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August 15 news, last week, NASA (NASA) has retired a nearly 15-year-old spacecraft, NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer), and will deorbit it in the coming months. This marks an important end to the agency's planetary defense program. During its service, the spacecraft discovered 400Near-Earth Asteroidsandcomet。

In low-Earth orbit, NEOWISE's infraredtelescopeIt scanned the entire sky 23 times, taking millions of images. Its original mission was to search for infrared radiation from galaxies, stars, and asteroids, and later focused onsunObjects within the system.

Exploring Near-Earth Objects

The Wide-field Infrared Survey Explorer (WISE) was launched in December 2009 with an initial mission duration of seven months. After WISE completed its checkout and ended its primary all-sky astronomical survey, NASA placed the spacecraft in hibernation in 2011 due to the depletion of its cryogenic hydrogen coolant, which reduced the sensitivity of its infrared detectors. But astronomers found that the infrared telescope could still detect objects close to Earth, so NASA reactivated the mission in 2013 for another decade of observations.

The restarted mission, renamed NEOWISE, aims to use the spacecraft's infrared telescope to detect tiny planets and comets that fly close to Earth.

"We never thought it would last this long," said NEOWISE principal investigator Amy Mainzer of the University of Arizona and UCLA.

On August 8, ground controllers at NASA's Jet Propulsion Laboratory in California sent the final command to the NEOWISE spacecraft. Currently, the spacecraft is in an orbit about 350 kilometers high and is gradually falling out of orbit due to atmospheric drag. NASA expects that due to the expansion of the upper atmosphere caused by increased solar activity, the probe will re-enter the atmosphere and burn up before the end of this year, a few months earlier than expected. The satellite does not have its own propulsion system to push it to a higher orbit.

"The sun has been very quiet for many years, but now it's starting to become active again, and this is the right time to let go," Mainzer said.

Most of the NEOs detected so far have been discovered using ground-based telescopes, but using space telescopes has its advantages because Earth's atmosphere absorbs most of the infrared energy from faint objects such as asteroids.

Astronomers using ground-based telescopes "primarily see sunlight that's reflected off the surface of an object," Mainzer said. NEOWISE measures thermal radiation from asteroids, giving scientists information about their size. "We can actually get a pretty good size estimate from relatively few infrared measurements."

The telescope on NEOWISE is relatively small in size, with a primary mirror of 40 centimeters in diameter, less than 1/16 the size of the James Webb Space Telescope. But its wide field of view allows NEOWISE to search the sky for infrared sources, making it well suited to studying a large number of objects. One of the mission's most notable discoveries is a comet formally named C/2020 F3, more commonly known as Comet NEOWISE, which was even visible to the naked eye in 2020. As the comet approaches Earth, large telescopes like Hubble are able to take even closer looks.

"The NEOWISE mission has been an incredible success, helping us better understand our place in the universe by tracking asteroids and comets that could pose a threat to Earth," said Nicola Fox, associate administrator of NASA's Science Mission Directorate.

What is in the universe?

According to the Center for Near-Earth Object Studies, the original WISE mission and the extended NEOWISE survey have discovered a total of 366 near-Earth asteroids and 34 comets. Of these, 64 are classified as potentially hazardous asteroids, meaning they are within 7.48 million kilometers of Earth (0.05 AU/average distance from the Earth-Sun) and are at least 140 meters in diameter. These are the objects that astronomers hope to discover and track to predict whether they are at risk of colliding with Earth.

There are about 2,400 known potentially hazardous asteroids, but there are many more lurking out there. Another advantage of space telescopes in searching for these asteroids is that they can observe them around the clock, while ground-based telescopes can only observe them at night. Hazardous asteroids, such as the one that exploded over Chelyabinsk, Russia, in 2013, approach the Earth from the direction of the sun; space telescopes are more likely to discover such asteroids.

WISE and its extended mission NEOWISE have helped scientists estimate that there are about 25,000 near-Earth objects.

"The vast majority of objects discovered by NEOWISE are very faint, and these are objects that ground-based telescopes are more likely to miss," Mainzer said. "This in turn gives us a better idea of ​​how many are really out there."

In 2010, using the original all-sky survey data from the WISE mission, scientists announced that they had discovered more than 90% of near-Earth objects larger than 1 km in diameter. If any of these objects hit the Earth, it would have a global impact.

In 2005, the U.S. Congress required NASA to discover at least 90% of near-Earth objects (NEOs) larger than 140 meters in diameter that could cause damage at a regional scale. So far, astronomers have found about 43% of these objects. A new probe, the Near-Earth Object Survey Telescope (NEO Surveyor), is scheduled to launch in 2027 to build on the work of NEOWISE. NEO Surveyor is designed to find two-thirds of the 140-meter-class NEOs within five years and 90% of these objects within a decade of launch.

"With NEO Surveyor, we're really focusing on the population of objects that are most likely to come close to Earth frequently," said Mainzer, who is also NEO Surveyor's chief scientist.

It is understood that the probe will orbit around a Lagrange point called L1, about 1.5 million kilometers from the Earth. In this special position, the gravitational forces of the Earth and the Sun cancel each other out, allowing the probe to remain stable near this point.

The $1.6 billion NEO Surveyor mission will have a wider mirror and more detectors than NEOWISE, increasing its sensitivity for detecting asteroids.

The new mission's observation station is far from the Earth to minimize interference from the Earth's thermal radiation. NEO Surveyor will also be equipped with a better viewing angle and a sunshield, allowing the spacecraft to turn its mirror toward the sun to search for asteroids that cannot be observed by ground-based telescopes.

"By hiding behind this tall sunshield, we have the opportunity to rotate and look in the direction of the sunlight, which is something NEOWISE cannot do because its sunshield is so small," Mainzer said.

Engineers at JPL are preparing to assemble the NEO Surveyor spacecraft, and Teledyne Imaging Sensors is producing camera chips for the mission.

"We're still more than three years away from launch," Mainzer said. "So we're in the busy phase of getting all the hardware built. The telescope is at JPL right now, getting aligned to test whether it can focus. The panels and instrument housings for the telescope are also at JPL."

For scientists like Mainzer, the prospect of new missions on the horizon also makes shutting down the NEOWISE mission less regrettable.

"The team involved in this project was very dedicated and they persevered to the end," Mainzer said. "Actually, this is not a sad ending. This is a very beautiful ending because we got a lot of data, it gave birth to a lot of great scientific results, and it also gave birth to another mission. So I feel very happy. We have no regrets." (Chenchen)