The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, is designing and will build the Solar Probe Plus spacecraft for NASA, on a schedule to launch no later than 2018. The compact, solar-powered probe will weigh about 1,350 pounds; preliminary designs include an 8-foot-diameter, 4.5-inch-thick, carbon-carbon, carbon foam solar shield atop the spacecraft body. The solar arrays will retract and extend as the spacecraft swings toward or away from the Sun during several loops around the inner solar system, making sure the panels stay at proper temperatures and power levels. At its closest passes the spacecraft must survive solar intensity more than 500 times what spacecraft experience while orbiting Earth.
Solar Probe Plus shown with solar array panels in stowed position.
Solar Probe Plus is being designed to launch on an Atlas V rocket - one of the most powerful launch vehicles NASA uses - with a variant of a STAR-48 solid-fuel third stage "kick" motor to achieve the required launch energy.
- Three-axis stabilized, using guidance and control sensors and attitude control thrusters to keep the solar shield pointed toward the Sun.
- A hexagonal bus (or six-sided body) with a central propellant tank.
- Science instruments mounted on the front and rear-facing panels, and extending from the lower deck.
- The Thermal Protection System (TPS), or solar shield, is attached to the spacecraft body through a transition structure, which includes the radiators for the solar array cooling system.
- Solar powered, with up to 343 watts at closest approach, delivered through actively cooled solar arrays. At closest approach, only the tip of the array extends into the partial shadow (or penumbra) created by the TPS; this provides power while minimizing solar array temperatures and thermal load into the cooling system. The remainder of the array is completely shadowed. As the spacecraft moves away from the Sun the array extends from the spacecraft, exposing more of the array to the Sun and providing the required power.
- X/KA-band telecommunications system provides high-speed downlink through a high-gain antenna when the spacecraft is away from the Sun (farther than .25 astronomical units, or ¼ the distance between Earth and the Sun) and low-speed command uplink and "health and status" downlink through low-gain antennas during solar encounters.
- Block-redundant processor suite with 256-gigabit solid state recorders and internally redundant power control and distribution systems.
Solar Probe Plus will be fortified with heat-resistant technologies developed for APL's MESSENGER spacecraft, which has already flown past Mercury three times in preparation for a yearlong orbit study of the planet that started in 2011. Solar Probe Plus' solar shield concept was partially influenced by designs of MESSENGER's sunshade.