This month marks the 10-year anniversary of a remarkable feat for NASA: the first time a spacecraft departed the orbit of one extraterrestrial body to successfully go on to orbit a second.
NASA’s Dawn mission set out to study the two largest bodies in the asteroid belt in one trip. This mission not only marked huge strides in aerospace engineering, but also helped scientists make leaps in our understanding of how the Solar System and planets formed.
The Dawn mission was the ninth mission of NASA’s Discovery Program, whose ethos was focused on answering specific scientific questions through the exploration of our Solar System. These missions tend to be low-cost and have fewer instruments than a robust, flagship-class mission like Cassini or the Mars Curiosity rover.
Carol Raymond, deputy principal investigator for Dawn, tells Inverse, “The amazing discoveries made by the mission are a testament to the wisdom of NASA in establishing the Discovery program, the ingenuity and dedication of the Dawn team, and the missions’ fantastic flight system and instruments.”
Dawn launched from Cape Canaveral on September 27, 2007. On February 18, 2009, it made a flyby of Mars, using the planet for a gravitational assist. Then, on July 16, 2011, Dawn entered Vesta’s orbit. On September 5, 2012, Dawn departed Vesta’s orbit as it headed to Ceres, where it entered orbit on March 6, 2015. The mission finally ended on November 1, 2018, and the spacecraft is now supposed to be in stable orbit, though there has been no contact since the end of the mission.
Living witnesses to the dawn and evolution of the Solar System
The Dawn mission is appropriately named — Ceres and Vesta are bodies relatively untouched since the early days of the Solar System, leaving planetary scientists a way to understand our origins. Ceres and Vesta are termed proto-planets, the sorts of objects that collided to form much larger worlds. Ceres is also considered a dwarf planet, and Vesta might have been had impact forces not given it a potato-like shape. Both have tremendous potential to reveal to us details about how the history of our Solar System unraveled.
“By examining two bodies that formed in the earliest epoch of Solar System formation and remained largely intact, the Dawn mission was able to shed light on the conditions under which bodies were accreting in the early Solar System, and the evolutionary processes that shaped them,” Raymond says.
Time zero in the Solar System is counted from the formation of the first solids — tiny grains found in meteorites with an age of ~4.6 billion years. These solids are formed by accretion, a gravity-driven process wherein particles accumulate into increasingly large masses as they accumulate more gravitational force. Accretion typically starts with dust particles. In the case of our Solar System, these particles formed smaller solids that then collided and coalesced, giving way to larger bodies — planetesimals, planetary embryos, and planets.
“This was all happening in a very dynamic process early on, but then the gas and dust started to dissipate. So the accretion slowed down. Then the idea was the big bodies, like the planets, you ended up with had swept up everything in their gravitational sphere of influence and it left a bunch of leftovers,” Raymond says. This upended the previous view that the asteroids were fragments of what should have been the fifth planet from the Sun.
The discovery of the first asteroids
Prior to the discovery of Ceres, there were thought to be seven planets rotating around the Sun, the most recent discovery having been Uranus by William Herschel in 1781. However, a group of astronomers called the “Celestial Police” understood that based on Kepler’s work and the Titus-Bode law (which predicts planetary spacing), the elliptical orbits of Mars and Jupiter indicated there was something (or many things) missing between them.
Giuseppe Piazzi’s discovered Ceres on New Year's Day in 1801; it was initially thought to be the missing planet, until Heinrich Wilhelm Matthias Olbrs, a member of the Celestial Police, discovered a second body in 1802, now known as the asteroid Pallas. Afterward, Herschel coined the term asteroid for the new breed of celestial body that did not fit the definition of a planet, and increasing bits of the asteroid belt were discovered — including Olbrs second asteroidal discovery of Vesta in 1807.
According to Raymond, once computers came along, so did more advanced models of the Solar System’s evolution. There started to be all kinds of problems with the pre-existing model.
“The results of the mission were able to test and support emerging models of distinct compositional reservoirs in the protoplanetary disk, and migration and dynamic rearrangement of planetesimals,” Raymond says.
Collectively, Vesta and Ceres were a fortunate pair for researchers. Combined, they account for ~45 percent of the mass of the material in the main belt.
Although they both reside in the asteroid belt, neither Ceres or Vesta are actually considered asteroids. Ceres, with a diameter of ~940 km and constituting 25 percent of the total mass of the asteroid belt, is actually considered a dwarf planet, the only one in our inner Solar System. All the other known dwarf planets like Pluto are located past the orbit of Neptune.
