digest2

NEO orbit classification from short-arc astrometric tracklets. Available as both a C command-line tool and a pip-installable Python package.

digest2 is a fast short-arc orbit classifier for minor planets, primarily used to identify Near-Earth Object (NEO) candidates from astrometric tracklets (groups of 2+ observations of the same object over a short time). It has been in operational use at the Minor Planet Center for over 15 years and is a key component of the NEO discovery workflow.

Given a set of observations, digest2 outputs a score (0-100) for each of 14 orbit classes, representing the pseudo-probability that the object belongs to that class. Objects scoring D2 >= 65 for the NEO class are posted to the NEO Confirmation Page (NEOCP) for follow-up.

References:

• Keys et al. 2019, "The digest2 NEO Classification Code" (PASP 131, 064501) -- arXiv:1904.09188
• Shober, Cloete, Veres 2023, "Improvement of digest2 NEO Classification Code" -- arXiv:2309.16407

Authors: Sonia Keys (original), with contributions from Carl Hergenrother, Robert McNaught, David Asher. ADES support added by Richard Cloete and Peter Veres.

License: Public domain.

---

Python Package (recommended)

pip install digest2

from digest2 import Digest2

with Digest2() as d2:
    results = d2.classify_file("observations.obs")
    for r in results:
        print(r.designation, r.noid.NEO, r.noid.MB1)

The digest2 Python package wraps a C scoring engine. Pre-built wheels are available on PyPI for common platforms (Linux, macOS, Windows), so most users get a ready-to-use binary with pip install digest2. If no wheel matches your platform or Python version, pip will automatically compile the C code from the source distribution - this requires a local C compiler (e.g., gcc or clang) but no external C libraries such as libxml2.

Python Tutorials

There are demonstration notebooks in digest2/notebooks/ that show how to use the Python API to classify tracklets, parse input files, and apply NEOCP filters. These are a great starting point for new users.

Python API

• Digest2(model_path=None, config_path=None, obscodes_path=None, repeatable=True, no_threshold=False) -- Stateful classifier; auto-discovers bundled model data. Set no_threshold=True to disable per-observation RMS ceiling clamping.
• d2.classify_tracklet(observations) -- Classify a list of Observation objects. Returns ClassificationResult.
• d2.classify_file(filepath) -- Classify all tracklets in an MPC 80-col or ADES XML file. Returns List[ClassificationResult].
• classify(input, ...) -- One-shot convenience function.
• parse_mpc80(line) / parse_mpc80_file(path) -- Parse MPC 80-column observations.
• parse_ades_xml(path) -- Parse ADES XML observations.
• digest2.filters -- NEOCP threshold filtering tools (requires pip install digest2[filters]).

All classification methods accept collect_orbits=True to return individual trial orbit elements alongside scores (see Trial Orbit Collection below).

Satellite and roving observers are supported in all input formats, matching the C CLI: ADES sys/pos1/pos2/pos3 fields (ICRF_KM positions are converted from km to AU; ICRF_AU used as-is; WGS84 roving positions converted to a geocentric vector) and MPC 80-column second lines (note2 s/v) are parsed into Observation.spacebased and Observation.earth_obs (geocentric equatorial vector in AU) and passed through to the scoring engine. Observations without position data fall back to the observatory's parallax constants; space telescopes with blank parallax entries in the obscodes file are scored from the geocenter.

Development Setup

cd digest2
pip install -e '.[test]'

# Download observatory codes (one-time)
curl -o digest2/digest2.obscodes https://minorplanetcenter.net/iau/lists/ObsCodes.html

# Run tests
pytest tests/ -v

Releasing to PyPI

Three GitHub Actions workflows handle CI and publishing for the Python package:

1. Tests (digest2-python-test.yml) -- Runs pytest on every pull request and on pushes to main that modify digest2/ files.

2. TestPyPI (digest2-python-testpypi.yml) -- On every push to a PR branch that modifies digest2/, wheels and an sdist are built for Linux, macOS, and Windows, version-suffixed with .devN, and published to TestPyPI. Reviewers can install a pre-release build with:

   pip install -i https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple/ digest2
   

3. PyPI release (digest2-python-release.yml) -- Triggered when a GitHub Release is published with a tag matching digest2-v*. Builds production wheels and sdist, then publishes to PyPI.

