Mos Eisley Spaceport Explosion, the Millennium Falcon transports our adventurers off Tatooine, bringing Luke Skywalker through the threshold into space. With the Imperial Star Destroyers shutting down, Luke bemoans Han Solo’s delay in jumping into hyperspace.
It takes time to perform these calculations via Falcon’s “Navicomputer”. Han explains that otherwise, they could “fly through a star” or “bounce too close to a supernova”. (Probably the same effect of each – are supernovas also bouncy?)
Heavenly calculations are needed to know where you are going. In Star Wars, these are done by on-board computers, or later by stalwart astromech droids like R2-D2. But, for the first time, simulations have been carried out on the ability of an unequipped ship to automatically navigate interstellar space.
Although not at hyperspace speeds, the simulations take into account speeds up to half the speed of light. Created by Coryn AL Bailer-Jones from the Max Plank Institute for Astronomy, these simulations can be our first step in creating our own “Navicomputers” (or R2-D2 if they have a personality).
Sail like Voyager 1 and 2
The farthest object we have sent into the Universe is the Voyager 1 space probe. Probes like Voyager update their position via radar and radio signals with Earth. You can actually track travelers real-time location online.
The location of the spacecraft is triangulated using two ground stations on Earth, then the position of a known bright object next to the apparent position (in the direction but not in the vicinity) of the spacecraft like a quasar. This tracking system is like a giant light-based umbilical cord connecting the craft to Earth.
But these ships do not have their own Navicomputers or R2 units. All advice depends on the connection to Earth. Once that spacecraft is out of signal range, or if the signal is interrupted, the spacecraft has no internal means of navigating.
Probes like Voyager will eventually lose connection with Earth and be left adrift for hundreds of millions of years. We may never know where they end up or who finds them – if at all.
Navigate a spaceship by pulsar
If we plan to send the craft into deep space, they need a way to navigate and make course corrections without instructions from Earth. One proposed method consists of referencing known pulsars.
Pulsars are the remnants of dead stars created from cataclysmic supernova explosions. As the stars collapse violently, their angular momentum or rotation is transferred to a smaller and smaller object – think of a figure skater retracting his arms. These pulsars rotate with known frequencies at known distances.
They could be used as interstellar GPS satellites to determine where you are in 3D space. However, there is some debate over the accuracy of this system, as you only have to rely on a handful of pulsars and space dust / gases, called the Interstellar medium, which could introduce an error in these pulsar calculations.
Bailer-Jones therefore proposes a method as old as maritime navigation. Use a sextant. Celestial navigation has already been done for centuries on the ocean. Ships would use a sextant to measure the angle or “angular distance” between a star or the Sun and the horizon in order to calculate the position on the Earth’s surface.
A spacecraft deep in interstellar space could use a similar technique measuring the angular distance between stars and extrapolating from their change in position over time when the ship is relative to those stars.
The stars move for two reasons when you travel in space. The first is parallax, the perceived movement of an object caused by your change in perspective. You can see this change in position if you hold one of your hands at arm’s length and look at your fingers with one eye closed and then the other. Your fingers seem to “move”. We see the sky moving in a similar way.
When our Earth revolves around the Sun, we witness the change in the position of the stars. When we are on one side of our orbit, it is as if we are looking at the sky with one eye open, as in the example of the hand. Six months later, we are looking across the Sun. The amount of offset of a star gives us a calculation of distance from that star in parsecs. (Ahem… Han Solo, are you paying attention? Parsec is a measure of distance.)
A star at a distance of one parsec will appear to change position in the sky by one “arc second” (a 3600e one degree in the sky) 6 months from our orbit around the Sun. One parsec translates to approximately 3.26 light years. Likewise, for a moving spacecraft, a star that is 1 parsec away will be displaced by 1 arc second for each AU (astronomical unit = average distance between the Earth and the Sun = about 150 million km) that the ship travels. in the space.
