The planetary line-up in this years early summer‘s sky was a rare event as shown in this Mike Frost find . . . .

Line-ups of the naked eye planets are quite rare and it was interesting to watch the dance of the bright lights in the evening sky during May as they changed places night after night.  Then in June, Jupiter and Venus swung pass each in a graceful sweep in the darking sky.  Even from cloudy UK it was a show worth seeing, for it will be some time before 5 planets grace the sunset skies again.  The next meeting of the five will be in 2040.  In that year the five planets will close up together during the closing days of August and into early September, so that on the evening of Saturday, September 8th they will be grouped in a tight bunch with the 2 day old crescent Moon close to Saturn, Venus and Mars, just above Mercury and Jupiter.  All close to the western horizon.  Lets hope for clear weather in that summer so that many can see the show.

---

 

I was in the pub, so it wasn‘t much of a surprise to bump into my old friend Clive, of the Interplanetary Dangerous Sports Club.  It was a little more of a surprise to see that he was in the company of a rather attractive young woman. Had he found someone to replace his ex-girlfriend (and nemesis) Clarissa?  That turned out not to be the case.

"Oh hi Frosty!" Clive said. "Have you met our Davina, our club president?"

I hadn‘t.  Where had she been all my life?

I offered to buy a round but Davina cut me short.  "Clive‘s buying all the drinks tonight", she purred, "he lost a bet with me."

"Really?  Over what?"

"You wouldn‘t be interested," said Clive, trying to be offhand.

Of course I was interested!  Clive was dispatched to buy me a beer, leaving me, briefly, alone with the beautiful Davina.  I tried to make conversation.

"So what was your wager about?"

Davina wasn‘t playing.  "I think Clive should tell you himself."

Clive proved curiously reluctant to begin his story, especially as he usually needed no prompting at all.  "Davina and I had a competition.  The winner would be the first person to land on, and provide photographic evidence of the landing, on a certain asteroid in the Kuiper Belt, way beyond the orbit of Neptune.  But not just any old asteroid."

Clive supped his pint for dramatic effect.  "Our asteroid," he announced, "has antigravity!"

* * * * * *

"Well that‘s not quite true," said Davina, "Let me explain.  Asteroids are the leftovers of the solar system, the material that didn‘t go to make up the Sun or the planets.  Like everything else in the solar system, most asteroids congealed under the influence of gravity.  And, the bigger the asteroid, the stronger the gravity.  Larger asteroids like Ceres and Vesta had enough gravity to form proper planetoids — spherical objects like the Moon or the Ganymede or Titan.  Smaller asteroids aren‘t big enough for gravity to form anything more than, basically, a heap of rubble.  But our asteroid was different. . ."

I was intrigued.  "How could that be?  As you say, small asteroids don‘t have enough gravity to hold together any more than loosely."

"Ah", said Clive," but our asteroid was a chip of the old block."

"Precisely," said Davina, "our asteroid was chipped off another, larger body in a collision billions of years ago."

"So your asteroid was a chuck of rock rather than a heap of rubble."

"Very good!  Most asteroids twenty miles in diameter are slag heaps, ours was solid rock.  But, more importantly, our asteroid was spinning, and spinning very quickly."

I began to understand.  "So there‘s a centrifugal force."

"Exactly.  No anti-gravity is involved.  There is a small gravitational pull, but it is easily overcome by the centripetal acceleration.  Anything on the surface doesn‘t stay there — it gets flung off because the asteroid is spinning so fast.  So there appears to be negative gravity. If our asteroid was simply a heap of rubble, it would have disintegrated billions of years ago. Instead it‘s a rapidly rotating rock that is devilishly difficult to land on. Isn‘t it Clive?"

Clive looked uncomfortable.  "Suppose so. Frosty doesn‘t want to hear the details, though"

"Yes I do!"

"Yes he does!"

So Clive had no choice but to tell his story.

* * * * * *

"Davina and I [said Clive] agreed to set off to the Kuiper Belt in separate solo spacecraft, so that we could each plan the orbital trajectory we felt was best for the asteroid.  A support craft would fly by a few days later to rescue us if need be.

