Protecting our home world: The Planetary Defence mission fleet

November 8, 2024 cosmos

Off

DESTINY+, Hayabusa2, International, ISAS, List, Small Bodies

On the 7th October 2024, the Hera mission launched into the sky. The destination is a binary asteroid that we last saw at close quarters moments before it was changed forever.

Named for the Greek goddess of marriage, Hera is the second part of humankind’s first demonstration of how we might protect the Earth from a deadly celestial impact. Concern for the safety of our planet has risen sharply over the last two decades, driven by the discovery of tens of thousands of closely orbiting asteroids, and the massive explosion over Russia that no one predicted.

In February 2013, an asteroid nearly 20m in size hit the Earth’s atmosphere. It exploded over Chelyabinsk in Russia with an energy equivalent to 440,000 tons of TNT. The resulting shock wave damaged thousands of buildings over 500 square kilometres, and approximately 1,500 people required medical treatment. The event took everyone by surprise, and underscored the vulnerability of our planet to a massive natural disaster.

The result was a concerted worldwide effort to seek nearby asteroids and determine if their trajectory was likely to collide with the Earth. The number of known “Near-Earth Objects” or NEOs soared from a few thousand into the tens of thousands and numbers continue to steeply climb. It is estimated that we currently know of less than 40% of NEOs with diameters over 140m, and less than 7% of NEOs with diameters over 50m. As demonstrated in Chelyabinsk, even asteroids below that size range pose a serious safety risk. Indeed, an asteroid with a size estimated at about 50m hit the Earth over Siberia in 1908. The “Tunguska Event” flattened 2150 square kilometres of forest. It is an area equivalent to that of Tokyo.

For even larger asteroids, the forecast is less bleak. The number of NEOs discovered with sizes over 1km has changed very little over the last twenty years. This suggests that we have discovered 95% of NEOs above this size, and likely 100% of asteroids larger than 10km. An asteroid in this size range is a possibility for the mass extinction that included the dinosaurs at the end of the Cretaceous Period 66 million years ago. None of the current population is posing that kind of threat to the Earth.

However, Chelyabinsk or Tunguska event is statistically likely every few hundred years. If the asteroid hits a populated area, the damage will be vast. Humanity needs to be ready.

The JAXA Planetary Defence team

Professor Yoshikawa Makoto is the leader of JAXA’s Planetary Defence team. Previously referred to as “Spaceguard” in Japan, JAXA has been involved in activities aimed at protecting our planet from impact since the early 2000s. The formal creation of a Planetary Defence team occurred in April this year with the goal of supporting existing programs and driving new initiatives.

Part of this program is identifying and tracking NEOs via ground based telescopes, such as that at the Bisei Spaceguard Center in Okayama. But should a threat be discovered, we need to be ready to divert the path of the incoming asteroid. For that, we need to go to space.

“The data we can obtain from observing asteroids from Earth is very limited,” explains Yoshikawa. “But if a spacecraft can reach an asteroid, the asteroid’s size and shape can be accurately determined, along with mass, density, and the type of surface material. This is information that we need to prevent a collision if the asteroid were to head towards the Earth.”

Multiple theoretical pathways exist for diverting an asteroid. But their effectiveness is heavily dependent on the properties of that space rock. So the best way to prepare for an incoming threat is to visit our local NEO population and uncover what we are likely to face.

Due to their small size and the vastness of space, intercepting asteroids is no small task. Fortunately, JAXA has some serious experience. With the Hayabusa missions, the space agency returned humanity’s first two asteroid samples, collected from asteroids Itokawa and Ryugu; both NEOs. Sample return missions require rendezvous, landing on the asteroid surface, and return detailed information about the asteroid structure and composition.

“Hayabusa, Hayabusa2, and NASA’s OSIRIS-REx missions were sample return missions from NEOs,” notes Yoshikawa. “NEOs were chosen because they are easier targets to reach when collecting samples. Planetary Defence was not the primary objective of these missions, but the results will be useful in this area.”

Hayabusa2 also performed an impact experiment on the surface of Ryugu, forming a new crater. The large extent of the hole revealed the asteroid to be a loosely-bound rubble pile. If a similar asteroid were to hit the Earth, the low mechanical strength would likely cause the asteroid to fragment into multiple pieces causing Chelyabinsk-like explosions over the planet. The density of rubble-pile asteroids is very low, and the force and technique needed to divert such an object would also be very different from a monolithic rock.

