The arrival of an apparent interstellar visitor, the comet now designated C/2019 Q4 (Borisov), invariably calls to mind the all too swift passage of ‘Oumuamua through our skies in 2017. Detected 40 days after perihelion, the object was headed out of the Solar system when discovered, making observation time limited and the prospects of visiting it with a probe problematic. Nonetheless, Andreas Hein and colleagues at the Initiative for Interstellar Studies put out a mission concept we reviewed in these pages. To refresh your memory, see Project Lyra: Sending a Spacecraft to 1I/’Oumuamua (formerly A/2017 U1), the Interstellar Asteroid).
Image: C/2019 Q4 (Borisov), in the center of the image. Note what appears to be a short tail extending from the coma. Credit: Gennady Borisov.
The mission the authors described stretched the boundaries of the technologically possible, not to mention the resources that would be available for such an attempt. But now we have a second interstellar wanderer, one detected well before perihelion. In a new paper assembled in what must be record time, Hein and two i4IS colleagues, Adam Hibberd and Nikolaos Perakis tackle the challenge of reaching the comet, noting several options with varying launch dates.
This is interesting stuff, because as the paper notes, the fact that we have found both ‘Oumuamua and C/2019 Q4 (Borisov) so close together, plus the fact that better observing technologies are coming online to find such objects, indicates that the population of interstellar materials coming into the Solar system is high enough to generate new discoveries, and in the not so distant future at that. If any kind of mission concept can be developed that is remotely feasible for both interstellar visitors, we should soon find more we can reach with a probe.
Adam Hibberd’s role in the new paper was to develop the needed algorithms for calculating the trajectories involved. Called Optimum Interplanetary Trajectory Software (OITS), the software is used to calculate a direct transfer from Earth to C/2019 Q4 (Borisov) with flight durations of up to 10,000 days, with the necessary Delta-V (change in velocity) coming out below 100 kilometers per second, but still too high to obtain with existing chemical propulsion.
The software shows that a mission with feasible Delta-V and direct transfer from Earth would have had to have been launched in July of 2018, using a Falcon Heavy launch vehicle, with arrival at the object in October of 2019. We’re reminded again of the benefit of early detection of objects like these, capabilities that new facilities like the Large Synoptic Survey Telescope (LSST) should be able to deliver in just a few short years.
We learn, however, that other strategies can be deployed beyond chemical propulsion, and here we’re into various kinds of gravitational assists coupled with propulsive maneuvers. Key to the concept is the so-called Oberth maneuver, in which a spacecraft going deep into the gravitational well of a large object like the Sun accelerates at the point when it has reached maximum speed. Firing the engine when orbital velocity is highest gives the mission maximum kick and relies upon creating the highest thrust possible in the shortest period of time.
From the paper, describing the Oberth maneuver in the context of this mission:
For later launch dates, the DeltaV increases to levels where no existing chemical propulsion system would be able to deliver the required DeltaV. One possibility to still reach an interstellar object is to use an Oberth maneuver. For an Oberth maneuver, the spacecraft is injected into a trajectory with a [perihelion] close to the Sun, where the spacecraft applies a boost. The closer the boost is applied to the Sun, the larger the gain in DeltaV. Additional flyby maneuvers are used to bring the spacecraft on the initial heliocentric trajectory.
The authors envision using the Oberth maneuver at the Sun in combination with a Jupiter flyby, thus leveraging the deepest gravitational wells available to us in the Solar System. The proposed trajectory is shown in the figure below, drawn from the paper. Notice that the Jupiter flyby is here used to decelerate the spacecraft toward the Sun. The Oberth maneuver at the Sun then flings the spacecraft outbound for its encounter with C/2019 Q4 (Borisov).
Image: This is Figure 3 from the paper. The green line is the trajectory from the Earth to Jupiter, with the deceleration at Jupiter and subsequent trajectory toward the Sun shown as a blue line. The solid red line is the trajectory following the Oberth maneuver at perihelion. Credit: Hibberd, Perakis & Hein.
As to the mission components, the authors’ calculations show that C/2019 Q4 (Borisov) could be reached in 2045 following a launch in 2030, using the Space Launch System, the requisite heat-shield (this draws on existing work with the Parker Solar Probe), and solid-fuel propulsion engines. The payload is envisioned as a CubeSat-class spacecraft with a mass of 3 kg.
Thus we have the outline of a way of reaching our second interstellar object. It’s helpful to know how this could be done no matter how it stretches our current methods because if we do find many more such objects, some are bound to present fewer challenges than the first two, with mission concepts that may prove less demanding. It pays us to be thinking now about early detection and fast implementation of such designs as we add these interesting objects into our list of high-priority destinations. After Ultima Thule, perhaps a comet from another star?
The paper is Hibberd, Perakis & Hein, “Sending a Spacecraft to Interstellar Comet C/2019 Q4 (Borisov),” available now as a preprint.