As you understand now, the task was fiendishly difficult — that of course was why we picked it.  We had somehow to land on a piece of rock, twenty miles across, that rotated once every ten minutes — a rock spinning so fast that anything on its surface, me included, would be immediately flung into space by the centrifugal force. Somehow I had to find a way of landing on the surface, taking a photograph, and getting back safely to the support vehicle.
I had a very good plan — so I thought.  I aimed to match my incoming speed so that, at closest approach of sixty miles or so, I was traveling at exactly the same speed as the asteroid‘s surface.  I would then launch a harpoon at the surface, with me attached to the other end, jettisoning myself from my approach craft.  I could then reel myself in along the tether line to the surface.  I couldn‘t fall off because I would be firmly secured to the rock all the time.  Take a few photographs, cut myself loose, and wait for the rescue ship to pick me up.  What could go wrong?"

"What did go wrong?" I asked.  Clive fingered his pint uncomfortably.

"I piloted my ship into a close approach.  At sixty miles distance the asteroid seemed very large - I could see jagged gashes and impact craters on the surface.  I fired off the harpoon and watched it streak towards the asteroid and then slam into it with a large explosion.  All the debris, of course, flew immediately into space, so the sight was very impressive.  The harpoon appeared to hold, so I cast myself off from the spaceship and waited for the slack to be taken up.  A few minutes later I could feel my tether going taut, and I switched on the reel to wind myself up the cable, towards the surface of the asteroid.

Almost immediately I know something was going wrong.  I could see the asteroid rotating beneath me, taking the harpoon landing sight out of my view.  I must have mis-judged the approach speed!  Worse, I was approaching the surface a lot more quickly than planned.  I had expected to take several hours to wind myself up to the surface, but instead the tether was wrapping itself rapidly around the asteroid, pulling me towards the surface.  This was going to be hairy!

I had barely ten minutes — just over one complete rotation — before I reached the surface.  In which time I had to decide what to do.  I figured that, whilst it wasn‘t quite the landing I had planned, all I had succeeded in doing was speeding up my journey to the surface.  Perhaps this might mean that I arrived just ahead of Davina rather than just behind.  My arrival might be less controlled, but I still had retro-jets on my spacesuit to give me some control over my landing.  So no change of plan.

I got closer and closer to the surface.  Now my tether line ran almost parallel to the surface — stretching away towards a horizon that was now miles away from me.  I could see my landing was unlikely to be soft — of course there was no dust on the surface of the asteroid, any impacts simply left holes in the rock.  So as the tether line ran out I turned my retro-jets towards the surface - switched them to full, and braced myself.
BANG!!!   OW!!!!"

Clive winced at the memory of his landing.

"So what happened next?"

"I fell off, of course.  I had just crash-landed on a planet with effectively negative gravity — what else was there to do?"

"But what about your tether?"

"There‘s the problem, you see.  I wasn‘t tethered directly to the surface, rather round it.  So I fell down, away from the asteroid‘s surface, for what felt like thirty or forty seconds — perhaps half a mile away from the surface.  Then the tether took up the slack again."

"Pulling you back towards the surface?"

"Exactly.  I was being wound round the asteroid again on the end of the tether.  A minute later I was heading back towards the surface — trying to figure out how to stop myself.  BANG!! OUCH!!  And I fell back off into space."

"Why didn‘t you just cut yourself free?"

"Two reasons," Clive explained, "First, I hadn‘t had time to take a photograph, as specified in the terms of the challenge. Second, I couldn‘t."

"What?!"

"One of the two collisions so far had damaged my escape release.  I couldn‘t break free from the tether.  At that point I began to get nervous."

"I‘ll bet.  You were trapped between a rock and a hard place."

Clive looked at me with disdain.  "I was like the end of a roll of cinefilm, flapping on the projector after the reel finished showing.  As soon as I hit the surface I lost momentum and dropped into space, just long enough for my tether to tighten up again and reel me back down to the surface.  And hitting the surface HURT!  All those jagged edges — I was pretty certain I would get a spacesuit breach if I hit too many times, and I was beginning to run out of fuel for my retro jets, which I was using to soften my landing."

"So how did you get out of it?"

"I was lucky, Frosty.  As I flapped back and forth into space, I could see a few miles around me.  And not far from where I was smashing down to ground I could see a thin line snaking across the surface.  It was my own tether!  I had wrapped my tether cable completely round the asteroid and a little more besides.  If only I could reach it, my own tether would give me something to hang on to.  So, after five or six painful collisions, I had a plan.  As I fell away from the asteroid, and waited for the slack to take up and pull me back to the ground, I switched my retro rockets to full, and tried to head for where I had seen my tether line pulled taut across the surface. I reckoned I only had one chance.  This time, as the line pulled me in for another crash landing, I could see I was just going to be able to reach the cable.  The jagged surface hurtled towards me, I reached out, and BANG!! OW!!   I held on!
When I opened my eyes I was hanging by my fingertips from the tether.  I used my last remaining strength to pull up a karabiner and clip myself onto the tether line so that I couldn‘t fall off.  Then I let go and hung free whilst I caught my breath.

