Helio and You: A Fresh Perspective on the Sun - NASA Science

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A Fresh Perspective on the Sun

What is the STEREO Mission?

Twenty years ago, NASA launched a groundbreaking mission to explore the Sun in ways never tried before. The Solar TErrestrial RElations Observatory (STEREO) mission consisted of two nearly identical spacecraft (STEREO-A and STEREO-B) which were designed to move into orbital locations both behind and in front of Earth in its own orbital track. This unique pattern allowed scientists to reveal the 3D structure of the Sun’s most powerful outbursts, called coronal mass ejections (CMEs) for the first time.

The STEREO mission helped to revolutionize our understanding of the Sun by pairing two spacecraft for stereoscopic rather than flat, 2D views of the Sun. The placement of the spacecraft also allowed scientists to take imagery of the Sun’s far side for the first time, which made it possible to generate more detailed predictions of space weather activity as previously hidden regions rotated into view. Although the STEREO-B spacecraft’s mission ended on Oct. 17, 2018, STEREO-A remains in service today and continues to be one of the key NASA heliophysics missions that helps to predict solar storms heading toward Earth, the Moon, and Mars.

STEREO spacecraft in orbit at the Sun.

Artist's concept of one of the STEREO spacract in orbit at the Sun.

NASA

What Did the STEREO Mission Make Possible?

Before the STEREO mission, scientists were restricted not just in the scope of their ability to see the Sun, but also in the ways in which they could examine its features. Pre-STEREO missions could not simultaneously image the Sun and remotely measure data about the material of which it is composed. But the STEREO mission changed all that by combining remote sensing with in-situ instruments. This, coupled with the stereoscopic vantage points offered by the mission as a whole, allowed STEREO to track CMEs from when they first emerged from the Sun (remote-sensing data), all the way to the STEREO spacecraft, where it physically measures the velocity and density of the solar material as they pass over the spacecraft (in-situ data).

Coronal mass ejections (CMEs) are large eruptions of plasma which emerge from the Sun’s corona. They can eject billions of tons of material at a time, which travels outward from the Sun at speeds ranging from slower than 155 miles per second (250 kilometers per second) to more than 1,860 miles per second (3,000 km/s). At those speeds, the fastest CMEs can reach our planet in as little as 15-18 hours. If they are directed at Earth, these events can cause major impacts on everything from satellites to the electrical grid.

But CMEs can also impact missions traversing between the planets, including future missions to the Moon, Mars, and beyond. If a powerful storm were to strike a crewed mission in deep space (such as one taking the multi-month journey to Mars), it could damage the spacecraft’s sensitive electronics, potentially impacting the crew’s mission. Beyond that, the intense flux of SEPs could harm the astronauts themselves, which has possible repercussions ranging from mild illness to higher chance of developing certain cancers.

By allowing scientists to track solar energetic particles (SEPs) and the solar wind throughout the solar system, the STEREO mission also helped shift the focus from just Earth-centric space weather monitoring to a model focusing on truly interplanetary, full solar system space weather. This in turn laid the groundwork for predicting space weather at the orbits of other planets, which is a crucial step in the path to future deep space exploration. After all, if we are to safely venture into space and set up outposts on the Moon and Mars, we will need to be able to detect and track CMEs and other space weather conditions across the entire solar system.

Why is Predicting Space Weather Important?

On July 23, 2012, a massive CME erupted from the Sun and into the solar system beyond. Some scientists described it as the largest solar storm since the 1859 Carrington Event, a solar storm so powerful that it created bright auroras as far south as Florida. Although this storm missed Earth, it washed right over STEREO-A. As a result, scientists were able to directly study the characteristics of this storm and learned more about the CME’s magnetic structure and the kind of shock waves and energetic particles it produced.

This movie presents an oblique view of Earth's magnetic field as it might have reacted during a solar storm on the order of the July 2012 storm or the 1859 Carrington Event.

NASA SVS

One of the dangers of CMEs is that the immense solar energy can overload everything from orbiting satellites to the pumps that power sewer systems. As a result, a major CME like the one on July 23, 2012, could prove disastrous if it were to hit Earth directly, with everything from small-scale GPS errors to major nationwide blackouts as possible consequences of such an event.

Solar storms like the one in 2012 are measured by a unit called Dst (disturbance–storm time), which is essentially an indicator of how much the storm in question would “shake” Earth’s magnetosphere if it were to hit our planet. A storm in March 1989 which knocked out power across Quebec measured in at Dst=-600 nT (nanoTesla), while the Carrington Event is estimated to have been somewhere between -800 and -1750 nT. The storm in July 2012 was measured at -1200 nT, meaning that it could have caused calamity if it had hit Earth.

Given that this could also threaten future missions to deep space, heliophysicists work hard to detect and track space weather events across the whole solar system. Just as meteorologists predict the weather here on Earth using a global network of sensors, scientists looking to accurately track space weather while conducting a multi-month journey to Mars need to know what the Sun is doing from all angles. The STEREO mission helped to prove the viability of this concept, opening the door for even more innovative missions to follow.

What Are Some Other Benefits of STEREO?

In addition to revolutionizing our ability to study space weather, the STEREO mission also led to the first ever 3D planetarium show. An IMAX 3D movie that has been shown at the Smithsonian was also produced based on STEREO data, as well as mobile apps that allow people to "hold the Sun in the palm of their hand" using stereoscopic glasses. These deliverables helped to bring the Sun to people here on Earth, making it possible for everyone to engage with our nearest star in ways that most people never have.

