Gelindril is not the only settled planet that supports life in the solar system. The closest neighbor to Gelindril is the planet Mersallia, which has an orbit similar in size and duration to Gelindril. The planet orbits the sun in an approximate 82 degree offset from the orbit of Gelindril. Twice each year the two planets come close enough that the gravitational fields of either planet disrupt the rotational cycle of both planets. Instead of a 24 hour day and night cycle on Gelindril and a 32 hour day and night cycle on Mersdallia, both are increased 3 times the normal duration. This essentially creates three days of daylight and three days of night on Gelindril. Each year the time in which this occurs drifts later by one month. The entire journey takes 21 days to get to Gelindril and about 52 to return.

The Mersallians possess much more advanced technology than Gelindril and are able to venture into space and visit Gelindril. These unusual beings are humanoid in appearance, but have slightly yellowed skin with iridescent quality, slightly pointed ears and bright blue, almond shaped glowing eyes, they otherwise have the same facial features of humans. They are slightly taller than humans and are a bit more slender. They wear a uniform consisting of a body suit that resembles a racecar driver, which is silver metallic or metallic red for officers, with accent stripes down the edges of the attire. They wear large armored helmets to protect against impact and also to allow them to breathe while in space. The Mersallians do not possess Mysidian on their planet, but it serves as a primary component in the manufacturing of medicines and for the making of a powerful narcotic. The products they make are ineffective while on Gelindril, and only gain their processed properties once they return home.

Mersallian visitors are generally referred to as "Mersallian spacemen" but Mersallia refers to its brave interplanetary travelers as "Terranaughts". Mersallia is only capable of visiting Gelindril because it is the only planet, which comes close enough to safely travel to. They use an enormous cannon on the surface of their planet to literally fire a projectile pod to the surface of Gelinril during the first day of the "tri-day" as the three days of light are referred to as. The gravitational pull of the planets causes less resistance and inertia, and the atmosphere of Mersallia is thin enough that the projectile can escape the planet and soar to the surface of Gelindril. The pod itself is a solid metal conveyance which is a perfect sphere and looks like a giant cannon ball. They have designated a specific drop zone or "impact zone" on the surface of Gelindril, which is easily noted because of the impact craters and lack of settlements near it. They use a giant telescope and advanced mathematics to plot the trajectory and have always been within 100 miles of their target zone.

Mersallia had always been on good terms with the Acidian people long before the great descent but mankind never knew of their existence. After the great descent occurred, Mersallia stopped coming to Gelindril and have only starting returning to their neighboring planet in the last 60 years. All they know of the great descent is what has been told to them by the Immortals and Humans.

The Mersallians do not share their technology because they fear it would unduly influence the advancement and development of Gelindril society, but they bring a wide array of interesting and useful items to trade for Mysidian, and have never shown any signs of hostility in their efforts to obtain the element. How the Mersallians arrive on Gelindril has become common knowledge, but how they return home remains a mystery as they do not have a cannon on Gelindril to return their pod home. Some time between their arrival during the tri-day, and their departure on the tri-night the Mersallians locate an Immortal to come to their aid. The Immortal uses his power to propel the Mersallian vessel into space where the Mersallian's giant electromagnet on their home planet can pull the pod back home. What the Immortals gain from this arrangement is unknown but under much speculation.

Despite their lack of animosity toward Gelindrians, they do come with protection because of the chance of conflict from some individuals and the savage creatures that roam the wilds around the impact zone. Mersallians use small hand held guns or rifles which fire radiation or microwave energy, which can deal enormous amounts of damage. They however do not have the equipment to recharge their weapons since their recharging units are powered by their primary fuel which is a very volatile liquid that cannot survive the exit velocity from their planet into space, and the few times it did, it always detonated on impact once the pod struck Gelindril, so the Mersallians only send an assortment of pre-charged weapons and equipment.

The first arrivals of the Mersallians were haphazard efforts to explore a new planet, and they never expected that they would ever find a way home. Even today, the Mersallians send an average of 25-50% more crew than what is needed for the mission because impact can injure and even kill some inside the pod when they hit Gelindril. The average crew size is about 12 individuals with an extra 3-6 people for a total of 15-18 crew members who hit down. On average, only about 9 walk away unwounded, and at least one person ends up dying from the impact most of the time.