A few minutes later I was ready to review my situation — and to be honest, it still didn‘t look too good.  I was suspended from a cable wrapped completely round an asteroid that I had spent the last hour banging into, with no means of setting myself free.  Twenty feet above me was the asteroid‘s surface, jagged and cruel, fault lines cleaved by collisions.  The closest ridge was flecked with paint scraped off my spacesuit helmet.

Below me: — I looked down, and wished I hadn‘t.  Now I had a fixed reference point above me, I could see the stars an infinite distance beneath my feet, the whole heavens moving rapidly beneath me as the asteroid spun on its axis.  It might sound spectacular but it was nearly enough to give me space sickness — and as I was going to spend at least the next few hours in my space suit, I wasn‘t keen on that.

I had a long hard think about my options.  My winding gear and release mechanism appeared to be damaged beyond repair, jammed shut.  I had plenty of air and water left, and a little food.  If I could break free from the asteroid the support crew could locate and rescue me.  Eventually I had an extraordinary idea.  The release mechanism connecting me to the cable was jammed — but what about the connection at the other end, between the tether and the harpoon?  Perhaps it might be possible to release the other end of the cable!

I realised that I had a difficult journey ahead of me.  I would have to pull myself, hand-over-hand, along the cable.  I had no clear idea for how far — I estimated a mile or two, but it might easily be ten.  I also had to spend some time deciding which direction to go in!  However, I could see no other way of escaping from the asteroid.
So I set off, hauling myself along the tether wire. One hand in front, pull myself along, then the other.  One hand, then the other.  The cable was pulled taut over successive ridges on the asteroid surface, but as I approached a ridge, my weight would pull the wire clear.  So I never got to touch the surface itself, always being suspended beneath it.  Gradually, very gradually, the end of the tether attached to my spacesuit began to slacken off and dangle beneath me, proving that I really was gradually unwinding myself from the asteroid.

It was exhausting work, Frosty.  Every half hour I would stop for a break, and every other hour for a sip of water and some refreshments. After twelve hours I snatched a few minutes of sleep — that wasn‘t easy either, it was very uncomfortable being suspended by a clip from a wire. My arms ached as though they were about to drop off.  Progress was tortuous, little more than a hundred yards an hour.  After twenty-six hours, the wire detached itself from the next ridge and suddenly lurched down — and joy of joy, I was able to slide down the wire a few yards.  You have no idea how good that felt!  As I passed beneath the ridge, I could see just why the wire had shifted — there above me was the harpoon crater!

The end of the harpoon was just visible — almost all the debris had fallen out of the crater, apart from some rocks jammed around the harpoon entry point.  But to reach it I had now had to climb up the tether wire.  This was even worse than moving along the wire.  Now I was moving inch by inch.  It took two hours of sheer agony to reach the top of the wire and the harpoon attachment.

I reached up to check the attachment of the harpoon, and dislodged the rocks, which promptly fell on my head.  OUCH! OUCH! OUCH!  I slid five yards back down the cable before I could stop myself.

Half an hour later I was back at the top of the cable.  This time there was an exposed face of rock, into which I was able to fire a bolt.  Finally, finally, I could attach myself to the surface of the asteroid.  I looked around to take stock.  Now it felt as though I was in a small cavern, pointing downwards towards outer space.  In front of my face was the harpoon attachment.  It had fared much better than the attachment at my end of the tether — of course, unlike the attachment at my end, it was built to survive an impact.  And I could see that it could be released.  Once I had regained my breath and my bearings, I reached over, undid the cable release, and watched as the other end of my tether dropped down and out of the impact crater.

Things were going to happen quickly now. The other end of my tether was now falling round the asteroid, unwrapping the cable, moving further and further away from the surface as the wire freed itself.  In about ten minutes it would complete an orbit of the asteroid, coming straight overhead, and at that point the combined weight of sixty miles of cable would rip me away from the asteroid‘s surface.  Best not to be attached to the rock face at that point!

