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Scientists choose an asteroid by balancing three questions: will it answer the mission’s science questions, can the spacecraft reach and operate around it, and does the body have the physical and surface properties the mission requires? A flyby, orbiter, impact experiment and sample-return mission can therefore choose very different targets. There is no universal asteroid score or fixed checklist; each mission sets its own priorities and then narrows the candidates with trajectory calculations, observations and engineering reviews.
1. Start with the question the mission must answer
Target selection begins before engineers plot trajectories. The science team defines what evidence the mission needs and what comparisons would make that evidence persuasive. An asteroid is valuable when observations of it can test a specific idea, fill a gap in a population or provide a useful contrast with other targets.
Lucy: seek informative differences among Trojan asteroids
NASA’s Lucy mission visits Jupiter Trojan asteroids because these bodies may preserve clues about the formation and evolution of the solar system. The spacecraft’s trajectory made several candidates reachable, but reachability alone did not decide the itinerary. The team looked for scientifically informative variation in color, size and history so that the targets could be compared rather than treated as interchangeable rocks. Eurybates, for example, is the largest remnant of a rare massive collision and has a neutral-colored C-type spectrum, making it an unusual member of the group. NASA explains the target-selection reasoning in its Lucy mission FAQ.
OSIRIS-REx: find material that can answer a laboratory question
OSIRIS-REx asked a different question: could a carbon-rich near-Earth asteroid provide material that a spacecraft could collect and return to Earth for detailed laboratory analysis? That goal favored an object with an accessible orbit, a suitable composition and a surface where a sampling mechanism could operate. Bennu was selected because it fit that combination, not simply because it was nearby or visually interesting. The mission’s 2016 press kit describes the scientific and engineering reasons in NASA’s OSIRIS-REx “Why Bennu?” document.
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These examples show why “best asteroid” has no meaning outside a mission brief. A target that adds diversity to a flyby tour may be a poor choice for an orbiter that needs stable proximity operations, and an object worth imaging may not expose collectable loose material.
2. Calculate whether the spacecraft can get there
Once science requirements produce a candidate list, trajectory designers remove objects that cannot be reached within the spacecraft’s launch opportunities, fuel budget, schedule or instrument plan. Accessibility is calculated, not guessed from an asteroid’s distance on a chart.
NASA/JPL’s Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) illustrates the kinds of quantities used to screen candidates. Its accessibility and observing-opportunity tool considers:
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| Trajectory or observation factor | Why it matters |
|---|---|
| Total delta-V | The change in velocity required for departure, course corrections and arrival; higher requirements can exceed propulsion capability or leave too little margin. |
| Mission duration | A longer flight affects spacecraft lifetime, operations cost and how long instruments must remain healthy. |
| Time spent at the asteroid | More time can support mapping or repeated observations, while a short encounter may favor a fast flyby. |
| Launch-date interval | A narrow launch window creates schedule risk; a wider interval can provide operational flexibility. |
| Asteroid absolute magnitude | This standardized brightness measure helps describe the object and its observability, although it is not a direct size measurement without assumptions about reflectivity. |
| Orbit condition code | A measure of how well the asteroid’s orbit is known; poorer knowledge increases navigation uncertainty. |
The values are mission-dependent. A spacecraft with a different propulsion system, launch vehicle or instrument lifetime will obtain a different accessible set. The NHATS criteria and observing opportunities are documented by NASA/JPL CNEOS.
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Bennu as an accessibility example
NASA’s OSIRIS-REx press kit described Bennu’s orbit as relatively Earth-like: it ranges from 0.9 to 1.4 astronomical units (AU) from the Sun, and its orbital inclination differs from Earth’s by 6 degrees. The same document notes that the most accessible sample-return candidates tend to have fairly circular, low-inclination orbits. Those figures explain why Bennu suited that mission; they are not universal limits that every asteroid mission must meet.
