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Interstellar travel: how to spot a ‘starman’ going by


Massive objects moving at near light speeds do not occur naturally in the universe as we know it. If we detect such objects it is a reasonable to assume they are artificial artifacts from advanced intelligent life. This according to Garcia-Escartin and Chamorro-Posada, authors of a recent paper, is a low-cost, sure-fire way of searching for intelligent life outside earth.


The habitable zone of Gliese 581 compared with our Solar System’s habitable zone. Image credit NASA.

Searching for life beyond earth is a grand and varied enterprise.

For a start we can look for exoplanets that fall inside the habitable zone of a star. A planet found in this zone may fulfill the requirements for life: liquid water, energy, elements and other nutrients, and appropriate physical conditions. Though we have located many exoplanets in recent times they are far from earth – many light years distant. For example one star system, Gliese 581, is 20.3 light years away (192,048,720,000,000 kilometres). With three planets in its habitable zone, we know nothing about conditions on them. The techniques used to find them can tell us nothing about their ecology – if any. being in a habitable zone does not guarantee life. It is only in recent years that we have realised how inhospitable Venus and Mars are to life – despite being in our habitable zone.

By looking for alien signals or transmissions, as in the SETI programme, we extend our search from ‘possible life’ to intelligent life. For advanced civilisations we look for artificial illumination or interstellar probes.

Let’s face it though, to know we are not alone will require quite good proof for most of us (apart from the misguided minority of UFO believers), and especially for the skeptical scientists.


Bussard ram-jet interstellar drive. Image credit NASA.

The intriguing proposition of Garcia-Escartin and Chamorro-Posada is based on three ideas. The first is that anything travelling faster than 3.3% of light speed (5,935,890 kilometres per hour) is artificial. All known natural objects travel slower than this speed, as do our current space probes. This speed was chosen as it is the estimated speed of the nuclear propulsion ship proposed by Freeman Dyson in the Orion project. Although the propulsion technology is feasible today the technological and economic hurdle of creating such a craft is way beyond our current means. Although it is certainly not inconceivable to achieve such interstellar travel in the next 100 years.


Scales of speed with respect to the speed of light in vacuum (logarithmic scale). The fastest man-made objects are in the range of velocities from 1/100,000 c to 1/1,000 c. Examples are the fastest manned ship, Apollo 10 on entry, the Galileo probe during its descent into Jupiter and the solar probe Helios 2. For comparison, we have included the average speed of Earth during its orbit around the Sun and the motion of the Solar System with respect to the cosmic microwave background frame. The fastest natural objects, like hypervelocity star HE 0437-5439 and neutron star RX J0822-4300, move in the scale of 1/1,000 c-1/100 c. We define a region of extraordinary propulsion (REP) for speeds which would point to an artificial object. The REP starts at the estimated speed for the nuclear propulsion Orion ship, which could be built with present human technology. Source original paper, Cornell University.

You are possibly thinking about now; “Doesn’t the mass of an object increases massively as its speed approaches light speed?” You would be correct, this consequence of Einstein’s theory of special relativity is demonstrated quite satisfactorily in particle accelerators around the world. To cover this the authors next identify a consequence of relativity theory: relativistic effects amplify the light reflected from a body travelling at near light speed – in some key situations. Allowing for the detection of ‘small’ objects.

This brings in the authors third criteria. Interstellar travel will be from one star system to another. The reflected-light magnifying effect would be greatest for the cases where earth is almost in line with the departure stellar system and the destination stellar system.


Earth’s position with respect to the ship’s trajectory. (a) Earth receives the light from the destination star reflected from an approaching ship. (b) Earth receives the light from the origin star reflected from an outbound ship. (c) Earth receives the light from a third star, which is reflected from the ship at an angle. Source original paper, Cornell University.

The authors propose to limit the first search to star systems that are reasonably close to each other (no further than 10 light years apart) to maximise the probability of stellar travel opportunities. Considering that Gliese 581, for example, is greater than 20 light years distance from us, I suggest that this criteria is too limiting.

The paper is an interesting, if not compelling, proposition. The authors do calculate what size an artifact would need to be, travelling at their minimum speed (3.3% light speed), to be detected at the distance of one of our closer stellar neighbours. Could such an artifact be detected by the Hubble or James Webb space telescopes, for example? What is the probability of success of such an experiment, compared to say the SETI experiments?

One idea I did find interesting is by focusing on detecting light reflected from ships, we do not need to assume any intention by the interstellar travellers to communicate with us. The ‘signal’ is independent of alien psychology. It is also independent of propulsion technology – we aren’t looking for any ‘signature’ of any particular technology, known or unknown.

It is an interesting paper. I’m not sure they have presented a compelling enough case to convince a funding body – yet.

This article was first posted on Australian Science you can read it here.

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