So I waited five minutes, then unhooked myself from the rock face and held on by my fingertips.  Six minutes passed, then seven, eight, nine, ten.  I was beginning to think that something had gone wrong when I began to feel a perceptible tug on my end of the tether.  Not long now!

Then I remembered — what about the photographic evidence?  I needed to take a picture!  It was too late to hook myself back on to the rock.  I let go with one hand and fished around for the camera.  Where was it?  It took twenty seconds to fish it out from my pockets.  I had to lift it up to my other hand so that I could open up the shutter protector.  I put it to my faceplate, checked that the auto-focus had worked, and pressed the shutter.  No click.  Oh no!  I had to wind the film on. . .  I raised the camera back to the hand I was holding on with. . .
Below me the free end of the cable came sailing past the crater.  The cable at my end yanked hard, snatching me off the rock face and juddering the camera out of hands and off into oblivion.  Sixty miles of cable snatched me out of the impact crater, banging my head on the lip as I exited.  OWWW!  Then I was tumbling, head over heels, head over heels, deep into outer space."

Clive took a sip from his beer.  "Then I was space sick."

* * * * * *

I took a moment to share in Clive‘s misery. But there was another pressing matter on my mind.

"So what happened to you, Davina?  How was your journey?"

Davina was about to speak, but Clive got in first.  "Oh, you don‘t want to know Davina‘s story. Very boring!"

"Yes I DO want to hear Davina‘s story!"

"Well," began Davina, huskily, but Clive interrupted.

"Nice weather we‘re having, don‘t you think?"

"CLIVE!" said Davina, very firmly, "isn‘t it about time that you bought another round? There‘s a dear."  And Clive shuffled off, muttering to himself.

"So were you successful in your mission?" I asked.

"Yes!" said Davina, smugly.

"You managed to land on the surface?"

"Yes I did.  No problems."

"And take a photograph?"

"Yep. Lots."

"But how did you manage to avoid all those problems with the centrifugal forces?"

"Easy peasy," said Davina, "I landed at the Pole."

  

Strange as it may seem, however, such asteroids do exist.  I came across one such example in the February 2002 edition of Sky and Telescope.  It was discovered in 2001 by the Czech astronomers Petr Pravek and Petr Kusnirak and has been given the provisional designation 2000 OE84 (the Czech astronomers will have the right to name it — perhaps I should suggest "Clive" or "Davina" to them).  It is an "Amor" class asteroid which means that it is in a potentially Earth crossing orbit, although its current orbit poses no threat to us, and so we will be spared the sight of Bruce Willis attempting to attach an atomic bomb.
I should make clear that neither this asteroid, nor the one in my story, possesses true anti-gravity.  As far as we know, gravity is always an attractive force.  (There‘s some evidence emerging for the universe having a non-zero cosmological constant, which would behave as a kind of repelling anti-gravity force over huge distances). On the scale of the solar system, gravity behaves much as Isaac Newton described it — masses always attract each other, with strength inversely proportional to the square of the distance between them.  Even Clive, who has landed on anti-matter planets in his time, has never encountered true anti-gravity.
Asteroid 2001 OE84 is 0.9 km across, and rotates once every 29.2 minutes.  We can estimate roughly how strong its gravitational pull is, and how strong the centripetal acceleration is on its surface (in the story, I call this a centrifugal force, which is technically not correct — but let‘s not worry about that).  My calculations suggest that the gravitational force is 0.001 m/s/s, and the centripetal acceleration 0.011 m/s/s.  On the Earth‘s surface, by comparison, the gravitational pull is around 9.81 m/s/s, and the centripetal acceleration at the equator about 0.03 m/s/s. Don‘t worry about the details; the important things are that, first, the acceleration due to OE84‘s spin outweighs the pull due to its gravity, and second, both accelerations are a lot less than the Earth‘s gravity.
So, if I had set my story on asteroid OE84, things would have been a little easier for Clive. The acceleration throwing him away from the asteroid‘s surface would be very gentle, and I suspect he would have no problem attaching himself to the rockface.  For the purposes of the story, therefore, I spruced things up.  I invented a much larger asteroid, twenty miles in diameter, and spinning rather faster.  This brings the centripetal acceleration up to about one fifth Earth‘s gravity. It means that Clive had bumpy landings, although not as painful as their equivalent on Earth.  Also hauling himself up the cable to the asteroid‘s surface would be rather easier than on Earth, and the impact of rocks on his head rather softer.  I knew you‘d be relieved to hear that.
Asteroid 2001 OE84 is the largest "anti-gravity" asteroid yet to be discovered, by some way.  It‘s unlikely that such an asteroid the size of Clive and Davina‘s remains to be found in the inner solar system.  So I situated it in the Kuiper Belt, the belt of recently discovered asteroids beyond the orbit of Neptune, where there are undoubtedly many wonderful objects to be discovered.
As I explain in the story, "anti-gravity" asteroids are very rare, because they can only form in collisions between larger bodies — they are, literally, chips off the old block.  They only hold together through their tensile strength — they are flying bricks, spinning through space.  If the acceleration due to their spin becomes too high, the asteroids will fly apart.  So I suspect the asteroid in my story is about as large as you can get without it falling to pieces of its own accord. And maybe firing a harpoon at the asteroid would be enough to fracture it.
So there you have it. It‘s possible, though unlikely, that somewhere out there is an asteroid big enough and spinning fast enough to pose a real challenge to Clive and Davina.  Of course, if like Davina, you were to land at the pole, the challenge would be much easier — there‘s no spin acceleration at all.
 
