogen isotope like deuterium) in a fusion reactor requires us to use a reaction, the proton-proton Empty Space Beta Canis Majoris is the B class variable Gomeisa, visible cycle, which as far as we know can only happen in the near Procyon. Antares is located in the neighbouring Loop I Bubble. awesome extremes of heat and pressure found come. Earth is not favourably positioned in the in the cores of stars. This fact alone makes the Galaxy as a home base for Bussard Ramjets. For original Bussard concept only a tiny bit less at least the previous three million years, the Sun impossible than FTL travel. has been passing through a huge void in space called `The Local Bubble'. Shaped roughly like Finally there is an astronomical problem to overa peanut, it is about 300 light years across and the Sun is currently located near the centre. The Local Bubble may have been created when a star went supernova (the neutron star Geminga may be a remnant of this cataclysm). The blast wave from the explosion swept through space, blowing away interstellar matter. The interstellar medium within the Local Bubble is about ten times lower than in the rest of the galaxy. This makes the interstellar matter in our Sun's vicinity a poor fuel source for a Bussard ramjet, limiting its performance. The craft would accelerate slowly, taking decades to reach relativistic velocities ­ until it escaped the Local Bubble!
Image Credit: Illustration by Dominic Harman via Amazon.co.uk

SF Classic Bussard ramjet starships have appeared in memorable novels by Larry Niven, Bob Shaw and Vernor Vinge. Greatest of all is Poul Anderson's `Tau Zero', the story of the strange fate of the crew of the Leonora Christine, a ramjet which runs out of control. The characters are carried helplessly into the very distant future.

The technical difficulties mean that humans will probably never build an interstellar ramjet exactly as Dr Bussard originally described it. Yet the idea of gathering fuel as you travel, rather than carrying it with you is so elegant that it is hard to imagine that it will not be applied in some fashion one day. Then, perhaps, the stars will be in our grasp.

Image Credit: NASA


12 Astronotes January 2008

Image of the Month
Image Credit: NASA

Can you identify this mystery object? (If you were a small boy in the late sixties or early 70s this may not be too difficult!) This is the Ascent Stage of an Apollo Lunar Module under construction in the Grumman plant at Bethpage, New York about forty years ago. Most of the vehicle's thin aluminium skin and mylar foil coating has yet to be attached, leaving the beautiful interior exposed. Beautiful? Yes, the engineering revealed is a masterpiece of functionality. Its clever, intricate design assembled by skilful craftsmen that tells as much about human beings and their dreams and potentials as the works of any Great Master. Leonardo would have loved it! Designed to function in a complete vacuum, the Lunar Module's design makes no concessions to aerodynamics and its outer skin's purpose is purely to protect the internal components and crew from

solar heat and micro-meteoroids. In the picture we can see the normally hidden aft electronics bay, the white sphere of a fuel tank for the ascent engine, the four nozzles of a set of reaction control thrusters and the dish of a communications antenna. Nine of these craft carried crews through space and six landed on the Moon. One saved the lives of the Apollo 13 crew. In December NASA announced that its new Lunar Surface Access Module would be the Altair spacecraft. Altair is planned to a be a much larger successor to Apollo's Lunar Module, hopefully in a little over a decade an Altair will carry four astronauts to the Moon's surface with many more to follow in the years to come. Successful development of the Altair, along with the Ares series of rockets and the Orion Crew Exploration Vehicle, will be vital for the future of human space exploration. (Caption by Colin Johnston, Science Communicator)

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Astronotes, Incorporating Friends' Newsletter is published monthly by Armagh Planetarium, College Hill, Armagh, Co. Armagh BT61 9DB Tel: 02837 523689 Email: cj@armaghplanet.com Editor: Colin Johnston ©2008 Armagh Planetarium All rights reserved