Interstellar Probe (1999) is the name of a space probe concept developed by NASA in 1999, intended to travel 200 AU (astronomical units) from the Sun within 15 years. The study was conducted by the Jet Propulsion Laboratory (JPL) and is notable for its proposed use of a large circular solar sail as the primary propulsion method.
Overview
The 1999 concept study was carried out by NASA's Interstellar Probe Science and Technology Definition Team during the spring of 1999. The mission was envisioned as the first dedicated spacecraft to explore the outer solar system, cross the boundaries of the heliosphere, and directly sample the nearby interstellar medium.
Propulsion: Solar Sail
The defining feature of the 1999 Interstellar Probe concept was its propulsion system: a circular solar sail measuring 400 meters in diameter (200-meter radius) with an areal density of 1 g/m². The sail would harness solar radiation pressure to accelerate the spacecraft. The mission profile called for a close solar flyby at 0.25 AU from the Sun to maximize light pressure, after which the sail would be jettisoned at a distance of 5 AU from the Sun.
The spacecraft was designed to achieve a velocity of approximately 14 AU/year (about 70 km/s), enabling it to reach 200 AU within 10 to 15 years and continue functioning up to distances of 400 AU or more.
Spacecraft Specifications
- Mass: Approximately 200 kg
- Power: Advanced radioisotope thermoelectric generator (RTG)
- Communications: Ka-band radio for data transmission to Earth
- Launch Vehicle: Delta II rocket
- Instrument Package: 25 kg, consuming 20 watts of power
Scientific Objectives
The mission's primary goals included investigating the outer heliosphere, crossing the heliopause (the boundary where the Sun's influence gives way to interstellar space), and making direct measurements of the local interstellar medium. The probe would have studied plasma, magnetic fields, neutral atoms, cosmic rays, and dust in regions beyond the reach of previous spacecraft.
Legacy and Subsequent Studies
While the 1999 Interstellar Probe concept was not built, it influenced later studies. Notable follow-on concepts include the Innovative Interstellar Explorer (published 2003), which proposed using RTGs powering an ion engine rather than a solar sail. The 1999 study also contributed to the broader field of solar sail propulsion, which later saw flight demonstrations by spacecraft such as IKAROS (2010), NanoSail-D2, and LightSail. The concept is distinct from the later Interstellar Probe study led by the Johns Hopkins University Applied Physics Laboratory (JHU/APL), which began in the 2010s and proposed a different mission architecture.
