You're making a lot of assumptions here and basing it on the limited knowledge of a specifies that has failed to make any significant strides into exploring anything other than its own solar system.

There's an easy answer that coincides with our current understanding of physics. Then there is "Aliens have discovered tech that makes it easier for them to fling a half mile long object vs a satellite sized object across the stars at a relatively slow speed."

Sorry if I take "explosion makes rock go far" as being the more plausible explanation than "Aliens want to wait a million years for a brief glimpse at a distant solar system".

It's not the size/volume that matters, but the mass. Even with our limited science and technology, we have designs for interplanetary ships that drag hundreds of meters of radiator surface with them. It's not hard to imagine - and it has been imagined many times - a ship that consists of a core with a gigawatt-range powerplant, reaction mass, efficient engines, and kilometer-long radiators to dump the powerplant heat into space. If anything, based on our knowledge and assuming no magitech breakthroughs in space propulsion, you should expect interstellar ships to be large but light, maximizing surface area while minimizing mass.

To be fair, there is no need for the probe to have been designed for a mission to our star system. It could be that it was a craft meant for something else entirely that left its star system, like our Voyager probes have, and drifted for eons before happening to pass through our star system and be observed.

Of course, that is still less likely than some unusual rock doing the same thing.

Sure, just a bunch of energy for such a large object. "To see if they can" may very well be a reason, but that doesn't seem likely for a species that advances far enough to be able to accomplish such an achievement.

> Sorry if I take "explosion makes rock go far" as being the more plausible explanation than "Aliens want to wait a million years for a brief glimpse at a distant solar system".

Playing Devil's advocate, a probe that size would likely visit many star systems, the Solar being a random one in the bunch. The brief pass would be necessary to avoid the energy expenditure of braking and reaccelerating.

"You don't know everything therefore aliens" is my favorite argument too

I don't think this was an alien probe but the size argument is not convincing to me. I would think that if you want to design a probe that can keep going for very long time spans and distances it would have to be big and work more like a factory that can constantly refresh its parts. Devices of the size of our satellites aren't and probably can't be designed to last thousands or millions of years. Something bigger on the other hand can be designed that way.

The object was actually in the rest frame of our local patch of the galaxy, so in essence, we came to it via our motion about the center of the galaxy.

Size is so relative. You're thinking in human scale. Who says all life in the universe is the same size as us?

All life in the universe faces the same kinetic energy equations to cross the stars.

And if the stars are the equivalent of our atoms to a much larger form of life, what would it's nanotech look like to us?

Probably not the same energy availability, though.

The specific reason he contends it may be an alien artifact is that he believes its characteristics match the profile of a light sail:

1) its shape may actually be more like a pancake than a cigar, a crumpled light sail would match this 2) it showed unexpected acceleration away from the Sun without outgassing. 3) its velocity is very close to the mean motion of our local stellar neighborhood, if you needed to drop a solar sail after deceleration so that it would be hard to work out what system you came from then this would be the velocity to do it at.

A size 1000m x 150m seems pretty small for something that would have taken tens of thousands of years to get to our system.

I think one theory is that it’s a huge “light sail” (hence the size). Just what I heard; no idea if this has any merit.