Elon Musk’s SpaceX has won a key preliminary ruling in its dispute with Blue Origin over a patent received by the Jeff Bezos-backed commercial space venture covering the landing of rockets at sea.

In one of two decisions issued this week in the case, the U.S. Patent Trial and Appeal Board granted SpaceX’s petition for what’s known as an inter partes review of 13 of 15 claims in Blue Origin’s rocket-landing patent — an important first step in a process that SpaceX hopes will ultimately result in the patent office invalidating the claims.

The judges wrote in their decision, “After considering the Petition, we conclude that SpaceX has demonstrated a reasonable likelihood that it would prevail in showing unpatentability of the challenged claims.”

They explained, “Our decision to institute [a review] acts as a preliminary measure of SpaceX’s evidence as having enough merit to take the case to trial. Blue Origin may now come forward with argument and evidence in response to SpaceX’s prima facie proof of unpatentability.”

The patent dispute is significant because the ability to recover booster rockets, for subsequent reuse, is key to improving the economics of commercial spaceflight.

Blue Origin applied in June 2010 for the patent, “Sea landing of space launch vehicles and associated systems and methods,” outlining a system for launching a rocket from a coastal launch site and recovering the booster by landing it vertically, tail-first, on a platform at sea — using the booster engines to control the descent. The patent was granted in March of last year.

SpaceX, which has already attempted to land a Falcon 9 rocket at sea, disputed the Blue Origin patent — citing prior work by researchers and scientists who proposed techniques similar to those in Blue Origin’s patent long before the patent was filed. “The ‘rocket science’ claimed in the ‘321 patent was, at best, ‘old hat’ by 2009,” wrote SpaceX in its petition.

Separately this week, the same judges denied SpaceX’s related petition over the final two claims in the Blue Origin patent — but that decision was based on a legal technicality that actually bodes well for SpaceX, as well. In that decision, the judges wrote that Blue Origin wasn’t clear enough in the patent about how the booster rocket would initially ignite, shut off and reignite for landing — signaling that those claims by Blue Origin aren’t enforceable as written.

The legal jargon in that second decision has caused confusion over the impact of the ruling, with some interpreting it as a win for Blue Origin. However, the related decision (see below), found by GeekWire in the Patent Trial and Appeal Board docket, states clearly and repeatedly that SpaceX has “demonstrated a reasonable likelihood of prevailing” on its petition seeking to invalidate a majority of the claims in the patent.

Blue Origin, based in Kent, Wash., is focusing heavily on vertical landings as part of its overall approach to commercial spaceflight.

SpaceX, which already conducts supply missions to the International Space Station, also has won a large chunk of the NASA contract to take human crews to the ISS. The California-based company recently announced plans to open a Seattle-area engineering office, better positioning itself to compete for engineering talent from the region’s aerospace and technology companies.

SpaceX plans to engineer satellites in the office to create a new commercial Internet service, the first step in Musk’s long-term plan to colonize Mars.

We’ve contacted both companies for comment on the ruling. Here’s the full text of the ruling granting SpaceX’s petition for review of the patent.

Patent ruling: SpaceX vs. Blue Origin