CubeSats to Study Asteroid Deflection Test
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 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.
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.
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 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.
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.