Anyone who has visited the small island of Venice, full of romantic canals and pedestrian paths with lots of dead ends, knows that distance does not always go hand in hand with ease of navigation. Fifteen years ago, NASA made one of the most complicated navigational routes to reach the smallest planet in the solar system: Mercury. The MESSENGER mission made its first flyby of Mercury 15 years ago on January 14, 2008, followed by two more flybys of the planet, and NASA finally put it into orbit on April 4, 2011.
Between its launch on April 3, 2004 at Cape Canaveral and its orbital entry in 2011, MESSENGER made a total of six flybys of Earth, Venus and Mercury. However, these were not just passive flights; they were gravity assists. Sean Solomon, principal investigator of the MESSENGER mission and former director/current assistant professor senior scientist at Columbia University’s Lamont-Doherty Earth Observatory, says Reciprocal that the challenge is not so much getting to Mercury as getting into orbit.
“According to celestial mechanics, if you send a spacecraft towards the sun and gain velocity from the sun’s gravitational well without slowing down along the way, the velocity will be around 10 km/s,” explains Solomon. “It’s too fast to do an orbital insertion with propulsion burn using any conventional propulsion system you might carry.”
Mariner 10, which made one Venus flyby and three Mercury flybys in the 1970s, was the first spacecraft to see the first rock from the Sun. Solomon explained that Mariner’s flyby data spurred the scientific community to send a spacecraft to conduct a more detailed study of Mercury. Unfortunately, at the time, no one knew how to get around the glaring problem of how fast a spacecraft would travel if its trajectory went straight from Earth towards Mercury.
No rocket system had the power necessary to slow the spacecraft enough to enter Mercury’s orbit with the sun’s overwhelming gravitational force emanating directly from the adjacent door. Without significant advances in rocket propulsion, such as solar-powered electric propulsion, another alternative would have to be discovered to study Mercury more thoroughly.
Mercury MESSENGER on the drawing board
Fortunately, in 1985, Chen-Wan Yen was a scientist at NASA’s Jet Propulsion Laboratory studying orbital dynamics. It was based on Gary Flandro’s fundamental idea for the Grand Tour of the Voyager program, which was to identify a rare outer planet alignment that would allow two spacecraft to visit four distant, large worlds in a relatively short time using gravity assists. As the spacecraft moves away from the sun, the force of gravity decreases, which means the spacecraft moves slower. Gravity assist, however, can launch a spacecraft by using the planet’s gravity to accelerate.
As it moves towards the Sun, the spacecraft will accelerate – too fast to orbit Mercury; but Yen realized that planetary flybys could also be used as gravity assists to slow down spacecraft. This was the basis of the MESSENGER mission.
Ralph McNutt, a physicist and chief scientist of space science at the Johns Hopkins University Applied Physics Laboratory, was the project scientist for the MESSENGER mission. He is also a member of the Parker Solar Probe team. The spacecraft will make its closest approach to the Sun of all missions. He explained that to prevent the spacecraft from getting too close, it would make a flyby of Venus after every third orbit around the sun.
McNutt was involved in brainstorming the idea for the MESSENGER mission, which was funded by the NASA Discovery Program. The program offers researchers the opportunity to submit new projects within budget limits. Many successful and well-known missions are part of the Discovery program, including the Dawn missions (which used a solar-powered ion drive), the InSight, Kepler and Pathfinder missions, and many others.
“There was always the question of how much would it cost and how would it be done? But it was really the beginning of the idea,” recalls McNutt. “So we said Discovery has a cost cap, we have to be able to do it cheaply. People talked about electric solar propulsion, but Dawn and Deep Space One didn’t exist. So we said, look, there’s no technology, we don’t know how to develop it. So we have to stick with the rocket launch and then try to see if we can make the gravity assist work.”
MESSENGER’s complex trajectory from launch pad to orbital entry involved one gravitational assist from Earth, two from Venus, and three from Mercury before the spacecraft finally slowed enough during its fourth planetary encounter to finally enter orbit.
Solomon explains that this meant that because each flyby required propulsion adjustments to reach the precise points needed to slow down the trajectory sufficiently to finally enter Mercury’s orbit, the spacecraft required a large amount of fuel. He estimated that of the launch weight of ~1100 kg, ~600 kg was fuel.
“So more than half of the launch mess was needed for all these maneuvers and for orbital launch,” says Solomon.
More specifically, McNutt says, “We launched MESSENGER and had the second largest fuel mass fraction of any spacecraft NASA has ever flown. Cassini had about 56 percent fuel at launch, and MESSENGER’s launch had about 52 percent fuel. This was because of all the velocity changes you needed from all the course corrections and to balance the gravitational systems. And then at the end you had to do a big burn at the end to actually insert the orbit.
