<Movie preview narrator voice> Asteroids have caused multiple mass extinctions. On Monday, Earth strikes back. </Movie preview narrator voice>

Space is mostly empty, but not completely so. As Earth barrels along its orbit around the Sun, it is peppered by impacts from near-Earth objects. Humans observe few such collisions. The ones we do see typically produce a pretty light show, rarely accompanied by noise and meteorites. However, the fossil record demonstrates that once in a while, collisions aren’t so benign; large asteroid impacts have dug craters, spawned tsunamis, caused climate changes, and wiped out life. What will we do if we find a previously undiscovered world on a path toward destruction?

With enough warning, a very small change in an asteroid’s orbital velocity could turn a certain future impact into a certain miss. There are lots of ideas for how to achieve such a change, but we’ve never tested any of them in space.

Artist's concept of DART spacecraft with asteroid in background
This illustration depicts NASA’s Double Asteroid Redirection Test (DART) spacecraft prior to impact at the Didymos binary asteroid system.
NASA / Johns Hopkins APL / Steve Gribben

The first such test happens on Monday, September 26th at 23:14 UTC (19:14 EDT/16:14 PDT, Earth received time), when NASA’s Double Asteroid Redirection Test (DART) smashes nearly head-on into Dimorphos, the satellite of asteroid 65803 Didymos. If the crash changes the velocity of Dimorphos’ orbit by a detectable amount, the mission will validate the notion that we can use a kinetic impactor to nudge a hazardous asteroid’s path — and keep Earth out of harm’s way.

NASA will host a live stream and press briefing during the three hours surrounding the impact. DART will stream photos from its DRACO camera down to Earth in real time, about one per second; space fans can watch those photos appear live on NASA’s uncommentated media feed beginning a little less than two hours before impact.

Fed into an onboard computer with software descended from anti-missile technology, those images will help DART autonomously guide itself to its crash. On its approach DART will initially target the binary system, then differentiate the larger and smaller members of the binary pair, and finally steer the spacecraft to strike the smaller Dimorphos. The mission recently tested this autonomous target selection capability by watching Europa emerge from behind Jupiter.

DART image of Jupiter and moons
This is a cropped composite from DART's Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) image, which centered on Jupiter during tests of the spacecraft's autonomous navigation system. From left to right are Ganymede, Jupiter, Europa, Io and Callisto.
NASA / Johns Hopkins APL

Once the DART spacecraft has smashed itself to bits, the DRACO camera feed will end, of course. On September 11th, an Italian-built minisatellite, LICIAcube (pronounced “lee-chee-ah kyoob”), separated from DART to establish a viewpoint on the carnage. LICIAcube will use two cameras (the high-resolution, monochrome LEIA and wider-angle, color LUKE) to shoot photos of Dimorphos throughout the approach, impact, and afterward. It will pass about 55 kilometers (34 miles) away from the moon 165 seconds after the predicted impact. LICIAcube’s images will be crucial for science but do not have the priority of DRACO’s real-time images, so they will trickle slowly down to Earth at the rate of a couple per day over the subsequent months.

Meanwhile, observers throughout space and across Earth will watch the immediate and long-term effects of the crash. Other in-space observers include the Lucy spacecraft, currently cruising toward its Jupiter Trojan asteroid mission, as well as the Hubble and James Webb Space Telescopes. They will be searching for the sparking plume of dust that may rise off the surface of Dimorphos in response to the crash. Both of the great observatories will image the Didymos system beginning about 15 minutes after the impact.

DART observing campaign map
A global observing campaign will follow the asteroid impact.
Johns Hopkins University Applied Physics Lab

Earth-based observers will also look for the brightening due to an impact plume, but the key result from the mission — detecting a change in Dimorphos’ orbital period — will take days to weeks to achieve. The Didymos system is near opposition and also near perihelion, and Dimorphos and Didymos mutually eclipse each other as seen from Earth, twice every 11-hour-55-minute satellite orbital period.

The impact, aimed nearly at the leading point of Dimorphos’ path around Didymos, should decrease that orbital period. Observers across Earth will look for the telltale dimming of the Didymos system’s light as these mutual events happen throughout the rest of 2021, while the pair are favorably positioned in the night sky. The period will need to decrease by at least 70 seconds for the change to be detectable above the uncertainty in the orbit.

Despite the favorable geometry, Didymos is still quite faint at 14th magnitude, out of reach for direct observation through an eyepiece from an 8-inch telescope. The mission organized no formal amateur observing campaign associated with the DART impact for this reason. However, astronomy clubs and other fans are still encouraged to observe Didymos and host watch parties.

Watch this space for an update on Monday after the scheduled impact! And while you’re waiting, enjoy this animation from Lawrence Livermore scientist Mike Owen, who asked: what will it look like when DART crashes into Dimorphos?



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