CubeSats to Study Asteroid Deflection Test
An artist graphic showing NASA DART impacting Didymos while the ESA Hera spacecraft observes. Hera will carry two CubeSats (APEX and Juventas) that will explore the Didymos objects in greater detail. Credit: ESA - ScienceOffice.org
As the dinosaurs figured out, the severity of asteroid impacts is Earth shattering. In an attempt to learn from their failure, NASA and ESA are collaborating to build and test planetary defense capabilities. The target is the Didymos binary asteroid system. NASA will impact the smaller object, Didymoon, with DART. ESA will observe the results with Hera. Together they will learn more about asteroids and deflection strategies. Taking a play from NASA's successful MarCO deep space CubeSats, ESA is outfitting Hera with two deep space CubeSats of its own, but with a unique twist. They will land on the asteroid.
The Didymos binary asteroid system is comprised of a larger object, Didymos and the smaller satellite object, Didymoon. The DART and Hera spacecraft will be the first to visit a binary asteroid system. Credit: ESA
The Didymos binary asteroid system contains two asteroids, Didymos and Didymoon. They are in both the Apollo and Amor groups, and are potentially hazardous asteroids (PHA). This means their orbit makes very close approaches to Earth and they could cause significant damage on potential impact. In size, Didymos is about 780 m (2560 ft) across and Didymoon is about 170 m (560 ft) across.
Diagram of the NASA DART mission that will impact Didymoon in a late 2022. This mission is studying the feasibility of asteroid deflection for planetary protection purposes. Credit: NASA
NASA and ESA are calling this international collaboration the Asteroid Impact Deflection Assessment (AIDA) mission. NASA is providing the Double Asteroid Redirect Test (DART) spacecraft that will impact Didymoon. ESA is providing the Hera spacecraft that will observe the results of the impact. Ideally, Hera would observe the impact in real time, but current plans only allow Hera to arrive at Didymos after DART's impact. In place of having a dedicated impact observation spacecraft, NASA will observe the impact's effects from ground based telescopes.
It is ideal for a spacecraft to be present when DART impacts Didymoon instead of relying on remote observations and a delayed visit. There are many time sensitive changes that will occur after the impact. One such change may be from the freshly exposed material revealed in the impact crater. It is unlikely that water ice will exist within Didymoon since it is so close to the Sun, but other materials may change when exposed directly to space. The impact plume will also be different by the time a delayed Hera arrives. Simulations may be able to recreate what likely occurred, but a direct observation reduces uncertainty in what happened.
The DART spacecraft is unique since is will carry not scientific payload other than a CCD for navigation. The high resolution CCD is derived from the New Horizon LORRI camera. It is a large challenge to impact Didymoon, requiring highly accurate autonomous homing to hit its center.
Diagram of the ESA Hera mission. Hera will study the physical, chemical, and orbital characteristics of the Didymos binary asteroid system. Two CubeSats will be deployed near Didymos that will each land on an object, providing additional information about the asteroid. Credit: ESA
Artist graphic of the ESA Hera spacecraft imaging the Didymos asteroid binary system. Hera will observe the results of the DART impact. In addition to the instruments Hera houses, it will carry two 6U CubeSats called APEX and Juventas. Credit: ESA - ScienceOffice.org
Hera will carry an asteroid framing camera to study the physical and dynamical properties of the asteroids. A laser altimeter will measure high accuracy shapes of both asteroids and determine mass estimates. Although not finalized, Hera may also carry an impactor similar to that used on Hayabusa2. This second impact would show how Didymoon reacts to two impacts using differing forces. As of January 2019, ESA announced the two CubeSats that will join Hera on its journey: Asteroid Prospection Explorer (APEX) and Juventas.
APEX and Juventas are 6U CubeSats (30 cm x 20 cm x 10 cm), the same size as the NASA MarCO and ESA GomX-4 CubeSats. Unlike the MarCO CubeSats that flew along InSight in deep space, Hera will deploy APEX and Juventas near Didymos.
APEX CubeSat that will survey the surface and interior structure of Didymos, along with landing on the surface. Credit: Swedish Institute of Space Physics
Juventas CubeSat that will measure the interior of the Didymos asteroids in high detail. It will also perform a landing on one of the objects. Credit: GomSpace
APEX has four main objectives. It will perform in-situ spectral analysis of the asteroids' surface, measuring how sunlight reflects off the surface. This will build a detailed map of the surface composition and the impact of space weathering. A magnetometer will pull out information on the interior structure of each asteroid. The navigation camera and LIDAR unit will make very close-up observations of one of the object's surfaces when APEX lands on it. Lastly, the inertial sensors will record the forces involved with landing on the asteroid.
Artists graphic of the ESA Hera spacecraft orbiting the Didymos asteroid. The APEX and Juventas CubeSats are orbiting close to Didymos, studying the surface and interior characteristics in high detail. Credit: ESA - ScienceOffice.org
Juventas will measure the internal structure of the Didymoon. Instead of a magnetometer, it will use satellite to satellite radio transmissions and low frequency radar to survey the interior. Not to be outdone by APEX, Juventas will also land on one of the asteroids. It is likely to bounce off the object at first, so this will be used to study the surface material in more detail.
DART is scheduled to launch in 2021, requiring 15 to 18 months of travel time before impacting Didymoon in 2022. Hera is currently scheduled to launch in 2023 with arrival at Didymos in 2024. However, DART may be launched later, allowing Hera to arrive at Didymos first, enabling it to watch the DART impact.
The AIDA mission with DART and Hera provides a unique opportunity to study near-Earth objects and how their orbits can be altered. Many questions will be answered by this mission, but many new questions will arise. This mission is extremely important for planetary protection and future asteroid mining. Planetary protection is an international issue, and we are happy to see international collaboration in this mission. We hope to see expanded cooperation related to this in the future.
REFERENCES
https://www.esa.int/Our_Activities/Space_Engineering_Technology/Hera/CubeSats_joining_Hera_mission_to_asteroid_system
https://www.nasa.gov/planetarydefense/dart
https://www.nasaspaceflight.com/2019/01/hera-objectives-planetary-defense-mission/
