Nasa's Dawn spacecraft takes a last look at Ceres on 1 September 2018
NASA’s Dawn spacecraft captured this view of Ceres on 1 September from 3,570 kilometers above the dwarf planet. Ceres’s youngest cryovolcano, Ahuna Mons, is visible just to the right of center at the top of the image. This picture is one of the last that Dawn transmitted before it ran out of fuel last week. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Shaped like a diamond, made of metal, or on a collision course with Earth, asteroids are strewn through the solar system like a trail of crumbs left over from planet formation.

Many asteroids are time capsules, keeping their secrets unchanged since the formation of the solar system.

The solar system hosts millions of asteroids, ranging in size from tiny specks a few meters across to potential planet destroyers many kilometers wide. They can be rich in carbon, filled with heavy metals, or packed with water and organics. Scientists think that many of these asteroids are time capsules, keeping their secrets unchanged since the formation of the solar system 4.5 billion years ago.

One asteroid, Ceres, is a dwarf planet, too. After NASA’s Dawn spacecraft visited this 950-kilometer-wide object, scientists learned that Ceres has a rocky core, an icy mantle, a crust, and a thin water vapor atmosphere. Ceres is geologically active, and it may even contain cryovolcanoes that spit out cold, organic-rich brines.

Dawn’s close-up study of Ceres showed us that visiting asteroids can yield valuable scientific discoveries. Last week, after 11 years in space, Dawn ran out of fuel, stopped communicating with Earth, and ended its mission.

The Dawn mission may be over, but here are six more ongoing or future asteroid missions that will continue its work uncovering our solar system’s origins. They might even help us prevent a deadly impact with our own planet.

1. Hayabusa2: A Diamond in the Sky

A large boulder on Ryugu’s surface, seen in part in the bottom left of this image, casts a shadow on the asteroid. Hayabusa2 was orbiting at an altitude of 64 meters when it took this photo on 21 September. Credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST

What might it be like to stand on the surface of an asteroid and watch the Sun rise? Hayabusa2 can tell you.

Hayabusa2 is an all-in-one asteroid mission: It will orbit, map, land, rove, and return samples to Earth. The Japan Aerospace Exploration Agency (JAXA) launched Hayabusa2 in December 2014. The craft arrived at its target, a diamond-shaped asteroid called Ryugu, this past June. It will remain there until December 2019, when it will return to Earth with samples from Ryugu’s surface and subsurface materials.

Ryugu is approximately 900 meters in diameter and travels in a 474-Earth-day elliptical orbit between Earth and Mars. It is made of carbonaceous material that likely dates back to the birth of the solar system.

“By exploring a C-type asteroid, which is rich in water and organic materials, we will clarify interactions between the building blocks of Earth and the evolution of its oceans and life,” according to the mission objectives.

The spacecraft has a suite of remote sensing equipment, including visible and infrared cameras and lidar, to map the asteroid’s surface properties. Hayabusa2 also carried four small landers. It released two of these rovers on 21 September and a third on 3 October. The rovers take advantage of Ryugu’s weak gravity to hop around the surface by repeatedly pushing off and landing in another spot about 15 meters away.

By combining data from the rovers and orbiter, the team selected a landing site for their first sample collection attempt, which will occur in early 2019.

Since landing, the rovers have been beaming back high-resolution photos from Ryugu’s rough terrain, a surface strewn with 10-meter-sized boulders. By stitching together a sequence of photos, the Hayabusa2 team demonstrated what it would be like to pass a 7.6-hour “day” standing on the asteroid’s surface.


2. OSIRIS-REx: Resources, with a Side Order of Risk

Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) launched in September 2016 and is scheduled to arrive at a large, near-Earth asteroid called Bennu this December. Bennu is interesting from a scientific point of view, but there’s another reason to give it a closer look: it’s 500 meters across, and there’s an uncomfortably high 1-in-2,700 chance that it could collide with Earth in the late 22nd century.

“OSIRIS-REx is an asteroid sample return mission, so our primary objective is to bring back pristine, carbon-rich material from the asteroid surface,” principal investigator Dante Lauretta said in an interview last September. Lauretta is a professor of planetary sciences at the University of Arizona in Tucson.

The craft will attempt three “touch-and-go” sample collections of asteroid material and bring its samples back to Earth in 2023.

Artist’s rendering of the OSIRIS-REx spacecraft orbiting above the asteroid Bennu
Artist’s rendering of the OSIRIS-REx spacecraft orbiting above Bennu. Credit: NASA Goddard Space Flight Center

“We believe this asteroid dates from the very dawn of our solar system and records the molecular evolution of organic molecules,” he said. “We’re hoping to gain insight into whether these kinds of asteroids delivered the building blocks of life to Earth, and maybe even the water that made our oceans and made our planet habitable.”