In addition, the STEREO mission is an international collaboration, with as much as 50% of the instrument payload contribution for STEREO having been provided by European Space Agency (ESA) member states. This is a prime example of global scientific cooperation, which other missions like Solar Orbiter (headed by the ESA) and the joint NASA-ESA Solar and Heliospheric Observatory (SOHO) have also typified.

This image is an infographic titled "Heliophysics Missions", illustrating the locations and primary operations of NASA's heliophysics mission fleet across the solar system. The graphic features a large illustration of the Sun on the left and Earth on the right, with various labeled spacecraft icons positioned in relation to them, such as the Parker Solar Probe near the Sun and others like the Solar Orbiter, STEREO, SDO, and TIMED shown in different orbits or locations. The text explains that these missions work together to provide a holistic view of the Sun and space weather, and lists missions under "Primary Operation" and "Extended Operation".

This graphic showing NASA's heliophysics fleet (featuring STEREO at the far lower left, near the Sun) demonstrates the importance of having a wide array of missions operating from multiple perspectives and with different, albeit overlapping scientific goals.

NASA

It is also important to note that while STEREO gave us a 3D view of the Sun from the plane of the ecliptic, missions like the Solar Orbiter are advancing our knowledge by moving up to 30 degrees off the ecliptic. The ecliptic is the flat plane that passes through the Sun’s equator and which most planets’ generally stick close to as they orbit. As missions like Solar Orbiter depart this orbital region, they give us better views of the Sun's poles, which only serves to further flesh out our understanding of the Sun in 360 degrees.

Since the STEREO mission has been going for 20 years now, giving scientists the chance not only to gather data on the Sun from multiple angles but also across over multiple 11-year solar cycles (a necessity if patterns are to be surmised), its success truly underscores why missions like STEREO are built to last.

What Does the Future Hold?

Observations STEREO made possible help us to better understand the potential impacts space weather can have on human society, which then makes it easier to prepare for and mitigate those impacts. Future missions can expand on this legacy by more closely examining the Sun from multiple angles and across multiple solar cycles.

We can only speculate about the results for now. But as NASA and its partners prepare to build bases on the Moon and put the first humans on Mars, studying space weather is more important than ever. It is clear that in this new era of space exploration, missions like STEREO, which push the boundaries of what is known by establishing new standards of scientific discovery, will be vital to determining the best course for our future in space.

Resources for Educators ↓

Heliophysics Resource Database

During the Aug. 21, 2017, total solar eclipse in Carbondale, IL, an eclipse watcher uses the small gaps in between their fingers to project several partial solar eclipse images on the ground. Credit: Sean Simmons

NASA Helio Club: Observing the Sun

In Session 2 of the NASA Helio Club study unit, students are led through activities that help them explore ways to safely observe the Sun.

Image of the Sun from the Student Helioviewer interface showing active regions labeled with NOAA numbers. The Sun appears in yellow ultraviolet light with swirling magnetic loops, and an Earth scale icon for size comparison is shown in the corner. A data panel at the top displays date, time, and observation settings.

STEREO Imaging Data at Helioviewer

Helioviewer.org is part of the Helioviewer Project, an open-source project which visualizes solar and heliospheric data from different missions.

Sungrazing comets are a special class of comets that come very close to the sun at their nearest approach, a point called perihelion. To be considered a sungrazer, a comet needs to get within about 850,000 miles from the sun at perihelion. Many come even closer, even to within a few thousand miles. Being so close to the sun is very hard on comets for many reasons. They are subjected to a lot of solar radiation which boils off their water or other volatiles. The physical push of the radiation and the solar wind also helps form the tails. And as they get closer to the sun, the comets experience extremely strong tidal forces, or gravitational stress. In this hostile environment, many sungrazers do not survive their trip around the sun. Although they don't actually crash into the solar surface, the sun is able to destroy them anyway. Many sungrazing comets follow a similar orbit, called the Kreutz Path, and collectively belong to a population called the Kreutz Group. In fact, close to 85% of the sungrazers seen by the SOHO satellite are on this orbital highway. Scientists think one extremely large sungrazing comet broke up hundreds, or even thousands, of years ago, and the current comets on the Kreutz Path are the leftover fragments of it. As clumps of remnants make their way back around the sun, we experience a sharp increase in sungrazing comets, which appears to be going on now. Comet Lovejoy, which reached perihelion on December 15, 2011 is the best known recent Kreutz-group sungrazer. And so far, it is the only one that NASA's solar-observing fleet has seen survive its trip around the sun. Comet ISON, an upcoming sungrazer with a perihelion of 730,000 miles on November 28, 2013, is not on the Kreutz Path. In fact, ISON's orbit suggests that it may gain enough momentum to escape the solar system entirely, and never return. Before it does so, it will pass within about 40 million miles from Earth on December 26th. Assuming it survives its trip around the sun.

Watching Comet ISON’s Journey

A collection of images and videos which illustrate how various spacecraft from NASA’s heliophysics mission fleet observed Comet ISON in 2013.

STEREO mission thumbnail

STEREO Science Center

A collection of galleries featuring images, videos, and other representations of the STEREO mission over the 20-plus years of its operation.

illustration of corona surrounding sun

Educator’s Guide – Exploring Magnetism

In this teacher’s activity guide, students can learn about magnetic fields and electromagnetism through inquiry and measurement.

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