 

Comparison of size, gravitational pull and centripetal acceleration

Planet          Radius (m)    Rotation        Accl due to     Accl due to
                                      Time (sec)     gravity           spin

Earth          6,400,000     86400            9.8100          0.0338

OE84          900              1752              0.0014          0.0116

Clive and    16,000          600                0.0245          1.7528
Davina‘s

---

Standing on terra firma, we can look up at the sky and amaze at its wonders in the visible wavelength (if one is into these sort of things).  With a few added instruments, we can extend the seeing in the infrared and in the ultraviolet range, albeit in a false colour.  We also have radio telescopes.  However a lot of information to be have in the shorter wavelengths of the electromagnetic spectrum, in the x-ray and gamma-rays.  These  two rays are not very environmental friendly, but the earth‘s environment absorbs these two radiations and shields us from the harmful rays.  So to observe in the x-ray or gamma ray range the telescope has to be in the space.  Currently two major x-ray telescopes, Chandra (NASA) and X-ray Multi-Mirror-Newton (ESA) are gathering data.  Now the time has come for the gamma -ray astronomy.  Spy satellites detected gamma-rays and thought these to be from enemy fire, but soon it became clear that these are true astronomical phenomenon.  In the spring of this year a new gamma-ray telescope, is to be launched.  Here is a brief preview.
 

Operating range
Our optical vision sees in the wavelength range of red light at 700nm (7x10 6m), and blue light at 400nm (4x10 6m).
The new gamma-ray telescope is to operate in the range of 0.2 to 0.0008nm, in the realm of x-ray and gamma-rays.  Production of x-ray and the gamma-rays are associated with high energy events, therefore it is customary to express x-ray and the gamma-rays with energy.  This energy is expressed in electron volts or eV.  One electron volt is the change in energy of an electron when it moves trough a potential difference of one volt.  The typical energy of the visible light is in the range of 2 to 3eV and energy of x-ray is of the order of a few keV; the new gamma-ray telescope will operate in the range of 15keV to 10MeV (10x103 to 10x106eV).  1eV is very small compared to a Watt, the energy unit we are used to, but in the temperature range of 1000° ™ 2000°K (700° ™ 1700°C), the equivalent energy is about 0.1eV per atom.  In the centre of the sun at a temperature of about 10 6K, the equivalent energy is about 10 3eV per atom.
Partners in the Project and a General Outline International Gamma Ray Laboratory, Integral, is a jointly funded project by the Czech Republic,  all 14 ESA members, NASA, Poland and Russia.
The satellite carrying the instruments is in two parts; the lower part is the service module, containing power plants, communication centre, control units like thermal, altitude, orbit control etc.; in short all the support system.  The upper part houses the instruments.  The whole structure is about 5m high, and weighs about 4 tons whichabout half is the weight of the instruments.  In order to increase the sensitivity of the instruments, they need to be shielded from the background radiation.  This increases the weight of the system.  The satellite structure is made from aluminium and carbon fibre and it is one of the spares leftover from the XXM-Newton project. This satellite would be placed in orbit by a Russian Proton rocket in April 2002.  Every third day, the satellite will orbit the Earth once, at the furthest point, it will be 40,000 km away.  Most of the orbit will be away from the Earth, so as to avoid terrestrial background radiation.
Italy will lead the team to build the main gamma-ray instruments.  There will be two instruments working in the gamma ray range, one for taking images an another working as a spectrograph.  In addition to the gamma-ray range, there will be x-ray monitor and optical camera to identify the source.  This is the sharpest gamma-ray instrument designed so far.  All these instruments will be co-aligned, which means that at a given time, all of these instruments will be looking at the same object.

