This was originally going to be 'Space Force 2017,' but issues with the timeline and the slow pace of my writing (and the difficulty of finding players), I really doubt that I'll be running this before the end of the year, so I decided to just set it in alternate 2010.
Space Force 2010
In 1989, aliens resembling creatures from myth and fantasy came to Earth in spacecraft that they called 'spelljammers.' Many stayed on Earth to teach, and to enjoy the relative peace and safety of this world. In the twenty-eight years since, humanity has learned much. Now, we send out our own spelljammers to the stars. Their mission: to explore strange new worlds, to seek out new life, and new civilizations, to boldly go where no-one has gone, before!
(In our history, TSR brought out an AD&D supplement called Spelljammer, about fantasy spacecraft, in 1989. In the timeline of the setting, actual spelljammers arrived on Earth, instead.)
3.5e Spelljammer on DandD Wiki
5e Spelljammer on DandD Wiki
I'm currently imagining this as a PL6 campaigne with few or no powers beyond some Devices and racial or 'class' abilities, though PL7 or 8 with limited powers is an option. Spellcasting will generally involve the Ritualist and Benefit (Readied Rituals) advantages.
The default idea is that the PCs are the senior officers (and perhaps one or two senior NCOs) of a Space Force/NASA armed explorer (blatantly Star Trek style, as implied above), but if the players prefer, junior officers (NCOs, enlisted) are an option, and civilians in this setting might also be explorers, possibly space merchants looking for new markets, or just curious folk out to go beyond Known Space. Likewise, they could be military personnel for some other space service, spelljamming crews from other worlds, colonists looking for/building a new home, and so forth.
The Tech Level and Equipment Availability chart in the Cosmic Handbook (p32) is one of those areas in which I feel that M&M goes too far in the direction of 'less granular,' so as an option, I suggest using the Progress Levels system from d20 Future (abbreviated 'PrL,' to avoid confusion with Power Levels), or the Technology Level system from GURPS (abbreviated TL), unless you and your players agree that the difference will not be that important in your game. Modern Earth in-setting is d20 PrL5 or GURPS TL8, with various technomagic items that in many cases imitate tech from higher PrLs. Most of the cultures in Known Space are PrL2 or TL3, roughly equivalent to the Middle Ages, which gives Earth a fairly significant advantage over them. (A few worlds are more advanced, like Eberron, which is roughly in the area of PrL4 or TL5, while still being pretty much Tech Level 0 - or 'between 0 and 1' - on the M&M scale.) On the other hand, living on monster-filled death worlds as they do has lead many alien races (and a number of individual aliens) to develop Powers of their own, rather than being dependent on equipment, rituals, and Devices.