Matthew Ward Agius
Matthew Agius is a science writer for Cosmos Magazine.
It could take two months for ground-based observation telescopes to determine whether the Double Asteroid Redirection Test (DART)’s successful crash into Dimorphos has successfully changed the asteroid’s trajectory.
Impact was confirmed as having taken place at 7:14pm on Monday evening (USEDT) around 17 metres from the preferred target area at more than 23,000 kilometres per hour.
That almost bullseye hit was the only complication; NASA and John Hopkins University’s Applied Physics Laboratory’s (APL) team describing a “straight down the middle” mission that was free of adjustments to successfully strike the asteroid.
NASA and APL spent seven years preparing for today’s destruction of the US$325 million spacecraft, however it only marks the midpoint of the experiment. While engineering and management teams will disband and join other projects, those involved in the analysis of the impact will continue working.
The kinetic impact should shift Dimorphos around one millimetre closer to Didymos, and astronomers at Lowell Space Observatory in Arizona and the Las Campanas Observatory in Chile will shortly recalculate its orbit around the larger asteroid.
That miniscule change should be enough to change the length of Dimorphos’ orbit – calculated at 11 hours and 55 minutes prior to the impact – by several minutes.
Eclipse measurement the key to DART’s success
The Didymos asteroid system is over 11 million kilometres from our planet. For context, that’s the distance you would cover in 507 days of non-stop commercial flight around Earth.
Even with their incredible power, Earth’s best ground-based telescopes are only able to make out these asteroids as ‘light points’ – star-like blips on the screen that dim as they pass in front of (eclipse) each other.
These eclipses were used by ground teams to calculate Dimorphos’ original orbit time.
A new, shortened period will indicate the mission’s success. If no change is detected, it will send NASA’s planetary defence team back to the drawing board.
“We cannot see the two bodies from the earth – they just appear almost like a star, a point source,” says APL’s Dr Caroline Ernst, who was the instrument scientist on the DRACO camera that fed live images of the crash back to Earth.
“Much like when you discover an exoplanet, you can tell that it’s there just by the light dimming as it goes in front of the star. It’s similar for the Didymos system: You get eclipses between the main [asteroid] and the moon. And so you’re measuring the timing of those eclipses.”
Italy’s LICIACube about to sweep the crash site
The DART spacecraft launched a tiny, Italian-made cubesat in the week before impact.
LICIACube’s job is to follow the spacecraft towards Dimorphos and capture the aftermath of the impact with its on-board cameras dubbed ‘LUKE’ and ‘LEIA’.
Sweeping through shortly after the crash, these cameras should see a plume of asteroid debris emanating from the surface. DART’s team also expect to find a crater measuring 10 to 20 meters in diameter.
Ernst expects, given the incredible low gravity of the asteroid, the ejecta could still be “coming out” from the impact site as the cubesat completes its pass.
DART mission systems engineer Dr Elena Adams also suggests there may be discolouration of the impact site owing to the evaporation of new fuel sources on board the craft’s experimental ion engine system.
“We were carrying a lot of hydrazine and Xenon on board,” Adams says.
“As engineers, we were discussing in the control room, would we actually see some sort of ‘brightening’, based on the fact that we just evaporated a whole bunch of Xenon?”
LICIACube will begin transmitting images shortly after its sweep, however they are not expected to be available until at least Thursday (Australian time).
“Some things will likely come out in even days, maybe weeks to say ‘This is what an observatory saw,’ or ‘This is what LICIACube saw,’” explains Ernst.
“I know that they plan to download images in the next couple of days. So we’ll get some pieces of the answer soon.”
Australia’s role in DART images, as experts respond to the DART crash
The DART impact has captured the imaginations of the public and experts alike.
With NASA streaming live pictures in the hour before the impact, Australia’s CSIRO was deeply involved in receiving and relaying images from DRACO back to mission control.
The Canberra Deep Space Communication Complex (CDSCC) was responsible for capturing the final images from the spacecraft prior to impact.
“This historic mission is a proud moment for our team at the tracking station,” says CDSCC’s outreach and visitor centre manager Glen Nagle.
“Any final commands to the spacecraft and all of the images and data were beamed down to our antennas and then immediately relayed to the waiting DART mission scientists.”
Professor Alan Duffy from Swinburne University says the ability to change the trajectory of an asteroid using kinetic impact as tested by the DART project would be an important step towards protecting Earth from the same event that led to the extinction of the dinosaurs.
Preventing an object the size of Dimorphos from hitting the Earth is key,” Duffy says.
“While they’re not common they are by no means rare, and are expected to hit every 20,000 years or so.
“Thanks to their great speed, even this small an object would impact with an energy release equivalent to thousands of nuclear bombs, leaving a crater three kilometres across and annihilating everything in a blastwave stretching hundreds of kilometres.”
While the investigation of the Dimorphos asteroid will commence with impact site imaging being undertaken by the LICIACube satellite, some experts are interested to see what the long-term effect of the crash may be.
The images beamed to Earth in the moments prior to impact indicate Dimorphos is a ‘rubble pile’ asteroid of coalesced rocks, similar to other asteroids like Ryugu and Bennu.
the targeted asteroid poses no threat to Earth, Professor Trevor Ireland from ANU’s Research School of Earth Sciences suggests agencies like NASA and APL may need to investigate different methodologies if spacecraft collisions cause ‘rubble pile’ asteroids to fragment and pose an added risk to our planet.
“The idea for planetary defence is that we nudge these objects by transferring momentum from a spacecraft to the body.,” says Ireland.
“But now we can see that if we hit these rubble piles too hard, it will fly apart. So then you end up with a shotgun scatter rather than a bullet heading at us.
“In terms of momentum transfer, we might want to have larger spacecraft with lower impact velocities. The modellers will have a field day with the DART experiment. Hugely successful and informative.”