New to the site, and like many others here, I've been bouncing around a few ideas for a hard sci-fi short story with some friends, centered around an interstellar voyage to colonize a habitable world. We figured that laser propulsion (a lightsail-equipped spacecraft being boosted and then decelerated by a powerful orbiting laser in the Sol system) is one of the better ways to propel an interstellar spacecraft; it's safer than antimatter, can get to relativistic velocities, and it's not limited by the rocket equation. It seems like the natural choice. But its dependence on an outside power source, back home in the departure system, is just something that gives me plenty of anxiety to think about. What if something went wrong with it? So that's the idea I was thinking of exploring.
About the mid 23rd century, the Sol system has been heavily colonized, and the various great powers within it are now racing against each other to colonize habitable worlds found around other stars (at sublight speed, no FTL here). The successful terraforming of Mars and Venus last century has left humanity with an enormous amount of energy infrastructure, namely several extremely powerful solar-pumped laser arrays. And I mean powerful; they were previously used to melt almost all of Mars's surface to liberate the oxygen and water in its soil, and to cut Saturn's moon Enceladus into pieces to be deorbited onto Venus to make its new oceans. With the two planets now habitable, the new use found for the laser arrays is to accelerate interstellar colony ships leaving the Sol system, and then to decelerate them once they're near their destination. By the point of the story this has already been done successfully several times, and a number of nearby star systems have been colonized through it.
The ships themselves are sleeper ships, at about a kilometer long are rather small by interstellar ark standards, but they carry an accelerator sail with a diameter of 1000 kilometers and a decelerator sail with a diameter of 300 kilometers. Reminiscent of Robert L. Forward's design, the laser array first boosts the ship away from the Sol system for six months at 1G by the accelerator sail (with the decelerator sail folded up inside it), until it reaches 50 percent of lightspeed and the laser beam shuts off (in the interim period, it's redeployed to accelerate/decelerate other ships elsewhere). Once it's near its destination, the laser array back in the Sol system again is shined towards it (or was a few years/decades earlier, with the beam taking that long to cross the lightyears to the ship). When the beam is detected, the accelerator sail detaches from the ship and is pushed ahead and away from it, and the decelerator sail is unfolded to receive the laser light reflected off the accelerator sail to slow the ship down (the accelerator sail has built-in servomotors to slowly change its shape as it races away from the ship, to keep the light reflected on the decelerator sail). After six months of deceleration, the ship has reached its destination system and the decelerator sail is detached, leaving the ship with only its built-in fusion engines intended for interplanetary flight (they have nowhere near the fuel capacity for interstellar velocities).
A standard colony ship carries a few dozen asteroid mining probes, surface construction equipment, 3D nanoprinters (many are built into hull-mounted robots that roll around the ship and repair any damage from the interstellar medium during the voyage), four VTOL SSTO spaceplanes for landing on and launching from habitable worlds (their built-in reactors can make their liquid hydrogen fuel from water or even just atmospheric humidity), and 10,000 humans in cryostasis (judged to be the minimum starting population needed to maintain a healthy gene pool in the long term). Food brought along can sustain all onboard for a year, and an onboard hydroponic farm can keep a few hundred of the crew fed during the six-month deceleration period (they're woken up early to monitor the deceleration and survey their destination) and in emergency situations (all others onboard are expected to be kept asleep until any emergency is resolved). Onboard gravity can come from either rotation or linear acceleration; the gimballed passenger compartments can either face forward or backward during the 1G acceleration or deceleration, or outwards for rotation during cruise and when stationary. An onboard fusion reactor powers the ship, and for contingency it carries enough fuel for twice the expected journey time.
The Ill-Fated Voyage
At the time of departure, it's been about fifty years since these interstellar colonization voyages have begun, and the Sol system has already received radio transmissions from the first missions in their destination systems up to 15-odd lightyears or so away, confirming they arrived safely and are settling into their new habitable worlds.
This particular mission is to an Earth-sized habitable exomoon found orbiting the gas giant Upsilon Andromedae d, named Majriti after 10th-century scientist Maslama al-Majriti (look it up, the IAU has already named it that). The moon is named Fatima after his supposed daughter. At 44 lightyears away, the journey will take nearly 90 years, and it's by far the longest interstellar journey attempted so far. But with interstellar colony missions having a perfect safety record so far, and any habitable world being too good a target to pass up on, they feel confident enough. The acceleration up to cruising speed goes off without a hitch, and the passengers and crew climb into their cryopods for the long journey.
About 87 years later, with the ship (I'll have to think of a name for it at some point) hurtling through interstellar space at 0.50 c, a skeleton crew wake up as planned in anticipation of the laser beam arriving, ready to monitor the deceleration, do some maintenance on the ship, and see what they can survey of Fatima before arrival.
Only, the laser beam never arrives. Something happened back in the Sol system, and the laser array never shined a deceleration beam at them. They don't know what happened; they didn't receive any radio transmissions since shortly after departure, and whatever it was, it was 44 years ago (maybe I'll explore what happened there in an eventual sequel story). Either way, the ship is still plugging along at cruising speed, rapidly approaching its destination and seemingly about to overshoot it, with nothing to slow it down.
So, with no laser beam coming from Sol, with the equipment they have, and with the few hundred crew that the hydroponics can sustain awake, what can they do, if anything, to slow down? Will they inevitably overshoot their habitable world? If so, would they be able to stop in another star system (not ideal for them given that it likely wouldn't have a habitable world, but better than endlessly coasting through space)? That's what I'm hoping to explore, and I'm hoping it's a good starting point for an interesting story.