Frequently asked.

Who wrote UFO@home?#

It is developed by RR0, which among other things built rr0.org, a French encyclopaedia of unexplained phenomena. That site uses UFO@home to illustrate its own case files — and RR0 has taken care to keep the tool usable by anybody.

What does the licence allow?#

UFO@home is under the MIT licence. That allows you to:

• use it on any site, including a commercial one;
• modify it, and keep your modifications private;
• redistribute it, bundle it into your own product, and charge for that product;
• fork it, and continue it in your own direction.

The one obligation is to keep the copyright notice and the licence text with the copies you distribute. There is no contributor licence agreement and no registration.

The source is on GitHub, including the scripts that generate its star, comet and satellite catalogues from public sources — so the data is reproducible as well as readable.

What does it send over the network?#

Recordings stay in your browser: there is no account and no server-side storage. Export writes a file to your disk, and that file is the whole recording.

What the editor fetches, and when:

Each is a picker in the interface, sitting where its data is reported and carrying the attribution its licence requires, so you can see what answered and choose another. The star, comet and satellite catalogues are generated ahead of time and shipped inside the bundle, so nothing is fetched for those.

A page that only replays a finished recording (<rr0-ufo>, <rr0-scene>, <rr0-sighting>) makes no lookup at all beyond the terrain and imagery tiles — a published recording carries its own weather and is never looked up again, which is also why it reads identically years later.

Can I host it entirely myself, with no external service?#

Yes, and nothing about it depends on RR0. The components are published on npm as @rr0/ufoathome and served from this domain; install them or copy the built .mjs bundles onto your own server and the reconstructions will play, with no runtime dependency on rr0.org. Recordings are your own files, on your own host, and a reconstruction asserts what its recording states and nothing more — no conclusion, no branding, no case number other than one you put there yourself.

Copy the .mjs bundles onto your server and the reconstructions will play. Skip the terrain and imagery providers (or point them at your own tile server) and the scene falls back to a plain horizon. The lookups exist for authoring; playback of a finished recording does not need them.

Why build this at all?#

Because a testimony written down loses almost everything about it. In 1968, at the AAAS symposium, the psychologist Roger Shepard argued, in the paper he gave there, that a visual reconstruction of a testimony is more faithful than a written or spoken one — witnesses are much better at recognising and adjusting a picture than at generating a description. UFO@home is that idea, built as software: draw it, move it, and let the witness correct it until it matches.

The project began in 2003 as a Java applet. It was rewritten from scratch in TypeScript in 2026, as web components, because the applet had become unrunnable and because the missing half had always been the sky: a reconstruction without the real sky of that night cannot be checked against anything.

Why is the object a flat shape rather than a 3D model?#

Because a 3D object placed in the scene is already a conclusion. It asserts a size, a distance and a solidity that no witness could perceive — and, worse, it silently rules out every explanation in which there was no object there at all: a halo, a planet, a satellite, an aircraft's landing light, a lenticular cloud, a reflection on a windscreen. Those links go to rr0.org's own catalogue of misperceptions, which is the reason this tool is built the way it is.

A flat shape on the witness's own field of view asserts exactly what they claimed: this is what reached my eye, this big, moving this way. Everything else stays open — which is the only way a reconstruction can be used to test a misperception rather than to rule one out by construction.

It also means the tool is honest about size. A recording stores an angle, never metres — but it does NOT mean distance is given up on. Where the witness saw the phenomenon cross something whose position is known, that crossing is stated in the recording (it passed behind that hangar, in front of that tree) and the scene raycasts the exact line of sight to measure it. Each crossing bounds its real width from one side, an angle plus a distance is a size, and a size does not change as it moves — so every other instant of the recording reads back as a distance too. The editor prints the range under the apparent size, reports a contradiction where the stated crossings cannot all be true, and says “unknown” where nothing crosses the line of sight at all, which is the honest answer for a light in an empty sky.

Is a reconstruction evidence?#

No. It is a way of stating a testimony precisely enough that it can be laid beside the record — and the record is what does the work: the Moon's phase that night, whether the sky was overcast, whether a comet was up, whether low orbit was even sunlit.

What a reconstruction is trying to do is agree with the witness. Not to illustrate their account, and not to correct it: to converge on it, one correction at a time, until they say that is what they saw. Every step of that convergence adds something that can be checked, so the closer it gets, the more there is to explain the sighting with — or to fail to.

