WISE, NEOWISE’s predecessor mission, imaged the entire sky in the mid-infrared range. Credit: NASA/JPL/Caltech/UCLA
NASA’s NEOWISE project, which gave astronomers a detailed view of near-Earth objects – some of which could hit Earth – ended its mission and burned up re-entering the atmosphere after more than a decade.
On a clear night, the sky is full of bright objects: stars, large planets and galaxies down to small asteroids flying close to Earth. These asteroids are commonly known as near-Earth objects and come in a wide variety of sizes. Some are tens of kilometers across or more, while others are only tens of meters or smaller.
Sometimes, near-Earth objects crash into Earth at high speeds, about 10 miles per second (16 kilometers per second) or faster. That’s about 15 times faster than the muzzle velocity of a rifle. An impact at that speed can easily damage the planet’s surface and anything on it.
Impacts of large near-Earth objects are generally rare over the course of a typical human lifetime. But they are more frequent on a geological time scale of millions to billions of years. The best example might be a 6-mile-wide (10-kilometer) asteroid that crashed into Earth, killed the dinosaurs, and created the Chicxulub crater about 65 million years ago.
Smaller impacts are very common on Earth, as there are more small objects near Earth. An international community effort called planetary defense protects humans from these space intruders by cataloging and tracking as many near-Earth objects as possible, including those that come very close to Earth. Researchers call near-Earth objects that could collide with the surface potentially hazardous objects.
NASA began its NEOWISE mission in December 2013. The primary objective of this mission was to use the Wide-field Infrared Survey Explorer space telescope to closely detect and characterize near-Earth objects such as asteroids and comets.
NEOWISE has contributed to planetary defense efforts with its research to catalog near-Earth objects. Over the past decade, it has helped planetary defenders like us and our colleagues study near-Earth objects.
Detection of near-Earth objects
NEOWISE was a groundbreaking mission, as it revolutionized the way we detect near-Earth objects.
The NEOWISE mission continued to use NASA’s WISE mission spacecraft, which ran from late 2009 to 2011 and conducted an all-sky infrared survey to detect not only near-Earth objects but also distant objects like galaxies.
The spacecraft orbited the Earth from north to south, passing over the poles, and was in a sun-synchronous orbit, where it could see the sun in the same direction over time. This position allowed him to scan the entire sky efficiently.
The spacecraft could observe astronomical and planetary objects by detecting signatures they emit in the mid-infrared range.
Human eyes can perceive visible light, which is electromagnetic radiation between 400 and 700 nanometers. When we look at the stars in the sky with the naked eye, we see their visible light components.
However, mid-infrared light contains waves between 3 and 30 micrometers and is invisible to human eyes.
When heated, an object stores that heat as thermal energy. Unless the object is thermally insulated, it releases that energy continuously as electromagnetic energy, in the mid-infrared range.
This process, known as thermal emission, occurs in near-Earth objects after the Sun has heated them. The smaller an asteroid is, the weaker its thermal emission. The NEOWISE spacecraft could detect thermal emissions from near-Earth objects with a high level of sensitivity, meaning it could detect small asteroids.
But asteroids aren’t the only objects that emit heat. The spacecraft’s sensors could detect heat emissions from other sources as well, including the spacecraft itself.
To ensure that heat from the spacecraft did not hinder research, the WISE/NEOWISE spacecraft was designed to be able to actively cool itself using state-of-the-art solid hydrogen cryogenic cooling systems.
Operational phases
Because the spacecraft’s equipment had to be very sensitive to detect distant objects for WISE, it used solid hydrogen, which is extremely cold, to cool itself and avoid any noise that could compromise the sensitivity of the instruments. The coolant eventually ran out, but not before WISE had successfully completed its scientific goals.
During the cryogenic phase, when it was actively cooling, the spacecraft operated at a temperature of about -447 degrees Fahrenheit (-266 degrees Celsius), slightly higher than the temperature of the universe, which is about -454 degrees Fahrenheit (-270 degrees Celsius). .
The cryogenic phase lasted from 2009 to 2011, until the spacecraft entered hibernation in 2011.
After the hibernation period, NASA decided to reactivate the WISE spacecraft as part of the NEOWISE mission, with a more specialized focus on detecting near-Earth objects, which is still feasible even without cryogenic cooling.
During this reactivation phase, the detectors did not need to be as sensitive, nor was the spacecraft kept as cold as during the cryogenic cooling phase, since near-Earth objects are closer than WISE’s distant targets.
The consequence of the loss of active cooling was that two of the four longwave detectors on board became so hot that they could no longer function, limiting the aircraft’s capability.
However, NEOWISE used its two operational detectors to continuously monitor in detail both previously detected and newly detected near-Earth objects.
The legacy of NEOWISE
As of February 2024, NEOWISE had made more than 1.5 million infrared measurements of around 44,000 different objects in the solar system. These included approximately 1,600 discoveries of near-Earth objects. NEOWISE also provided detailed size estimates for more than 1,800 near-Earth objects.
Despite the mission’s contributions to science and planetary defense, it was decommissioned in August 2024. The spacecraft eventually began falling toward Earth’s surface, eventually reentering Earth’s atmosphere and burning up on November 1, 2024.
NEOWISE’s contributions to near-Earth object hunting have provided scientists with much deeper information about asteroids around Earth. It also gave scientists a better idea of the challenges they will have to overcome to detect faint objects.
So, has NEOWISE found all near-Earth objects? The answer is no. Most scientists still believe that there are many more near-Earth objects that have yet to be identified, especially smaller ones.
To carry on NEOWISE’s legacy, NASA is planning a mission called NEO Surveyor. NEO Surveyor will be a next-generation space telescope capable of studying small near-Earth asteroids in more detail, primarily to contribute to NASA’s planetary defense efforts. It will identify hundreds of thousands of near-Earth objects that are as small as about 33 feet (10 meters) in diameter. The spacecraft is scheduled to launch in 2027.
Toshi Hirabayashi is an associate professor of aerospace engineering, Georgia Institute of Technology. Yaeji Kim is a postdoctoral associate in Astronomy, University of Maryland.
Toshi Hirabayashi is part of several planetary mission teams with NASA, ESA, and JAXA through the Georgia Institute of Technology. Yaeji Kim does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has not disclosed any relevant affiliations beyond his or her academic appointment.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
