5 Planets line up
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.
Hang On In There
by Mike Frost
(As always, I try to base the stories in known physics. There are asteroids
that behave pretty much as I describe the one in the story — see the notes
at the end of the story)
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.
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."
"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
"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
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
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
"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?"
"But how did you manage to avoid all those problems with the centrifugal
"Easy peasy," said Davina, "I landed at the Pole."
By Mike Frost
Please read the story above before reading these notes.
In "Hang On In There" I have my hapless hero Clive (no connection to
our esteemed librarian and webmaster) trying to land on an asteroid which
is spinning so fast that anything on it‘s surface immediately flies off.
Is this feasible? At first sight, it would appear to be very unlikely —
how could such a body form, and how would it hold together?
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
OE84 900 1752 0.0014
Clive and 16,000 600 0.0245
Seeing in Gamma Rays
by Paritosh Maulik
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.
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
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.
Coded mask camera
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
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.)
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
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.
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.
Coded mask and the detector assembly
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
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
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,
About Integral http://sci.esa.int/content
by Ivor Clarke
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!