You can take "objective chances" to just be "sufficiently well-grounded subjective chances". E.g., assigning a 50% chance to a coin flip is very different (and more robustly justified) than assigning some arbitrary 1/Y chance to the wild claims of Pascal's mugger.

Real-life cases that are parallel in structure to the asteroid case come up all the time when thinking about "collective harm"-type cases. Voting in a close election is another example: in a population of X voters, the chance of placing a decisive vote is typically on the order of 1/X, and affects at least X people. So whenever the total benefits of the better candidate winning are sufficient to justify the time costs of up to X voters, it will generally be worth it for any individual to vote (for the better candidate), no matter how high X is, and hence no matter how small a chance (1/X) their vote has of "making a difference".