To release a new version:

1. Update the version in pyproject.toml (e.g., version = "2.2.0").
2. Open a PR with the version bump (and any other changes) and merge it into main.
3. Go to the repository's Releases page and create a new release:
   • Tag: digest2-v2.2.0 (must start with digest2-v)
   • Target: main
   • Title/notes: Describe what changed in this release.
4. Publishing the release automatically triggers the workflow, which builds wheels for all platforms and uploads them to PyPI.

Python Package Structure

src/digest2/
├── __init__.py           # Package init, exports Digest2, classify, Scores, ClassificationResult, TrialOrbit
├── _extension.c          # CPython C extension (score + score_orbits bindings to d2lib)
├── core.py               # High-level API: Digest2 class, classify() function, collect_orbits support
├── result.py             # ClassificationResult, Scores, TrialOrbit dataclasses
├── observation.py        # Observation dataclass, MPC80/ADES parsers
├── model.py              # Model/obscodes/config path resolution
├── filters.py            # NEOCP filter tools (from NEOCP_filters/)
└── data/
    └── MPC.config        # Bundled per-site observatory errors

C Library API (d2lib)

The files d2lib.h / d2lib.c provide a clean, non-threaded library interface used by the Python extension:

• d2_init(model_csv, obscodes) -- Load model and observatory data
• d2_score_observations(obs, n, classes, n_classes, is_ades) -- Score a tracklet
• d2_score_observations_ext(obs, n, classes, n_classes, is_ades) -- Score a tracklet and collect trial orbit elements (returns d2_result_ext)
• d2_free_result_ext(result) -- Free memory from extended result
• d2_cleanup() -- Release resources
• d2_configure() -- Set observatory errors, repeatable mode, noThreshold flag
• No libxml2 or pthreads dependency (XML parsing done in Python)

Key Design Decisions

• Existing CLI is unchanged. make in digest2/digest2/ still builds the same binary.
• Preprocessor guards (D2_NO_LIBXML, D2_NO_REGEX) allow building without libxml2/regex.
• Global state is managed by d2_init()/d2_cleanup() -- single-threaded library mode.
• Band correction matches C code exactly (common.c updateMagnitude).
• MJD calculation uses C-style truncation-toward-zero integer division.

Trial Orbit Collection

The collect_orbits=True parameter exposes the individual trial orbits generated by the statistical ranging engine. Normally these orbits are immediately discarded after binning; this feature captures them for analysis.

Memory warning: collect_orbits=True can use substantial RAM because it stores every collected trial orbit. In typical runs this can be hundreds of MB per tracklet, and for difficult/large solution spaces can approach ~1 GB. We recommend to keep collect_orbits=False (default) for production scoring unless you explicitly need orbit-level diagnostics.

Python usage:

from digest2 import Digest2

with Digest2() as d2:
    results = d2.classify_file("observations.obs", collect_orbits=True)
    r = results[0]
    print(f"Orbits collected: {len(r.trial_orbits)}")
    elems = r.orbit_elements  # dict of numpy arrays
    print(f"q range: {elems['q'].min():.2f} - {elems['q'].max():.2f}")

Data returned per trial orbit (TrialOrbit dataclass):

Field	Type	Description
q	float	Perihelion distance (AU)
e	float	Eccentricity
i	float	Inclination (degrees)
H	float	Absolute magnitude
d	float	Geocentric distance used to generate the orbit (AU)
an	float	Angle parameter in orbit solution (radians)
iq	int	q bin index (0..28)
ie	int	e bin index (0..7)
ii	int	i bin index (0..10)
ih	int	H bin index (0..17)
new_tag	bool	True if this orbit tagged a previously unvisited bin

ClassificationResult.orbit_elements property returns a dict of numpy arrays keyed by field name, for convenient vectorized analysis.

Design notes:

• Opt-in via collect_orbits=False default -- zero performance/memory impact when not used (NULL pointer check in C)
• All valid orbits collected (those passing stability tests and within model bounds), not just those tagging new bins
• Scores are deterministic -- score() and score_orbits() produce identical scores for the same input
• Score reconstruction -- digest2 scores can be exactly reconstructed from trial orbits + population model data, since scores are based on which (q,e,i,H) bins are reachable, weighted by model populations. See the trial orbits notebook for a worked example.

