This article originally appeared on Van Kane's blog and is reposted here with permission.
Last week, Santa in the guise of NASA managers brought the solar system small bodies science community a sack full of belated Christmas presents. The Venus science community was unfortunately left with no presents under the tree.
An artist’s conception of the Lucy spacecraft flying by the Trojan Eurybates and the Psyche spacecraft in orbit around asteroid 16 Psyche.
The losers were a Venus mapping mission and Venus atmospheric probe mission. Their rejection will continue a drought in NASA launches to our sister world that followed the Magellan mission’s launch in 1989. When asked why neither Venus mission was selected, the head of NASA Planetary Science division, Jim Green, answered that the competition for selection was among mission proposals and not between destinations. He said that the review teams found that the proposals for the Venus missions scored less well than the proposals for the selected missions.
Dates for key events in the Lucy and Psyche missions:
The Lucy mission, named after the famous humanoid fossil, will survey two asteroid fossil beds for clues to the early history of the solar system. It will study the Trojan asteroids that share Jupiter’s orbit, either preceding (the “Greek” camp in L4 Lagrangian orbits) or trailing (the “Trojan” camp in L5 Lagrangian orbits) the giant planet. Telescopic observations suggest these bodies have primitive compositions, several of which don’t appear to be represented in our meteorite collections and that haven’t yet been visited by spacecraft.
The planned orbits and asteroid encounters for the Lucy mission
The origin of these asteroid populations is a mystery, and its solution would tell scientists much about the dynamics of the young solar system. Planetary scientists now believe that the orbits of the giant planets migrated in toward the sun and then out again soon after their formation. In the process, they scattered asteroids and comets hither and thither. Jupiter’s Lagrangian orbits may have been sticky gravitational traps that caught a diverse sample of bodies that originated from throughout the outer solar system to its fringes. Another theory suggests that the Trojans originated in the same region as Jupiter and followed it in its movements and are therefore samples of conditions where Jupiter formed. Either way – and it’s possible that the present population represents a mixture of sources – these bodies hold clues to conditions and processes from the infancy of our solar system.
The most recent planetary decadal survey emphasized the importance of these bodies by prioritizing a mission to explore them. “Trojan asteroids, at the boundary between the inner and outer solar system, are one of the keys to understanding solar system formation. Originally thought to have been captured from the outer parts of the asteroid belt, Trojan asteroids are proposed in new theories to have been captured instead from the Kuiper belt during a phase of extreme mixing of the small bodies of the solar system. In-depth study of these objects will provide the opportunity to understand the degree of mixing in the solar system and to determine the composition and physical characteristics of bodies that are among the most primitive in the solar system.”
The report listed three key questions related to the study of Trojan asteroids in context with other bodies throughout the outer solar system:
What were the initial stages, conditions, and processes of solar system formation and the nature of the interstellar matter that was incorporated? Important objects for study: comets, asteroids, Trojans, and Kuiper belt objects.
“What governed the accretion, supply of water, chemistry, and internal differentiation of the inner planets and the evolution of their atmospheres, and what roles did bombardment by large projectiles play? Important objects for study: Mars, the Moon, Trojans, Venus, asteroids, and comets.
“What were the primordial sources of organic matter, and where does organic synthesis continue today? Important objects for study: comets, asteroids, Trojans, Kuiper belt objects, Enceladus, Europa, Mars, Titan, and Uranian satellites.”
The Lucy looks to the New Horizon Pluto mission for two of its instruments with near copies of that mission’s LORRI high resolution camera and the RALPH color camera and imaging spectrometer. The third instrument is a thermal emission spectrometer derived from an instrument on the OSIRIX-REx asteroid mission. Data from these instruments will provide information on the processes that shaped these worlds, their composition, and physical properties of the surface material such as the average size of particles. Tracking of the spacecraft’s radio signal will provide information on each asteroids mass and therefore density which provides clues to their composition and to whether they are solid objects or rubble piles.
The creativity behind the Lucy mission is that its proposers found a trajectory that over 12 years encounters seven asteroids (two in a binary system). The Lucy mission will encounter its targets using two large solar orbits that take it out to the orbit of Jupiter to encounter the Trojan swarms. In the first of these orbits, it will fly by a tiny main belt asteroid (DonaldJohanson, named after the paleontologist who led the team that found the Lucy fossil) and then four diverse asteroids in the Greek population. The next orbit takes it into the Trojan population for a single encounter with a binary asteroid system whose characteristics are similar to those of comets suggesting they may be refugees from the distant outer solar system. After this second long orbit, the spacecraft should have sufficient fuel for further encounters with main belt and Trojan asteroids in a third orbit if NASA approves funding for an extended mission. (Each of these extended orbits appear to take approximately six years, so any encounters from an extended mission seem likely to occur in the late 2030s.)
The Lucy mission will study a variety of asteroids through brief, but intense flybys. It will be something like photographing boulders along the roadside while speeding by on a freeway for later analysis. The second Discovery mission selected, by comparison, will be like parking your car next to one especially intriguing boulder for a nearly yearlong examination.
The single destination for the Psyche spacecraft will be the relatively large asteroid of the same name. This world is the largest of the rare (type M) metallic asteroids. Psyche could be unique remnant of a class of asteroids that formed so close to the sun that only metals could condense out of the early solar nebula and was later flung into the main belt of the asteroids. Or it could be the inner, metallic core of a once larger protoplanet that had its overlying layers of rock and possibly ice blasted off by impacts with other asteroids.
