Ceres

In 2006, the International Astronautical Federation formally defined both planets and dwarf planets.

“A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.”

One of the first objects designated a dwarf planet by NASA was Ceres, the 25th largest body between the Sun and Neptune. Because of its similarity to the large icy moons found in the outer solar system, Ceres can help shed light on small body formation in the early solar system. With this goal in mind, NASA launched the Dawn spacecraft in 2007, which arrived at Ceres in 2015, after making a 1 year stop at the large asteroid Vesta. There have been entire books written about the mission. While at Ceres, Dawn produced the following false color, but still gorgeous, topological map.

I’m particularly struck by both how moon like it is, and the interesting patterns atop its north pole.

But this body is not our moon. As it lies in the asteroid belt, we’re not even the planet with the closest semi-major axis to it; that distinction goes to Mars. Here, we can see it’s location on the day Dawn arrived.

With a trajectory colored in gray, much like the body itself, we can see how inclined it’s orbit is compared to the rest of the inner solar system.

One of the Dawn spacecraft’s defining features was that it used 3 solar pannel powered xenon ion thrusters, which could only produce a max thrust of 0.09 Newtons of thrust at any given point. Just to put that in perspective, if you took an average apple and held it, it would be exerting about 1 newton of force on you. That’s two orders of magnitude more force than Dawn’s thrusters. This is why Dawn, despite having gravity assist from Mars (and a 1-year break at Vesta) took 7 years to get to Ceres.

As an astrodynamics PhD student who works with low thrust spacecraft, another item of note stands out to me. At Dawn’s arrival to Ceres, it was 2.87 astronomical units (Where an astronomical unit is the distance between the Sun and the Earth), away from the sun. The energy from sunlight reaching a point, the solar flux, drops off proportionally to the inverse distance squared.

This means that at Ceres, Dawn’s solar panels, could only receiving, about 12% of the energy it could receive at earth. Being a low thrust spacecraft allowed dawn to rendezvous with two separate bodies.

If we wanted to launch a second spacecraft, powered by chemical propulsion, directly to Ceres, our best bet would be to launch in about January 2023, and it would take approximately 400 days to arrive there.

If our new spacecraft was launched atop either a Delta IV or Falcon Heavy, it could have a similar dry mass to dawn, while if it launched on the most powerful Atlas V, it would only have about two thirds the mass. This is without investigating any mass saving maneuvers, like gravity assists, which would decrease the required propellent, but increase the time of flight.

Tooling

I hope you enjoyed this more free-form post exploring Ceres and Dawn. In order to create the solar system plots and initial mission design plots, I used JPL’s small-body mission design tool and Small-Body Database Browser you should check them out sometime. I recommend looking at Eris, another dwarf planet, for some highly inclined fun.

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