“Bullseye” — DART Changes An Asteroid’s Orbit

NASA’s DART spacecraft did better than expected altering Dimorphos’s orbit around its parent asteroid Didymos. Calling it a “bullseye,” NASA Administrator Bill Nelson announced today the orbit changed by 32 minutes, demonstrating that humans can alter an asteroid’s course. To use this method to actually deflect an asteroid that threatens Earth, the course change would have to happen years in advance. That’s why the most important next step in planetary defense is to find and catalogue hazardous asteroids. For that, the NEO Surveyor space telescope is needed.

The Double Asteroid Redirection Test (DART) was launched in November with a singular goal — crash into the 525-foot (160-meter) wide asteroid Dimorphos, which orbits a somewhat larger asteroid Didymos. The binary asteroid pair, or double asteroid, is a remnant of the formation of the solar system.

For all those eons, Dimorphos orbited Didymos once every 11 hours and 55 minutes. Now it’s 11 hours and 23 minutes. For the first time, humans have changed the motion of a celestial object.

Nelson called it a “watershed moment for planetary defense and a watershed moment for humanity.”

Observations by ground-based optical telescopes and radars agree the orbit changed by 32 minutes plus or minus 2 minutes. The minimum change for NASA to declare success was 73 seconds.

The point was to determine whether kinetic energy imparted by sending the refrigerator-sized DART spacecraft hurtling into Dimorphos at 14,000 miles per hour could change its orbit. On September 26, DART did just that, cracking a sizeable chunk off  Dimorphos in the process. An image taken by the Italian Space Agency’s (ASI’s) LICIACube cubesat showed the result.

LICIACube rode along with DART and separated 15 days before the impact so it could send back images of what happened. DART itself was destroyed. At today’s briefing, ASI President Georgio Saccoccia said a total of 720 images have been received.

The impact released a lot of debris that was spread out into space by the solar wind, creating a comet-like tail. The National Science Foundation’s SOAR ground-based telescope in Chile captured an image on September 28, two days after the impact. NASA’s Hubble Space Telescope took another look on Saturday, October 8, showing how the shape of the debris field already is changing.

Dimorphos is so small it can barely be detected from Earth and little was known about it until DART arrived. Images from DART prior to the impact and from LICIACube before and after are providing lots of data for scientists to study. LICIACube got as close as 59 kilometers (37 miles) traveling at a relative velocity of 6 kilometers per second (4 miles per second) according to Saccoccia.

For right now the focus is on how much the orbit changed as an indicator of whether this kinetic energy method would work if we ever actually had to divert an incoming asteroid.

Nancy Chabot, DART coordination lead at the Johns Hopkins Applied Physics Lab that manages the mission, emphasized that DART caused just a 4 percent change in the orbital period, a “nudge.” To use this technique in a real-world situation, an asteroid on a collision path with Earth would have to be nudged years in advance. “Warning time is key.”

Didymos and Dimorphos pose no threat to Earth. In fact, in the next 100 years no known asteroid poses a threat to Earth that would cause regional or global consequences. Those are asteroids 140 kilometers (460 feet) or more in diameter, about the size of Dimorphos or larger.

NASA’s planetary science division director Lori Glaze points out that only 40 percent of asteroids that size have been located, however. The next step in planetary defense is finding the rest of those asteroids as well as comets, known collectively as Near Earth Objects (NEOs).

In 1998, Congress directed NASA to locate 90 percent of NEOs 1-kilometer (0.6 miles) or more in diameter within 10 years. That size asteroid is thought to have caused the extinction of the dinosaurs. NASA accomplished that task and in 2005 Congress set another goal of locating 90 percent of hazardous NEOs 140-meters or more in diameter within 15 years in the George E. Brown Near-Earth Object Survey Act, part of the 2005 NASA Authorization Act. The 15 years have elapsed and while progress has been made, there is a long way to go because it is difficult to find such small, dark objects using only ground-based telescopes.

Glaze reiterated today what’s needed is a space-based infrared telescope dedicated to searching for asteroids, the Near Earth Object Surveyor or NEO Surveyor. The University of Arizona’s Amy Mainzer, Survey Director for NEO Surveyor, estimates it will take 10 more years to fulfill the congressional requirement once NEO Surveyor is in orbit. Without NEO Surveyor, make that 30 years.

The program had trouble winning support for years, but the recent Decadal Survey on Planetary Science and Astrobiology strongly endorsed it. Nevertheless, NASA proposed delaying NEO Surveyor for two years, from 2026 to 2028, in its FY2023 budget request, which is currently pending in Congress. House and Senate appropriators urged NASA to keep the launch date in 2026 or at least launch it before 2028 and added money to that end. What will be in the final appropriations bill remains to be seen. The recently enacted 2022 NASA Authorization Act directs NASA to launch it in 2026 or as soon as practicable.

Meanwhile, scientists are eagerly studying what happened to Dimorphos. Tom Statler, DART program scientist at NASA Headquarters, showed this intriguing depiction of the post-impact Dimorphos with rectangles superimposed over an image from LICIACube. The rectangles are not real, but each increases the contrast by a factor of two to highlight changes.

“The rectangles you are seeing are not real. It’s just that in each successive rectangle the contrast has been boosted by another factor of two in order to bring out that faint structure. And this is a visually stunning image. And every little wiggle in the streamers, every little blob, every little particle is a clue to something that happens on the surface of an asteroid when an object impacts it. And if you’re looking at this image and a dozen new questions are popping into your head that you would never have thought to ask before seeing this, well, that’s just one of the hallmarks of great science. It opens up new questions that we would never have thought to ask.

“But in addition to the science value of this image, I really love it because it is artistic. It is poetic. And even though those rectangles aren’t real, they’re suggestive of windows, of windows of understanding, we’re opening new windows of understanding looking deeper and deeper and deeper to gain a better understanding of not just how to defend our planet against this natural hazard of asteroid impacts, but also to better understand how our solar system works and how we got to be where we are now.” — Tom Statler

The data and images released today are just the beginning. Scientists will continue to refine precisely how much the orbit changed — the 32 minutes has an error bar of plus or minus two minutes — with data from four telescopes in Chile and two radars in the United States. They are just part of the dozens of telecopes on Earth and two in space (Hubble and James Webb) continuing to monitor the after effects and NASA’s Lucy mission enroute to the Trojan asteroids also may have gotten images yet to be received on Earth.

Didymos and Dimorphos soon will be getting another visitor, too. The European Space Agency’s Hera mission will arrive in 2026. DART and Hera are part of a NASA-ESA joint effort called the Asteroid Impact and Deflection Assessment (AIDA).