TheSolar Sentinels was a series of proposedspace missions to theSun.^[1] Solar Sentinels was proposed in 2006 in conjunction with other Sun missions, and another simpler proposal was submitted in 2008.^[1]

It would have studied the Sun during itssolar maximum, the last before the beginning of theOrion program. Six spacecraft would have been launched, which would separate into three groups. The Solar Sentinels were proposed to be part of theNASA programLiving With a Star.

The goals of the Solar Sentinels are:

1. Understand the acceleration and transit ofsolar energetic particles
2. Understand the initiation and evolution ofcoronal mass ejections (CMEs) and interplanetary shocks in the innerheliosphere

This mission's driving purpose was that thesolar maximum ofsolar cycle 24 would have been the final one before the first upcoming crewed missions to theMoon andMars. This was the last chance in 11 years to understand thesolar storms and the deadlyradiation of asolar maximum.

There would have been six spacecraft: four identical spacecraft which would explore the innerheliosphere, one spacecraft which would take its post nearEarth, and the final sentinel to trail slowly behind Earth.

The majority of the sentinels are the Inner Heliospheric Sentinels (IHS), which would observe theSun at distances of 0.25AU. This would be a challenge to scientists and engineers working on these probes, as this is one-fourth the distance between the Earth and the Sun. These probes would makein-situ measurements ofenergetic particles andplasma. Instruments to measureX-ray,radio, andneutron emissions would be included. Part of the mission concept includesVenus flybys.

The Near Earth Sentinel (NES) would study thesolar corona inUV andvisible light.

The Farside Sentinel (FSS) would study thephotosphericmagnetic field. As three spacecraft are needed to completely monitor this magnetic field, partnerships with two other spacecraft would be made:NASA'sSolar Dynamics Observatory and theEuropean Space Agency'sSolar Orbiter.

The three Inner Heliospheric Sentinels would have been launched together. The proposed launch dates were 2014, 2015, or 2017.^{[citation needed]} The nominal mission lasts three years, with an extension to five years if possible.

Several other solar spacecraft would have helped with this mission, such asSTEREO,Japan'sHinode, theSolar Dynamics Observatory, andESA'sSolar Orbiter. Ground-basedtelescopes would also assist the mission. It would have been part of a group of four missions, including theSolar Dynamics Observatory and the Geospace missions (Radiation Belt Storm Probes andIonosphere-Thermosphere Storm Probes), from theLiving With a Star program.^[2]

• Reports on Solar Sentinel at theWayback Machine (archived May 28, 2010)
• The Importance of the Solar Sentinels at theWayback Machine (archived July 12, 2016) (NASA 2006)

• Missions to the Sun
• Cancelled spacecraft

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