And that is where it earns its keep. Suppose the reconstruction can put a known phenomenon — a planet, a halo, an aircraft — in the exact place the witness saw something, at the right size, at the right hour. Show it to them and ask the only question that matters: does that look like what you saw? If the answer is yes, investigator and witness have converged on the facts rather than argued about the conclusion, and the explanation stands on something firmer than anyone's opinion of the other.

If the answer is no, that is a result too, and a better one than a written note saying the witness disagreed — because the thing they are rejecting is on screen, at the right place and the right size, and the next person can look at it.

How is this different from Sitrec?#

Sitrec (“situation recreation”, Mick West, from 2022) is an excellent tool and it does a different job. It starts from instrument data — a video, an ADS-B track, a sensor's own metadata, orbital elements — and reconstructs the geometry that could have produced that footage. It is the right tool when there is footage.

UFO@home starts where there is none: a person, an account, and a date. It reconstructs what a witness described, and it is built around what a testimony can and cannot say — angles rather than metres, a stated appearance rather than a placed object, and an explicit record of who supplied every non-testimonial fact.

Their licences differ too. Sitrec was MIT-licensed; it was archived in March 2026 in favour of Sitrec2, whose licence permits personal or non-commercial academic use only, and neither redistribution nor modification. UFO@home is MIT.

Isn't this what SIMOVNI did?#

It is the same idea, fifty years earlier and in hardware. SIMOVNI was an optical simulator built in 1976 within France's GEPAN (the CNES's own UFO study group) by Jean-Jacques Velasco: the witness looked through a viewfinder at the real landscape with a virtual UFO superimposed, whose shape (on slides), colour, brightness and size could be adjusted until it matched. It is, as far as we know, the first serious attempt at visual rather than written testimony. It was also a single physical bench in Toulouse, it was used on only one or two occasions, and Dominique Caudron — who had built his own simulators earlier — thought its results unreliable.

SimOvni 2, presented by Laurent Chabin at CAIPAN 2 in 2022, revives the idea with a head-mounted display and real calibration work behind it.

What UFO@home adds is not a better optical bench: it is that a reconstruction becomes a small file anyone can open, replay, embed, check and disagree with — instead of a session that happened once in a room, in front of one investigator, and left a written summary.

Why not just use Stellarium?#

Stellarium is a planetarium, and a very good one — we have read its source to learn how it does things. But it answers “what was in the sky?”, not “what did this person see, and could the sky account for it?”. It has no testimony format, no witnessed phenomenon, no weather, no decor around the observer, no long-exposure instrument, and nothing to embed in a page.

Where the two overlap, UFO@home sometimes takes the harder road on purpose: the Milky Way is a texture in Stellarium and a line-of-sight integral here, which is why its dark rift comes out of a dust model rather than out of an image. Where Stellarium renders something we do not, the working rule of this project is to go and find out how they do it.

How do I ask for a feature, or report something wrong?#

Open an issue: github.com/RR0/UfoAtHome/issues. A GitHub account is free and takes a minute.

What makes a request easy to act on:

• A real case, if there is one. “Witnesses often say it was behind a hill” is a feature; “here is a sighting where that matters, dated, placed and sourced” is a feature that gets built right.
• What the record says. If the thing you want reconstructed is measured somewhere — a dataset, a catalogue, a published measurement — say where. This project's rule is that nothing gets invented: something reproducible needs data or physics behind it, and knowing which one it is settles most of the design.
• What you would conclude from it. A feature that cannot change anyone's reading of a case is decoration.

Pull requests are welcome on the same terms as any MIT project. If you would rather not use GitHub, the maintainer's contact is on rr0.org.

Can I put a reconstruction in a forum post?#

If the forum allows raw HTML and a module script, yes — the two lines are on the editor page, and every published reconstruction hands them out itself. Most forums do not allow that, for good reasons. Two things that usually work instead: an <iframe> pointing at a page of your own that holds the component, or simply a link to ufoathome.org/editor/?sighting= followed by the URL of your recording, which opens it here for anyone.

What if I don't know the exact date, or the exact time?#

Say so, and the tool will say so too. Dates are stored in EDTF, so “1954”, “June 2025”, “around 05:00” and “uncertain” are all things the format can state. What you lose is only what genuinely depends on the missing part: with no full date and place there is no sky to compute, and the interface says that rather than drawing a plausible one.

What languages does it speak?#

The components and this site are in English and French, picked from your browser's own preferences with English as the fallback. Adding a language means adding one typed messages module per component — no markup changes, no build configuration. It is one of the easiest contributions to make.

What does it cost?#

Nothing: no price, no account, no quota and no paid tier. The external services it uses while authoring are public ones whose usage policies it respects — which is why place search runs only when you ask for it.