Final Orbit: How and why did NASA kill its spacecraft?

Final Orbit: How and why did NASA kill its spacecraft?

NASA/JPL

For more than a decade, NASA’s Near-Earth Object Infrared Survey Explorer (NEOWISE) mission has been searching the skies for near-Earth objects. Using its infrared vision, the spacecraft, which is in orbit above Earth’s surface, searched for asteroids and comets throughout the solar system, and used them to identify those that could come close to Earth.

You may recognize the name because it was used for one of the mission’s discoveries, Comet NEOWISE, which was the brightest comet in more than 20 years when it passed by Earth in 2020.

But the NEOWISE mission will soon come to an end, as the spacecraft will fall into Earth’s atmosphere and burn up. The team is preparing to this end early next year and obtaining the last science it can from the spacecraft before it becomes unusable.

The task can be completed in several ways. If the spacecraft relies on solar energy, as is often the case, it may fade away as its panels lose their effectiveness. That’s what happened to the InSight lander on Mars, where its solar panels were covered in wind-blown dust that built up over time until it eventually couldn’t generate enough power to continue operating. Or an orbiting spacecraft might run out of propellant, so it can no longer orient itself and return information to Earth, as happened with the Kepler space telescope.

The solar panels of the InSight Mars lander were too covered in dust by 2022 to continue working. NASA/JPL-Caltech

Sometimes, the task is no longer necessary because it has been replaced by newer and better technology. For example, the Spitzer Space Telescope was a pioneer in the field of infrared astronomy, but with the launch of the more powerful James Webb Space Telescope on the horizon, it is no longer necessary.

But what happens to the spacecraft once its mission is over? How do you go about stopping these things? We spoke to team members at NEOWISE, a NASA mission preparing to retire next year, to find out.

Great ending

Some spacecraft can be left to safely launch into space as long as there is no chance of them hitting anything or causing any damage. That’s what happened to Spitzer, which will gently float away from the planet for 50 years or so as it orbits the sun and tracks Earth.

However, planetary orbiting spacecraft must be disposed of carefully. When Cassini’s mission to Saturn ended, it plunged so far into the planet’s atmosphere, or out of orbit, that it disintegrated and was intentionally destroyed. This ensures there is no chance of contaminating any of Saturn’s moons, which could be habitable. The final landing also allowed for a range of stunning new research into Saturn’s atmosphere.

The NEOWISE spacecraft orbits Earth, so it must also be disposed of safely. The concerns are that the spacecraft does not become space debris, an increasing problem in some orbits, and that deorbiting does not pose a risk to anyone on Earth.

This last point is not certain. There have been cases in which pieces of deorbited vehicles have landed on Earth, as is the case with the Skylab space station, which fell from orbit in 1979, pieces of which landed in Australia.

Large pieces of Skylab, such as this oxygen tank, broke up in the Australian desert in 1979. Craigboy/Creative Commons

Today, missions must plan how they will deorbit before launch.

A specialist team at JPL (NASA’s Jet Propulsion Laboratory) is analyzing what the crashes would look like based on factors related to the spacecraft, such as its mass and speed. This makes the effects of deorbit predictable.

“There is very little, if any, possibility of any debris falling and injuring someone,” said Joseph Hunt, NEOWISE project manager at JPL. The spacecraft’s orbit will decrease, causing it to burn up in Earth’s atmosphere early next year.

“We want to finish this job safely,” Hunt said.

WHY ENDS NEOWISE

NEOWISE did not run out of fuel — nor does it have any propellant on board — nor did it become an artifact. But the activity of the sun forces its demise.

The spacecraft is located in low Earth orbit, about 300 miles from the planet’s surface, which means it is affected by the planet’s atmosphere.

“The Sun is constantly emitting charged particles and radiation at multiple wavelengths. What happens when it hits the Earth’s atmosphere is that it can cause it to inflate. That can cause the atmosphere to expand a little bit further,” explained Amy Mainzer, principal investigator for the NEOWISE mission. From his normal height.

A set of images shows differences in the sun’s activity levels over 10 years. David Chennett, Joseph B. Gorman, Lorraine W. Acton

The Sun operates on an activity cycle of approximately 11 years, sometimes more active and sometimes less. NEOWISE got lucky and began its extended mission during a period called solar minimum, when the Sun’s activity was low. The solar minimum was particularly quiet, so the atmosphere was less expanded than usual. All of this allowed the spacecraft to survive longer.

