Changing the orbit of an asteroid – NASA’s Dart mission

On 26th September 2022 NASA will intentionally crash a spacecraft into an asteroid. Why are they doing this, you ask? To test, if it is possible to change its orbit. Still not sure why this is important? Let’s take a closer look at the mission and its goals.

Why would you slam a spacecraft into an asteroid?

To sum up the mission in a few words. NASA built a spacecraft with an ion-engine and considerable solar panels. Then they launched it on a Falcon 9 and send it on a course to the double asteroid Didymos. On 26th September, it will impact the minor-planet moon Dimorphos.

This seems like a waste of a spacecraft, which only launched on 24. November 2021 from Vandenberg Space Force Base. So, why would they do that? To test if it is possible to change the orbit of an asteroid. To understand why this might become essential, let’s take a step back and start with asteroids.

You might have heard of asteroids impacting Earth in the past. The latest notable impact happened in Chelyabinsk in 2013^[1]. In fact, there have been more such impacts and even bigger ones in the past. You probably have heard of the asteroid that impacted Earth 65 million years ago, killing much of life on Earth. The probability of impacts like these might be pretty low, they can, however, happen any time again. NASA has a database with over one million known asteroids in our solar system^[2]. Though none of them is a threat to Earth at the moment.

How devastating an impact of one of them on Earth would be, depends mainly on its mass, the speed of impact and its composition. The consequences of an impact can reach from a local destruction only, up to global changes and destruction. So, wouldn’t it be great, if we could prevent the impact in the first place? This is very difficult. However, there are three concepts how it might be possible^[3]:

• Impulsive deflection by a stand-off nuclear explosion
• Gradual orbit change with a nearby massive spacecraft (the “gravity tractor” concept)
• Impulsive deflection via a sudden addition of momentum (the “kinetic impactor” concept)

NASA’s Dart is the first mission, which will try the last one. Let’s take a closer look at the concept.

Impulsive deflection via a sudden addition of momentum

The name of the concept sounds pretty complex, but if you take a look at it, it’s actually pretty simple. To change the orbit of an asteroid, you need to change its speed or direction. All the concepts above try to achieve just that. The only difference is how they are attempting it.

In this case, the plan is to crash a spacecraft into the asteroid, thereby transferring the momentum of the spacecraft to the asteroid. Thus changing the speed and direction of the asteroid. The bigger the asteroid gets, the bigger or faster the spacecraft has to be. As it currently is difficult and expensive to launch such a spacecraft from Earth, the concept can only do small changes to the speed of an asteroid. We are talking about accelerations of mm/s or cm/s. Therefore, the concept is only useful for small asteroids or if we know years or even decades in advance about the impact.

Nonetheless, it is a concept worth testing. Especially as there are a couple of unanswered questions with this concept. This is where the Double Asteroid Redirection Test (DART) mission comes into play.

What is the Double Asteroid Redirection Test?

As mentioned before, the DART spacecraft will impact an asteroid. In this case, Scientist choose Dimorphos as the target. It is an asteroid with a diameter of about 160 m. It was picked because it’s orbiting the bigger – 780 m diameter – asteroid Didymos. Dart has a mass of about 560 kg and will impact Dimorphos at 6 km/s. The impact will change the orbit of Dimorphos. Take a look at the picture below, or watch https://youtu.be/aNSYuY6N1Rs.

One orbit of Didymos takes Dimorphos 11.9 hours. The impact will reduce this by 7 minutes. But how can we confirm and measure this change? By observing Didymos with telescopes on Earth. This is how the orbital period of Dimorphos was measured. After the impact, the period will be measure again to confirm its change. It will be done by basically taking the time between two transits of Dimorphos in front of Didymos (as seen from Earth).

Let’s get back to the spacecraft, or should I say spacecrafts as there are two?

What spacecrafts are involved in the mission

The DART spacecraft is the main spacecraft of the mission and the one that will impact on Dimorphos. Therefore, it isn’t equipped with scientific instruments. It only carries, want it will need to get to its target. Nonetheless, it is testing some new technologies:

• A camera (DRACO) for optical navigation for autonomous navigation to impact at the center of the asteroid
• 22 m² solar cells to power the NASA Evolutionary Xenon Thruster-Commercial (NEXT-C) engine
• Transformational Solar Array technology, which has very-high-efficiency solar cells and reflective concentrators providing three times more power than current solar array technology
• A new type of high gain communication antenna

The DART spacecraft is accompanied by a nano-satellite called LICIACube. The name stands for Light Italian CubeSat for Imaging of Asteroids. It is a 6-unit CubeSat and was built by the Italian Space Agency. Its mission is to take images of the impact, the plume and make observation of the asteroids after the impact of DART. This includes, for example, observations about the composition of the asteroid. Therefore, it is equipped with two cameras:

• LEIA (LICIACube Explorer Imaging for Asteroid)
• LUKE (LICIACube Unit Key Explorer)

LICIACube will only have a couple of days to make its observations, as it will only fly by the asteroids and not enter into an orbit.

What to expect on 26th September?

DART will impact the asteroid on 26 September 2022 at 23:14 UTC. NASA will provide a live stream from mission control on YouTube and cover the event on other channels. I expect it to be similar to events like the rover landings on Mars. We might even get to see some images.

Let’s see, if we indeed can change the orbit of an asteroid.

Do you want to know more about what’s happening beyond low earth orbit? Take a look at these posts.