NASA spacecraft on track for asteroid deflection experiment

Artist’s concept of the DART spacecraft, with its LICIACube ride along spacecraft, approaching asteroids Didymos and Dimorphos. Credit: NASA

A NASA spacecraft is aiming to slam into a stadium-size asteroid at more than 14,000 mph Monday in a planetary defense experiment to test a technique that could be used in the future to divert threatening asteroids off a collision course with Earth.

The Double Asteroid Redirection Test, or DART, mission will try to change the orbit of the asteroid Dimorphos, the smaller object in a binary pair of asteroids circling the sun close to Earth’s orbit. Dimorphos and its larger companion, named Didymos, pose no near-term threat to Earth, according to NASA.

Scientists will measure how the DART spacecraft’s collision changes the course of Dimorphos around Didymos, validating models of how a kinetic impactor in the future could knock an asteroid off a path that would strike Earth.

“This inaugural planetary defense test mission marks a major moment in human history,” said Bobby Braun, head of space exploration at the Johns Hopkins University Applied Physics Laboratory in Maryland, which built, manages, and controls the DART spacecraft for NASA. “For the first time ever, we will measurably change the orbit of a celestial body in the universe. Doing so has clear benefits in ensuring humanity’s ability to deflect a potential threatening asteroid in the future.”

The DART spacecraft launched last November on a SpaceX Falcon 9 rocket from Vandenberg Space Force Base in California. The spacecraft is about the size of a vending machine with a mass of less than 1,300 pounds, or 600 kilograms. It has two roll-out solar array wings spread out to a span of more than 60 feet (19 meters), which have generated electrical power through the spacecraft’s 10-month cruise to set up for Monday’s impact.

If all goes according to plan, DART will strike asteroid Dimorphos at 7:14 p.m. EDT (2314 GMT) Monday at a relative velocity of more than 14,000 mph, or about 4.1 miles per second (6.6 kilometers per second). The impact will pulverize the DART spacecraft, and scientists expect about 2 million pounds of rock and dust from Dimorphos will be thrown into space, equivalent to about 0.025% of the asteroid’s estimated total mass.

“We’re doing this test at a double asteroid, a binary asteroid, with the larger asteroid being orbited by its smaller moon Dimorphos,” said Tom Statler, DART’s program scientist at NASA Headquarters. “This is the perfect natural laboratory for this double test because there are two tests in DART. The first test is the test of our ability to build an autonomously guided spacecraft that will actually achieve the kinetic impact on the asteroid. The second test is the test of how the actual asteroid responds to the kinetic impact because, at the end of the day, the real question is how effectively did we move the asteroid? And can this technique of kinetic impact be used in the future if we ever needed to?”

Scientists classify Didymos and Dimorphos as near-Earth asteroids, although scientists say there is no near-term threat from the pair. No space mission has ever explored the asteroids, but scientists who have observed them through telescopes say the asteroids are about a half-mile (780 meters) and 525 feet (160 meters) in diameter, respectively. They are located about 0.6 miles (1 kilometer) from each other.

This graphic illustrates the major elements of the DART mission, showing the spacecraft’s approach and collision with asteroid Dimorphos, while the Italian LICIACube ridealong satellite and ground-based telescopes observe the impact. Credit: NASA/Johns Hopkins University APL

Experts estimate there should be around 25,000 near-Earth asteroids the size of Dimorphos. An asteroid of that size that impacts Earth could wipe out a metropolitan area, causing mass casualties.

NASA says surveys have discovered around 40% of similar-sized near-Earth asteroids. Scientists have found more than 95% of the population of larger 1-kilometer-class (0.6-mile) near-Earth asteroids, which could wreak global damage if they hit our planet. The percentage is much lower for the smaller asteroids, but they pose a more limited risk.

The $330 million DART mission is the first project by NASA’s Planetary Defense Coordination Office, set up in 2016 to help detect, track, and potentially defend Earth against potentially hazardous asteroids.

NASA plans to launch its second planetary defense mission, an infrared telescope and follow-on to DART, in the late 2020s to find most of the undetected dangerous near-Earth asteroids.

“We’re not aware of a single object right now within the next 100 years or so that is really threatening the Earth,” said Thomas Zurbuchen, head of NASA’s science mission directorate. “But I’ll also guarantee to you that if you wait long enough, there will be an object.”

“Our work right now with the DART mission is one possibility of what we might do if we found an asteroid on an impact course with the Earth,” said Lindley Johnson, NASA’s planetary defense officer, before the mission launched last year. “So we’re testing this kinetic impactor technique, where we just ram a spacecraft into the asteroid at high velocity to shave a little bit of speed off of its path, and that changes into the future.”

A small speed adjustment could result in large changes in the asteroid’s location years or decades into the future, meaning that with enough warning, a relatively compact spacecraft could be all that is needed to safeguard Earth from an impact.

“Our objective is to find these objects far way in time and far away from Earth, and to be able to enact this change in their orbit many years in advance, so it doesn’t take much to change them at all,” Johnson said.

“This demonstration will start to add tools to our toolbox of methods that could be used in the future, and we need several of them because the circumstances that we might face could be quite different,” Johnson said. ”

The most effective deflection method would depend on the size of a potentially threatening asteroid, along with its orbit and when it might hit Earth,

“Some of the other things that have been studied are what we call a gravity tractor, which is just taking a spacecraft and station keeping with the asteroid and using nature’s tug rope, gravity, the mutual attraction between the spacecraft and the asteroid will slowly tug that asteroid out of its impacting trajectory into a more benign one,” Johnson said. “Of course a technique like that takes longer to implement, so we would have to have more warning time to be able to implement it.”

Other options include ion beam deflection, where an ion engine could fire particles into an asteroid to gradually push it off course. And there’s the more violet nuclear option, which could involve a detonation near the asteroid.

The kinetic impact technique will be put to the test Monday. In order for it work, the DART mission fuses military-grade missile defense guidance technology to help the spacecraft home in on its asteroid target. And then scientists are ready with ground-based telescopes and space-based observatories — including the James Webb Space Telescope — to watch the sky for signs that DART hit the faint asteroid, then measure how the collision altered the trajectory of Dimorphos.

Scientists predict the impact will change the speed of Dimorphos around its companion Didymos by about 1%, likely reducing the time it takes to complete one orbit from 11 hours and 55 minutes to around 11 hours and 45 minutes. But the DART experiment will try to confirm those estimates, and it could take several weeks for scientists to precisely measure the orbit change.

The DART spacecraft itself won’t be around for that phase of the mission. The probe will be destroyed as it slams into Dimorphos, but will downlink near-real time images from its navigation camera at a rate of one frame per second until the impact.

Radar observations from Arecibo Observatory in 2003 showed the shape of asteroid Didymos, but little detail about its companion Dimorphos. Credit: Arecibo Observatory/NASA

The spacecraft’s Didymos Reconnaissance and Asteroid Camera for Optical navigation, or DRACO, imaging system, will take pictures of the Didymos and Dimorphos asteroids just before impact, collecting information on the asteroids’ locations to help DART navigate toward an aim point at the center of Dimorphos.

The final phase of the approach Monday will play out quickly. DART’s on-board computer will take control around four hours before impact, using sophisticated on-board navigation algorithms derived from missile guidance systems, called Small-body Maneuvering Autonomous Real Time Navigation, or SMART Nav.

The corrections needed to guide DART in toward Dimorphos will be too fast for mission control to command, and there will be a 38-second communication delay from the asteroid’s location to Earth, a distance of around 6.8 million miles (11 million kilometers).

Twelve hydrazine-fueled thrusters will steer DART on its final collision course.

DART will stream live video back to Earth from its DRACO cameras. Because of the high-speed approach and the small size of Dimorphos, the target asteroid will only be revealed in DRACO’s view finder in the final 60 to 90 minutes before impact. Didymos, somewhat larger than Dimorphos, is already resolved by DART’s DRACO camera.

About 50 minutes before impact, with SMART Nav at the wheel, DART’s navigation system will start to adjust its target from Didymos to Dimorphos.

“That’s a very sweaty time for us,” said Evan Smith, deputy mission systems engineer for DART at the Applied Physics Laboratory. “We have a lot of contingencies built right around that 50-minute transition and we’re going to be watching the telemetry like hawks, very scared but excited.

“And then from there, at 20 minutes to impact, we’re going to something called precision lock, where we totally ignore Didymos and we just go for Dimorphos only,” Smith said. “We expect to be thrusting quite a bit at that period. At two-and-a-half minutes to impact, we cut off all thrusting and we’re going to coast in.

“We’re going be streaming images the whole time, so images are coming in through DRACO, through our avionics and right out of the radio,” Smith said.

DART’s data handling system is designed to capture pictures, process them, and then downlink them to Earth in about 2 seconds, ensuring that the final image received on Earth will be taken just before impact. With the processing time on the spacecraft and the ground, coupled with the 38-second light travel time from DART back to Earth, the imagery received on the ground will be displayed with a lag of about 45 seconds behind real time, according to Elena Adams, DART’s mission systems engineer at APL.

Going into Monday’s approach, scientists know little about the shape of Dimorphos, which was discovered with a ground-based telescope in 2003. Didymos, the larger asteroid of the pair, was discovered in 1996, and scientists have some basic knowledge of its shape, thanks to radar observations.

“At about four minutes out … we’re finally starting to see the shape of Dimorphos, and then in four minutes we slam into it,” Adams said. “So there’s really not much time to react, and we’ve got to be right the first time.”

“It’s going to start off as a little point of light, and then eventually it’s going to zoom in and fill the entire field of view,” said Nancy Chabot, a planetary scientist at APL who is coordinating telescopic observations of DART’s impact and its aftermath.

The DART spacecraft released a small ride along spacecraft called LICIACube on Sept. 11. The stowaway spacecraft was provided by the Italian Space Agency, and is about the size of a small briefcase. It will fly a few minutes behind DART and attempt to capture images of the impact before sailing past Dimorphos at a range of around 34 miles (55 kilometers).

LICIACube’s images won’t come down live. They will be downlinked back to Earth slowly over the days following DART’s impact.

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