A Brief History of ALCDEF

Below is the background into why ALCDEF was created and a short primer on the format itself. This database has become an important tool for those doing asteroid research where raw time-series photometry data are required.

The Beginning

In the last two decades, the number of asteroid lightcurves has grown almost exponentially, mostly due to the efforts of backyard astronomers and students working with small college observatories. Papers involving professional surveys (Waszczak et al. 2015; Chang et al., 2015; Chang et al., 2019) added almost 20,000 lightcurves to the asteroid lightcurve database (LCDB; Warner et al., 2009).

The rotation periods derived from these lightcurves have helped change the picture of the Solar System dramatically by providing conclusive evidence of the YORP effect (Yarkovsky–O'Keefe–Radzievskii–Paddack; Rubincam, 2000), which is the thermal re-radiation of sunlight that affects the rotation rates (e.g., Pravec et al., 2008) and spin axis orientations of asteroids (e.g., Hanus et al., 2013). The availability of the raw data that led to the lightcurves has also allowed a significant increase in the number of shape models (which include spin axis orientation). Beyond the mere curiosity of knowing the shapes of various asteroids, these models have also helped refine theories and ideas on YORP, asteroid densities, and binary formation mechanisms, among others.

These advancements are due in large measure to the availability of the underlying raw time-series data, which are required for the modeling process. However, there remains a fundamental issue: the lack of a universally-accepted, central repository similar to the one for astrometric data as managed by the Minor Planet Center. Without such a repository, a researcher may have to contact a number of people and scour several web sites in order to find all available lightcurve photometry. This can be a poor use of resources.

First Tries

The idea of a central repository for asteroid time-series photometry is not new. Magnusson et al. (1993) made one of the earlier significant efforts towards this goal by outlining the Uppsala Asteroid Database, which stored time-series data going back to the 1950’s for hundreds of asteroids. The final version of this was produced by Lagerkvist et al. (1993). An update was done in 2011 (NASA, 2011) where the data lines were put into their own ASCII tables, apart from the metadata tables. However, not all data in the Uppsala catalog can be taken at face value.

While the 2011 update appears to have standardized all dates to be light-time corrected and magnitudes to unity distances using geo- and heliocentric distances, many other pitfalls are to be found. Quoting from the metadata table for one asteroid but which appears to be common to many if not all asteroids,

Some observation sets … may have an artificial Julian date, which may be set near 0.0, since the observation time is unknown or the lightcurve is a composite of several observation sets made at different times … Some observation sets have unknown constants added to their magnitudes.

In other words, some of the dates may be offsets from a zero point that are based on a presumed period that may be wrong. This is a critical problem when modeling since the asteroid-Sun and asteroid-Earth geometries are an essential element in the process. Having the wrong dates, especially if based on incorrect periods, will lead to bad models.

The issue with the magnitudes can be mitigated by using the values as relative magnitudes, not absolute, when modeling. However, for those data that are actually absolute, i.e., standardized apparent magnitudes reduced to unity distances, their value is diminished. The lightcurve inversion modeling process cannot constrain the z-axis of an xyz ellipsoid unless all the data are absolute. It might be possible to “unscrew” the catalog data into original data, but this may involve guess work or, at the very least, referring to the original paper, if available, to try to understand the data’s pedigree. This takes time, lots of time, which is a rare commodity for most researchers.

Along Comes ALCDEF

At the Division of Planetary Sciences meeting in 2010 (Stephens et al., 2010; Warner et al., 2011), a renewed effort for establishing a central repository was introduced along with a standardized reporting format: the Asteroid Lightcurve Data Exchange Format (ALCDEF). The critical elements of ALCDEF were that:

1. Metadata and data were in a single, simple ASCII file (ASCII values < 128)
   This has been updated to now allow UTF-8 encoded text files without a byte order mark (BOM).
2. Information for more than one observing run for a given asteroid and even multiple asteroids could be contained in a single file.
3. The format followed the familiar and easily-parsed keyword = value structure, including the photometric data.
4. The data were raw photometry, i.e., they were not reduced to unity distances nor were arbitrary offsets to magnitudes added, such as might be done during period analysis. In other words, if the data were not differentials from a common zero point, they represented the apparent sky magnitude of the asteroid at the time of the observation. If the magnitudes were offsets, then the common zero point could be indicated in a comment line.
5. The data used full Julian Dates for the mid-time of each observation. The JD were not offsets from an arbitrary zero point nor based on a presumed period (the S-ALCDEF standard – see below – allows importing partial or full JD or MJD values).
6. The metadata section included a number of required keywords, e.g., object number and name, mid-date of observations, data set contributor name and contact information, etc. The standard also included a number of optional keywords that would further define the data for use by other researchers.

Where in the World Wide Web is ALCDEF?

Initially the data were stored on a public web site maintained by the author but this was not an adequate long-term solution. A conversation in 2009 with then MPC Directory Timothy Spahr eventually led to a web page on the MPC site in 2010 that was developed by MPC staff member Michael Rudenko with the assistance of Assistant Director Gareth Williams.

In 2014, the author began work with MPC staff members Dr. Jose-Luis Galache, Jim Davies, and Michael Rudenko to produce a new version of the web site that would be developed by the author using PHP code and MySQL database. The new code would include additional cross-checks of incoming data to assure its quality and avoid contradictions, e.g., if trying to submit data for (8132) Vesta (there's no such critter). Data retrieval of only newly added objects or a subset of lightcurves was another enhancement to the original site.

Home At Last

As of 2019 February, the site is again being hosted by the author, but this will be only for a short time. Work has already been done and is continuing to have the ALCDEF database hosted by the NASA Planetary Data System (PDS). A trial peer-review of PDS submission was made in 2018 which allowed fine-tuning what information would be stored and used by the PDS.

In early 2019, observations from the ATLAS project (Tonry et al., 2018) were added to the ALCDEF database. That raised the number of observations to well more than 32 million for more than 180 thousand distinct asteroids.

Where ever the files reside, it should always be possible to access the site by visiting http://alcdef.org