Gamma-ray Detectors
Gamma-rays because of its penetrating powder, can not be focused by conventional lenses or mirrors, so the gamma-ray detectors to be used in this project are glorified pin-hole cameras called coded mask technique.  Instead of one pin hole in the conventional pinhole cameras, this instrument uses an opaque metal plate (mask) with multiple holes.  This produces multiple images on the detector and a computer decodes the image, see Box 1 above.
The kinetic energy of the gamma-ray photons are converted into charged particles and semiconductor detectors measure the charge.  The detectors used are cadmium-telluride and germanium.  These detectors have very good energy resolution.  The gamma-ray imaging devise is called IBIS (Imager on Board the Integral Satellite).  The expected angular resolution is 12 arcmin in the energy range of 15 keV to 20 MeV. Sources can be located to 1 arcmin.  The angular resolution of the instrument depends on the spatial resolution of the detector.  This in turn depends on the number of the pixels or the picture elements.  For a rough sketch of the detector see Box 2 below.  The two layer array of detectors allows a three dimensional image analysis.  The coded mask, that is the plate with multiple holes, is made from tungsten and the gap between the detector and the mask plate is about 3.2m.  (As a side note, coded mask instruments are now being developed for medical imaging.)

Gamma-ray Spectrometer
The Gamma-ray Spectrometer on Integral (SPI) is also a coded mask devise similar to the imaging camera above.  The mask consists of 127 hexagonal elements; 64 of which are transparent and 63 are opaque.  The mask is made from 3 cm thick tungsten.  About 1.7m below this mask, there are 19 hexagonal germanium detectors, cooled to -188°C.  The total detection area is 500 cm sq.  Again like the previous instrument, it is also shielded by Bismuth Germanate Oxide crystals.  All these heavy elements makes the total weight of the instrument to about 1,300 kg.  The field of view is 16°, with a resolution of 2°.  The detection range is 20 keV to 8 MeV and the resolution is 2 keV at 1 MeV.

X-ray Monitor
The imaging in the x-ray range will be done by the Joint European X-ray Monitor JEM-X.  It is also a coded mask instrument.  The distance between the coded mask and the detectors is about 3.2m.
The detector is a multi-wire proportional counter.  It consists of two identical gas chambers filled with xenon gas at 5 bar (about 75 psi).  When a x-ray hits the gas, it knocks off a electron, which are accelerated by the electric field.  This causes more electron releases from the gas.  The charge developed during this process, is a measure of the energy of the x-ray.  There are crossed wires as well, which determines the location of the x-ray.  The total detection area is 1000 cm sq. and the resolution is 3 arcmin.  The imaging covers a range covers a range of 3 to 35 keV.

Optical Imaging
The Integral facility also carries an optical imaging devise is called OMC, Optical Monitoring Camera.  It is capable of seeing objects down to magnitude 19.7.  This is a standard 50 mm refractor, with a CCD for imaging which operates at -80°C.

Gamma Ray and Astronomy
Most of the gamma-ray sources last from a few hundredths of a second to a few minutes, hence this phenomenon is called Gamma-ray bursts (GRB) and these are almost detected daily. Some of these could be very powerful and a few of these are of a lower energy level or soft gamma-rays.  In some cases the energy of the gamma-ray may be close to that of the x-ray range.  Initially the astronomers could detect a gamma-ray burst, but failed to locate the source precisely.  Eventually supernova remnants were tied down to be the possible location of the gamma-ray sources.  It was only in 1997 that a Dutch ™ Italian x-ray satellite, Beppo-SAX, was the first to locate a gamma-ray source so these could be further examined in other wavelengths.  The simultaneous multi-wavelength capability of the Integral offers observations, both in the high and low energy range (seven orders of magnitude), and this will help to pin point the source as it happens.