Unlike the spacecraft’s ground observation, distant quasars will not work in this scenario because they are astronomically too far away. The closest quasar to Earth is half a billion light years away, so the parallax effect is virtually invisible. Instead, the craft would observe the closest and brightest stars to take measurements throughout its journey, as those stars will demonstrate the greatest parallax effect.
The stars will also appear to change position because they themselves move in the Milky Way. The closer we are to these stars in a moving spaceship, the more noticeable their own movement will be over time. The change in the apparent position of the star in the sky due to its real movement in space relative to the ship is called an “aberration”.
The spacecraft can distinguish changes in the position of a star either by parallax or by aberration. The two types of motion, parallax and aberration, taken together can tell us two things about the spacecraft that we need to know. Parallax gives us a real-time position of the spacecraft in 3D space. The aberration gives us the speed of the spacecraft relative to the movement of these stars.
For the system to work, the spacecraft would carry a star map of the known positions and speeds of stars that have already been mapped from Earth using data from star mapping missions such as Gaia and Hipparcos. Gaia alone maps 1% of the galaxy … which doesn’t seem like much until you realize it’s 1 BILLION stars. If our ship is to travel even a few light years in space – much farther than we’ve ever been – this map is more than enough.
Simulate an on-board computer
Some assumptions have to be made about the virtual spaceship we send into the universe Bailer-Jones chooses for the simulation. Gaia can achieve precision over angular distances between stars down to sub-milliarcseconds. Really fine measurements. But to be sure, this simulation assumes that the spacecraft can at least measure up to an arc second.
We don’t know how powerful the craft’s navigation instruments can be. Keep in mind that an interstellar probe probably needs to be compact and carry other detection equipment. More precise angular measurements mean larger telescopes for navigation.
The spacecraft, using existing star maps, has access to the expected directions and speeds of the stars relative to the spacecraft. The machine measures the angular distances between a selection of these stars and a reference star towards which an on-board sextant is always pointed. In this case, that star could be our own Sun, but any star could be used and this is an important note because the purpose of this system is for navigation to work regardless of your starting point.
The simulations placed the craft between 0.1 and 10 light years from Earth – a higher estimate of the distance traveled by our first attempts at interstellar travel. Remember that the star closest to us, Proxima Centauri, is only 4.2 light years away. Even that would be amazing.
The ship is also simulated at speeds ranging from 0 to 500 km / s as well as relativistic (approaching the speed of light) down to 0.5c (0.5 times the speed of light – NOT 0.5 past speed of light). If we’re going to go to another solar system, we’ll probably have to travel at a good fraction of the speed of light and the simulation wants to capture how that affects our navigation, if at all.
The simulation results: Yes, you can know where you are in space! Second, Bailer-Jones determined with what degree of precision. For example, using 10 stars as a reference point with an angular measurement accuracy of 1 ”moving at .39c, the spacecraft can determine where it is with a position accuracy of 5AU and a speed accuracy of 5 km. / s. Not bad. 5AU, however, is a large space bubble.
However, using 100 stars, the craft can locate within 1.2 AU and determine its speed at 0.6 km / s. Additionally, traveling at relativistic speeds does not change the craft’s overall ability to know where it is. (We will leave the problem to the next generation of FTL vessels)
If you increase the accuracy of the angular distance measurement to 0.1 arc second, the location of the craft could be measured to around 0.3 AU and the speed to 200 m / s using just 20 stars. . So any additional ability to increase measurement accuracy reduces the total number of calculations you need to perform. Hope Han knows that.
Reading Bailer-Jones’ research, I felt a connection to our little virtual spaceship flying through the stars. It’s still far from hyperspace, and we don’t fly fast enough to worry about flying through other stars, but we might be about to fly at other stars. I just hope the ship’s navigation computers get at least some sort of sci-fi themed name. R2? L3? Chewy? … Chekov? Any of these would do.