C implementation:

• d2_trial_orbit struct (11 fields, ~72 bytes/orbit) defined in d2lib.h
• d2_orbit_buffer (dynamic array with geometric growth, initial capacity 1024) attached to tracklet.orbit_buf
• Orbit capture happens in d2math.c:tagAngle() after qeiToBin() succeeds -- guarded by if (tk->orbit_buf) so the CLI path is unaffected
• d2_score_observations_ext() in d2lib.c allocates the buffer, calls score(tk), and transfers ownership to the result
• _extension.c:py_score_orbits() converts to Python tuples and is registered as score_orbits in the module

Files involved:

File	Role
digest2/d2lib.h	d2_trial_orbit, d2_orbit_buffer, d2_result_ext types
digest2/digest2.h	Forward-declares d2_orbit_buffer; orbit_buf pointer in tracklet struct
digest2/d2math.c	~20 lines in tagAngle() to capture orbits when buffer is active
digest2/d2lib.c	d2_score_observations_ext() and d2_free_result_ext()
src/digest2/_extension.c	py_score_orbits() C-to-Python bridge
src/digest2/result.py	TrialOrbit dataclass; trial_orbits field and orbit_elements property on ClassificationResult
src/digest2/core.py	collect_orbits parameter threaded through all API methods
src/digest2/__init__.py	Exports TrialOrbit

Tutorial notebook: notebooks/mpc_tutorial_digest2_trial_orbits.ipynb demonstrates orbit collection, element distributions, correlations, bin occupancy analysis, and exact score reconstruction from trial orbits.

---

C Command-Line Tool

Building from Source

Requirements:

• C99-capable C compiler (e.g., gcc or clang)
• make
• libxml2 development headers (libxml2-dev or libxml2-devel)
• Optional: internet connection to download latest observatory codes from the MPC website.

cd digest2/digest2
make                          # Produces the `digest2` executable

Ensure the runtime data files are alongside the executable:

• digest2.model (or digest2.model.csv), and MPC.config (found under digest2/population/; updated versions can be copied in when available).

Platform notes:

• The provided Makefile is minimal and may require small tweaks for non-Linux platforms (see digest2/digest2/BUILDING.md for details).
• libxml2 is required for ADES XML parsing. Install the development package for your platform:
  • Debian/Ubuntu: sudo apt-get install libxml2 libxml2-dev
  • RHEL/CentOS/Fedora: sudo dnf install libxml2 libxml2-devel
  • macOS: brew install libxml2 (ensure headers are on your include path via pkg-config)

Usage

# MPC 80-column format
./digest2 sample.obs

# ADES XML format
./digest2 sample.xml

# With config file (per-observatory errors)
./digest2 -c MPC.config sample.obs

# From stdin
cat sample.obs | ./digest2 -

# Generate binary model from CSV (faster subsequent loads)
./digest2 -m digest2.model

Run ./digest2 sample.obs to verify the build. Expected output (small differences may occur):

Desig.    RMS Int NEO N22 N18 Other Possibilities
K16S99K  0.73   0   2   1   0 (MC 2) (MB1 93) (MB2 3) (JFC <1)

Notes:

• If you have an internet connection, digest2 will download the latest observatory parallax data from the Minor Planet Center. Otherwise, download ObsCodes.html and place it in the current directory as digest2.obscodes.
• To execute from a different directory, use the -p option: ./digest2 some.obs -p '/path/to/data'.
• Run ./digest2 --help for full command-line help.
• More detailed options can be found in digest2/OPERATION.md.

Input Formats

1. MPC 80-column (.obs): Fixed-width format with packed designation, date, RA/Dec, magnitude, observatory code
2. ADES XML (.xml): Rich format with per-observation uncertainties (rmsRA, rmsDec), roving/satellite observer support

Satellite/roving observations in the standard MPC 80-column form — an observation line followed by a separate 's'/'v' second line carrying the observer position — are fully handled by parse_mpc80_file(), which pairs sequential lines. Purely as a convenience, the Python parse_mpc80() also accepts the two lines concatenated into a single 160-column record (the form the MPC's database happens to store in its obs80 column), applying the second segment's observer position automatically. Users are not required to use the concatenated form — sequential two-line input is equally supported.

