Rhizomatica, a non-profit that's spent years building open-source communications tools, just officially released Mercury V2, a completely free, open-source software modem for HF data.
If you've used VARA for Winlink, Mercury V2 is apparently very similar. It uses the same OFDM foundation, supports ARQ connections for reliable file/email transfer, and this is the part I really like, it mirrors VARA's TCP TNC interface (separate control and data ports), so the software that already talks to VARA has a much easier path to talking to Mercury V2 too. The big difference: Mercury V2 is fully GPL-3.0 open source. No license fee and the whole program is on GitHub for anyone to read, audit, or improve. It's part of Rhizomatica's larger HERMES project, which they've been developing since 2017 with backing from ARDC.
In Rhizomatica's own early testing against VARA, Mercury V2 comes in roughly neck-and-neck in good SNR conditions and actually pulls ahead when conditions get noisy. Rafael Diniz, the project's lead developer, has said that Mercury holds real advantages over VARA, including better performance on high-SNR links. ( https://www.arnewsline.org/news-text/2026/5/14/open-source-software-modem-called-a-vara-replacement ) And Steve Stroh (N8GNJ), who writes the Zero Retries newsletter (https://www.zeroretries.org/p/zero-retries-0250 ) and saw an early demo at Hamvention, called it a big deal for the evolution of HF data comms, because it's open source and already runs on Linux, something VARA and PACTOR IV have never offered.
Where to Get It
- Source code: github.com/Rhizomatica/mercury
- Demo videos: YouTube – Rhizomatica Communications
- Mailing list: HERMES General
- HERMES project overview: hermes.radio