3. Match the body to the mission architecture
A mission type determines what “usable” means. NASA/JPL’s catalog of small-body targets of spacecraft missions includes flybys, orbiters, impact experiments and sample-return missions, each with different operational demands.
| Mission architecture | Target characteristics that become important | Typical operational priority |
|---|---|---|
| Flyby | A favorable encounter geometry, adequate illumination and enough observing time as the spacecraft passes. | Maximize scientifically useful measurements during a brief, precisely timed encounter. |
| Orbiter | A gravity field and rotation state that permit controlled proximity operations, plus an orbit that can be navigated safely. | Maintain repeated coverage while managing collision, communications and fuel risks. |
| Impact experiment | A body whose size, shape, composition and observing geometry allow the impact and its aftermath to be measured. | Make the change produced by the impact detectable and interpretable. |
| Sample return | A navigable approach, a surface with accessible material, acceptable thermal and lighting conditions, and a spacecraft design that can collect and depart. | Obtain uncontaminated material without exceeding hazard, navigation or departure limits. |
A flyby can tolerate a rugged or rapidly changing surface because the spacecraft never has to descend. A sample-return vehicle cannot: it must approach slowly, determine its position relative to local terrain and keep its collection hardware clear of hazards. The architecture therefore changes the weighting of every other criterion.
4. Examine size, rotation and other physical properties
Physical measurements help engineers predict gravity, lighting, thermal behavior and navigation conditions. They also shape the scientific interpretation of images and spectra.
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For Bennu, NASA’s 2016 OSIRIS-REx press kit listed a diameter of 492 meters and a rotation period of a little over four hours. The mission considered size and rotation because a very small, rapidly rotating body can make close operations and sample collection more difficult. These are characteristics of Bennu, not universal minimum or maximum values: a large body may be ideal for one investigation, while a small one may be the only way to test another.
Shape and reflectivity matter as well. An asteroid that appears bright may be small and reflective or larger and darker; telescopic brightness alone does not settle the question. Better shape, size and rotation estimates improve both the trajectory model and the interpretation of surface observations.
5. For sample return, choose a site after choosing the asteroid
“The target asteroid” and “the sampling site” are separate decisions. OSIRIS-REx first selected Bennu as the mission destination. After arrival, the spacecraft mapped the entire body and the team evaluated specific places where a sampling maneuver could succeed.
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The site review combined four requirements:
- Scientific interest: the location had to contain material relevant to the mission’s carbon-rich, primitive-asteroid goals.
- Safety: the surrounding terrain needed to avoid boulders, steep slopes and other hazards that could damage the spacecraft.
- Navigation feasibility: the team needed reliable landmarks, lighting and measurements to guide the spacecraft during its approach.
- Sampleability: loose or weakly bound material had to be available to the collection mechanism in the area reached by the maneuver.
NASA describes this mapping-led process in “NASA’s OSIRIS-REx Mission Will Have a Map for That.” Deputy principal investigator Ed Beshore summarized the explorer’s first task this way: “I like to say the first thing any explorer does upon reaching a new land is to start making maps.”
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The scale of a sampling decision
NASA’s 2016 mission-planning article described the OSIRIS-REx touch-and-go target area as 164 feet (50 meters) in diameter. That number refers to the planned sampling area, not Bennu’s overall diameter and not a general rule for asteroid missions. It illustrates why a globally promising asteroid can still present a difficult local choice: the spacecraft must find a safe patch large enough for the maneuver, with material the sampler can actually acquire.
6. Improve the evidence before and during the encounter
Ground-based observations continue to change the candidate’s measured properties. Asteroids often appear as points of light through a telescope, so teams use complementary methods to estimate dimensions and shape.
Stellar occultations reveal a silhouette
During a stellar occultation, an asteroid passes in front of a distant star and briefly blocks its light. Observers at several locations record when the star disappears and reappears. Combining those timed “chords” produces a silhouette and improves estimates of the body’s size and shape. NASA explains how Lucy teams predict the shadow path and coordinate observations in “Watching the Blink of a Star to Size Up Asteroids for NASA’s Lucy Mission.”
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During a spacecraft encounter, tiny changes in the spacecraft’s motion can reveal an asteroid’s mass. Combining that mass with independently measured size and shape gives an estimate of bulk density. Density helps scientists compare composition among targets, while the improved shape model helps navigators predict how the asteroid’s gravity and surface geometry will affect operations.
