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1. Start with the question the mission must answer
Mission planners begin by stating the scientific questions in testable terms. A target is valuable when measurements at that object can distinguish between competing ideas, reveal an unusual case or provide a useful comparison with other bodies. NASA describes the Lucy mission’s Jupiter Trojan targets as potential records of how the solar system formed and evolved. Lucy’s team therefore sought a set of objects with informative differences in color, size and history, rather than simply the largest or brightest asteroid. NASA’s Lucy mission FAQ explains this target-selection logic.
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Why Lucy selected Eurybates
Eurybates illustrates how a distinctive object can strengthen a mission. It is the largest remnant of a rare massive collision and has a neutral-colored C-type spectrum. Including it gives Lucy a scientifically unusual member in a broader tour whose targets span colors and sizes. The point is not that every mission should seek a collision remnant; it is that the object’s properties must add information relevant to that mission’s comparisons.
Why OSIRIS-REx selected Bennu
OSIRIS-REx asked a different question: could a carbon-rich near-Earth asteroid supply material for return to Earth and laboratory analysis? Bennu’s composition made it scientifically attractive, but that alone would not make it a viable target. The spacecraft also needed an orbit that could be reached and a body that could support months of close operations and a controlled sampling attempt. NASA’s 2016 OSIRIS-REx mission press kit describes the reasons for choosing Bennu.
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2. Calculate whether the spacecraft can get there
After generating scientifically promising candidates, trajectory specialists determine which ones fit the spacecraft, launch vehicle and schedule. A trajectory is not just a line from Earth to an asteroid: it has an energy cost, a departure date, a flight time and an encounter geometry. NASA/JPL’s Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) evaluates candidates using total delta-V, mission duration, time spent at the asteroid, launch-date interval, asteroid absolute magnitude and orbit condition code. The NHATS accessibility and observing-opportunity tool documents these measures.
What the main trajectory terms mean
- Delta-V: the total change in velocity the mission must provide. Lower requirements generally leave more performance margin, but the acceptable value depends on the launch system and spacecraft.
- Launch-date interval: the period during which a suitable departure is possible. A candidate may be reachable in principle but unusable if its window does not fit the mission’s funding, development or planetary-assist schedule.
- Mission duration: the time from launch to arrival and, where relevant, the return leg. Longer flights affect consumables, reliability and the time available for other targets.
- Time at the asteroid: a brief flyby and a prolonged orbital or proximity campaign impose very different navigation and propulsion demands.
- Orbit condition and absolute magnitude: indicators used in accessibility studies to describe how well the orbit is known and how faint the object appears. They help planners judge both reachability and the effort needed to observe it.
Bennu’s accessible orbit was mission-specific
NASA’s 2016 OSIRIS-REx press kit says Bennu travels from about 0.9 to 1.4 astronomical units (AU) from the Sun and that its orbital inclination differs from Earth’s by 6 degrees. The same material notes that relatively circular, low-inclination orbits tend to be among the more accessible for sample-return planning. These numbers explain why Bennu fit that mission; they are not requirements that every asteroid must meet.
3. Match the body to the mission architecture
The spacecraft’s operating mode determines what “suitable” means. NASA/JPL’s Small-Body Targets of Spacecraft Missions list shows that missions may perform flybys, enter orbit, create an impact, or collect and return a sample.
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| Mission type | What the target must support | Typical selection emphasis |
|---|---|---|
| Flyby | A predictable encounter at a useful speed, distance and lighting geometry | Scientific contrast among multiple bodies, instrument viewing conditions and a trajectory that passes each target |
| Orbiter or prolonged proximity mission | Reliable navigation and propulsion operations near the body over an extended period | Well-characterized gravity, rotation, shape and hazards, plus enough science return to justify the longer campaign |
| Impact experiment | A target whose size, composition and observing geometry allow the collision and its effects to be measured | Physical properties, impact visibility and the ability to observe before and after the event |
| Sample return | A reachable body with a safe approach, navigable surface and material the collection system can acquire | Composition, surface conditions, touchdown risk, return trajectory and contamination control |
Lucy’s flyby tour can visit several bodies along one planned trajectory. OSIRIS-REx, by contrast, had to remain near Bennu, map it in detail and execute a controlled touch-and-go maneuver. A target that is excellent for a fast flyby may be unsuitable for hovering near the surface or collecting a sample.
4. Check size, rotation and other physical constraints
Physical properties affect both the science and the spacecraft’s ability to operate. In the OSIRIS-REx example, NASA described Bennu as 492 meters in diameter and rotating once in a little over four hours. Size and spin mattered because a very small, rapidly rotating body can make close navigation and surface collection more difficult. Bennu’s values are mission-specific examples, not universal minimum or maximum limits.