What Dawn learned about Ceres and Vesta
Dawn was the first mission to orbit a dwarf planet, and that dwarf planet turned out to be a gold mine of scientific data for researchers. Ceres, Raymond says, “is a carbon-rich body with evidence of vigorous aqueous alteration in its early history that inevitably led to a global subsurface ocean,” making it a water world in line with moons in Jupiter and Saturn’s systems.
Ceres is still geologically active as well. Cryovolcanism leads to bright hydrothermal brine deposits on the surface. Organic material was also identified and is understood to be produced within Ceres.
While this could lead some to surmise that it hosts potentially habitable conditions like those at Jupiter’s moons Europa and Ganymede or Saturn’s moons Enceladus and Titan, it’s unlikely that life took hold there, Raymond says. At least not so far. But this finding might suggest there may be “a plethora of ocean worlds in the outer Solar System,” as Raymond puts it, in other large planetesimals or dwarf planets.
According to Raymond, evidence from Ceres can constrain its formation to around 1.5 million years after Vesta, which is curious given that it is more analogous to the outer planets. In the scheme of the Solar System’s history, 1.5 million years is the blink of an eye.
Models suggest that Jupiter’s formation opened a gap in the accretionary disk that prevented small ice-rich planetesimals from drifting towards the Sun, which likely enhanced the divide in composition between the inner and outer Solar System.
Meanwhile, Vesta is much smaller than Ceres with a diameter of ~530 km. It has a differentiated interior, similar to Earth, though thanks to the Dawn mission, scientists learned that its structure is a bit messier than the “layer cake” model we have on Earth (e.g. crust, upper mantle, lower mantle, outer core, inner core).
“Ceres changed our view of the evolution of large volatile-rich planetesimals and provided evidence in support of its origin in the outer Solar System,” said Raymond. “Vesta pinned down conditions in the inner Solar System in much finer detail, and both provided evidence to test dynamic models.”
How ion propulsion works
The Dawn mission was only possible due to a relatively recent advancement in aerospace engineering: ion propulsion. This is what Ion propulsion allowed Dawn to exit Vesta’s orbit, enter solar orbit, and then enter Ceres’ orbit.
In order to do harder missions, John Brophy — an engineering fellow at NASA, explains to Inverse how the ion engines worked. Historically, space missions have used chemical rockets, which are limited because there is only so much energy in the propellant before you burn through them. Ion propulsion, a type of electrical propulsion, is not limited in the same way.
“They even call (chemical propulsion) the tyranny of the rocket equation, because it's so limiting. But for ion propulsion, it breaks that tyranny,” Brophy says. “Now you can apply electric forces and accelerate those ions to very, very high speeds.” This makes dawn 10 times more fuel efficient than the best chemical rocket engines, Brophy says.
Dawn, and Deep Space 1 – a test experiment performed by NASA prior to Dawn, used xenon to generate the ion thrusting. The use of xenon is nontrivial; a noble gas is required for this flavor of propulsion. The thrusting of the engine occurs through the acceleration of ions using a consistent input of electricity, which in this case comes from solar power. This makes the ion propulsion incredibly fuel efficient.
Deep Space 1 was technically the first successful spacecraft to change velocity during its flight with ion thrusters. Using less than 74 kg of xenon, the spaceship changed its velocity a total of 2.7 miles per second (9720 miles per hour). Dawn set a new record, though, more than doubling that of Deep Space 1, by generating a total velocity change of over 7.1 miles per second (25560 miles per hour).
The power of ion propulsion is not to be underestimated. Brophy says it could be used to deflect comets or asteroids in the future. Sorry Bruce Willis and Ben Affleck, we don’t need you now.
“So, you can rendezvous with the asteroid, and you blast the ion beam into the asteroid,” Brophy says. “You can actually push it and control it to have a very precise control over where that asteroid goes. You can just change its orbit so that if it were on a collision course with Earth, you just push it off into a different orbit so it wouldn't hit the Earth anymore.”
Brophy and Raymond both mentioned the upcoming Psyche mission, whose launch was delayed, but will launch sometime next year. It will also use ion propulsion to visit another asteroid in the asteroid belt that is hypothesized to be an exposed metallic core of a very early planet in our Solar System’s formation.
“We can and should continue this type of planetary exploration for the immense new knowledge returned and the sheer joy of discovering new worlds for humanity,” Raymond says.