Config File Keywords

Keyword	Description
headings/noheadings	Toggle column headers in output
rms	Output great-circle RMS of tracklet
rmsPrime	Output RMS from ADES-provided uncertainties
raw	Output raw population scores
noid	Output no-ID scores (default)
repeatable/random	Deterministic vs stochastic Monte Carlo seeding
obserr	Default observational error (arcsec), default 1.0
obserrXXX	Per-observatory error (e.g., obserrF51=0.3)
poss	Show "Other Possibilities" column
noThreshold	Accept ADES uncertainties without floor/ceiling clamping

---

Algorithm Overview

1. Motion vector & photometry: Computes a motion vector from the first and last observation, and a composite V magnitude from available photometry (default 21 if none).

2. Statistical ranging: Generates many trial orbits consistent with the observed motion, each with an absolute magnitude (H) consistent with the apparent brightness.

3. Histogram binning: Each trial orbit is located in a 4D binned model of the Solar System (dimensions: q, e, i, H). Bins are tagged as "reachable" for each orbit class.

4. Iterative search: Orbits are generated to explore the full solution space, terminating when diminishing returns in discovering new bins.

5. Population scoring (raw): The population of tagged bins for a given class, divided by the total population of all tagged bins, gives the raw score (x100).

6. No-ID scoring (noid): Same as raw, but uses the estimated undiscovered population (total minus known objects with sky uncertainty < 1 arcmin). This is the default score and represents the probability that an unidentified tracklet belongs to a class.

Model Dimensions

The population histogram is binned in 4 dimensions:

• q (perihelion): 29 bins
• e (eccentricity): 8 bins
• i (inclination): 11 bins
• H (absolute magnitude): 18 bins

Total: 29 x 8 x 11 x 18 = 45,936 bins per class, times 15 classes.

Orbit Classes (14 + "MPC Interest")

Abbr	Description
Int	MPC Interest (q<1.3 OR e>0.5 OR i>=40 OR Q>10)
NEO	Near-Earth Object (q < 1.3 AU)
N22	NEO with H <= 22
N18	NEO with H <= 18
MC	Mars Crosser
Hun	Hungaria group
Pho	Phocaea group
MB1	Inner Main Belt
Pal	Pallas group
Han	Hansa group
MB2	Middle Main Belt
MB3	Outer Main Belt
Hil	Hilda group
JTr	Jupiter Trojan
JFC	Jupiter Family Comet

---

NEOCP Filters

Post-processing tools that apply threshold-based filtering to digest2 output to separate likely NEOs from non-NEOs, using the methods documented in Veres et al. (2025).

Requirements: Python 3.6+, pandas (pip install pandas or pip install digest2[filters])

Tools:

• find_filter.py -- Trains optimal per-class thresholds from labeled NEOCP data. Reads a digest2 CSV and produces optimal_thresholds.json.
• neocp_filter.py -- Applies thresholds to new digest2 output to flag likely non-NEOs for removal from NEOCP.

Sample data:

• digest_data_19-24.csv -- Training data (2019-2023 NEOCP)
• digest_data_24.csv -- Test data (2024 NEOCP)
• optimal_thresholds.json -- Pre-computed thresholds

End-to-End Example

1. Create a config file digest2.conf in the directory where the digest2 binary lives:

   repeatable
   norms
   raw
   noid
   Int
   NEO
   MC
   Hun
   Pho
   MB1
   Pal
   Han
   MB2
   MB3
   Hil
   JTr
   JFC
   

2. Run digest2 on an ADES XML file and save the output:

   ./digest2 sample.xml -c digest2.conf > sample.digest2
   

3. Convert the output to the CSV format expected by the filter tools:

   with open("sample.digest2", "r") as infile:
       lines = infile.readlines()[2:]  # skip header lines
   
   output_lines = []
   for line in lines:
       fields = line.strip().split()
       fields.append("0")  # Add class column (placeholder)
       output_lines.append(",".join(fields))
   
   with open("sample.digest2.csv", "w") as outfile:
       outfile.write("trksub,Int1,Int2,Neo1,Neo2,MC1,MC2,Hun1,Hun2,Pho1,Pho2,"
                     "MB1_1,MB1_2,Pal1,Pal2,Han1,Han2,MB2_1,MB2_2,MB3_1,MB3_2,"
                     "Hil1,Hil2,JTr1,JTr2,JFC1,JFC2,class\n")
       for line in output_lines:
           outfile.write(line + "\n")
   

4. Apply the filter:

   python NEOCP_filters/neocp_filter.py sample.digest2.csv NEOCP_filters/optimal_thresholds.json
   

   If the input contains non-NEO tracklets, neocp_filter.py will output those flagged as likely non-NEOs. NEO tracklets produce no output.

