Skip to main content

Discovery of the First Interstellar Comet and the Spatial Density of Interstellar Objects in the Solar Neighborhood


The discovery of the first interstellar comet 2I/Borisov confirmed the astronomers’ speculation that the passage through the solar system of relatively large objects (asteroids and comets) formed outside of it is a common occurrence. There may be many such objects, but their discovery is still a rare case. Large professional telescopes and existing observation programs with these telescopes have limitations that can be overcome with more modest instruments. The paper describes for the first time how comet 2I/Borisov was discovered with the 65-cm telescope designed by G.V. Borisov and those features of the observation program that made it possible to make this discovery. Recommendations are given for constructing future programs for detecting such bodies. The spatial density of interstellar objects (such as asteroid 1I/'Oumuamua or comet 2I/Borisov) in the solar neighborhood has been estimated. According to our estimate, there may be about 50 interstellar bodies >50 m in size in the Solar System in a sphere with a radius of 50 AU at any given time. Since the speeds of possible approach to the Earth of such objects can be very high, such probable approaches require special attention. The use of special space projects for their study is discussed.

This is a preview of subscription content, access via your institution.

Fig. 1.
Fig. 2.
Fig. 3.
Fig. 4.


  1. Bannister, M.T., Opitom, C., Fitzsimmons, A., Moulane, Y., Jehin, E., Seligman, D., Rousselot, P., Knight, M.M., Marsset, M., Schwamb, M.E., Guilbert-Lepoutre, A., Jorda, L., Vernazza, P., and Benkhaldoun, Z., Interstellar comet 2I/Borisov as seen by MUSE: C2, NH2 and red CN detections, arXiv e-prints arXiv:2001.11605, 2020.

  2. Bertaux, J.L., Blamont, J.E., Tabarie, N., Kurt, W.G., Bourgin, M.C., Smirnov, A.S., and Dementeva, N.N., Interstellar medium in the vicinity of the sun: A temperature measurement obtained with Mars-7 interplanetary probe, Astron. Astrophys., 1976, vol. 46, pp. 19–29.

    ADS  Google Scholar 

  3. Bertaux, J.L. and Blamont, J.E., Possible evidence for penetration of interstellar dust into the Solar System, Nature (London), 1976, vol. 262, pp. 263–266.

    Article  ADS  Google Scholar 

  4. Bodewits, D., Noonan, J.W., Feldman, P.D., Bannister, M.T., Farnocchia, D., Harris, W.M., Li, J.Y., Mandt, K.E., Parker, J.W., and Xing, Z., The carbon monoxide-rich interstellar comet 2I/Borisov, arXiv:2004.08972, 2020.

  5. Bolin, B.T., Lisse, C.M., Kasliwal, M.M., Quimby, R., Tan, H., Copperwheat, C., Lin, Z.-Y., Morbidelli, A., Abe, L., Bendjoya, P., Burdge, K.B., Coughlin, M., Fremling, C., Itoh, R., Koss, M., Masci, F.J., Maeno, S., Mamajek, E.E., Marocco, F., Murata, K., Rivet, J.-P., Sitko, M.L., Stern, D., Vernet, D., Walters, R., Yan Lin, Andreoni, I., Bhalerao, V., Bodewits, D., Kishalay, D., Kunal, P., Bellm, E.C., Blagorodnova, N., Buzasi, D., Cenko, S.B., Chang Chan-Kao, Chojnowski, D., Dekany, R., Duev, D.A., Graham M., Juric, M., Kramer, E.A., Kulkarni, S.R., Kupfer, T., Mahabal, A., Neill, J.D., Ngeow, Ch.-Ch., Penprase, B., Riddle, R., Rodriguez, H., Rosnet, P., Smith, R.M., Sollerman, J., Soumagnac, M. T., Characterization of the nucleus, morphology and activity of interstellar comet 2I/Borisov by optical and near-infrared GROWTH, Apache Point, IRTF, ZTF and Keck observations, arXiv e-prints arXiv:1910.14004, 2019.

  6. Bolin, B.T., Bodewits, D., Lisse, C.M., Fernandez, Y.R., Helou, G., and Cenko, S.B., Possible fragmentation of interstellar comet 2I/Borisov, Astron. Telegram, 2020, no. 13613, p. 1.

  7. Ceballos, O., Howell, E.S., Mendez, A., Fernandez, Y.R., Lovell, A.J., and Schambeau, C.A., Observing interstellar comet 2I/Borisov for radio OH lines with the Arecibo observatory, Lunar Planet. Sci. Conf., 2020, no. 2326, p. 3078.

  8. Charnoz, S. and Morbidelli, A., Coupling dynamical and collisional evolution of small bodies: An application to the early ejection of planetesimals from the Jupiter-Saturn region, Icarus, 2003, vol. 166, pp. 141–156.

    Article  ADS  Google Scholar 

  9. Engelhardt, T., Jedicke, R., Veres, P., Fitzsimmons, A., Denneau, L., Beshore, E., and Meinke, B., An observational upper limit on the interstellar number density of asteroids and comets, Astron. J., 2017, vol. 153, pp. 133–143.

