I've never created a transformation/import script that worked right the first time - which makes me happy! These scripts reflect your assumptions and understanding of your data and if you've goofed up, you'll know!

Here's the final, fixed script:

drop schema if exists enceladus cascade;
create schema enceladus;
set search_path='enceladus';

create table inms(
  id serial primary key,
  created_at timestamp not null,
  date date not null generated always as (created_at::date) stored,
  year int not null generated always as (date_part('year', created_at)) stored,
  flyby_id int references flybys(id),
  altitude numeric(9,2) not null check(altitude > 0),
  source text not null check(source in('osi','csn','osnb','esm')),
  mass numeric(6,3) not null check(mass >=0.125 and mass < 256),
  high_sensitivity_count int not null check(high_sensitivity_count >= 0),
  low_sensitivity_count int not null check(low_sensitivity_count >= 0),
  imported_at timestamptz not null default now()
);

update inms
set flyby_id = flybys.id
from flybys 
where flybys.date = inms.date;

create table chemistry(
  id serial primary key,
  name text not null,
  formula text not null,
  molecular_weight int not null,
  peak int not null
);

copy chemistry(name, formula,molecular_weight, peak) 
from '[Absolute path to]/csvs/chem_data.csv'
with delimiter ',' header csv;