Planners also need estimates of shape, mass, gravity, surface relief, temperature and reflectivity. Those estimates may be uncertain when the asteroid is still only a point of light. Uncertainty is itself a planning factor: a target with poorly known dimensions may require more fuel margin, more optical navigation or a revised encounter plan than an otherwise similar object.
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5. For sample return, select a site after selecting the asteroid
Choosing a mission target and choosing a sampling location are separate decisions. OSIRIS-REx first selected Bennu as the asteroid, then mapped the body to find a place where the spacecraft could safely collect useful material.
Four tests for a sampling site
- Scientific interest: the material should address the mission’s questions about carbon-rich or otherwise relevant asteroid matter.
- Safety: the approach and departure path must avoid boulders, steep terrain and other hazards.
- Navigation feasibility: landmarks and surface geometry must allow the spacecraft to determine its position accurately during the maneuver.
- Sampleability: loose regolith or other collectable material must be available to the sampling mechanism in sufficient quantity.
NASA’s Goddard account, “NASA’s OSIRIS-REx Mission Will Have a Map for That,” describes teams mapping hazards, terrain, temperature, navigation conditions and potential sample material before narrowing the candidate sites. The planned OSIRIS-REx touch-and-go target area was described as 164 feet (50 meters) in diameter. A scientifically exciting patch that cannot be approached or sampled safely does not win the selection.
Ed Beshore, OSIRIS-REx deputy principal investigator at the University of Arizona, summarized the mapping phase this way: “I like to say the first thing any explorer does upon reaching a new land is to start making maps.” NASA published the quotation in the same mapping article.
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6. Reduce uncertainty with observations before and during the encounter
Ground-based astronomy continues to refine a target after a mission has been proposed. One technique is a stellar occultation: as an asteroid passes in front of a star, observers at multiple locations record the brief disappearance. The timing of each disappearance traces a chord across the asteroid’s silhouette. Combining the chords produces a better estimate of size and shape than a single image of a point-like object.
NASA describes Lucy teams predicting an asteroid’s shadow path, coordinating observations from several sites and combining the results into silhouette and shape estimates in “Watching the Blink of a Star to Size Up Asteroids for NASA’s Lucy Mission.”
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How candidate asteroids are compared
There is no published, portfolio-wide weighting that assigns the same percentage to science, cost, risk and accessibility for every mission. Instead, teams compare candidates along several axes and adjust their importance to the mission architecture.
| Comparison axis | Questions planners ask | Why it can change the decision |
|---|---|---|
| Science value | Does the asteroid test the mission’s hypotheses, represent an unusual class or add a useful comparison? | A less accessible object may be worth pursuing if it answers a uniquely important question; a scientifically similar object may be dropped if another is easier to reach. |
| Trajectory accessibility | What delta-V, launch dates, flight time and stay duration are required? | A target outside the available launch window or performance margin cannot be selected, regardless of its scientific appeal. |
| Physical suitability | Are size, rotation, gravity, shape and lighting compatible with the instruments and operations? | Flyby imaging, orbit insertion and sampling each tolerate different risks and uncertainties. |
| Characterization quality | How well are the orbit, dimensions, shape, mass and surface properties known? | Better knowledge reduces navigation surprises and helps teams design observations that will answer the science questions. |
| Operational safety and sampleability | Can the spacecraft approach, navigate and, if required, collect material without unacceptable hazard? | This axis is decisive for prolonged proximity operations and sample return, but may matter less for a distant flyby. |
A practical decision sequence
Although missions differ, the selection process can be understood as a sequence of gates rather than a single ranking:
- Write the science requirements. Specify what measurements, comparisons or returned material would count as success.
- Build a candidate list. Use known orbits, spectra, sizes and mission-specific target catalogs to identify bodies that could answer those questions.
- Run trajectory studies. Test launch dates, delta-V, transfer time, encounter speed and time available at the asteroid.
- Match operations to the body. Check whether the planned flyby, orbit, impact or sampling system can work with the asteroid’s size, spin, gravity and shape.
- Improve the target model. Obtain observations that refine orbit, dimensions, shape, rotation and composition; plan for remaining uncertainty.
- Trade science against risk. For sample return, map candidate sites and require scientific value, safety, navigation feasibility and collectable material together.
- Retain contingency options. A mission may keep alternate targets or sites until observations and rehearsals show that the primary choice is robust.
What asteroid selection is not
- It is not a contest for the largest, nearest or most visually dramatic asteroid.
- It is not governed by one universal scoring formula. The NASA examples provide criteria and trade-offs, but no cross-mission set of weights.
- It is not the same as sample-site selection. Bennu was chosen as a mission target before OSIRIS-REx selected a specific surface area.
- It is not finished when a spacecraft launches. New observations can refine shape, density, hazards and navigation plans during approach and encounter.
The result is a target that is scientifically meaningful and operationally achievable for one particular mission. Change the science question, launch date, spacecraft or encounter mode, and the preferred asteroid can change as well.
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