What are examples of spatio-temporal data sets such that subjects are observed only once and are, thus, indexed in time only once?

Here's a hypothetical example in the ecology realm: consider a tree's first year of life as the subject of interest. Each tree can only have one first year of life. Additionally, trees cannot move in space. So, for each subject, we can only have one observation, indexed in space and time.

What are some astronomy data sets that you have come across that are similar to the format of this data set?

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    $\begingroup$ Asteroids and comets come to mind. It has happened multiple times that certain asteroids were observed once (or twice), but never seen again. It happened even to a friend of mine: He positively discovered an asteroid, but only saw it one evening, so it wasn’t enough to calculate an orbit, and the next few evenings were cloudy, so he never saw it again. (Fortunately for him, he managed to discover four more whose orbits were tracked, including 74503 Madola. $\endgroup$ May 25, 2023 at 4:53
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    $\begingroup$ Are you asking for lists of short lived events? $\endgroup$
    – James K
    May 25, 2023 at 6:33
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    $\begingroup$ Any catalog of classical novae or supernovae. Or gamma-ray bursts, or gravitational-wave sources, or … $\endgroup$ May 25, 2023 at 6:39
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    $\begingroup$ OK, most gravitational-wave sources are not very well indexed in space... $\endgroup$ May 25, 2023 at 7:19
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    $\begingroup$ @PierrePaquette Thank you for the recommendation! $\endgroup$
    – Ron Snow
    May 25, 2023 at 13:19


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