Credit: MPIfR/Ralph Eatough.

Once we have 'boldly gone where no man or woman has gone before,' the one question that will likely come up first will be: Where are we? The answer might be provided by zombies, that is, the undead stars we all know and love -- called pulsars. The idea of using them for navigation through space was suggested almost as soon as they were discovered (to mixed reception). So in 2017, an actual device will be installed on the ISS that will give us an idea about the viability of using them to illuminate our path in deep space.


The NICER/SEXTANT (acronyms for "Neutron-star Interior Composition Explorer & Station Explorer for X-ray Timing and Navigation Technology") mission will consist of a device about the size of a small refrigerator. Mainly containing 56 separate X-ray telescopes and silicon detectors, it will essentially be doing two separate studies for the price of one (something we all can appreciate). NICER's focus will be on studying the nature of neutron stars (especially their inner structure). While SEXTANT, on the other hand, has an altogether different goal --  the clever thing still uses the exact same hardware. The differences between the two are in how the data is treated.


Artist rendition of NICER/SEXTANT (Credit: NASA)



Currently, as we explore our own backyard - with only Voyager having poked its head through the hedge, so far -  we mostly have Earth-centric systems that allow for positioning and speed determination. Typically, for spacecraft that are farther away; we use the Deep Space Network (DSN), and it is doing a great job. Yet, what if we get farther out into interstellar space and lose our line of sight for some reason? We are at a point where we can make robots that are more than clever enough to take care of business, and that business includes navigation. The only tricky part is in giving them the tools to be able to do so independently. This takes us back to the old days when humans first saw Terra Firma drop beneath the horizon, as if it were a pebble in an incomprehensibly vast shoreline. We had to find our way with a clock... and a sextant.

Image of a sextant (via: Wikimedia Commons)


By measuring the angles of a few stars and knowing the exact time at which those measurements were taken, we can calculate a position on Earth. Logically, this is also one of methods we can use for deep space travel--measure the relative angles of stars in visible light and triangulate. This is not an unusual technique and many devices are available, but each has its drawbacks. Ultimately, the same triangulation could be done with pulsars, however, the likely method for positioning using them will be quite different.





Pulsars, especially the millisecond ones, are exceptionally accurate clocks. Best viewed in X-ray, these pulses can be utilized in a way similar to GPS satellites. Of course, GPS signals have a timestamp that allows a receiver to determine the delay with which it arrives and calculate the distance to the satellite. Pulsars are not that cooperative but with them we know the time between the pulses extremely accurately. There will be a difference in the time between them due to Doppler shift. They, in turn, can translate into speed very easily. Calculating a position is more math intensive though. Furthermore, until not that long ago, to derive these measurements, we had to compare the pulsar signal with a relayed signal from a known position in the solar system. But by now, further study has come up with the math that will allow a spacecraft to plot its own position in space truly independent of anything else. Practically, with a minimum of four pulsars, an initial position can be determined. After that this initial point becomes a reference for subsequent positions, much as the older methods work.



Caveats remain, of course. For example: The measured distance to any pulsar will have some margin of error involved, which will affect the perceived direction of a pulsar. Gravitational bending of the light from a pulsar - for example; if it has to pass close to the sun - also needs to be considered. Since pulsars themselves are not stationary in space, the measured Doppler shift will depend on the position of the observer in relation to the pulsar's direction of travel. However, the researchers are confident and expect the accuracy to be within one kilometer, near or far  -- no matter the distance. Pretty nifty considering most of these objects are hundreds of thousands of light-years away.



Source: Wikimedia Commons


I haven't gotten my hands on the exact nitty-gritty of how the team at Goddard plan to go about it, but the available science seems sound. In my opinion, all it needs is some of that all-important experimentation. After that; there is nothing holding us back from strapping on a warp drive and taking it for a spin... well, maybe that warp drive. Our Galactic Positioning System will get us home to boast. Still, I do feel there is the need for proper naming, as it's technically not very much a sextant (Plus, "GPS" is already taken). I would humbly like to coin "Milky Way Finder".

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