This text is an intellectual property of Vinicio Coletti, Rome, Italy. |
| Published on November 16, 2007 |
I think it's time to plan
the launch of a probe directed outside the solar system.
Such a probe would have as natural destination the star system more close
to the Sun, that is the Alpha Centauri one.
This system is actually composed by three different stars.
The first one, α1, has a luminosity
about 50% greater than the Sun, and is a star of the same kind.
The second, α2, is reddish and about 50% fainter than the Sun; it orbits around the first in about 80 years.
The third, α3 but called also Proxima Centauri, is thought to orbit
around the other two with a period of around half a million years.
It is a very faint and red star, of the kind named properly dwarf red star.
The closest star at the moment is Proxima Centauri (4,22 light years), but the
rest of the system is not too far away (4,36 ly).
At the moment, the only two objects, still working, that are approaching the limits of the
solar system are the Voyager 1 and Voyager 2 probes, that were launched in 1977 and
are now travelling at the speed of about 17 km/s, at about 0.001 ly away
from the Sun. At this speed, they would need about 77,000 years to reach
the Alpha Centauri system.
It's clear that if we want to get there, something else must be used.
We need thus a very high travelling speed to reach another star in times compatible
with the human lifespan (and with the duration of man made objects...).
Such high speeds can be reached in two ways.
One way is through a very powerful engine,
able to give a big thrust to the probe in a short time. This is actually the
method used so far with chemical rockets, but no known technology of this kind
could give relativistic speeds to a probe.
The second method is through an engine able to give a faint thrust, but for
very long times. The constant acceleration will eventually give the probe
the high speeds needed to the interstellar journey.
Happily enough such engine do exist already: it is the ionic engine.
Intead of expulsing matter as a result of a chemical reaction, as in usual
rockets, ionic engine uses a strong electromagnetic field to accelerate
ions to much higher speeds.
In reaction engines, the more fast is the exhaust, the more efficient is
the engine, for the simple fact that cinetic energy is linked to the speed
by a square law. This means that is you double the speed of the exhaust,
you get a fourfold cinetic energy. If you have 10 times the speed, you get
100 times the energy.
This highly efficient use of matter means also that you would need to load
the probe with much less fuel for a given journey, resulting in
a lighter probe that would then accelerate even more.
By the way, there is no magic involved with these engines. If you want
to expell matter at high speed, thus with a very high energy, you need to
have this high energy available on board and since you need electricity
to fuel the powerful EM field of the ionic engine, you must have
a strong and durable electric generator available.
I have always been, and I am still, a convinced oppositor of nuclear
weapons and nuclear power plants here on earth. But for such a long
journey away from the sun, I have to admit that the only possible
generator would be a nuclear one, in the form of either an
isotopic battery or a real tiny nuclear reactor.
It would give all the energy needed by the engine and by the other
systems of the probe, for very long times.
It's time now for some calculations.
What matter here is most of all the thrust of the engine compared
to the mass of the probe.
I've begin calculating very faint thrusts, starting from 1/1000 of the probes's mass,
but I got travelling times still too long.
In my proposal, the ionic engine should provide a thrust about
equal to the mass, if measured in Newton by kg-mass. Otherways stated,
the thrust would have to be about 1 tenth of the mass, if measured
in kg-weight by kg-mass. A probe of 1 metric ton would thus require
a ionic engine with a thrust of 100 kg, kept active for many
years. The acceleration would be 0.98 m/s2 or 0.1 g.
I assume here than the probe would have an initial speed of about
36,000 km/h as a result of the launch and the abandon of earth orbit,
through conventional chemical rockets. This is equivalent to 10 km/s and
even if this is less, this in not really important for the final figures.
The constant acceleration of 0.98 m/s2 will result in a speed
of about 2,550 km/s after a month and 30,900 km/s (1/10 of the speed of
light) after 1 year.
After about 3 years the probe will reach 30% of the light speed and will
automatically turn off its engine. More speed is not needed and this will
spare energy and ions for the final phase of the journey.
