You will probably have heard of the old tradition (before GPS!) of using the stars to help navigate around the earth especially at sea.  But when you’re actually travelling among the stars, and they are no longer ‘fixed’ in the sky, how would you navigate then?  This was the question asked by the Daedalus team in the 1970s as they planned their flight out of the solar system to Barnard’s star.  Despite noting that the interstellar probe presented problems of navigation far beyond those that had to be solved for the current interplanetary probes,  their original investigation concluded that there were no fundamental problems that could not be overcome through the normal process of technological development.  So one might ask what has happened over the last 30-40 years and what are the implications for the Icarus Team?

Those fundamental issues were discussed in the original Daedalus navigation paper but if you don’t want to search out your historical JBIS here are the main conclusions: they determined that an angular measuring accuracy of a microradian or so (0.2” – arc seconds) and a 5 metre telescope using sensors in the visible to thermal bands (0.5-100 micrometres) would do the job.  But there was a caveat that currently known stellar positions were not accurate enough for interstellar navigation although they supposed that this problem would be resolved by taking astrometric observations from Daedalus flight trails.

Well, in the first instance let’s check out one of the most recent missions that might offer some comparison …The NASA New Horizons Mission to Pluto and the Kuiper Belt.  The guidance and control system on New Horizons is capable of providing attitude knowledge to better than +/-471 microradians.  That relates to how accurately you can point your communications system back to earth; there’s more on this in the earlier blog by Pat Galea.  The Daedalus paper stated 250 microradians would suffice for this during its cruise phase so at least it is getting close to that requirement.  Interestingly, Galileo Avionica, the manufacturer of the star trackers often used for guidance and control in interplanetary missions including New Horizons, states the accuracy of their A-Str star tracker as better than 10” which is about 48 microradians.  So, still not that near the requirements set out by the Daedalus study.  Perhaps that was good enough for navigating around the outer solar system so hence there was no real need to go for greater accuracy.

Suppose we take a look at a mission specifically designed for accurate stellar measurements such as the ESA Hipparcos mission in the 1990s?  Hipparcos was designed to measure stellar parallax (and positions) using a main mirror of only 29cm…In this case the error was given as between +/-0.7 to 0.9” (for star magnitudes down to the 8^th magnitude).   That’s not a whole lot more than a microradian.  The follow-on ESA mission of Gaia promises even greater accuracy albeit with larger mirrors.  So it seems that the Daedalus team were correct in their assumption and sufficiently accurate systems are now possible.

You might recall that the Deadalus mission profile was to carry out a fly-by of the target system and the only complication was for the sub-probes to not fly too close to the targets (remember the fly past velocity was to be 12% of the speed of light – if you get too close the relative motion becomes too great).  Interestingly, it now looks possible that Icarus may include some deceleration or perhaps some elements might even go in to orbit which will remove some of the problems identified by the Daedalus team but will add a significant amount of complexity to a fully automated system.  In the near future and with the plans for exoplanet detection it is likely that there will be another advantage for Icarus: that of having prior knowledge of the planetary system before it sets off.  Indeed, there are some concerns that by the date of a possible Icarus launch/arrival at the target, future solar system based observations may well exceed the capability of the Icarus onboard systems until the probe gets quite close, but at least that will mean there would be a high level of knowledge about the target system.

This all sounds fairly promising for the Icarus study and there is more good news.  Recent studies have been made into using x-ray pulsars to help navigate space.  In effect, the suggestion is that by using 4 x-ray pulsars and their precise timing signals then you can effectively set up an interstellar GPS system.  You can find more information about this at http://www.centauri-dreams.org/?p=136. 

Finally, after all that promise from developments of the last 30-odd years there still seems to be one nagging thought that was also raised by the Daedalus team…how are we going to make the system reliable enough to last the anticipated mission time i.e. perhaps 50-70 years!  Modern star trackers are rated for around 15 years although some spacecraft out there, for example the Voyagers, are already over 30 years old and still under guidance and control and still expected to go on for another 10 years! 

So there is real hope and the Icarus team will be taking on the challenge of all these issues for their new interstellar design.  The review of the navigation problem will consider all the relevant assumptions of the Daedalus team and will be taking a closer look at the technology and the real missions that can be related to it.  Overall, it seems possible that rather than leaving it to any future technological development Icarus may be able to use current technology and meet the requirements for interstellar navigation.  But, as my partner, Sue always says, if all else fails we could always stop and ask for directions!

Next time I shall take a look at the scientific benefits of sending a probe to the nearest stars with specific reference to a related navigation issue raised by Project Daedalus: improving the accuracy of measuring stellar positions and distances and how it could help calibrate the scale of the universe.

If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambitions of creating a credible starship design.

Alternatively take a look around the remainder of the Icarus site.

References:

Fountain, Glen H., et al, ‘The New Horizons Spacecraft’, arXiv: 0709.4288v1 (26 Sep 07)

Perryman, M.A.C. et al, ‘The Hipparcos Catalogue’, Astron. Astrophysics, 323, L49-L52, (1997)

Richards, G.R.  ‘Daedalus Project: The Navigation Problem’, JBIS, ppS143-148, (1978)