NASA's NEOWISE Mission Ends After 15 Years, Making Way for New Explorations
On August 15, it was reported that last week, the US National Aeronautics and Space Administration (NASA) retired a spacecraft that had been in service for nearly 15 years, NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer), and will leave orbit in the next few months. This marked an important end to the agency's planetary defense program. During its service, the spacecraft discovered 400 near-Earth asteroids and comets.
In the near-Earth orbit, NEOWISE's infrared telescope scanned the entire sky 23 times and took millions of pictures. Its original mission was to search for infrared radiation from galaxies, stars, and asteroids, and later focused on objects within the solar system.
Exploring Near-Earth Objects
The Wide-field Infrared Survey Explorer (WISE) was launched in December 2009, and the original mission was designed to last for seven months. In 2011, after WISE completed its inspection and ended its main all-sky astronomical survey, due to the depletion of its cryogenic hydrogen coolant and the resulting reduction in the sensitivity of the infrared detector, NASA put the spacecraft into hibernation. But astronomers found that the infrared telescope could still detect objects close to the Earth, so NASA reactivated this mission in 2013 for another ten years of observations.
This restarted mission was renamed NEOWISE. Its purpose is to use the spacecraft's infrared telescope to detect tiny planets and comets with flight paths close to the Earth.
We never thought it would last this long, said Amy Mainzer, the chief investigator of NEOWISE from the University of Arizona and the University of California, Los Angeles.
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 leaving the orbit due to atmospheric drag. NASA expects that due to the expansion of the upper atmosphere caused by increased solar activity, the detector will re-enter the atmosphere and burn up by the end of this year, a few months earlier than expected. The satellite does not have its own propulsion system to boost it to a higher orbit.
Mainzer said, The sun has been very quiet for many years, but now it's starting to get active again, and this is the right time to let go.
So far, most of the detected near-Earth objects have been found through ground telescopes. But using a space telescope has its advantages because the Earth's atmosphere absorbs most of the infrared energy from faint objects such as asteroids.
Mainzer said that astronomers using ground telescopes mostly see the sunlight reflected on the surface of the celestial body. NEOWISE can measure the thermal radiation from asteroids, providing scientists with information about their sizes. We can actually get quite good size estimates with relatively few infrared measurements.
The telescope on NEOWISE is relatively small in size, with a main mirror diameter of 40 centimeters, less than 1/16 of the James Webb Space Telescope. But its wide field of view enables NEOWISE to search for infrared light sources in the sky, which is very suitable for studying a large number of objects. One of the most famous discoveries of this mission is a comet officially named C/2020F3 and more commonly known as Comet NEOWISE, which was even visible to the naked eye in 2020. As the comet approaches the Earth, large telescopes such as Hubble can conduct closer observations.
Nikola Fox, deputy director of NASA's Science Mission Directorate, said, The success of the NEOWISE mission is remarkable. By tracking asteroids and comets that may pose a threat to the Earth, it helps us better understand our place in the universe.
What's in the universe?
According to the data of the Center for Near-Earth Object Studies, the original mission of WISE and the extended survey of NEOWISE have discovered a total of 366 near-Earth asteroids and 34 comets. Among them, 64 are classified as potentially hazardous asteroids, which means they are less than 7.48 million kilometers (0.05 astronomical units/average Earth-Sun distance) from the Earth and have a diameter of at least 140 meters. These are the objects that astronomers hope to discover and track to predict whether they have a risk of colliding with the Earth.
There are currently about 2,400 potentially hazardous asteroids known, but there are many more lurking out there. Another advantage of space telescopes in searching for these asteroids is that they can observe around the clock, while ground telescopes can only observe at night. Dangerous asteroids like the one that exploded over Chelyabinsk, Russia in 2013, approached 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 approximately 25,000 near-Earth objects.
Mainzer said, The vast majority of the celestial bodies discovered by NEOWISE are very faint, and these are the ones that ground telescopes are more likely to miss. This in turn gives us a better understanding of the true number.
In 2010, with the original all-sky survey data of the WISE mission, scientists announced that they had discovered more than 90% of near-Earth objects with diameters larger than 1 kilometer. If any of these objects hit the Earth, it will have a global impact.
In 2005, the US Congress required NASA to discover at least 90% of near-Earth objects with diameters of 140 meters, which may cause damage in a regional range. So far, astronomers have discovered about 43% of such objects. The new detector, the Near-Earth Object Survey Telescope (NEOSurveyor), is planned to be launched in 2027 to further explore on the basis of the work of NEOWISE. NEOSurveyor is designed to discover two-thirds of the 140-meter-class near-Earth objects within five years and find 90% of such objects within ten years after launch.
Mainzer said, With NEOSurveyor, we're really focused on those groups of celestial bodies that are most likely to approach the Earth frequently. Mainzer is also the chief scientist of NEOSurveyor.
It is understood that this detector will orbit around the Lagrange point called L1, which is about 1.5 million kilometers from the Earth. At this special position, the gravitational forces of the Earth and the sun cancel each other out, allowing the detector to remain stable near this point.
The $1.6 billion NEOSurveyor mission will have a wider mirror and more detectors than NEOWISE, thereby increasing its sensitivity in detecting asteroids.
The new mission's observation site is far from the Earth to minimize the interference from the Earth's thermal radiation. NEOSurveyor will also be equipped with a better viewing angle and a sunshade, allowing the spacecraft to turn its mirror towards the sun to search for asteroids that ground telescopes cannot observe.
Mainzer said, By hiding behind this tall sunshade, we have the opportunity to rotate and look towards the sunlight, which NEOWISE couldn't do because its sunshade is very small.
Engineers at JPL are preparing to assemble the NEOSurveyor spacecraft, and the Teledyne Imaging Sensors company is producing the camera chips for the mission.
There are still more than three years until the launch, Mainzer said. So we are now in the busy stage of manufacturing all the hardware. The telescope is currently at the Jet Propulsion Laboratory, ready to be aligned to test whether it can focus. T he panels and instrument housings of the telescope are also at JPL.
For scientists like Mainzer, the upcoming new mission prospects also make the closure of the NEOWISE mission less regrettable.
Mainzer said, The team involved in this project is very dedicated, and they stuck to the end. In fact, this is not a sad ending. This is a very beautiful ending because we got a lot of data, gave birth to many great scientific results, and also gave birth to another task. So I feel very happy. We have no regrets left.