What are We Looking for
Soon after the beginning of the universe, there was only hydrogen and helium.  During the stellar evolution, nuclear fusion reaction synthesised heavier elements.  As stars died, these elements were scattered, by the stars and became the raw material for the next generation of the stars and planets.  Carbon is produced in the low mass stars; in these stars hydrogen has run out, the diameter of the star is about 25 times the size of the sun, temperature is about 4000°K (~3700°C).  Massive stars on the other hand contain heavier elements like oxygen, gold, iron, sodium or chlorine; when these stars use up their fuels they end up as a supernova explosion.  These elements are dispersed during supernova explosions, forming the next generation of stars and planets and eventually the living beings.  In the supernova explosion radioactive elements can form with well defined energy levels.  Studies of these elements gives an good indication of the process of synthesis of elements.  In the Milky Way aluminium -26 has been detected, this isotope emits a gamma-ray at 1.809MeV and has a half life of 1.1 million years.
Integral will look at the sites of the known recent supernova explosions.  It may also detect some unknown supernova sites.  Rosat, the German x-ray satellite and the US gamma-ray satellite Compton-observatory detected a 1.156MeV gamma-ray line in the constellation Vega, only 700 light years away.  This supernova explosion took place about 900 years ago.  This gamma-ray line comes from radioactive titanium -44.  This isotope has a relatively short life indicating that the supernova remnant is young.

Strange Compacts Objects
During a supernova explosion, although most of the matter of the star is ejected, the inner core collapses on itself.  If the mass of the core is about 2 to 3 times the solar mass and the radius is about 20 km, the gravitational forces overcome the mutual electrostatic repulsion of protons and the electrons; the star now is composed of essentially neutrons and are called neutron stars.  However on the other hand, if the mass is more  than the 2 to 3 solar mass, the gravitational forces take over the star end up as a black hole.
These compact objects, due to their high gravitational forces, accretes material.  These materials spins very fast.  This process can lead to the generation of high energy radiations.  The Integral will look for these sources with high precision, trying to understand the physical processes involved.
An extremely powerful gamma-ray burst occurred on 27th August 1998.  This was associated with a neutron star with a very high magnetic field called magnetar.  If the strength of an ordinary fridge magnet is around 100 or 10 2 units, these stars has a magnetic field of 10 15 units!  Ordinary neutron stars (magnetic strength 5x10 12 units) rotate very fast, in the order of a few hundred rotations in one second.  Neutron stars loose their energy by magnetic radiation and after a few million years of energy loss, the rotation may drop to one rotation in several seconds.  Highly magnetic magnetars, on the other hand, louse energy very fast and in a few thousand years, the rotation drops to about one rotation in a few  seconds.  Some of the gamma-ray bursts also show similar fluctuation of intensity.  This led to the suggestion that gamma-ray bursts may be associated with the highly magnetic neutron stars or magnetars.  At present only a theoretical model exits for the magnetars.  The Integral will attempt to study these objects.

Giant Black Holes
The centre of the Milky Way is very active in the radio and infrared range.  It is also active in the gamma-ray range as well.  Integral hopes to make a detailed study of this region.
Active Galactic Nuclei (AGN) are very bright sources.  These are brighter than the host galaxy. Quasars are very bright and often very distant objects.  Quasars are believed to be powered by black holes.  AGNs are variable and emit a wide a range of radiation.  The fluctuation can vary between seconds to several months.  If these sources were very large, the time taken by the light to travel across the width of these objects would be very long, which means that these objects would stay brighter for a long period.  This indicates that these objects are very small.  Both AGNs and quasars perhaps harbour black holes.  These objects give out jets of gas at a relativistic speed, i.e. comparable to the speed of light.  These will be the targets for Integral.
According to theoretical model, if two neutrons stars collide, the result is a formation of a black hole.  If a star of several hundred solar masses collapses under its own weight, it produces a hypernova and the star also ends up a black hole.  Both of these phenomenon are predicted to cause, gamma-ray bursts.  These models need to be verified.

The origin of Cosmic Gamma-rays
On the earth nuclear reaction is the main source of gamma-ray and these could be from natural radioactive decay or reaction in a nuclear reactors.  In the cosmic background, apart from the radioactive elements there are other mechanisms, which can also cause production of gamma-rays.  Nuclear decay of radioactive elements produce gamma-rays of well defined energy levels; this called nuclear spectral line emission.
Nuclear interaction:  Proton-proton or a proton-nucleon collision can produce gamma-rays of different energy levels.  Nuclear fusion powers the stars.  In the sun, proton-proton reaction forms helium.  Cosmic ray ™ interstellar gas interaction can also produce gamma- rays.
Matter ™ Antimatter Reaction:  Electron and positron are a matter ™ antimatter pair.  Their annihilation produces an energy of 511 keV.  This corresponds to the mass of the particles converting to energy.  This radiation has been observed at the centre of the Milky Way.
Inverse Compton Scattering:  If a particle of light, photon, interacts with fast electron, the photon may gain energy and it may be boosted up in the gamma-ray range.  If the temperature of the accretion disc around a compact star is high enough, it may give off x-ray; now if the star ejects some charged particles, these x-rays can get an energy boost and may to be converted to gamma-rays.
Synchrotron Radiation:  When electron move in a magnetic field, at relativistic speed, electrons can emit a wide range of radiation, including gamma-rays in extreme cases.  Stars can have strong magnetic field and charged particles, hence this could also be a possible source of cosmic gamma-rays.
 