At this point the probe would be at about 0.93 light years from us and would
keep going at the speed of 0.3 c for another 8 years and 4 months.
Then it would rotate 180 degrees on itself and will fire up the engine
again, using it as a brake now. In another 3 years of constant deceleration
it would travel the last 0.93 light years and would reach the Alpha
Centauri system at low speed, exactly that needed to enter a stable orbit
around one of the stars, the one chosen as the primary target of the mission
(I would suggest Alpha-1).
What's amazing in these computations is that the total travelling time
will be just 14 years and 4 months. To receive some information back
from α Cen, we would then need another 4.36 years, the time needed
by light to reach us. So the actual time from the launch to the first
signals coming back from another star would merely be about 18 years and
8 months.
Accelerating the probe more, for example up to 0.5 c, would not be
as effective as you can think, for such a short journey. The probe
would accelerate and decelerate for 10 years and a total of 3.46 ly.
The 1 ly left would be covered in 2 years, for a total travelling time
of 12 years, quite close to the original 14 years, and for this to happen
the probe ought to afford the increasing dangers of going 60% faster.
Not a good bargain. It is for this reason that I think that a top speed
of 0.3 c is close to the ideal for such a journey.
There are really many experiments that could be done during an interstellar journey.
First of all, the probe will be the quickest and most far object ever built by mankind and this will
allow a tought verification of the relativity and of gravitation theories.
Then it would see the border region of the solar system, were solar wind interacts with
the outside space.
More far and out, it will go deep into the interstellar medium and it will analyse radiation and cosmic
rays that normally don't reach the earth, because we are someway shielded by the solar wind.
This will probably gather important information about the solar system and the formation and evolution
of the whole universe. Some hints could even come about dark matter and dark energy. This information
alone could make the probe worth building and launching.
But it is the arrival on Alpha Centauri that will probably reserve the bigger surprises. It will be
the first time that we will have a close up on a different star system and we will have a lot of
new data about those three stars and on planets that orbit around them.
The probe should carry, of course, a load of sensors to take pictures, measure electric and magnetic fields,
timings, cosmic rays, etc. It would take data about the
close stars, sending them to the earth. Then is should automatically scan the area in search of planets,
and for each one detected it should take as many data and pictures as possible, with a particular emphasis
on atmospheres and their composition, looking also for radio signals and signs of artificial activity of
any kind.
Because, yes, it could even found other life forms, either intelligent or not.
An interstellar probe requires of course an accurate design that will face many new problems to solve.
First of all there is the ionic engine to build, then the tiny nuclear reactor, able to work without
intervention for 15 years or more.
Then, the probe will have to decide itself what to do in case something goes wrong, since the stellar
distance forbids a remote command schema. A signal from the probe would require more than 4 years and
the command in response an equal time. Clearly, it will be the software on board to direct all operations
and this mean that this software is required not to show important bugs in more that 14 years of operation.
Also, high speed and cosmic rays will expose all electronics to a big stress. The hardware should be thus
shielded some way and largely redundant. Software will be complex, as we have seen, and should be thoroughly
tested to avoid problems along the long road.
Communication is another interesting arena, because we will have to send a signal at more that 4 light years,
a distance noticeably greater than the ones we are used to inside the solar system. Radio is a great media,
but to cover a so big distance the beam should be powerful and as tight as possible. Fortunately the
nuclear reactor will provide all the power is needed and high frequencies coupled with relatively large
antennas will direct the beam to the Sun and its suburbs, where we live.
I know also that the maser was invented before the laser and it sould be a mean to send a coherent and tight microwawe beam.
I don't know if masers have ever been used in radio communications, but if this is possibile, it would be the way to go.
Even a conventional radio source, if powerful enough, could be received on earth by our large radio telescopes, although
only those located between the south pole and the 30 north latitude, because α Cen system lays at a declination
of about 60 south.