The challenge mode is simply survive while killing 75 rocks/UFOs. Then it's over, and you continue.
The rocks get faster, more numerous, different spawning behaviors, and later versions of them add a more dangerous "explode towards you" behavior if you're too close when you shoot them. For end of wave, the game doesn't wait for you to clear everything before launching the next wave.
The rock+UFO looks like a UFO crash-landed askew into an asteroid, shooting it frees the embedded ship which then attacks you like other UFOs. I've a Vectrex with Minestorm and it really isn't the same.
One subtle difference that I loved... when a rock traverses the side of the screen, it vanishes completely for a second or two and "pops" back in on the other side, quickly growing to its normal size. That's neat for experienced players because you can hang out on the edges with a bit more confidence knowing you have a sec to get out of the way.
Oh, that's cool, I watched an older video of it (when it was first announced) and it seemed like the asteroids
always moved the same slow speed regardless of size or if they'd been shot or not. Sounds like they changed that? Good.
Did they vary the UFO speeds? That was another thing from the older video that was very noticeable and annoying, seeing them crawl along and not be very challenging at all.
I wasn't a fan of the power-ups because, based on that initial video, there seemed to be too many of them. I commented at the time that there should be gaps where you're on your own, you better survive long enough to see the next ones.
I don't like the idea of endless power-ups, that's new school bullshit that makes these marginal newer gamers even weaker players. This is a common issue with these newer "gamers" who can't handle real game challenges or steep difficulty curves (they would have hated being in an '80s arcade, hahaahahaa). On another forum (AtariAge) there was some younger player who was talking up how certain home versions of Centipede were better than the arcade, and how the new Recharged version was the best ever. After a few back-and-forths it became obvious that he had never actually played an arcade cabinet, never even
seen one. Which means he'd never played the damn game with a trak-ball. What?!? No wonder he thought the Recharged version was better (he hadn't even played the 5200 version with the trak-ball). I told him that until he found a way to play the original version correctly (MAME or cabinet, but with the right control scheme) that he was talking out of his ass about how much better the 2600 version of Centipede was, to say nothing of the plodding Recharged version. He seriously didn't see the problem even though a few us were trying to patiently explain to him how important the controls are for certain games (Defender, Star Trek: SOS, Tron, Missile Command, Tempest), how not everything is a goddamn joystick or joypad. Not sure it ever truly sank in, but he understood how far he was from the real experience.
What I'm saying is that I hope this Alan-1 version gets enough tweaks so that it gets close
enough to original Asteroids in terms of the asteroids and the UFOs (closer than that early video), I know they were saying that they welcomed feedback and would make adjustments for the arcade version. My view is that if there are power-ups, especially lots of them and with different abilities, then there should be more/better/harder UFOs as the game goes on, have higher hurdles to have to get over. I would expect at some point that a version of one of those linked Space Duel UFO pairs should show up. I also suggested that there could be a menu option when you first start the game, you want this regular new version or a more classic (harder, more varied, less power-ups) experience? Sounds like at least they got the message about not having all the asteroids move at the same speed.
I can't remember, do the big asteroids split into 2 smaller asteroids when shot? It seemed from the initial video that they didn't, they just got smaller (but not faster). From what I recall that bothered me, too.