NEOWISE

FIELDS OF STUDY: Astronomy; Asteroid Impact Avoidance; Space Technology

ABSTRACT: The National Aeronautics and Space Administration’s Wide-field Infrared Survey Explorer (WISE) mission used infrared telescopes to find asteroids and other space objects in the solar system and the Milky Way galaxy that were invisible to light-dependent instruments. A follow-up project, NEOWISE focuses on gathering additional data and preparing for potentially threatening near-Earth objects, such as undiscovered asteroids. The data revealed that there are fewer such objects than previously believed. Future projects, such as a proposed mission to Mars, would also benefit from the data gathered by NEOWISE.

The NEOWISE Missions

Following the successful Wide-field Infrared Survey Explorer (WISE) mission, which ran from December 2009 until October 2010, scientists at the National Aeronautics and Space Administration (NASA) decided to continue to use the spacecraft to explore the galaxy. By that time, the cryogen, or frozen hydrogen coolant, that had kept some of the instruments working had run out. NASA personnel used the two remaining operational cameras to complete the scan of the main asteroid belt and continue to seek asteroids and comets. The spacecraft was put into a state of hibernation after four months.

In September of 2013, NASA reactivated the WISE spacecraft and renamed it Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE). This marked the beginning of a three-year planned study on near-Earth objects (NEOs). The focus on NEOs resulted in new data that included millions of measurements and images collected through 2014. Data from December of 2013 to December of 2014 was released online for public use in March of 2015.

An extension of WISE, NEOWISE was designed to find asteroids, determine their size, and place them into family groups. The project provided data on numerous celestial objects in the asteroid belt between Jupiter and Mars. It revealed 34,000 previously unknown asteroids and twenty-one comets, in addition to some hundreds of NEOs.

Because some asteroids were considered potentially threatening to Earth, NASA continued to closely study their sizes, orbits, and probable future paths. NASA requested and Congress approved increased spending for the Near-Earth Object (NEO) Observations Program in 2012 and again in 2014. The collection of data on 158,000 additional objects was followed up with close studies of the discoveries. Scientists use the data from NEOWISE to compute objects’ physical properties for inclusion in NASA’s Planetary Data System.

Mission Background

While there were many early attempts to detect infrared emissions in the galaxy, most provided insufficient information due to the limitations of early equipment. More success was achieved with the launch of the Infrared Astronomical Satellite (IRAS), a project to map the sky that began on January 25, 1983. It was a mission shared by the United States, the United Kingdom, and the Netherlands. IRAS carried 62-pixel cameras, which was state-of-the-art technology at the time. Even so, the satellite found some 500,000 sources of infrared light, including six comets and more than two thousand asteroids. It also revealed 12,000 variable stars and the core of the Milky Way galaxy. Variable stars change in brightness.

As successful as IRAS was, improved technology later inspired another look at the galaxy. The WISE mission, which NASA launched in 2009, was driven by an increased concern for near-Earth asteroids and the possibility of collisions. Its telescope and four cameras provided overlapping images that were hundreds of times more sensitive than those taken by IRAS. After scanning the sky twice, WISE had transmitted 2.7 million images. But the mission was designed to run for only a limited time, because the cryogen used to maintain the sensitivity of some of the instruments would run out.

When the WISE mission ended, NASA decided to extend the hunt for near-Earth objects using the two remaining operational cameras. Named NEOWISE, the extension of the mission was expected to complete a full scan of the solar system’s main asteroid belt. Since being reactivated, NEOWISE has completed sixteen full scans of the sky and was 93 percent into its seventeenth scan in early 2022. NASA gave the mission a two-year extension in 2021. Its successor, the NEO Surveyor, was scheduled for launch in 2026.

Data Findings

Although examining and interpreting the images and data sent back from WISE and NEOWISE is an ongoing project, some of the early findings surprised NASA scientists. First, infrared images showed that there are fewer midsize NEOs than previously believed. The number of asteroids larger than one hundred meters wide was reduced from 35,000 to just under 20,000.

NEOs, such as near-Earth asteroids (NEAs), are defined as objects that orbit within 1.3 astronomical units (AU) of the sun and whose orbits swing toward Earth. One astronomical unit is equal to about 149.6 million kilometers (about 93 million miles). Because the instruments WISE used do not rely on visible light, they are able to detect many more objects and judge their sizes more accurately. For example, to a person using a telescope that relied on light, a small, very reflective object would look the same size as a larger, darker mass. The infrared survey supplied more information on the numbers, sizes, and reflective properties of objects in space.

However, the number of large asteroids, one kilometer (0.62 miles) wide or larger, did not change significantly. The previous estimate had been 1,000 and the new finding was 981. Most of those had already been found, including all those 10 kilometers (6.2 miles) or larger, which would be a serious threat to life on Earth should they collide with the planet.

NEOWISE data also helped researchers study asteroid families. When a large space object is smashed in a collision with another body, the resulting pieces move together through space. The original mass is called the "parent body," and all the fragments belong to a single family. The NEOWISE data helped scientists group about one-third of the examined asteroid population into seventy-six families. Of those, twenty-eight were newly discovered families. The study also showed that some belonged to a different family than originally believed.

By 2021, NEOWISE had made infrared observations of more than 39,000 objects and estimated the size of more than 1,850 NEOs. The mission also detected more than 248 comets, including one named Comet NEOWISE that was visible to the naked eye from Earth in 2020.

NEOWISE and Future Missions

An additional purpose for NEOWISE was to provide information to use for future NASA missions. For example, NEOWISE collected data on NEOs that could be targets for the possible Asteroid Redirect Mission, which would involve the capture of a boulder from an asteroid. The boulder would then be placed in orbit around the moon for later study. That project was part of a much longer-term proposal for a journey to Mars. The boulder capture would test the robotic spacecraft and advanced solar electric propulsion (SEP) needed for the Mars mission.

NASA also planned to test techniques for intercepting asteroids on an Earth-bound path. Humankind has long been aware of the possibility of a collision between Earth and a large asteroid. It is believed that such a collision was responsible for the extinction of dinosaurs from the planet. That makes the information on the size and location of space objects provided by NEOWISE vital in helping researchers plan for future threats and possible interceptions.

PRINCIPAL TERMS

• asteroid: a relatively small, irregularly shaped, rocky space object that orbits the sun.
• infrared: electromagnetic wavelengths that are slightly longer than red visible light waves but shorter than microwaves.
• near-Earth object: a celestial object that orbits the sun within 1.3 astronomical units (AU) and that potentially poses a collision hazard to Earth.
• Wide-field Infrared Survey Explorer telescope: an instrument used to collect images and data from throughout the Milky Way.

Bibliography

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