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Sending a Probe to Alpha Centauri

by Wade D. Hobbs, Jr.

First published on January 31, 2005.

Latest Revision on May 19, 2005.

New Paper added on March 17, 2005.

New Paper added on March 18, 2005.

New Note added on May 18, 2005.

New Note added on May 19, 2005.

New Note added on May 20, 2005.

It is now physically possible to send a probe, albeit a tiny one, to Alpha Centauri. The matter is no longer a topic of scientific theory or science fiction. Unlike the huge probes that are currently in use, such a probe is based on micro-level or even nano-level technology, sizes that are generally applicable to semiconductors. The power source is based on the concept of laser propulsion, which employs a laser beam to propel a spacecraft. In this case, a laser, such as the laser source at the U.S. National Ignition Facility, is beamed onto the shiny surface of the miniature craft, accelerating the craft to relativistic speed. The probe travels at relativistic speed, here defined to be .9c or greater, through the vacuum of space to Alpha Centauri. At this speed, the probe would pass Alpha Centauri in about five years. It is my hope that this technology will be used for peaceful purposes.

How to Build It

The first consideration in design of the probe is to keep it as small as possible. "Microdevices" (measured in micrometers, or 10 to the -6 power), or "nanodevices" (measured in nanometers, or 10 to the -9 power) are the primary candidates. Components of these sizes are typically used in semiconductor applications. If the mass of a microdevice is too big for acceleration with the available lasers, a nanodevice should be used.

With regard to functionality, the first probe to Alpha Centauri does not need any special instruments - a "dumb probe" will be sufficient. Such a design simplifies matters tremendously, eliminating such considerations as how to design components to account for length contraction, which is predicted by Einstein's Theory of Relativity for objects moving at relativistic speeds. The surface of the probe should be coated with a reflective material such as mylar to enhance acceleration through the technique of laser propulsion.

The tiny probe can be sprayed into the projected path of the laser beam, much like semiconductor components are created through spraying of layers of material onto a substrate. The laser beam is instantaneously activated, accelerating the probe to relativistic speed. In fact, because of the tiny size of the probe, thousands can be mass-produced and then sprayed into the path of the laser beam if desirable, sending thousands of the probes accelerating to relativistic speeds. Should friction from our atmosphere be an obstacle to a surface launch, the probe(s) can be dropped in space and then accelerated by laser propulsion.

Calculating Trajectory

Several things must be considered when calculating the trajectory of the probe. First, one must account for the effects of gravity from the Sun, the Earth, the other planets and Alpha Centauri. The speed of the Earth and the speed of Alpha Centauri must be considered as well. In the event of a surface launch, the possibility of wind gusts must be considered. I do not anticipate any collision with dark matter in the space between Earth and Alpha Centauri, though such a collision is theoretically possible. By my estimate, the odds of such a collision are low. A trajectory that avoids the planets, meteors in the meteor belt and comets in the Oort cloud will be advantageous.

Utility of the Probe

This probe would be the first human-made object to move at relativistic speeds. A flyby of Alpha Centauri would be a first as well, which alone is enough to justify such a mission. This first mission, employing a dumb probe, would allow us to confirm the effects of length contraction. It would be a first step in sending a probe with a clock, or other components such as a camera and other sensors.

All of the other proposals for interstellar travel are currently impractical. The other proposals are numerous: Fusion rockets, other types of nuclear-powered vehicles, travel through worm holes, and solar sails of massive size, to name but a few. All of these ideas, though they have promise, are currently too expensive to implement and require years of further research.

The cost of my proposal would be far less than current projects which require the lifting of heavy payloads into space. The power source, a laser beam or a matrix of laser beams, is positioned on the earth's surface, so it is not necessary to send it to space by rocket. The probe itself is tiny, so costs are further reduced regardless of whether the probe is accelerated from the surface of the earth or is positioned in space for initial acceleration.

Future Projects

For future projects, it will probably be necessary to attempt to engineer probes that will account for length contraction. It will then be possible to use the type of sensors that are used on current probes which operate under the conditions of classical mechanics. Some means of sending communications from the probe would be useful as well, though such a design is beyond the scope of this article.

It is further possible that laser propulsion, combined with miniature devices, will effect a broader distribution of work and exploration opportunities throughout the space community. It is quite foreseeable that space hobbyists, individuals and science organizations may have a larger role to play in using these technologies to explore our solar system and the space beyond it. Again, I truly hope that this technology will be used for peaceful objectives, such as the exploration of Alpha Centauri.