    Article  ADS  Google Scholar 

  10. Eubanks, T.M., Efficient searches for galactic stream interstellar asteroids, Lunar Planet. Sci. Conf., 2019, no. 2132, p. 3262.

  11. Gibbs, W.W., Alien comets may be common, analysis of 2I/Borisov suggests, Science, 2019, vol. 366, p. 558.

    ADS  Google Scholar 

  12. Hibberd, A., Perakis, N., and Hein, A.M., Sending a Spacecraft to Interstellar Comet C/2019 Q4 (Borisov), arXiv e-prints arXiv:1909.06348, 2019.

  13. Hui, M.-T., Ye, Q.-Z., Fohring, D., Hung, D., and Tholen, D.J., Physical characterisation of interstellar comet 2I/2019 Q4 (Borisov), arXiv e-prints arXiv:2003.14064, 2020.

  14. Jewitt, D., Luu, J., Kleyna, J.T., Hainaut, O.R., Jedicke, R., Wainscoat, R.J., Chambers, K.C., Keane, J.V., Petric, A., Denneau, L., Magnier, E., Berge, T., Hube, M.E., Flewelling, Heather., Waters, C., Schunova-Lill, E., and Chastel, S., Initial characterization of interstellar comet 2I/2019 Q4 (Borisov), Astrophys. J., 2019, vol. 886, pp. 29–35.

    Article  Google Scholar 

  15. Laughlin, G. and Batygin, K., On the consequences of the detection of an interstellar asteroid, Res. Notes Am. Astron. Soc., 2017, vol. 1, p. 43.

    ADS  Google Scholar 

  16. Levison, H.F., Morbidelli, A., Van Laerhoven, C., Gomes, R., and Tsiganis, K., Origin of the structure of the Kuiper belt during a dynamical instability in the orbits of Uranus and Neptune, Icarus, 2008, vol. 196, pp. 258–273.

    Article  ADS  Google Scholar 

  17. Mann, I. and Jessberger, E.K., The in-situ study of solid particles in the Solar System, in Astromineralogy, Henning, T., Ed., Lecture Notes in Physics, 2010, vol. 815, pp. 233–257.

    Google Scholar 

  18. Manzini, F., Oldani, V., Ochner, P., and Bedin, L.R., Interstellar comet 2I/Borisov exhibits a structure similar to native Solar System comets, arXiv e-prints arXiv:2004.02033, 2020.

  19. Meech, K.J., Weryk, R., Micheli, M., Kleyna, J.T., Hainaut, O.R., Jedicke, R., Wainscoat, R.J., Chambers, K.C., Keane, J.V., Petric, A., et al., A brief visit from a red and extremely elongated interstellar asteroid, Nature, 2017, vol. 552, pp. 378–381.

    Article  ADS  Google Scholar 

  20. Opitom, C., Fitzsimmons, A., Jehin, E., Moulane, Y., Hainaut, O., Meech, K.J., Yang, B., Snodgrass, C., Micheli, M., Keane, J.V., Benkhaldoun, Z., and Kleyna, J.T., 2I/Borisov: A C2-depleted interstellar comet, Astron. Astrophys., 2019, vol. 631, pp. 8–12.

    Article  ADS  Google Scholar 

  21. Postberg, F., Srama, R., Hillier, J.K., Sestak, S., Green, S.F., Trielo, M., and Grun, E., Simulating STARDUST: Reproducing impacts of interstellar dust in the laboratory, European Planet. Sci. Congress, 2008, p. 873.

  22. Seligman, D. and Laughlin, G., The feasibility and benefits of in situ exploration of Oumuamua-like objects, Astron. J., 2018, vol. 155, pp. 217–229.

    Article  ADS  Google Scholar 

  23. Shustov, B.M., On the mass spectrum of asteroids, meteoroids and space debris, INASAN Sci. Rep., 2019, no. 4, pp. 356–364.

  24. Shustov, B.M. and Tutukov, A.V., The initial mass spectra of astronomical objects, Astron. Rep., 2018, vol. 62, pp. 724–732.

    Article  ADS  Google Scholar 

  25. Xing, Z., Bodewits, D., Noonan, J., and Bannister, M.T., Water production rates and activity of interstellar comet 2I/Borisov, Lunar Planet. Sci. Conf., 2020, no. 2326, p. 2275.

  26. Yang, B., Kelley, M.S.P., Meech, K.J., Keane, J.V., Protopapa, S., and Bus, S.J., Searching for water ice in the coma of interstellar object 2I/Borisov, Astron. Astrophys., 2020, vol. 634, pp. 6–10.

    Article  ADS  Google Scholar 

  27. Zhang, Q., Ye, Q., and Kolokolova, L., Interstellar comet 2I/Borisov is single again, Astron. Telegram, 2020, no. 13618, p. 1.

Download references

Author information

Authors and Affiliations


Corresponding author

Correspondence to B. M. Shustov.

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Borisov, G.V., Shustov, B.M. Discovery of the First Interstellar Comet and the Spatial Density of Interstellar Objects in the Solar Neighborhood. Sol Syst Res 55, 124–131 (2021).

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: