“We’re moving an asteroid,” said Tom Staller, NASA’s program scientist for the DART mission. “We are changing the motion of a physical celestial body in space. Humanity has never done this before.” Here’s what you need to know about this mission. The DART spacecraft is about the size of a school bus. It is traveling to reach its asteroid target from launch in November 2021. The spacecraft will reach the asteroid system on September 26. Impact is expected at 7:14 p.m. ET.
Where is he going;
The spacecraft is headed for a binary asteroid system, where a tiny asteroid “moon,” named Dimorphos, orbits a larger asteroid, Gemini. Twin. meaning “twin” in Greek, it is about 2,560 feet (780 meters) in diameter. Dimorphos, meanwhile, is 525 feet (160 meters) wide and its name means “two forms.” At the time of impact, Gemini and Dimorphos will be relatively close to Earth — within 6.8 million miles (11 million kilometers). Neither Dimorphus nor Gemini are in danger of colliding with Earth — before or after the collision.
What will DART do?
DART goes down in a blaze of glory. It will target Dimorphos, accelerate to 13,421 miles per hour (21,600 kilometers per hour) and crash into the moon almost head-on. The spacecraft is about 100 times smaller than Dimorphos, so it won’t obliterate the asteroid. Instead, DART will try to change the asteroid’s speed and path in space. The mission team compared this impact to a golf cart crashing into one of the Great Pyramids — enough energy to leave an impact crater. The impact will change Dimorph’s speed by 1% as it orbits Gemini. It doesn’t sound like much, but doing this will change the Moon’s orbital period. The boost will shift Dimorphos slightly and make it more gravitationally bound to Gemini — so the collision won’t change the binary system’s path around Earth or increase its chances of becoming a threat to our planet.
What will we have to see?
The spacecraft will share its view of the binary asteroid system through an instrument known as the Gemini Asteroid Identification and Camera for Optical Navigation, or DRACO. This imager, which serves as DART’s eyes, will allow the spacecraft to identify the binary asteroid system and discern which spacecraft it is supposed to hit. This instrument is also a high-resolution camera that aims to capture images of the two asteroids that will be transmitted back to Earth at a rate of one image per second in what will appear almost like video. You can watch the live stream on NASA’s website starting at 6 p.m. ET Monday. Gemini and Dimorphos will appear as spikes of light about an hour before impact, gradually growing larger and more detailed in the frame. Dimorphos has never been observed before, so scientists can finally get a sense of its shape and what its surface looks like. We could see Dimorphos in exquisite detail before DART crashed into it. Given the time it takes for the images to return to Earth, they will be visible for eight seconds before signal loss occurs and the DART mission ends — if it was successful. The spacecraft also has its own photojournalist along for the ride. A briefcase-sized satellite from the Italian Space Agency took a ride on DART in space. Called the Light Italian CubeSat for Imaging of Asteroids, or LICIACube, it was detached from the spacecraft on September 11. The satellite travels behind DART to capture what’s happening from a safe perspective. Three minutes after impact, LICIACube will fly by Dimorphos to take images and video of the impact plume and perhaps even spy on the impact crater. The CubeSat will turn to keep its cameras pointed at Dimorphos as it passes. The images and video, while not immediately available, will be transmitted to Earth in the days and weeks after the collision.
How will we know if the mission was successful?
LICIACube won’t be the only observer watching. The James Webb Space Telescope, the Hubble Space Telescope and NASA’s Lucy mission will observe the impact. The Didymos system may glow as its dust and debris are ejected into space, said Statler, the NASA program scientist. But ground-based telescopes will be key to determining whether DART successfully altered Dimorphos’ motion. The Gemini system was discovered in 1996, so astronomers have many observations of the system. After impact, observatories around the world will watch Dimorphos pass in front of and move behind Gemini. Dimorphos takes 11 hours and 55 minutes to complete one orbit of Gemini. If DART is successful, that time could be reduced by 73 seconds, “but we actually think we’ll change it by about 10 minutes,” said Edward Reynolds, DART project manager at Johns Hopkins University’s Applied Physics Laboratory. Statler said he would be surprised if a measurement of the period change was made in less than a few days, but even more so if it took more than three weeks.
What if the DART fails and doesn’t hit the asteroid?
“I’m very confident that we were going to hit on Monday and it was going to be a total success,” said Lindley Johnson, NASA’s planetary defense officer. But if the DART loses its proverbial dartboard, the team will be ready to ensure the spacecraft is safe and all its information will be downloaded to figure out why it didn’t hit the Mayor. The Applied Physics Laboratory’s Mission Operations Center will intervene if necessary, although DART will operate autonomously for the last four hours of its journey. It takes 38 seconds for a command to travel from Earth to the spacecraft, so the team can react quickly. The DART team has 21 contingency plans that it has rehearsed, said Elena Adams, a DART mission systems engineer at the Applied Physics Laboratory.
Why should we try it and why on this asteroid?
Dimorphos was chosen for this mission because its size is comparable to asteroids that could pose a threat to Earth. An asteroid the size of Dimorph could cause ‘regional catastrophe’ if it hits Earth. The asteroid system is “the perfect physical laboratory” for the test, Statler said. The mission will allow scientists to better understand the size and mass of each asteroid, which is critical for understanding near-Earth objects. Near-Earth objects are asteroids and comets with orbits that place them within 30 million miles (48.3 million kilometers) of Earth. Detecting the threat of near-Earth objects that could cause serious harm is a primary goal of NASA and other space agencies around the world. No asteroids are on a direct collision course with Earth, but there are more than 27,000 near-Earth asteroids of all shapes and sizes. The valuable data collected by DART will contribute to planetary defense strategies, especially in understanding the kind of force that can shift the orbit of a near-Earth asteroid that could collide with our planet.
Why don’t we just blow up the asteroid, like in “Armageddon”?
The movies make combating asteroid approaches seem like a rushed battle to protect the planet, but “that’s not the way to do planetary defense,” Johnson said. Blowing up an asteroid could be more dangerous because then pieces of it could be on a collision course with Earth. But NASA is looking at other methods of changing the motion of asteroids. The DART spacecraft is thought to be a kinetic impactor that could alter Dimorphos’ speed and trajectory. If DART is successful, it could be a tool to deflect asteroids. Another option is a gravity attractor, which relies on the mutual gravitational pull between a spacecraft and an asteroid to pull the space rock from the impactor orbit to a more benign orbit, Johnson said. Another technique is to deflect an ion beam or shoot an ion engine at an asteroid for long periods of time until the ions change the asteroid’s speed and orbit. But both take time. “Any technique you can imagine that changes the orbital velocity of the asteroid in orbit is a viable technique,” Johnson said. An international forum called the Space Planning Commission brought together 18 national space agencies to evaluate what might be best to deflect an asteroid, depending on its size and trajectory. Finding populations of hazardous asteroids and determining their sizes are priorities for NASA and its international partners, Johnson said. Design for a space telescope called the Near-Earth Object Surveyor mission is currently under review.
Will any other spacecraft fly from Dimorphos in the future?
The Gemini system won’t stay lonely for long. To investigate the impact’s aftermath, the European Space Agency’s Hera mission will launch in 2024. The spacecraft, along with two CubeSats, will arrive at the asteroid system two years later. Hera will study both asteroids, measure the physical properties of Dimorphos, and examine the DART impact crater and the moon’s orbit, with the goal of creating an effective planetary defense strategy.