Testing Einstein with future probes

All theories are subject to empirical confirmation. Beyond the initial probe which contains no sensors or communication devices, I propose to create a miniature probe that will include a "Laser Emitting Diode" (LED) to be used to test Einstein's Theory of Relativity. The probe will be accelerated to relativistic speeds. Most students of modern physics have studied the famous example of a vessel moving in a relativistic frame of reference. In the example, a light is emitted inside the vessel. Einstein's second principle of special relativity is that light moves at a constant speed in a vacuum, estimated to be 186,000 miles per second. The test probe, which will test this principle, will include an LED, a sensor to receive the light signal and to record the time required to receive it and some means of communicating the results back to the earth.

In matters of physics, I always bet on Einstein, so I expect relativity to be confirmed in this context. This is not to say, however, that c will always be constant in all contexts. It may well be that, in other branches of science and philosophical endeavor, faster-than-light travel may be possible. As a matter of fact, much of the research in the area of superluminosity is quite fascinating and intriguing, and I encourage readers to pursue it as rigorously as possible.

Credits

Of course, this article would be incomplete without recognizing the pioneers in the field of laser propulsion, including the late Robert Forward, Geoffrey Landis, Leik Myrabo, G. Marx, H. Harris, A. Kantrowitz and their teams at Rensselaer Polytechnic Institute, the University of Alabama at Birmingham, the Jet Propulsion Lab in Pasadena, and NASA.

I would further like to recognize Sal Rodriguez. Sal and I had several conversations about this topic in 2002, when he was a PhD student at George Washington University.

An article from either Popular Mechanics or Popular Science, circa 1998-99, was also helpful.

After I had formulated my thoughts, I ran across a website at "centauri-dreams.org" (World Wide Web) on the day that I published this article. That site, too, has information of relevance to this topic.

I had also read Jordin Kare's proposals of February, 2002, from "space.com" (World Wide Web). I thought that the estimated duration of 30-40 years for travel to Alpha Centauri was too pessimistic.

References

1. R. Forward, “Pluto: Gateway to the Stars”, Missiles and Rockets magazine, Vol. 10, April, 1962, pp. 26-28. (Article)

2. G.A. Landis, "Optics and Materials Considerations for a Laser-propelled Lightsail”, NASA Lewis Research Center, 1989, available online at www.sff.net/people/Geoffrey.Landis/lightsail/Lightsail89.html. (Paper)

3. G. Marx, Nature magazine, July 7, 1966. (Article)

4. G. Marx, Astronaut. Acta, 1963, pp. 19, 131. (Article)

5. L. Myrabo, Scientific American magazine, February, 1999. (Article)

6. H. Harris, Scientific American magazine, February, 1999. (Article)

7. Popular Mechanics or Popular Science, 1998-1999.

8. P. Gilster, "Imagining and Planning Interstellar Exploration", at http://www.centauri-dreams.org/2004.10.17_arch.html, (Article and Book by the same title)

9. L. David, "New Space Sail Concept Rides Stream of Laser Driven Bomblets" (discussing Jordin Kare's "Sailbeam" concept), www.space.com, Feb. 11, 2002. (Article)

Please send comments to wadehobbs@emailaccount.com.

W.D. Hobbs, Jr.

METHODS FOR DETECTING A MINIATURE PROBE MOVING AT RELATIVISTIC SPEEDS

First published on March 17, 2005.

It is now possible to accelerate a miniature probe to relativistic speeds, but detection of such a probe is being debated. Two methods of detecting such a probe are presently identifiable: (1) Creation of an artificial signal from the probe itself, or (2) monitoring of signals created naturally by the probe as it travels through space.

The first of these methods, creation of an artificial signal from the probe itself, quite possibly will require engineering that accounts for length contraction. Since such engineering presents many new and complex problems, it will not be immediately addressed.

The second method holds immediate promise. As the probe travels through space, it will produce a radio signal that is unique. We know of few objects, if any, that are 10^(-6) in size that move at relativistic speeds through space. Perhaps one exception is a dust particle gravitating around a black hole, but since the probe is not in the vicinity of a black hole this consideration is irrelevant. Since a relativistic-speed probe creates a unique signal (and for that matter, a unique pattern), it is simply a matter of determining the frequency of the signal that will be produced.

After determining the signal frequency, the problem of signal detection can be furthered simplified by focusing the direction of the receiver along the path of the calculated trajectory of the probe. By listening for a unique signal from a small area of space, the signal can be detected, regardless of its faintness. This method should work at least until we approach the immediate vicinity of the object star, when other signals created by the star in this frequency may interfere with the probe signal. Mr. Daniel Junker, my esteemed colleague and a physics expert, has pointed out the difficulties of detecting a signal against the background of another star, and for this information I am truly indebted.

By analogy, we detect very faint signals from distances that are far greater than four light years when we analyze the signals created by black holes. With Very Long Baseline Interferometry, we routinely analyze faint signals from distances that are far greater than four light years. It seems quite possible then that the faint signal of an object that is relatively nearby can be detected.

W.D. Hobbs, Jr.

Detecting a probe as it approaches Alpha Centauri: a radioactive beacon, a microwave mirror and a Big Square

by W.D. Hobbs, Jr.

first published March 18, 2005.

Detection of a probe, once it is on its way to Alpha Centauri, is still an open issue. Three techniques for detection are presented and explained in this paper. First, it is proposed that we send a gram-sized object having a radioactive emission to Alpha Centauri. Second, it is suggested that we use the probe as a microwave mirror to reflect microwaves back to earth. As a third alternative, it is noted that a meter-size square of reflective material itself would be detectable by two techniques.

A radioactive beacon

One recommendation for detecting a miniature, interstellar probe is to create the probe from radioactive material, which emits an obscure wavelength on the electromagnetic spectrum. As the probe flies through space, it would emit a signal, which could be detected. My esteemed colleague and physics expert, Mr. Dan Junker, and I agree that the signal from such a radioactive probe might be a bit small. We believe further, however, that a custom-built telescope could detect the signal even if such sensing is beyond the scope of existing telescopes.

Though the signal would be faint, we have detected fainter fluctuations in the past. One need only consider the COBE satellite that detects faint fluctuations in radiation from billions of light years in the past.

Even as it approaches Alpha Centauri, a probe might be detectable if we selected a unique frequency on the electromagnetic spectrum, doped the probe material so that it emits that frequency, and monitored the probe for that signal. Since most stars burn mostly Hydrogen and Helium, it would be advisable to select a frequency that would greatly contrast to the waves emitted by these two elements.

It is a matter of predicting the spectrum of faint radiation, looking in the direction of the distant signal, and detecting the fluctuation pattern. Or, restated in other terms, we predict the spectrum of the radiation caused by the probe, we direct the sensor in that direction (a known), and we detect the faint signal.

A microwave mirror

As a second alternative, it is proposed that we use the probe itself as a mirror to reflect microwaves back to the earth. Microwaves, unlike waves from a laser beam, do not diverge as they travel through space. Therefore, it is quite possible to both propel the probe with a microwave beam and to receive reflected signals back on earth or in the vicinity of earth.

A Big Square

As a third alternative to detecting an interstellar probe, it is noted that a meter-size square of reflective material, which has been described as a candidate for a solar sail, would itself be detectable as it approaches Alpha Centauri. In fact, with the right equipment, it would be detectable by two means.

First, we could use a variation on a technique that is used for detecting “extrasolar” planets (Planets that orbit stars other than the sun.) One way of detecting such a planet is to record the amount of radiation that is being emitted by the star, and then compare that to the amount of radiation we can sense when a planet orbits between the star and the earth. We know that a smaller amount of radiation is sensed when the planet is between the distant star and the earth. In a like matter, a one-meter sheet of mylar or reflective material would block the radiation from Alpha Centauri. In fact, when it is first launched, if the sheet were to be positioned directly between the telescope and Alpha Centauri, it would block all of the radiation from the star entirely! As it approaches the star, it would still be, in effect, a Big Square that blocks a significant part of the radiation from the star!

As it approaches even nearer to the star, the sheet would of course appear increasingly small. Nevertheless, it will block at least some of the radiation from Alpha Centauri at all times along its path to the star. We can certainly model and predict how much of the radiation would be blocked by the sheet, and, in the event that our current telescopes are unable to detect the fluctuations up to within 98% of the distance to the star, we can certainly build new detectors.

A second technique for detecting The Big Square would be to analyze the light passing through it. As light passes through mylar or diamond, for example, it creates a different signal than light that is coming directly from an ordinary star. Consider a sheet, or Big Square, which consists of mylar or diamond. Some of the radiation from the star passes through the sheet of mylar or diamond. The altered signal that has passed through the sheet could be detected here in the vicinity of earth.

Conclusion

For these reasons, it is quite probable that a small probe – either a miniature or a meter-sized square – can be detected as it approaches Alpha Centauri. A gram-size probe that emits a unique, radioactive signal could be detected. Further, it is possible to detect microwaves reflected from the probe back to the vicinity of earth. Finally, it is possible to detect a meter-sized sheet as it approaches the star, either by measuring the change in the amount of radiation or by analyzing the light from the star after it passes through a sheet made of mylar or diamond, otherwise known as The Big Square.