After delivering the samples to Earth in 2020, the Hayabusa2 spacecraft returned to deep space with a focus on planetary defence, renaming as the Hayabusa2♯ mission. The destination of Hayabusa2♯ is asteroid 1998 KY26 with a planned rendezvous in 2031. The asteroid has an estimated diameter of 30m, placing it in the class of asteroid with the approximately one-per-century risk of hitting the Earth and causing a major natural disaster. Asteroids of this size have never previously been visited by spacecraft, so their physical properties, structure, and composition, and thus the best methods for deflection, remain unknown.

“Around 1998 KY26, a very strange astrodynamical environment is formed by its small gravity and large centrifugal force due to its rapid spin,” describes Mimasu Yuya, Operation Lead for Hayabusa2♯. “This will be a challenging environment for Hayabusa2, because the spacecraft was not originally designed for such an environment. We hope to take on this unknown by devising an operation scheme and adding new functionality to the spacecraft guidance and navigation control.”

While en-route to 1998 KY26, Hayabusa2 will attempt a close flyby of asteroid Torifune (formerly 2001 CC21). A flyby is another tricky task, as the spacecraft will pass the asteroid at relative speeds of 5km/s (18,000 km/h). Navigation must be done with incredible accuracy to approach the asteroid close enough to snatch a few moments where images and data can be collected.

It is a skillset that will also be practiced by the JAXA DESTINY+ mission, which plans to flyby asteroid Phaethon. Like Hayabusa2, DESTINY+ is not primarily designed as a planetary defence mission but will use the incredibly high speed flyby of 36 km/s to take data on Phaethon and the asteroid’s unusual dust production.

This proximity imaging during rapid flybys could be employed to investigate a potential incoming asteroid. Moreover, if the trajectory of the spacecraft can be successfully controlled to such precision, then the spacecraft could also be placed on a collision course itself.

“The flyby of Torifune in 2026 by Hayabusa2 aims to perform a high-speed flyby with high precision navigation,” says Yoshikawa. “If we can do this, we will be able to crash a spacecraft into a small asteroid and change the asteroid’s orbit.”

Crashing a spacecraft into an asteroid to deflect its trajectory away from the Earth is one possibility for saving our planet from an incoming catastrophic collision. It is the one method that humanity has tested.

The Asteroid Impact and Deflection Assessment collaboration

In 2022, the NASA Double Asteroid Redirection Test (DART) slammed into asteroid Dimorphos. Dimorphos is the smaller companion of a binary asteroid system, orbiting its bigger sibling asteroid Didymos. While the DART spacecraft did not survive the impact to report on the results, the rotation period of the binary pair around one another could be measured from Earth and was observed to change after the impact with the spacecraft.

DART proved that colliding a spacecraft into an asteroid could definitely create a change in trajectory, but the details of the impact could not be determined from the Earth. The properties of Dimorphos, such as its density and cohesion, and the effect of the impact from the size of the crater, the location of the impact relative to the asteroid’s centre of mass, and any global distortion, remains unknown. Without this information, it is not possible to generalise the DART result to predict what would be needed to knock a different asteroid away from the Earth.

Left: A mosaic of the asteroids Dimorphos (left) and Didymos (right) created from images captured by the DART spacecraft. Middle: view of Dimorphos from the final 10 full-frame images obtained by DART camera. Right: the last complete image of Dimorphos, taken by DART from a distance of about 12km, and 2 seconds before impact. The image shows a patch of the asteroid that is 31m across (NASA/Johns Hopkins APL).

Hera was therefore launched this year to review the damage wrought by DART. Together, DART and Hera form AIDA: the Asteroid Impact and Deflection Assessment collaboration, which is an international initiative to explore the practicality of a redirecting an asteroid away from Earth using kinetic impact. NASA led the DART mission that performed the initial impact into Dimorphos. ESA is leading the Hera mission to review the result, carrying onboard a thermal infrared imager that was developed at JAXA.

The last view we had of the Didymos – Dimorphos binary system was a close-up of the rocks littering the surface of Dimorphos before the images returned by DART filled with a dramatic red. Hera’s arrival at the end of 2026 will give the first view of the crash site, and the spacecraft will begin to conduct detailed observations of both asteroids.

