Interorbital Systems (IOS) is developing a new generation of low-cost, rapid-response manned and unmanned orbital launch vehicles. NEPTUNE Modular Series rockets are designed for minimum cost and maximum reliability. Unnecessary, expensive, complex, failure-prone, and performance-limiting systems such as wings and turbopumps have not been included in their design. Since the NEPTUNE Modular Series launch vehicles are designed to be deployed from a private island launch site, launch costs will be relatively insignificant (compared to standard spaceport fees), and launch scheduling will be based on customer demand (not on placement in a spaceport's launch rotation). The IOS modular rocket system is an evolved version of a similar system developed by OTRAG in the 1970's. Lutz Kayser, the former head of the OTRAG team, is a primary consultant on the IOS project.  

The NEPTUNE 1000 is a four stage (parallel staged), medium-lift launch vehicle capable of placing a 1000-Kg payload into polar low-earth orbit or accelerating a 190-Kg payload to Earth-escape velocity. The rocket is composed of 33 Common Propulsion Modules. The engine count breaks down to 24 booster engines, 6 stage two engines, 2 stage three engines, and 1 stage four engine. The NEPTUNE 1000 is slated to launch the Google Lunar X PRIZE Synergy Moon lander/rover to the Moon. It will also be utilized to launch a two-person crew module into low earth orbit for short orbital tourism missions. The crew module (CM-2) is presently in development. 

NEPTUNE Modular Series rocket components have been undergoing ground and flight tests since 1999. Testing includes ongoing static rocket engine firings as well as launches of the IOS Neutrino sounding rocket. The IOS Neutrino sounding rocket (see photo below) has provided valuable data on rocket engine and hypergolic propellant performance in flight, Inertial Measurement Unit and Guidance Computer operation under high acceleration and vibration conditions.

Interorbital Systems will be carrying out further static rocket engine testing as well as at least three low-altitude (50,000 ft) flight tests of a single Common Propulsion Module (see below) and a single low-altitude (50,000 ft) test of a full-up NEPTUNE 30 rocket with a dummy core stage and dummy Satellite Module. The flight tests will verify the performance of all rocket systems before the first orbital launch attempt. Flight tests will begin in January 2010.