W.D. Hobbs, Jr.

Monitoring the Natural Signals of the Alpha Centauri Probe

by W.D. Hobbs, Jr.

first published May 18, 2005.

"Two methods of detecting such a probe are presently identifiable: (1) Creation of an artificial signal from the probe itself, or (2) monitoring of signals created naturally by the probe as it travels through space."

From "METHODS FOR DETECTING A MINIATURE PROBE MOVING AT RELATIVISTIC SPEEDS", by W.D. Hobbs, Jr.

It has been noted in earlier papers that one method of detecting an interstellar probe is to monitor the signal that it creates as it travels through space. A gram-size probe moving at relativistic speeds will encounter significant amounts of matter, particularly in the vicinity of a star, because the star's gravity attracts more matter.

As it approaches Alpha Centauri, a gram-size probe will effectively create a measurable amount of radiation in a matter that is analogous to bremsstrahlung radiation, which is radiation that is emitted by charged particles as they are deflected. Bremsstrahlung radiation refers to a phenomenon that occurs at the level of simple photons. One can expect not only that bremsstrahlung radiation will be created by the probe as it nears Alpha Centauri at relativistic speeds, but that radiation on a larger scale will be created as well.

Given a means of measuring such radiation, the problem of detection is overcome.

In addition, as the probe moves through space, it will create a "bow shock" that is analogous to the bow shock created by a boat as it displaces water in front of it. The probe's bow shock will be significant as it departs from the solar system because of the relative abundance of matter near the sun. Since there is less matter in the interstellar medium, the bow shock will be less significant in this phase of the voyage. The bow shock will increase dramatically as it encounters the heliosphere of Alpha Centauri since the abundance of matter is much greater near the star. Consequently, the bow shock of the probe can be measured in order to track it as it enters the Alpha Centauri system.

Reference:

1. "Centauri Dreams - Imagining and Planning Interstellar Exploration", Paul Gilster, 2004.

W.D. Hobbs, Jr.

One More Method for Detecting the Alpha Centauri Probe

by W.D. Hobbs, Jr.

first published May 19, 2005.

Another technique for detecting a probe as it approaches Alpha Centauri can now be described. This note is based on the following assumptions: (1) The probe will have mass of one gram; and (2) it will be accelerated to at least .9c and will continue at relativistic speeds until it reaches the vicinity of Alpha Centauri.

Based on these assumptions, the probe trajectory should be calculated so that it will collide with either a planet in the Alpha Centauri system or a comet in the vicinity. Such an impact would create radio signals or radiation that could be detected here in the vicinity of Earth. In addition, the impact might be detectable through a visual telescope of sufficient power.

In the event that it is determined that the planet's atmosphere would burn-up the probe without creating a detectable signal, a smaller rock - a comet - that has no atmosphere should be selected. The trajectory calculations for such a mission provide a new challenge for engineers and scientists alike; however, the reliability of this detection method renders it well worth the effort.

For those who insist on using acronyms, the author suggests the label "Direct Planetary Impact Technique", or DPIT.

W.D. Hobbs, Jr.

The Finalé: One More Way of Detecting the Alpha Centauri Probe

by W.D. Hobbs, Jr.

first published May 20, 2005.

In his book, "Centauri Dreams - Imagining and Planning Interstellar Exploration", Paul Gilster describes the efforts of James Lesh and two of his associates. Lesh and friends have apparently described a mission to Alpha Centauri in which a 20 watt laser would be used to communicate back to the Earth.

Applying that logic to the proposed methods presented above, it is quite possible that a miniature laser having a power capability of 20 watts can be sent to Alpha Centauri at relativistic speeds. At a pre-established time and point in space, it can be made to activate, sending a signal back to Earth. The Hubble Space Telescope - or a modified version of it - can be used to detect this 20-watt signal.

This proposed method presumes that the effects of length contraction on the probe can be ignored. (Such an issue can easily be tested on shorter flights and tests in the solar system.) This method is not quite as simple as the Direct Planetary Impact Method (DPIT), but it is certainly a practical technology. Further, it provides the additional benefit of allowing tests on three issues: (1) The effects of length contraction on a manufactured component; (2) The Einstein test mentioned in the paper, "Sending a Probe to Alpha Centauri"; and, (3) Testing a means of interstellar communication.

It is this author's belief that this idea should be implemented.

Sayonara!

Reference:

1. "Centauri Dreams - Imagining and Planning Interstellar Exploration", Paul Gilster, 2004, p. 191.

W.D. Hobbs, Jr.