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These observations do not merely refine a press release. They can change whether a candidate remains viable, alter the safest approach path or determine how confidently scientists can compare two asteroids.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.7. Compare candidates on mission-specific axes
Teams commonly organize trade studies around five axes. The axes are broadly reusable, but their relative weights are not fixed:
| Axis | Questions the team asks |
|---|---|
| Science value | What hypothesis can this object test? Does it add a distinct composition, age, color, size or collision history to the mission’s comparisons? |
| Trajectory accessibility | What delta-V, launch dates, flight time and stay time are available with this spacecraft? |
| Physical suitability | Are size, rotation, gravity, shape, thermal conditions and illumination compatible with the planned encounter? |
| Characterization quality | How well are the orbit, dimensions, rotation and composition known, and can observations reduce the remaining uncertainty? |
| Operational safety and sampleability | For close operations or collection, are navigation landmarks, terrain and usable material adequate? |
A candidate can lead on one axis and lose on another. For example, an object with exceptional scientific novelty may require more propulsion than the spacecraft carries. A very accessible asteroid may have a surface too hazardous for a sampler. The selected target is the one that offers the strongest mission-level case after these conflicts are made explicit.
8. Lucy and OSIRIS-REx show two different selections
| Feature | Lucy | OSIRIS-REx |
|---|---|---|
| Primary science purpose | Compare Jupiter Trojan asteroids that may preserve evidence about solar-system formation and evolution. | Collect carbon-rich near-Earth asteroid material for return to Earth and laboratory study. |
| How diversity matters | The target set spans useful differences in colors, sizes and histories; Eurybates is an unusual collision remnant with a neutral C-type spectrum. | The asteroid had to provide both the desired primitive composition and a practical collection opportunity. |
| Operational emphasis | Encounter geometry and observing time for a multi-body flyby tour. | Extended proximity operations, detailed mapping, safe approach and departure, and a workable sampling site. |
| Decision after arrival | Encounter observations further refine size, shape, mass and density comparisons. | Mapping Bennu led to a second decision: which local site could be sampled safely and productively. |
The contrast is the key lesson: mission design determines which asteroid properties count as decisive.
9. Why there is no universal asteroid score
The reviewed NASA mission materials provide criteria and examples, but they do not establish a portfolio-wide formula that assigns fixed percentages to science, cost, risk and accessibility. Nor do they give a single statistic for how many asteroids every mission screens. Those quantities vary with launch vehicle, spacecraft capability, funding, schedule, instruments and scientific objectives.
Instead, teams use mission-specific analyses. Early telescope data may identify a promising orbit and spectrum. Trajectory studies then test launch opportunities. As observations improve the shape, rotation or orbit, engineers update navigation and hazard models. For sample return, global mapping is followed by a local site trade. A target can therefore be rejected late if new evidence reveals unacceptable risk, or retained despite a difficult orbit if its science value is unique and the mission can afford the required capability.
10. A practical decision sequence
- Write the science requirements. Define the composition, history, population comparison or physical process the mission must investigate.
- Generate a candidate population. Use known orbital, brightness, spectral and observational data to identify objects that could address those requirements.
- Run trajectory screens. Calculate delta-V, launch-date intervals, flight duration and possible time near each asteroid using the actual spacecraft and launch assumptions.
- Check architecture-specific constraints. Test whether the body’s size, rotation, gravity, shape and illumination support a flyby, orbit, impact or collection operation.
- Improve target knowledge. Obtain additional astrometry, spectra, light curves, occultation measurements and other observations, then update orbit and shape uncertainties.
- Trade science against risk. Select the candidate that provides the strongest defensible science return within navigation, safety, schedule and propulsion margins.
- For sample return, map before sampling. After arrival, evaluate local terrain, temperature, navigation conditions and available loose material before choosing the final site.
This sequence explains why asteroid selection is an iterative engineering-and-science decision rather than a popularity contest. The target must be interesting, reachable, measurable and operable under the same mission plan.
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What readers should remember
- Science goals define what counts as a valuable asteroid.
- Trajectory accessibility is quantified with delta-V, launch timing, mission duration and time at the body.
- Flybys, orbiters, impact experiments and sample-return missions need different physical and operational conditions.
- A sample-return target is chosen before its specific sampling site, which requires a second safety and sampleability assessment.
- Occultations, spacecraft tracking and detailed mapping reduce uncertainty about shape, size, mass, density and terrain.
- There is no universal weighting formula; the best target is the one that satisfies a particular mission’s science and operational constraints together.
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