The Double Asteroid Redirection Test (DART) – NASA's first in-space planetary defense demonstration – will get one chance to hit its target, the small moon in the binary asteroid system Didymos. The asteroid is an ideal test target: measuring the change in how the smaller asteroid orbits its larger partner is much easier than observing a change in a single asteroid's orbit around the Sun.
APL leads and is developing the DART mission for NASA. Despite the challenges of a global pandemic, work is humming along at APL and other locations across the country, as the mission heads toward its summer 2021 launch – and attempts to pull off a feat so far seen only in science fiction movies.
To direct the DART spacecraft to its intended target – a binary asteroid that consists of a small moon, Dimorphos, orbiting a larger body, Didymos – scientists first need to understand how the system behaves. They’ve been watching Didymos from Earth since 2015, and now, an international campaign coordinated by Cristina Thomas, DART's Observing Working Group lead from Northern Arizona University, is making critical observations using powerful telescopes to understand the state of the asteroid system before DART reaches it. Observations today will help researchers better understand the extent of the dent DART makes when it slams into Dimorphos in September 2022.
The most recent observation campaign took place on Cerro Paranal in northern Chile, where scientists viewed Didymos using the Very Large Telescope, which is run by the European Southern Observatory. The VLT comprises four telescopes, each with 8.2-meter mirrors, two of which were used to look at Didymos.
"The Didymos system is too small and too far to be seen from Earth as anything more than a point of light,” said APL's Andy Rivkin, DART investigation team co-lead, who participated in the observations. “But we can get the data we need by measuring the brightness of that point of light, which changes as Didymos rotates and Dimorphos orbits.”
Quick dips in brightness occur when the smaller object – in this case, Dimorphos -- passes in front of or is hidden behind a larger object (Didymos) from Earth’s point of view. These observations will help scientists determine the location of Dimorphos about Didymos and inform the exact timing of DART's impact – and maximize the deflection.
The investigation team will observe Didymos through next spring. Final ground-based observations will occur as the spacecraft speeds toward the asteroid, as well as after impact.
THE FIRST PLANETARY DEFENSE MISSION
Research with Impact
While the telescope observations are key to understanding the Didymos system, they're not quite enough to fully understand DART's target.
"Even though we are performing ground-based observations, we don't know much about composition and structure of Dimorphos," said Angela Stickle, DART's Impact Simulation Working Group lead from APL. "We need to anticipate a wide range of possibilities and predict their outcomes, so that after DART hits Dimorphos we'll know what our measurements are telling us."
Structure is essential to the equation; in Didymos, researchers aren't sure whether DART will impact an asteroid made of solid rock, loose rubble or something "softer," more akin to sand. A softer surface would absorb the punch and keep the asteroid from moving as drastically as a solid rock with a harder surface.
Extensive modeling and simulation, part of a large international campaign that started in 2014, is being done in conjunction with Lawrence Livermore National Laboratory and other members of the investigation team to help researchers predict what will happen to DART's target after impact. They've considered these various factors—along with the added momentum from DART's impact and the resulting debris ejected from the crater it creates – as they've run various simulations. These simulations help the team shape its expectations for impact.
We don't know much about composition and structure of Dimorphos. We need to anticipate a wide range of possibilities and predict their outcomes, so that after DART hits Dimorphos we'll know what our measurements are telling us.
Eyes on DART and Didymos
Researchers will have the ability to eventually see Dimorphos close-up – albeit briefly – thanks to DART's onboard camera (named DRACO) and a planned ride-along CubeSat.
Released just before impact, the shoebox-sized LICIACube, provided by the Italian Space Agency, would document DART's impact and its aftermath.
DRACO – the Didymos Reconnaissance and Asteroid Camera for Op-nav – is DART's only onboard instrument. It will serve primarily as an optical navigation system, capturing images that help the spacecraft reach its target.
Those images will feed the APL-developed Small-body Maneuvering Autonomous Real-Time Navigation (SMART Nav) algorithm – the system that, in the spacecraft's final hours, will automatically and precisely guide DART into Dimorphos. The team has used SMART Nav in a series of tests on simulated spacecraft avionics, which will boost engineers' confidence that the system will be ready to operate successfully when the mission is really relying on it.
Wired for Success
DART has gone through extensive development since the primary spacecraft structure arrived at APL this past spring. Jumping from modeling and simulation to assembling cables and connectors onto a wiring harness, the engineering team then quickly transitioned into full-on integration and testing.
The spacecraft, about the size of a refrigerator, sits in the middle of a vast clean room on APL’s campus, its panels and siding splayed out showcasing a colorful array of wires, cables and boxes – DART’s “guts.” All of the spacecraft’s meticulously tangled innerworkings, which have been tested from beyond the walls of the clean room, will get a final inspection in preparation for panel closeout. Once DART is buttoned up, it will be onto the environmental testing phase, which will subject the spacecraft to the rigors of launch and the harsh conditions of space.
DART will have every opportunity to test all of its capabilities and run through various scenarios in anticipation of what it will face when it finally reaches Dimorphos and Didymos in 2022. And while that sort of approach is par for any mission course, DART’s challenge is unlike any other. For a mission that only gets one shot, practice makes perfect.
DART is the first mission developed for NASA's Planetary Defense Coordination Office, and one piece of NASA's wider planetary defense planning. In 2016, NASA established the Planetary Defense Coordination Office (PDCO) to lead U.S. government efforts to detect and warn of potentially hazardous asteroids and comets and to study means to mitigate the hazard when possible.
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