From the http://www.imperial-library.info/content/pocket-guide-empire-third-edition-magic-aetherius, we know that travel to Aetherius was possible in the Reman dynasty. Doing so presumably requires travelling to the stars via Oblivion; the moons and god-planets too can be reached through Oblivion, as shown in http://www.imperial-library.info/content/cosmology. In "http://www.gamesas.com/topic/1327177-tatterdemalion-the-lunar-province-of-secunda/", we are given an explanation as for how Reman Cyrodiil I was able to colonize the moon, which required travel through "the most dangerous of outer realms":
...iconoclasts feared daedric vexation for unritualized trepass into the Void, and perhaps rightly so. However, this was all quickly dissolved when the Sixteen-Plus Princes of Tumult lent their nymic oaths in their first display of coalition since the Fall of Lyg in the previous kalpa. Conjecture points to some machinations of Nocturnal, who took on her mantle of Ur-Dra of Oblivion, and it was by her primogeniture that Reman was able to pursue his cosmic acquisitions without further censure.
Things are substantially different in the Second Era: with the potentate (or one of a series of warlords from across Tamriel) at the head of the "empire", and with Cyrodiil's wealth and influence vastly diminished, there was no possibility of exploiting the void travel institutions put in place since Reman. More importantly, the Daedric prince Molag Bal http://www.facebook.com/notes/the-elder-scrolls-online/introduction-to-the-lore-of-the-elder-scrolls-online-en/486381251371913 through the "planemeld", or merging of Mundus with his own Daedric plane. It is of particular interest that this particular Daedra would be the cause of perhaps the greatest supernatural threat of the Second Era, when Tiber Septim himself is descibred as having a pact with Bal in his http://imperial-library.info/content/tiber-septim%E2%80%99s-sword-meeting-cyrus-restless:
What do you want? Doesn’t work that way. I always want. I’ll give you ten years under my name, but not this skin. Deal. That skin is looking pretty haggard, anyway. Now get up and fight, dummy, we’re all counting on you. Thank you, Bal, I won’t forget this.
An examination of space colonization in the Second Era should not pass over the Sunbirds of Alinor, which are mentioned with the Reman expeditions in the Pocket Guide. However, this claims that the Altmer were victim to the same issues as Man: untenable expenses. Again, in the Sword-Meeting, the Sunbirds are shown to have a presence in the void. It is therefore difficult to place the Aldmeri spacefaring fleet on a timeline: the http://www.imperial-library.info/content/pocket-guide-empire-third-edition-pyandonea shows that most of the Alinoru navy was destroyed by Maormer forces in 2E 486.
However, the Altmer seem to have been able to recover from this in around 90 years, establishing themselves as the first Aldmeri Dominion over Valenwood and Elsweyr as well as Summerset. I don't know whether the destruction of Alinor's navy would include the Sunbirds - the report from the surviving ship in 486 is apparently the first report of Pyandonea, suggesting that the Sunbirds had never been employed in combat against the Maormer. This is likely the most important event in the known history of Second Era Altmer exploration. So, my second point to speculate on is: Were the sunbirds of Alinor active during the middle of the Second Era, and would they have been affected by the magical catastrophes of the Soulburst in 2E 579 and Planemeld four years later?
There's potentially a third group of mortals reaching through Oblivion in the Second Era. Namely, the Khajiit. The http://www.imperial-library.info/content/pocket-guide-empire-first-edition-elsweyr-confederacy http://www.imperial-library.info/content/pocket-guide-empire-third-edition-sugar-and-blood-cats-south describe how in 2E 309 the conflicting states of Anequina and Pellitine united briefly as Elsweyr, and soon after the Mane put in place a stable system of government enabling the sharing of power, whilst promising a paradise called "Llesw'er" to the Khajiit people. The interesting part arises from a fragment of the supposed http://www.gamesas.com/topic/1327271-return-false/page__view__findpost__p__19959461 posted by Temple Zero. According to this, Llesw'er was indeed established on the moons, and word sent to the Imperial Province around 2E 310:
Khajiit saw reason in these words [of the Mane] and so it climbed and climbed, cat upon cat, for a hundred days. Much sugar was brought there to support the climbers and in the end Khajiit climbed high, so very high that it was in fact closer to Jo'Segunda than to Nirni below. At that moment, little Alfiq fell upwards and from there on Khajiit helped Khajiit up, which was down, until all were gathered there. This is where Khajiit intends to stay from now on, for who could know strife when walking sugar and not sand?
Shortly before the crucial period of the Interregnum in which the Planemeld occurs, the Knahaten Flu infects eastern Elsweyr, particularly Senchal. According to the First Aldmeri Dominion, the Altmer saved many lives at this time, and claims that it is this debt that causes Elsweyr to join the Dominion. Of course, the Imperial Geographical Society takes the exact opposite view of this event, talking of "whole neighborhoods [...] razed" and "charred skylines". In any case, it is not clear how much damage was done to the nation by the Flu. Since this is arguably the most important event in Second Era Khajiit history, I shall frame the third part of my speculation in a similar manner to the previous two sections: If such a place as Llesw'er were to exist, could its formation have been aided by the collapse of the Reman empire and their lunar colonies? Could it have endured throughout the Second Era? Would it have been affected by the Soulburst and Planemeld?
And a final notion: the actual "physical" state of the moons differs in reports of it. Seen from Mundus, it is decaying divine flesh; to the Khajiit, it is sugar; to Tiber Septim, it is something altogether less pleasant. Since they too are divine plane(t)s, does the composition and appearance of Masser and Secunda vary with its inhabitants and their means of getting there?
So, Daedric pacts to ease space travel, catacysmic events resulting from the merging of two planes, endangered sunbirds, defunct mananauts and moon-cats. Any ideas?