Even with solar-powered electric propulsion, BepiColombo, ESA and JAXA’s mission to orbit Mercury, which launched in October 2018, will not orbit Mercury until 2025 and will still require several gravity assists. MESSENGER paved the way for those who wanted to conduct an in-depth study of the solar system’s deepest rocky mystery.
The MESSENGER team was inspired by Mariner 10’s flybys of Mercury. When proposing the mission, the MESSENGER team had six goals in mind. Combining these elements would help the MESSENGER team better understand the light gray planet’s evolutionary history and what sets it apart from other inner rocky planets in the solar system.
First, Mercury is made of denser material than any other planet in the solar system, so the planet’s core is a huge part of the planet’s interior, 85 percent by volume. Scientists wanted to understand why.
“It was really brand new territory to get down to chemical remote sensing instruments and go to Mercury, measure the composition of the crust and learn something about how a very dense planet like mercury has been folded and differentiated,” Solomon told Inverse. .
Solomon then recalled that there were many similarities between the Moon and Mercury. Both have heavily cratered areas and large smooth flat areas. But they also have many others – lunar marias are volcanic plains that have been triggered by asteroid impacts on the far side of the moon, which have a dark pigment due to their iron and titanium content. On Mercury, the color of the plains is brighter.
Solomon explains that Mercury also has distinctive tectonic features. “All the major tectonic features that Mariner 10 imaged appear to be contraction features; string errors. So the hypothesis was born [the Mariner 10] representing 45 per cent of the planet, Mercury has shrunk after most of the surface material has been deposited by cooling the interior, and this contraction has led everywhere to these shrunken landforms.”
“We wanted to significantly improve our understanding of geology,” he says.
Mariner 10 also detected a weak magnetic field. The researchers wanted to know if Mercury’s magnetic field was generated in the same way as Earth’s magnetic field – with a liquid outer dynamo core – or if it had a different mechanism and if the orientation was distinctive.
The team was interested in the polar deposits identified by Mariner 10, which suggested the presence of water ice as well as the planet’s unique exosphere. Mercury even has a comet-like sodium tail that can shine from the Sun and can illuminate details of changes in the planet’s exosphere.
MESSENGER retrieved all the data the mission had determined and remained in orbit collecting images and data until April 30, 2015, when it crashed into the surface of Mercury after an inevitable orbit breakup. Solomon says the biggest surprise was probably two chemical remote sensing measurements. He explained that one of the measurements suggested that the oxidation-reduction state was different from the materials of the other inner planets.
“So it says that Mercury comes from a different part of the solar nebula than the other inner planets.” Solomon says.
This would suggest that there were significant chemical changes in the gaseous, swirling, pancake-like cloud that predate the solar system and are ultimately responsible for its formation.
The second surprise was that all the models used to explain the metal-rich composition of Mercury showed depletion of volatiles (a type of substance that can easily go from solid or liquid to gas), but to the surprise of the MESSENGER team, Mercury has plenty of volatiles that have not yet been clarified.
Looking more closely at the polar ice caps, Solomon says that directly at the poles you can see an almost perfectly reflective surface that was ice – probably directly exposed, but just beyond the pole the ice was covered by about 20 cm of very dark material that does not reflect light. Surprisingly, he said, this kind of dark matter is most likely to be encountered in the outer system in places like small satellites of the major planets, off the main belt, and in Kuiper Belt objects.
“The hypothesis that has long been put forward to explain these very low reflectance values is that they are covered by some complex organic material,” he said.
He likened them to comets, which are a combination of ice and other materials and are often described as dirty – the idea being that on Mercury, these complex organic “dirty” materials keep the ice stable.
MESSENGER also found elemental carbon (probably graphite) on the surface, which Solomon says is likely contributing to the darkening of some of the darkest parts of Mercury’s surface.
“We associate carbon with compounds that only condense in the further part of the solar system” – he says. “So Mercury is an anomaly in many ways, and yet it appears to be in the innermost solar system.”
Will we ever land on Mercury?
MESSENGER paved a winding trail from Earth to Mercury and answered many of the scientists’ questions about the nature of the planet, but in so doing it may have raised more questions than it answered.
However, McNutt pointed out that while MESSENGER may be out of service, MESSENGER’s crash site may still be useful. It can act as an end member data point for BepiColombo if they want to see a new crater on the planet’s surface and compare it to older craters.
McNutt is also a proponent of the next step: a lander.
“Does the ice we see now in the polar regions of Mercury come from comets that have bombarded Mercury in the past? And if so, were there organics in some of these comets? As a result, did some of it remain in some of the materials we see near the poles? he asked rhetorically. “One way you go and solve this problem is you put a ladder on one of the poles and you actually look at it spectroscopically and try to figure out what you’re looking at.”