April 24, 2024

When an object like ‘Oumuamua appears again, we could be ready with an Interstellar Object Explorer (IOE)

On October 19, 2017, Pann-STARRS survey astronomers observed an Interstellar Object (ISO) passing through our system – 1I/2017 U1 ‘Oumuamua. This was the first time an ISO had been detected, confirming that such objects regularly pass through the Solar System, as astronomers predicted decades earlier. Just two years later, a second object was detected, the interstellar comet 2I/Borisov. Given the unusual nature of ‘Oumuamua (still a source of controversy) and the information ISOs can reveal about distant star systems, astronomers are eager to get a closer look at future visitors.

For example, multiple proposals have been made for interceptor spacecraft that could reach future ISOs, study them, and even conduct a sample return (like ESA’s Comet Interceptor). In a new paper from a team at the Southwest Research Institute (SwRI), Alan Stern and his colleagues studied possible concepts and recommended a purpose-built robotic ISO flyby mission called Interstellar Object Explorer (IOE). They also demonstrate how this mission could be accomplished on a modest budget with current spaceflight technology.

The study was led by Alan Stern, principal investigator of NASA’s New Horizons missions and their colleagues at the Southwest Research Institute (SwRI) in Boulder, Colorado. This included lead scientist Silvia Protopapa, manager Matthew Freeman, researcher/director Joel Parker, and systems engineer Mark Tapley. They were joined by Darryl Z. Seligman, a research associate at Cornell, and Caden Andersson, a researcher at the Colorado-based company Custom Microwave Inc. (CMI). Their article was published on February 5, 2024, in the journal Planetary and Space Science.

The Vera C. Rubin Observatory is under construction on Cerro Pachon, Chile. The observatory should be able to detect interstellar objects like ‘Oumuamua. Credit: Wil O’Mullaine/LSST

Interstellar objects (ISOs) are abundant!

Since ‘Oumuamua first touched our system, scientists have placed a high value on ISOs, which represent ejecta from other solar systems. By obtaining samples and studying them closely, we could learn a lot about the formation of other stars and planets without actually sending missions there. We could also learn a lot about the interstellar medium (ISM) and how organic material, and perhaps even the building blocks of life, are distributed throughout the galaxy (also known as the Panspermia Theory). As they state in their article:

“ISOs represent the remains of the formation of planetary systems around other stars. As such, their study offers critical new insights into the chemical and physical characteristics of the disks from which they originated. Furthermore, a comprehensive analysis of their composition, geology and activity will shed light on the processes behind the formation and evolution of planetesimals in other solar systems.

“Close encounters with small bodies in our solar system have greatly improved our understanding of these objects, contextualized our terrestrial observations, and advanced our knowledge of planetesimal formation models. Likewise, a close flyby of an ISO promises to be equally transformative. It is the next logical step in exploring the early history of our Solar System and exoplanetary systems.”

Furthermore, population studies of ISOs have indicated that about seven pass through our Solar System every year. Meanwhile, other research has shown that some are captured periodically and are still here. As next-generation instruments come online, scientists predict there will be a significant increase in the rate of ISO discoveries in the late 2020s and 2030s. This includes the Vera C. Rubin Observatory, currently under construction in Chile , which is expected to gather its first light in January 2025.

Researchers predict that the observatory will gather data on more than 5 million Asteroid Belt objects, 300,000 Jupiter Trojans, 100,000 near-Earth objects (NEOs), and more than 40,000 Kuiper Belt objects. They also estimate that it will detect around 15 interstellar objects in its first ten-year run, known as the Legacy Survey of Space and Time (LSST) – although other estimates say up to 70 ISOs per year. For their study, Stern and his colleagues assume that any ISOs within a distance of about twice the distance between Earth and the Sun (2 AU) would be bright enough to be detectable by LSST.

‘Oumuamua (l) and 2I/Borisov (r) are the only two ISOs we know for sure. Image credit: (left) ESO/M. Kornmesser; (right) NASA, ESA and D. Jewitt (UCLA)

Objectives and Instruments

As Stern and his colleagues explain in their paper, the IOE proposal would have two main scientific goals. This includes determining the “composition of ISO to provide insights into its origin and evolution.” As noted, these studies would provide valuable information about the initial conditions of the ISO host solar system. In this regard, the OIE would provide information similar to that which the New Horizons revealed about the Kuiper Belt Object (KBO) Arrokoth or how ESA’s Rosetta mission detected the building blocks of life on comet 67P/Churyumov – Gerasimenko.