“It’s ultimately up to Mother Nature.”

But this reduced solar activity does not last forever, and we are now entering a period of increased solar activity, with more radiation from events such as solar flares affecting the atmosphere. This means the end of the spacecraft.

“You would think we’re in space, about 500 kilometers up in orbit, you would think that’s completely devoid of atmosphere. But not quite,” Mainzer said. “There’s very little left, and that’s enough to create drag forces that pull and drag the spacecraft.” “Eventually into the atmosphere.”

With no onboard propulsion system, the spacecraft cannot push back or move to another orbit. He can only rotate in place, so it’s a “battle we can’t win,” Mainzer said.

Knowing that the end is coming, the NEOWISE team sets a date when they feel comfortable finishing the job. The problem is that as drag from the atmosphere increases, the spacecraft becomes more difficult to control, and controllability decreases dramatically and rapidly.

“The laws of physics, in the end, always dictate what happens,” Mainzer said. “It’s ultimately up to Mother Nature.”

A second life

NEOWISE has been running since 2013, so it has been doing well for 11 years. But in reality, he is already in his second life. That’s because it used to be a completely different mission called WISE.

Workers prepare the WISE spacecraft for launch in 2009. Doug Kolko/NASA

WISE, or Wide-field Infrared Survey Explorer, was launched in 2009 and was designed to study objects inside and outside the solar system, including the Milky Way and distant galaxies. The mission discovered thousands of small objects called planetesimals, in addition to discovering many star clusters. It also discovered Earth’s first Trojan, an asteroid that shares an orbit with Earth.

The mission used infrared sensors, which must be cooled to extremely cold temperatures in order to work. The original temperature was only 8 K, and was achieved using solid hydrogen as a coolant.

But the coolant is consumed. By 2011, its mission completed, the coolant ran out, so the spacecraft went into hibernation. “I thought that was the end of the story,” Mainzer said.

But later, in 2013, NASA was interested in using a spacecraft to search for near-Earth objects, and the team realized that the WISE spacecraft might be able to do just that. It was reactivated and cooled to its new operating temperature over several months. Mainzer said the spacecraft stabilized at a temperature of 75 Kelvin, “which is considered a warm summer’s day for the cooling world.”

A series of mysterious red dots represents comet C/2020 F3 NEOWISE, which was discovered by the NEOWISE spacecraft in 2020. NASA/JPL-Caltech

At warmer temperatures, two of its four channels will not function properly, but the remaining two can still be used for scientific purposes. Over the past decade, the spacecraft has scanned the sky, searching for and finding thousands of near-Earth objects using millions of infrared measurements.

“Getting that amount of time outside the spacecraft was an amazing cosmic jackpot,” Mainzer said. “It wasn’t supposed to happen.”

Goodbye

Although NEOWISE is now coming to an end, it won’t be long before it gets a successor. The NEO Surveyor mission, which is working to launch in 2027, is also an infrared spacecraft inspired by the success of NEOWISE. It would have the benefits of improvements in infrared detectors, which have become more powerful in recent decades thanks to technology based on mobile phone cameras.

Technical rendering of NEO Surveyor. NASA

Mainzer will also be the main intermediary for NEO Surveyor, and is already busy overseeing the arrival of the instruments to the spacecraft ahead of integration.

Ahead of this launch, the team is still working on finalizing the NEOWISE mission and everything it entails. NASA doesn’t like to showcase the end of missions, preferring to focus on new launches or newly acquired data, Hunt said. But they want to make sure the mission ends with the right fanfare, which includes speaking to the media and celebrating mission accomplishments, as well as preparing internal reports and documents.

Not to mention, the spacecraft is still doing scientific research until the last possible minute, so mission operations must be maintained. “You never know on these missions, when the greatest possible piece of science could be published, or the greatest observation could be made,” Hunt said. “It might have been the day before it was shut down!”

When the final day of the mission comes, it will be a bittersweet experience for the team. NEOWISE has achieved far more over his many lifetimes than he ever expected, but it will still be sad to see him go.

“In some ways, these spacecraft are little extensions of you. They’re like your eyes in the sky. You can see what you see,” Mainzer said. But she added that what motivates her to take on the next mission is how much fun she and the team have had with NEOWISE. And now Moving on to the next adventure, she said, “Let’s go see more of the good stuff!”

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(tags for translation)space

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