Given Bennu’s close proximity to Earth and its likely composition, another purpose of this sample return is to test the feasibility of mining aluminum, iron, platinum, and rocket fuel materials from asteroids in the future. That’s the “resource identification” aspect of the mission.

Bennu may also help us understand how sunlight heating the asteroid’s dark surface affects its orbit, something called the Yarkovsky effect (see the video below). The asteroid’s orbit has drifted about 280 meters since its discovery in 1999, and studying this effect up close may help scientists to better predict the odds of a too close encounter between Bennu and Earth (thus the “security” part of the acronym).

YouTube video


3. Lucy: Six Trojans and a Rock

Why visit one asteroid when you could visit seven in the same mission?

That’s just what NASA’s upcoming Lucy mission is set to do. It plans to visit seven asteroids that live in three distinct areas of space: in front of Jupiter, behind Jupiter, and in the main asteroid belt between Jupiter and Mars.

Six of Lucy’s seven targets are Trojans, the two clusters of asteroids that orbit in the stable gravitational points along Jupiter’s orbital path ahead of and behind the planet. Jupiter’s Trojan asteroids span a range of composition, size, and reflectivity. This variability implies that they likely didn’t all form in the same location.

“Trojans likely harbor objects that formed throughout the outer solar system,” Lucy principal investigator Harold Levison of the Southwest Research Institute in Boulder, Colo., said in a January 2016 presentation.  “Because the Trojans are remnants of the primordial material that formed the outer planets, they hold vital clues to deciphering the history of the solar system,” Levison later added in a statement.

Spacecraft Lucy’s orbital path will take it past six Trojan asteroids and one main-belt asteroid
Lucy’s orbital path, seen here in green, will take it on a complicated route past six Trojan asteroids and one main-belt asteroid by using gravity assists from Earth. This diagram shows a frame of reference where Jupiter remains stationary. Credit: Southwest Research Institute

Lucy, the first mission to visit Trojan asteroids, will visit at least one Trojan of each classification. Its seventh target is a main-belt asteroid. The mission has ambitious scientific goals for each target: catalog the asteroid’s geophysical and geochemical properties, measure its morphology and cratering, calculate its mass, estimate its bulk interior composition, look for undiscovered rings or satellites, and compare it to other asteroids of the same class and grouping.

The spacecraft will be an orbiter-only mission, visiting each asteroid without landing. Its instrument payload will include visible and infrared cameras, a radio science instrument, and spectrometers to measure surface compositions.

Lucy received approval for development on 30 October. It is currently scheduled for launch by NASA in October 2021, and it will take 12 years for it to reach all of its target asteroids.

“Lucy, like the human fossil for which it is named, will revolutionize the understanding of our origins.”

The time spent will be worth it, Levison explained. “Lucy, like the human fossil for which it is named, will revolutionize the understanding of our origins,” he said.

4. Psyche: A Heavy-Metal Experience

Asteroid 16 Psyche orbits in the main asteroid belt between Mars and Jupiter, along with the vast majority of asteroids. What makes this speck special enough for its own dedicated NASA mission? It is made almost entirely of metal and may be the leftover core of a planet that never formed.

“This is an opportunity to explore a new type of world—not one of rock or ice, but of metal,” Lindy Elkins-Tanton, Psyche principal investigator and professor of Earth and space exploration at Arizona State University in Tempe, said in a statement.

The mission that will explore this metal world, also called Psyche, will be a small spacecraft with large solar panels that will help it speed toward its target. Psyche will be equipped with magnetometers, multispectral imagers, and high-energy and neutron spectrometers. Magnetometers and high-energy spectrometers are not often included on missions to carbonaceous or silicate asteroids, but they are perfect for characterizing this mostly metal object.

Unlike the targets of other asteroid missions, 16 Psyche is almost entirely made of nickel-iron metal, similar to the composition of Earth’s core.

Artist’s rendering of the Psyche spacecraft orbiting asteroid Psyche
Artist’s rendering of NASA’s Psyche spacecraft orbiting the asteroid of the same name. Spikes like those depicted here might ring one of the asteroid’s large impact basins, formed from melted and quickly refrozen surface metal. Credit: NASA/JPL-Caltech/Arizona State University/Space Systems Loral/Peter Rubin

“Psyche is a unique body because it is, by far, the largest metal asteroid out there; it’s about the size of Massachusetts,” David Oh, an engineer at the Jet Propulsion Laboratory in Pasadena, Calif., and Psyche’s project systems engineering manager, said in September 2017. “By exploring Psyche, we’ll learn about the formation of the planets, how planetary cores are formed and, just as important, we’ll be exploring a new type of world.”