Second, the IOE would determine or constrain the “nature, composition, and sources of ISO coma activity and determine the processes responsible for [the] observed activity.” Typically, coma activity results from the sublimation of ice as objects approach a star, which releases dust grains and refractory organic molecules from the core. As previous observations have shown, the activity of comets depends on solar heating and the comet’s own physical characteristics. As Stern and colleagues expressed in their paper:

“By characterizing the composition and spatial distribution of an ISO’s coma, IOE can directly determine the primary components of its target ISO, identify the mechanisms behind the coma’s activity, and deepen our insights into the composition and processes existing within its disk. of protoplanetary formation, where planetesimals as if they were forming… Furthermore, comparing the physical properties (i.e., chemical composition, size distribution, type of mixing) of ices and refractories in the coma with those on the surface can provide insights into possible processes that may have modified the surfaces. ”

Based on these scientific objectives, Stern and his colleagues listed which instruments the IOE would need. These include:

  • A visible-wavelength panchromatic imager with arcsecond-class angular resolution and high dynamic range
  • A visible wavelength imager with three filters (min) and an infrared imaging spectrometer covering the wavelength range 1–2.5 µm (possibly up to 4 µm) with a resolving power of at least 100
  • An ultraviolet (UV) spectrometer covering the wavelength range 700–1970 angstrom (Å) with a spectral resolution of 20 Å or greater
  • A visible-wavelength panchromatic imager and UV and infrared imaging spectrometers
Artist’s impression of a swarm of spacecraft with laser sails arriving at ‘Oumuamua, the interstellar asteroid. Credit: Maciej Rebisz

Mission Profile

The next step is the design of the spacecraft itself, which is dictated by the ephemeral nature of ISOs. As ‘Oumuamua and Borisov demonstrated, the speed of the ISOs means they are likely to remain undetected until they are near the inner edge of the Main Asteroid Belt. Furthermore, their hyperbolic trajectories mean they are likely to circle our Sun and become inaccessible shortly after being detected. Lastly, there is the positioning of the intercept mission itself, which directly affects the spacecraft’s ability to position itself and reach the ISO target.

For their study, Stern and his team selected a “storage orbit” location at the Earth-Moon Lagrange Point L1, located between the Earth and the Moon. This location has several advantages, namely the fact that a deployed spacecraft would need to generate very little thrust to reach escape velocity – meaning that most of your available acceleration (delta-v) will be put into your intercept trajectory. This storage orbit also means less propellant and less time needed to launch, and allows for rapid gravity assist in a near-Earth flyby.

For their study, Stern and his team set a detectability limit of 2 AU and simulated ISOs with an average speed of 32.14 km/s (~20 mps) and a solar closest approach of 10 AU or less. Other constraints that were considered included the positions of the Earth and the ISO at the time of its detection, the ISO’s orbit parameters, the maximum distance that a mission could intercept an ISO (also known as the “heliocentric intercept radius”), and the relative speed between spacecraft and ISO. To analyze this data effectively, the team generated an algorithm to optimize the interception trajectory and establish a small subset of ISOs that could feasibly be intercepted.

They simulated all of these calculations over a 10-year period and (using previous missions as precedents) derived several key parameters. As established, the mission would need to be capable of an acceleration (delta-v) of 3.0 km/s, establish a minimum flyby altitude of 400 km (~250 mi), intercept the ISO within 3 AUs of the Sun, and reach a hover speed of 100 km/s (62 mps). With this “detection sphere” established, they found that the chances of a successful interception increased considerably at higher speeds – 3 to 3.9 km/s (1.86 to 2.4 mps) – and at distances closer to 3 AU.


The study of ISOs is a burgeoning field of astronomical research that encompasses next-generation observatories (such as Vera Rubin) and proposed interception missions. In addition to IOE, similar concepts have been proposed since the detection of ‘Oumuamua and 2I/Borisov – including Project Lyra, a proposal made by the Institute for Interstellar Studies (i4is). While such a mission could take years to accomplish, detailed studies like this one will help inform the next phase of development – ​​the design and testing of the mission concepts themselves.

Stern and his colleagues acknowledge that more research is needed before that happens, but they emphasize that their work is an important first step. “More detailed work will be needed below to better prepare the mission concept to be proposed for a future NASA mission opportunity,” they write, “but this report provides the basic mission objectives, key requirements, and attributes as a point of departure”.

Further reading: Planetary and Space Science

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