The Solar Sentinels was a series of proposed

space mission Spaceflight (or space flight) is an application of astronautics to fly objects, usually spacecraft, into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such a ...

s to the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

.Synergies of Orbiter & Probe+ (2009)
/ref> Solar Sentinels was proposed in 2006 in conjunction with other Sun missions, and another simpler proposal was submitted in 2008. It would have studied the Sun during its

solar maximum Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle take ...

, the last before the beginning of the

Orion program Orion (Orion Multi-Purpose Crew Vehicle or Orion MPCV) is a Reusable spacecraft, partially reusable crewed spacecraft used in NASA's Artemis program. The spacecraft consists of a Crew Module (CM) space capsule designed by Lockheed Martin that ...

. Six spacecraft would have been launched, which would separate into three groups. The Solar Sentinels were proposed to be part of the

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

program

Living With a Star Living With a Star (LWS) is a NASA scientific program to study those aspects of the connected Sun-Earth system that directly affect life and society. LWS is a crosscutting initiative with goals and objectives relevant to NASA's Exploration Initiat ...

Goals

The goals of the Solar Sentinels are: # Understand the acceleration and transit of

solar energetic particles Solar energetic particles (SEP), formerly known as solar cosmic rays, are high-energy, charged particles originating in the solar atmosphere and solar wind. They consist of protons, electrons and heavy ions with energies ranging from a few tens ...

# Understand the initiation and evolution of

coronal mass ejections A coronal mass ejection (CME) is a significant ejection of plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understanding ...

(CMEs) and interplanetary shocks in the inner

heliosphere The heliosphere is the magnetosphere, astrosphere, and outermost atmospheric layer of the Sun. It takes the shape of a vast, tailed bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding ...

This mission's driving purpose was that the

solar cycle 24 Solar cycle 24 is the most recently completed solar cycle, the 24th since 1755, when extensive recording of solar sunspot activity began.Kane, R.P. (2002).Some Implications Using the Group Sunspot Number Reconstruction. ''Solar Physics'' 205(2), 3 ...

would have been the final one before the first upcoming crewed missions to the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

and

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

. This was the last chance in 11 years to understand the solar storms and the deadly

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

of a

Spacecraft

There would have been six spacecraft: four identical spacecraft which would explore the inner

, one spacecraft which would take its post near

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, and the final sentinel to trail slowly behind Earth.

Inner Heliospheric Sentinels

The majority of the sentinels are the Inner Heliospheric Sentinels (IHS), which would observe the

at distances of 0.25 AU. 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 make ''in-situ'' measurements of energetic particles and plasma. Instruments to measure

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...

, and

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

emissions would be included. Part of the mission concept includes

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

flybys.

Near Earth Sentinel

The Near Earth Sentinel (NES) would study the

solar corona In astronomy, a corona (: coronas or coronae) is the outermost layer of a star's Stellar atmosphere, atmosphere. It is a hot but relatively luminosity, dim region of Plasma (physics), plasma populated by intermittent coronal structures such as so ...

in UV and

visible light Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...

Farside Sentinel

The Farside Sentinel (FSS) would study the

photospheric The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esca ...

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

. As three spacecraft are needed to completely monitor this magnetic field, partnerships with two other spacecraft would be made:

Solar Dynamics Observatory The Solar Dynamics Observatory (SDO) is a NASA mission which has been observing the Sun since 2010. Launched on 11 February 2010, the observatory is part of the Living With a Star (LWS) program. The goal of the LWS program is to develop the ...

and the

European Space Agency The European Space Agency (ESA) is a 23-member International organization, international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 ...

Solar Orbiter The Solar Orbiter (SolO) is a Sun-observing probe developed by the European Space Agency (ESA) with a National Aeronautics and Space Administration (NASA) contribution. Solar Orbiter, designed to obtain detailed measurements of the inner heli ...

Launch

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

Cooperation

Several other solar spacecraft would have helped with this mission, such as

STEREO Stereophonic sound, commonly shortened to stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configurat ...

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

's Hinode, the

, and

ESA The European Space Agency (ESA) is a 23-member international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 in the context of European ...

. Ground-based

telescopes A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

would also assist the mission. It would have been part of a group of four missions, including the

and the Geospace missions (

Radiation Belt Storm Probes The Van Allen Probes, formerly known as the Radiation Belt Storm Probes (RBSP), were two robotic spacecraft that were used to study the Van Allen radiation belts that surround Earth. NASA conducted the Van Allen Probes mission as part of the Li ...

and

Ionosphere-Thermosphere Storm Probes The Ionosphere-Thermosphere Storm Probes (I-TSP) is a NASA mission which will study the ionosphere and the thermosphere. This mission is part of the Living With a Star program, the second mission in a pair of geospace missions. The first mission ...

), from the

program.

References

External links

* * (NASA 2006) {{Sun spacecraft Missions to the Sun Cancelled spacecraft