On board the spaceship headed towards them, among many others, were Olivia's parents, Sarah and Jacob.  Their relationship had always been contentious, but they had reconciled enough to board the ship and consign themselves to several months close quarters with each other.  This was, they knew, a risky decision, as they had never been able to live together more than about a month before splitting apart, either in heated argument or sullen silence.  But, bound by a common desire to leap at the first opportunity in almost 20 years to see their daughter Olivia in person, they swallowed hard (or gritted their teeth), and committed.  Unlike when they had been living on the Mars Colony, with Jacob leaving for many months at a time to go back and forth from Earth while Sarah stayed on Mars with young Olivia, this time they had no second option, no way to avoid each other's company; living quarters were obviously tight on the spaceship.

They had mostly survived each other's company, by not talking too much.  Every previous time, they had fallen into each others arms and spent every waking hour together, fascinated in everything the other had to say (mostly Sarah talking, but Jacob talking as much as she could convince him to).  Then, eventually, they would grate on one another, but not part until they could not stand one another's company any more and it had come to a loud argument, at which point Jacob would move out, or at least spend the minimum amount of time possible at home.

This time, as if there were a limited quantity of peaceful and happy conversation that they could count on before the storm-tossed ship of their relationship wrecked on the shoals, they spent their time mostly watching saved dramas from Earth (in Sarah's case) or playing video games (in Jacob's case).  It helped that they were older, and their lower energy levels made it easier to keep things on an even keel.  But, they did have a few tasks they needed to do, during the spaceflight; it was not all just waiting for time to pass.  One of these tasks, was to care for the lab rats.

The spaceship spun slowly on its central axis as it made the long journey from Earth to Mars.  This was not enough to impart more than about 1% of Earth's gravity, but it had one desirable outcome; dirt fell down to the floor (since the "floor" of each room was the one closest to the outside of the ship).  Crumbs in a meal, flakes of skin shed over time, anything which, on a planet, would end up on the floor and then (hopefully) vacuumed up or mopped away, would not float forever in zero-gravity, because although the centrifugal force was not very strong, it was enough to make solids "fall" towards the outside of the ship (eventually).  While the ship was hurtling forward at the beginning of the trip, things were different, but once it had achieved cruising speed in outer space and had only enough fuel to decelerate at the end, it began to rotate, and the outside edge of every room became "down".  This made cleaning, immensely easier than if it had been truly zero gravity.  Every time Sarah needed to clean the rats' containers, she was especially thankful for this.

Sarah had left Mars twenty years before, thinking she was leaving her daughter in the care of several dozen responsible and capable adults, and that she would be returning back relatively soon with Jacob, who would fix all the things that had stopped working in the Colony.  She had realized, almost as soon as the trip back to Earth began, that this was a mistake.  It was not as if Jacob would not have been willing to come back anyway even if Sarah had not gone to ask him; his routine job involved shuttling back and forth between Mars and Earth, with each shipment of new equipment, getting trained on it on Earth, maintaining it en route, and setting it up on Mars.  He had always been a responsible father, as far as his job allowed, and even if Sarah had not been there to ask him to return there was little doubt that he would have done so.  It was just that Sarah, a good-hearted and in some ways insightful person but not one who was naturally calm in a technological emergency, was accustomed to respond to any technical problem which was beyond her skill to fix, by running to ask Jacob for help.  As things were starting to fall apart on the Colony, she had joined the stampede of people who were leaving, out of a confused impulse to follow this normal response to technical problems.

She also felt, looking back on it, that the crew of the ship which had come to take back to Earth anyone who wanted to leave the Colony, had been trying to persuade her to go.  They seemed to have been trying to convince everyone who was willing to leave Mars, to do so.  She had berated herself daily (or nightly) for panicking and leaving, from the very first night she spent on the ship headed back to Earth.  Communications had been cut off from Earth during the trip (for reasons involving a "malware complex", which she did not really understand, or care to), but when they arrived at Earth they heard the horrible news that all of the adults on Mars were dead, and the children were left there alone.  She was devastated, and inconsolable.

She heard news from Mars mostly through Jacob, who knew the fellow, Gerard, who maintained the comm link to Mars throughout the Long Blackout (when the malware complex had wrecked the internet and nearly all electronic communications).  To her astonishment and relief, they heard that the children were still alive, and seemed to be likely to continue surviving.  Eventually, after several years, the malware complex was tamed, and she was able to send and receive text messages (and the occasional picture or very short video) from the Colony.  It was many years, though, before the opportunity came to return.  She had to work hard, and use every social skill and trick she knew, to get both herself and Jacob on the return trip.

