diff --git a/src/layup/data/ObsCodes.json.gz b/src/layup/data/ObsCodes.json.gz
new file mode 100644
index 00000000..88ba7098
Binary files /dev/null and b/src/layup/data/ObsCodes.json.gz differ
diff --git a/src/layup/utilities/data_processing_utilities.py b/src/layup/utilities/data_processing_utilities.py
index f55128bd..0014a308 100644
--- a/src/layup/utilities/data_processing_utilities.py
+++ b/src/layup/utilities/data_processing_utilities.py
@@ -1,7 +1,12 @@
+import gzip
 import logging
+import os
+import tempfile
 from concurrent.futures import ProcessPoolExecutor
+from importlib.resources import files
 
 import numpy as np
+import requests
 from sorcha.ephemeris.simulation_geometry import barycentricObservatoryRates
 from sorcha.ephemeris.simulation_parsing import Observatory as SorchaObservatory
 from sorcha.ephemeris.simulation_setup import furnish_spiceypy
@@ -17,6 +22,25 @@
 logger = logging.getLogger(__name__)
 
 
+def write_fallback_obscodes():
+    """Decompress the observatory-codes file bundled with layup to a plain JSON
+    file and return its path.
+
+    The MPC observatory-codes file is normally downloaded from
+    minorplanetcenter.net on first use.  When that download fails (e.g. the MPC
+    server is unreachable, as has happened on CI runners), we fall back to the
+    copy shipped in ``layup/data/ObsCodes.json.gz`` instead of failing the run.
+    Observatory codes change rarely, so a slightly stale fallback is far better
+    than a hard failure.  Returns a path suitable for ``Observatory``'s
+    ``oc_file`` argument (which reads the decompressed JSON directly).
+    """
+    compressed = files("layup.data").joinpath("ObsCodes.json.gz").read_bytes()
+    dest = os.path.join(tempfile.gettempdir(), "layup_obscodes_fallback.json")
+    with open(dest, "wb") as f:
+        f.write(gzip.decompress(compressed))
+    return dest
+
+
 def process_data(data, n_workers, func, **kwargs):
     """
     Process a structured numpy array in parallel for a given function and keyword arguments
@@ -315,7 +339,22 @@ def __init__(self, cache_dir=None):
         # Furnish the spiceypy kernels
         layup_furnish_spiceypy(cache_dir)
 
-        super().__init__(FakeSorchaArgs(cache_dir), config.auxiliary)
+        try:
+            super().__init__(FakeSorchaArgs(cache_dir), config.auxiliary)
+        except requests.exceptions.RequestException as exc:
+            # The observatory-codes download from the MPC failed (server
+            # unreachable, timeout, etc.).  Fall back to the copy bundled with
+            # layup so we don't fail the run over a transient network outage.
+            logger.warning(
+                "Could not fetch observatory codes from the MPC (%s); "
+                "falling back to the copy bundled with layup.",
+                exc,
+            )
+            super().__init__(
+                FakeSorchaArgs(cache_dir),
+                config.auxiliary,
+                oc_file=write_fallback_obscodes(),
+            )
 
         # A cache of barycentric positions for observatories of the form {obscode: {et: (x, y, z)}}
         self.cached_obs = {}
diff --git a/tests/layup/test_data_processing_utilities.py b/tests/layup/test_data_processing_utilities.py
index 00078e32..7a363610 100644
--- a/tests/layup/test_data_processing_utilities.py
+++ b/tests/layup/test_data_processing_utilities.py
@@ -11,6 +11,7 @@
     has_cov_columns,
     parse_fit_result,
     process_data,
+    write_fallback_obscodes,
 )
 from layup.utilities.data_utilities_for_tests import get_test_filepath
 from layup.utilities.file_io.CSVReader import CSVDataReader
@@ -560,3 +561,43 @@ def test_parse_fit_result_missing_some_cov_columns():
         assert len(fit_result.cov) == 36
         assert np.all(np.array(fit_result.cov[:-1]) != 0.0)
         assert fit_result.cov[-1] == 0.0
+
+
+def test_write_fallback_obscodes():
+    """The obscodes copy bundled with layup decompresses to valid JSON that
+    sorcha's Observatory can read directly (network-outage fallback)."""
+    import json
+
+    path = write_fallback_obscodes()
+    with open(path) as f:
+        obs = json.load(f)
+    assert len(obs) > 2000
+    assert "X05" in obs and "500" in obs  # Rubin + geocenter
+
+
+def test_layup_observatory_falls_back_on_mpc_failure(monkeypatch):
+    """If the MPC obscodes download fails (e.g. server unreachable), the
+    LayupObservatory should fall back to the bundled copy instead of raising."""
+    import requests
+
+    import layup.utilities.data_processing_utilities as dpu
+
+    orig_init = dpu.SorchaObservatory.__init__
+    calls = []
+
+    def fake_init(self, args, auxconfigs, oc_file=None):
+        calls.append(oc_file)
+        if oc_file is None:
+            # Simulate the MPC download failing.
+            raise requests.exceptions.ConnectTimeout("simulated MPC outage")
+        # The fallback path supplies a local oc_file; let the real init read it.
+        orig_init(self, args, auxconfigs, oc_file=oc_file)
+
+    monkeypatch.setattr(dpu.SorchaObservatory, "__init__", fake_init)
+
+    observatory = LayupObservatory()
+
+    # First attempt (download) raised; the second used the bundled fallback.
+    assert calls[0] is None and calls[1] is not None
+    assert "X05" in observatory.ObservatoryXYZ
+    assert len(observatory.ObservatoryXYZ) > 2000