More about gamma-ray astronomy;
Magnetic Monster Stars, Mark Garlic, Astronomy Now, vol. 15, January, 2001, p 2
Blasts from Past, Matthew Cox, Astronomy Now, vol. 15, March, 2001, p 62
About Integral  http://sci.esa.int/content

While reading a book on holiday about the calendar (Mapping Time by E G Richards, Oxford University Press), I was struck by how old some of the everyday conventions we use are.  Every day items such as the number of days in a week and the length of the months, the names of the days and months go back to early Roman and Greek times.  But even older than these, are the number of degrees in a circle. 
The length of the month is of cause a legacy of the old lunar month.  The trouble is that the orbit of the Moon and the length of the year are very inconveniently not in agreement.  The lunar month is 29.53 days long and the year is 365.25636 days long and it has proved to be an headache for millennia trying to get them both to žt in well with each other.  Over the years many have tried to find a way of fitting the lunar month to the year by adding and dropping days, weeks and even whole months in some years and not in others.  Little can be done to keep the year and lunar month in step over a long period and the arrangement we have now is only one day out in 400 years.  But there are other problems too. 
The Moon orbits the Earth in 27.32 days relative to the stars and only appears to go round the Earth in 29.5 days from our standpoint on earth.  Likewise the day is 24 hours long from our point of view, but the Earth spins round in 23h 56m 4s, the sidereal day, with respect to the fixed stars.  Each of these differences are due to the Earth moving further along its orbit by 2.5 million kilometres a day, and being in different place from when the time period started, so extra time is required for the Moon and the sun, to return to their apparent start positions. 
In the early days of civilisation, folk made only a crude estimate of the lengths of time passing and one of the earliest was the number of days in a year.  After people started to become farmers rather than hunter-gathers it was necessary to know when to plant the seeds for the next years crop and when to reap the harvest.  As people had more time on there hands being farmers, a record of each year was advisable so progress could be made. 
Counting the 12 lunar cycles took 354 days, but it was soon realised that that was a little short of a full year so 360 was used as it fitted better.  This figure is one of the oldest numbers used in the world and represents the first guess at the number of days in a year and it served the early people for a long time.  It is only in the warmer countries where the sun shines more than here that it is possible to tell just when that year has ended (most times at the shortest day, as from then on the days are longer) and when the sun has started its way back from its lowest point in the winter sky or its highest mid point of summer.  Accurate records are needed and a constant watch on the lengths of shadow around midday are necessary to determine the exact time and date of midwinter or the longest day in summer. 
And so it was that the Sumerian and early Babylonian civilisations in the fertile regions of the Tigris and Euphrates rivers started to count the days that made up a year from before 5000BC.  This was some time before writing was invented and pictographs must have been used  to keep track of days and years.  So keeping accurate records was difficult, and so an early estimate of 360 days in a year wasn‘t too bad. 
360 is also a very useful number, it is divisible by 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120 and 180.  Not many numbers have such a large even range of  numbers by which they can be divided evenly.  The Babylonians divided the day into 12 beru and each beru into 30 ges, the same way as the year was divided into months and days.  So it was natural that as the sun circled the Earth in 360ish days, each part of a circle could be seen to be equal to one day, therefore 1 degree equalled a day.  It was easy to divide up a circle with a useful amount of sections (degrees) in various ways and most folk will always use an easy way if possible.  So early on geometry was born even if it did take a long while to work out p.  It did not take long for the peoples in these regions to find their calendars going out of synchronisation with the sun in the sky after a few years as the real year was 365¼ days long.  Because life expectancy was only 20 to 40 years most people would never realise how much the discrepancy was until writing and records were kept.  So next time you use your setting circles on the telescope think how old that number is!