Jacob, on the other hand, had little to do with bureaucratic affairs such as the decision about who could go on the first trip to Mars in 20 years.  It was not that he had no opinion on the matter; he earnestly desired to see his daughter again, and meet his grandchildren.  It was that he had no talent for dealing with administrators, and he knew it, so he stayed well away lest he say (or fail to say) something, and wreck their chances.  He knew himself well enough to leave such matters to Sarah.  In a way, it was the mirror image of her response to technical crises; if it involved dealing with people to get something done, he immediately turned to her.

The lab rats did not require a lot of work, but Sarah and Jacob were quite conscientious about it.  This was not due to any prodding of conscience or work ethic, although they were both reasonably ethical people.  Rather, it was because the trip from Earth to Mars took a long time, and there was precious little to do.  Really, all the lab rats even required was that their food be replenished, their water source refilled, their cages cleaned, and all of it be sterilized and checked for leaks or damage from time to time.  But, Jacob and Sarah were extraordinarily thorough, stretching what could have been 10 minutes of work into at least an hour each day.

The rats were being sent to replicate an experiment which had taken place many years before, in which rats were conceived, gestated, and born on Mars, and then after growing to adulthood were sent back to Earth.  They had all died, in manners which suggested that their skeleton and muscles, having developed entirely in Martian gravity, simply could not adapt to Earth's gravity, which was about 3 times as great.  However, there were questions raised, by some issues with the experiment.  For one, some of the rats died during the trip back to Earth, when they were in nearly zero gravity, and this suggested that there were problems with them not related to gravity, perhaps an infection or some damage sustained during blastoff from Mars.  So, the experiment was being repeated.

Obviously, the primary question that this related to, was whether or not someone born on Mars (like their own daughter) could ever travel to Earth, and survive.  They both tried not to think about that part.  They finished the feeding, watering, cleaning, and checking, and then took the rats out of the steel boxes into which they were put temporarily (sorted by sex), and afterwards reintroduced them to their enclosures (mostly transparent plastic tunnels, so that they could more easily manage to move around in near-zero gravity).  They then went back to their own enclosures, which were (compared to their body size) just as small as the rats'.

One of the rats, a smaller than average one with bands of red and blue dye on its tail, floated as it pushed itself along the tunnel.  Moving in the tunnels when on board the spaceship, was a little like swimming.  "Red-Blue" did not, in some sense, understand why there was no gravity, but it also was not particularly bothered about not understanding.  It was able to find a way to move around, it had plentiful food with no predator smell to alarm it, and the temperature was always pleasant.  If it could have found a mate (Red-Blue was male), it would have been in rat paradise.

Except...

Except that this time, when the humans came to feed it, there had been a slight scent of predator.  Not enough to panic it; just a hint.  Red-blue's whiskers twitched nervously.  He had smelled the humans many times by now, and knew from experience that they were not predators (on rats, anyway).  He also came from a long, long line of lab rats that had been selected for willingness to tolerate close proximity to humans; his kind found the smell of human comforting.

Not for any good reason, it should be said; humans had done a lot of awful things to his ancestors.  However, the ones who were most naturally averse to humans, were constantly culled from the gene pool, until his kind had an instinctive willingness to tolerate, or even seek out, human contact.  For lab rats, letting humans handle you might get you killed, but not letting them handle you would get you killed before you had a chance to have offspring, and if there was one thing which Mother Nature (even in the un-natural environs of a lab) did not tolerate, it was not having offspring.  So, Red-Blue and all his comrades were comfortable with the smell of humans.

This, though, this was a different smell.  Red-blue had never smelled it before, but he knew, from an even more ancient instinct that went back to ancestors who had never been in a lab, that he did not like it.  He began grinding his teeth, anxiously.  Another rat, Green-Red, came up to him, and they squeaked together.  Not language, in the human sense; there was no grammar, no syntax, no declination or conjugation to memorize.  But, as clearly as humans speaking (in fact, more clearly, since less prone to subtlety), they both spoke that they were worried.  Not panicked, not terrified, but worried.  Their lives had been turned upside down (literally) when they were placed on board the spaceship and blasted off towards Mars, but they had adapted to their new lives, and were even happy most of the time.  But now, the scent of a predator smelled like change in the air.  They did not like it.