Yesterday I wrote about the theory of neutron stars and pulsars, and about the NICER mission that is about to begin examining them in more detail. After fifty years of theoretical study and limited astronomical data, we are soon to have a dedicated x-ray telescope with the primary purpose of studying the composition and properties of pulsars in the galaxy.

However the mission has a second goal, and one with a more practical purpose than studying distant neutron stars and pulsars. The mission will also be testing out the idea of x-ray navigation in space, with an experiment known as Station EXplorer for X-ray Timing And Navigation Technology (aka SEXTANT). 

Over the past thirty years, Global Positioning Systems and SatNav have become common in our cars, cameras, and cell phones. Whenever you turn on one of these devices, an antenna in the device begins receiving a signal from a number of satellites that are orbiting the Earth. By carefully measuring the time it takes for the signal to travel from each of the satellites to the device, and knowing the exact position of each satellite, a computer inside the device can triangulate its exact position on the surface of the Earth.

The problem is that we are now starting to travel beyond the Earth, to other planets and eventually to other stars. GPS satellites orbiting the Earth will be less effective as we travel further away from the planet, and it would be too expensive and technically difficult to place similar signal satellites elsewhere in the solar system and the galaxy. And so for that reason, we need to develop a new technology for navigating space.

The answer might be pulsars. Each pulsar sends a signal of a specific pulse width and period, and therefore we can identify a pulsar by its x-ray pulse signature. In theory it would be possible to identify the direction that each signal is coming from, and then by analyzing its properties determine which pulsar emitted it. By comparing this information to a catalog of known pulsars and their locations, it would be possible to determine a location anywhere in the galaxy. Such a system could be used for both manned missions and for autonomous probes to plot their own trajectories.

The goal of the SEXTANT mission is to increase our knowledge of the local pulsar population, and to provide an initial test of the possibility of navigating using pulsar signals.

As part of this same mission, scientists and engineers also are operating the XCOM mission, with the goal of transmitting data from the probe to the Earth using x-rays instead of traditional radio waves. At present the probes that are exploring our solar system and beyond communicate with their controllers by emitting radio waves of various frequencies and intensities, and large antennae on Earth receive the weak signals and process them. However the amount of data that can be transferred in this way is limited. It is believed that using higher frequency electromagnetic waves, such as x-rays, would allow scientists to transmit much larger packets of data at a higher rate, and increase our ability to control future probes and receive significantly more data from them in return. 

If XCOM is successful, then future missions will be able to provide far more data and control in a more efficient manner. And if SEXTANT is successful, then future probes might not need controlling at all, as they plot their own trajectories and locate themselves using distant pulsars.

It is a very interesting use of pulsars as a cosmic scale GPS, and should provide the first data just in time for the fiftieth anniversary of the first discovery of pulsars. It will be fascinating to watch where this new technology takes us in the coming years.