JAXA’s Thermal InfraRed Imager (TIRI) onboard Hera is the successor to the TIR imager that flew with Hayabusa2. A thermal imager looks at hot and cold regions on the asteroid, and can see how quickly these heat and cool as day rolls to night on the tiny worlds. This is a property known as “thermal inertia” and it is linked with density, as high density material cools slower than a more porous construction. When the TIR examined asteroid Ryugu, it revealed that the asteroid was covered in low density boulders. This was a surprise, as initial expectations had been that the boulders would be dense like the meteorites found on Earth.

Left: Asteroid Ryugu captured with the Optical Navigation Camera Telescopic (ONC-T) from about 9km. Right: Asteroid Ryugu now images with the Thermal Infrared Imager (TIR).

(ONC-T image: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST. TIR image: JAXA, Ashikaga University, Rikkyo University, Chiba Institute of Technology, University of Aizu, Hokkaido University of Education, Hokkaido Kitami Hokuto High School, AIST, National Institute for Environmental Studies, University of Tokyo, German Aerospace Center (DLR), Max Planck Society for the Advancement of Science, Stirling University.)

In addition to improved sensitivity and resolution compared to the Hayabusa2 TIR, the TIRI introduces a six-band multicolour spectral imager. This allows images of the asteroids to be taken using six different wavelengths of infrared light. The results can identify different mineral compositions amongst the rocks.

“The TIRI will be able to determine whether the boulders around the impact site are consolidated or highly porous,” said Associate Professor Okada Tatsuaki, JAXA Project Team Leader of Hera. “And whether the material inside the impact site is the same or different from that outside.”

This information on the construction and composition of Dimorphos will be highly valuable for understanding the crater size and resulting structural changes from the DART impact. It will also be important to compare with the TIR results from asteroid Ryugu, and later that of Torifune and 1998 KY26. While Didymos is through to be an S-type asteroid, Ryugu is a C-type asteroid and may have a distinctly different structure.

Experience gained with the impactor experiment onboard Hayabusa2 will also be useful as Hera examines the collision site of DART. The 15m diameter crater generated on Ryugu was much larger than the team anticipated, showing that the asteroid was a rubble pile of the porous rocks identified by the TIR.

“Before the Hayabusa2 Small Carry-on Impactor (SCI) experiment, the Hera team considered the asteroid to be made of monolithic rock and a several metre sized crater would be formed by the DART impact,” Okada remembers. “But after Hayabusa2, we considered that the asteroid was more likely to be a loosely-bound rubble pile under micro-gravity, and easily fragmented by high velocity impacts. A 15m diameter crater was formed by the impact of a 2kg copper projectile at 2km/s during the impact experiment on Ryugu. This suggests that a huge crater of more than 100m could be formed by the impact of the 550 kg DART spacecraft, travelling at 6km/s. A catastrophic event.”

Onward to Apophis

Despite being the subject of a hard bump by a spacecraft, asteroid Dimorphos is not expected to pass close to Earth. However, the same is not true of asteroid Apophis. The 340 m asteroid is due to pass within 32,000 km of our planet on 13th of April 2029, making it the first time in recorded history that an asteroid of that size has approached so close. The asteroid should even be visible to the naked eye over some parts of the Earth’s surface.

Apophis does not present a threat to the Earth, but the proximity provides an exciting opportunity to see how the Earth’s gravity warps and changes the asteroid. This, like the information about the impact of DART on Dimorphos, will help planetary defence plans should we need to provide the force to push an asteroid ourselves.

After delivering its own asteroid sample back to Earth in 2023, the NASA OSIRIS-REx spacecraft headed for asteroid Apophis, becoming the OSIRIS-APEX mission. OSIRIS-APEX will intercept Apophis after the close approach to Earth, providing data on the asteroid after it has been squeezed by the Earth. However, ideally we would like to see the asteroid before and during the approach for comparative studies.

ESA is therefore considering launching the Rapid Apophis Mission for SpacE Safety (RAMSES), which would intercept Apophis before the close approach and accompany the asteroid during the flyby.

While current asteroid encounters pose us no harm, the prospect of a major celestial impact is a chilling scenario. However, the fleet of spacecraft currently exploring asteroids close to our Earth are gathering the information that we need should a day dawn when we need to fight to protect our planet.

Further information:

Hayabusa2 website
DESTINY+ website

Hera website (JAXA)
Hera website (ESA)

OSIRIS-REx website (NASA)
RAMSES mission (ESA)