---

Directory Structure

digest2/
├── digest2/                 # C source code and build system
│   ├── Makefile             # Build: `make` produces `digest2` executable
│   ├── digest2.c            # Main program: CLI, threading, I/O orchestration
│   ├── d2math.c             # Core algorithm: orbit generation, scoring, statistical ranging
│   ├── d2cli.c              # Command-line parsing and config file reading
│   ├── d2model.c            # Population model: orbit class definitions, bin partitions
│   ├── d2modelio.c          # Model I/O: read CSV, read/write binary model
│   ├── d2mpc.c              # MPC 80-column format parser, observatory code loading
│   ├── d2ades.c             # ADES XML format parser (uses libxml2)
│   ├── common.c             # Shared utilities (obscode parsing, magnitude conversion)
│   ├── digest2.h            # Main header: observation, tracklet, perClass structs
│   ├── d2model.h            # Model dimensions: QX=29, EX=8, IX=11, HX=18; 15 classes
│   ├── d2ades.h             # ADES optical observation struct
│   ├── common.h             # Shared function declarations
│   ├── ALGORITHM.md         # Algorithm description
│   ├── BUILDING.md          # Build instructions
│   ├── OPERATION.md         # Usage, config file format, orbit class list
│   ├── sample.obs           # Sample MPC 80-column input
│   ├── sample.xml           # Sample ADES XML input
│   └── MPC.config -> ../population/MPC.config
│
├── population/              # Solar System population model data
│   ├── digest2.model.csv    # 4D histogram of SS populations (q, e, i, H bins)
│   ├── MPC.config           # Config with per-observatory observational errors
│   ├── README.md
│   └── make_population/     # Tools to regenerate model from SSM + astorb.dat
│       ├── s3mbin.c         # Processes Pan-STARRS Synthetic Solar System Model
│       ├── muk.c            # Combines s3m.dat + astorb.dat -> digest2.model.csv
│       └── README.md
│
├── NEOCP_filters/           # Post-processing filters for NEO/non-NEO classification
│   ├── find_filter.py       # Derives optimal thresholds from labeled digest2 output
│   ├── neocp_filter.py      # Applies thresholds to flag likely non-NEOs
│   ├── optimal_thresholds.json  # Pre-computed thresholds
│   ├── digest_data_19-24.csv    # Training data (2019-2023 NEOCP)
│   ├── digest_data_24.csv       # Test data (2024 NEOCP)
│   ├── MPC.config
│   └── README.md
│
├── src/digest2/             # Python package source
├── tests/                   # Python package tests
├── pyproject.toml           # Python package build configuration
└── README.md                # This file

Key Data Structures (C)

• observation -- Single astrometric observation: MJD, RA/Dec (radians), V magnitude, site index, rmsRA/rmsDec
• tracklet -- Group of observations of one object: designation, observation list, motion vector, scoring arrays, per-class results. Contains orbit_buf pointer (NULL unless collecting trial orbits)
• perClass -- Per-class scoring data: raw/noID scores, tagged bin arrays, population sums
• site -- Observatory parallax constants and observational error
• d2_trial_orbit -- Single trial orbit: q, e, i, H, geocentric distance, angle, bin indices, new_tag flag (defined in d2lib.h)
• d2_orbit_buffer -- Dynamic array of d2_trial_orbit with geometric growth (defined in d2lib.h)

Dependencies

• C CLI: C99 compiler, libxml2 (for ADES XML parsing), pthreads, math library
• Python package: Python >= 3.8, C99 compiler for building from source (no libxml2 or pthreads needed)
• Filters (optional): pandas
• Runtime data: digest2.model.csv, MPC.config, digest2.obscodes (auto-downloaded from MPC)