NASA has scheduled Psyche to launch in 2022 and arrive at its target in 2026.

“This is the only way humans will ever visit a core. We learn about inner space by visiting outer space.”

According to Elkins-Tanton, “16 Psyche is the only known object of its kind in the solar system, and this is the only way humans will ever visit a core. We learn about inner space by visiting outer space.”

5. DART: Diverting Disaster

Some 1,000 asteroids are about 1 kilometer in diameter, large enough to cause a global catastrophe if they collided with Earth. If we spot one on a collision course with Earth, could we nudge it out of the way?

NASA’s Double Asteroid Redirection Test (DART) mission, scheduled to launch in December 2020 at the earliest, aims to be the first to measurably change an asteroid’s orbital motion.

DART’s target is 65803 Didymos, a double-asteroid system. The 800-meter Didymos A and its 170-meter partner, Didymos B (nicknamed “Didymoon”), will pass within 11 million kilometers of Earth (about 29 times the distance from Earth to the Moon) in October 2022.

Artist’s rendering of asteroid Didymoon’s crater and ejecta following the DART spacecraft impact
Artist’s rendering of Didymoon’s crater and ejecta following the DART impact. Credit: ESA–

That’s when DART will crash into Didymoon at a speed of about 6 kilometers per second and change its orbital speed around the primary asteroid by less than 1%.

“A binary asteroid is the perfect natural laboratory for this test,” Tom Statler, program scientist for DART at NASA Headquarters in Washington, D. C., said in June 2017. “The fact that Didymos B is in orbit around Didymos A makes it easier to see the results of the impact, and ensures that the experiment doesn’t change the orbit of the pair around the Sun.”

Before its deliberate crash, DART aims to provide high-resolution images of the surface and field-test its autonomous guidance system.

“With DART, we want to understand the nature of asteroids by seeing how a representative body reacts when impacted, with an eye toward applying that knowledge if we are faced with the need to deflect an incoming object,” Andrew Rivkin, leader of the DART investigation team and a planetary astronomer at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., said in a press release. Data from DART, the team says, will help design a future mission to redirect a larger object.

6. Hera: The Forensic Analyst

Plans are afoot to return to Didymos with the Hera spacecraft to inspect DART’s effects. This European Space Agency–led component of the Asteroid Impact and Deflection Assessment (AIDA) should arrive at the Didymos system about 4 years after the DART impact.

Hera, which is currently in development, aims to survey the fresh impact crater and characterize how Didymoon changed postcollision. It is scheduled to launch in late 2023 and arrive at Didymos in 2026.

“This will enable us to assess the effectiveness of asteroid deflection as a planetary defense technique.”

“We’ll have a crater for which we have the initial conditions of its formation, offering us a documented impact experiment at actual asteroid scale,” Patrick Michel, director of research at France’s National Center for Scientific Research and lead scientist for Hera, said in a press release. “This will enable us to assess the effectiveness of asteroid deflection as a planetary defense technique and allow us to infer many things about collisions more generally and their fundamental role in all the stages of the history of the solar system.”

Hera will contain visible and near-infrared cameras similar to those on the Dawn spacecraft, a lidar instrument to conduct precision altimetry, a spectral imager to measure surface composition, and radio science instruments to measure Didymoon’s motion and communicate back to Earth.

The spacecraft also will insert two CubeSats into orbit around Didymoon, which will be able to get much closer than the main craft to the asteroid’s surface, survey the impact crater, and measure the asteroid’s altered dynamics.

“At just 160 meters across, the smaller component of the Didymos binary asteroid is too small to truly orbit around,” Michael Kueppers, Hera project scientist and a space scientist at the European Space Astronomy Centre in Madrid, said in September. “Instead, these CubeSats will fly Rosetta-like hyperbolic arcs, maintained by maneuvers every few days, hopefully culminating in landings.”

The video below shows how Hera will provide key information about how DART affected Didymoon and will help validate asteroid impact models.


The Era of Asteroids

On 1 November, NASA announced that Dawn, the longest continuous asteroid mission, came to a close after more than a decade spent exploring the two largest asteroids in our solar system, Vesta and Ceres. These and other asteroids, however, haven’t yielded the last of their secrets.

Planetary scientists around the world are preparing for the fresh influx of information on our diverse population of asteroids that will come over the next few decades. Where else could the trail of planetary breadcrumbs lead? We won’t have to wait long to find out.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Correction, 16 November 2018: This article has been updated to reflect the true diameter of Ceres.


Cartier, K. M. S. (2018), Exploring planetary breadcrumbs one asteroid at a time, Eos, 99, Published on 09 November 2018.

Text © 2018. The authors. CC BY-NC-ND 3.0
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