Telescopic observations reveal that Psyche’s surface is 90% metallic and 10% silicate rock. The spacecraft’s instruments should distinguish between these scenarios by measuring the composition in detail and looking at the arrangement of the silicate material. The mission’s principal investigator wrote me, “If the silicate material is primarily high-magnesian pyroxene or olivine, then these silicates are likely the remnants of a crystallizing magma ocean, and indicate that Psyche started as a differentiated planetesimal and had its mantle stripped, validating the mission’s prime hypothesis for this body. If the silicates are all primitive chondritic material, then they were likely added as later impacts, and Psyche may have started life as a highly reduced metallic body without a significant silicate mantle, or, the nature of impact flux and its consequences are far more significant than our current models indicate. The numbers and shapes of craters on Psyche’s surface may help decipher that story.” The spacecraft’s gamma-ray and neutron spectrometer (derived from an instrument on the MESSENGER Mercury orbiter) will help determine the asteroid’s bulk elemental composition.
Psyche the asteroid won’t be an unchanged relic. Its original surface will have been battered by numerous impacts over the subsequent billions of years. The hydrated materials recently discovered on its surface with telescopic studies, for example, are likely to have been delivered by impacts of other asteroids. It’s possible that by now, the body is a jumbled rubble pile. The cameras on the spacecraft (near copies of the cameras that the Mars 2020 rover will carry) will be tasked with taking the images that will allow geologists to reconstruct its history. By using filters tuned to specific wavelengths of visible and near-infrared light, the camera’s images also will help map the surface’s fine-scale composition.
Psyche Mission Team
The planned orbits for the Psyche spacecraft around its namesake
The Psyche asteroid’s sits deep within the asteroid belt at 3.3 times the Earth’s distance from the sun. (By comparison, Vesta is at 2.6, the asteroid Ceres is at 3, and the Trojan asteroids average 5.5 times the Earth’s distance from the sun.) To reach this world, the Psyche spacecraft, like the Dawn spacecraft that has explored Vesta and Ceres, will use solar electric propulsion to slowly but methodically reach its namesake world. The gentle thrust of its engines will deliver the spacecraft to Psyche approximately seven years after launch and will allow it to spiral down to progressively lower orbits. The mission’s planners expect the spacecraft to orbit as close as 105 kilometers from the surface where the cameras will have a resolution of 5 meters.
While not yet selected as an approved mission, the NEOCam telescope was awarded an additional year’s funding to mature its design. For the team proposing this mission, this is the third time it has vied for selection. It was originally proposed in 2006 and not selected as a finalist and reproposed in 2010 when it was awarded funding to mature the technology of its sensors. If the mission eventually is funded by NASA, it would have two goals. The first would be oriented toward protecting our planet by discovering a large number of the small (from a few tens of meters across up to a kilometer), near Earth asteroids that have evaded detection by other means. The second goal would be more scientifically oriented with the NEOCam telescope expected to also observe more than a million main belt asteroids and about a thousand new comets. The resulting database would allow sophisticated analyses on the sizes, compositions, and orbital dynamics of the population of small worlds.
So far as I can recall, this is the first time that a Discovery mission finalist has been awarded additional funds to mature its design to be ready for a future funding opportunity. (Two other missions from the 2010 competition were also given funding to mature instrument technologies, but neither were finalists.) NEOCam’s (this is an acronym for Near Earth Object Camera) focus on small bodies whose orbits lie close to and often cross that of Earth’s places it at the junction of several of NASA’s programs. From finding and mapping the location of these objects, there is good science, there is planetary protection, and there is finding potential worlds for future human exploration or mining. As a result, says NASA’s Green, the additional funding awarded to mature NEOCam’s design is seen as a strategic investment.
Unfortunately, missions to Venus are not seen as a strategic investment and both finalists for this planet are simply left as unselected. I was very disappointed to see that neither was selected. (I had hoped for the selection of one Venus and one asteroid mission.) I believe that this world can tell us much about the evolution of terrestrial planets in our solar system and represents what is likely to be a relatively common class of larger rocky worlds around other stars.
So for fans of Venus and for all the other solar system destinations, what are the opportunities for selection of future missions? The European Space Agency is currently reviewing proposals for its fifth medium class science mission, which would enable planetary missions roughly the same capability as NASA’s Discovery program. I know that there is a proposal for a Venus mapping mission and a Saturn orbiter to study the moons Titan and Enceladus. Based on proposals for the last competition, there are likely to be other missions proposed to study other solar system bodies including orbiting main belt asteroids. The planetary mission proposals are in competition not only with each other but also with astrophysics and heliophysics missions. The selection of finalists for this competition is expected by June, the selection of the final winner is expected around 2019 with a launch around 2029.
NASA has just begun the process to select its next New Frontiers mission, which will have a total budget (likely $1.2 billion or more) 80-100% larger than the Discovery missions (likely $675 million or more). These missions are selected from a pre-approved list of high priority missions. For this competition, this list is:
Comet Surface Sample Return
Lunar South Pole-Aitken Basin Sample Return
Titan and/or Enceladus
Saturn Atmospheric Probe
Venus atmospheric probe and lander
Trojan Asteroid Tour and Rendezvous
We don’t know what the selection of the Discovery Lucy mission, which will study Trojan asteroids, will have on the chances for the selection of a New Frontiers Trojan mission. The selection of the finalist proposals is expected in November, the final selection in mid-2019, and launch by the end of 2025.
Finally, NASA plans – subject to the generosity of the President and Congress with future funding – to launch Discovery missions approximately every three years. With the launch for the Psyche mission in 2023, the 15th mission in this series should launch around 2026. Working backwards from that date, we might see the start of the next completion late this decade and selection of the next mission(s) in the early 2020s. There were many exciting missions proposed for this just completed competition; many are likely to be re-proposed. And we are likely to see new ideas put forth.
As the selection of Lucy and Psyche shows, these competitions among scientists result in creative and scientifically rich missions. By the mid-2020’s we should have another two or more new missions to look forward to.
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