Since my original posting on Starman, there have been so many observations made of it as it recedes, mainly by amateur astronomers, that JPL has produced multiple updates for the orbit of the Tesla. The latest orbit is sufficiently good that JPL has also calculated future encounters with the Earth, the Moon and Mars. Although light pressure and outgassing are still not modelled, the orbit solution is not now going to change much, with a large database of astrometric observations and an orbit arc of more than ten days.

The last published image was taken a distance of 0.02496 AU – 3.73 million kilometres, or 9.71 Lunar Distances – using a robotic telescope in Chile. This is the wonderful image from Michael Keith at the top of this page.

The calculated orbital period is 1.5399 years, so there is no 3:2 orbital resonance with the Earth. There will, at least initially, be a relatively close pass to the Earth every 3 years, but the Tesla will slip rapidly further and further away from the Earth with each successive encounter. The orbit also confirms that Starman will pass approximately half way between the orbit of Mars and the distance of the innermost, Hungaria, asteroids.

The table, below, summarises the last three orbit solutions:

JPL #6 | JPL #7 | JPL #8 | |

Length of arc | 4.5 days | 5.2 days | 10.1 days |

Number of observations | 269 | 330 | |

Perihelion distance | 0.986063 AU | 0.986063 AU | 0.986062 AU |

Eccentricity | 0.261849 | 0.261849 | 0.260562 |

Semi-major Axis | 1.335855 AU | 1.335855 AU | 1.333529 AU |

Aphelion distance | 1.685648 AU | 1.685648 AU | 1.680996 AU |

Period | 1.544 years | 1.544 years | 1.5399 years |

My colleague, Ivan Valtchanov pointed out a paper on Astro-ph on the long-term orbit evolution (https://arxiv.org/abs/1802.04718) of the Tesla. The paper makes various interesting predictions such as a very close Earth encounter in 2091 and that there is a negligible change of a Mars impact in the next million years but, not unreasonably as the car started from a point close to Earth, a quite high probability of Earth impact.

Incidentally, in one million years orbiting, the car would run up around 10^{15}km – around 100 light years – on the clock, which may invalidate its warranty and roadworthiness (my thanks to my colleagues Jon Brumfitt, Leo Metcalfe and Paolo Pesciullesi for those gems). One suspects though that solar UV will have seriously degraded the paintwork and tyres long before the end of the century.

It would be interesting to see if their prediction of a 2091 close encounter with Earth still stands after the publication of the JPL #8 ephemeris, given that they used an orbit based on a very short arc – apparently little over 1 day of observations – although it would be surprising if the major conclusion of the paper, that a Mars impact is unlikely, were to change. The interesting part is that the study finds a very interesting consequence of the future close approaches to the Earth. Not unexpectedly, their study finds orbit flips, in which a close approach to Earth changes the orbit’s argument of perihelion by 180 degrees, changing the orbit from having perihelion at the Earth’s orbit to having aphelion at this point and perihelion around the orbit of Venus.

JPL have calculated future planetary encounters and find, not unnaturally, that Starman’s first interplanetary encounter was an approach to 0.36 Lunar Distances to the Moon immediately after the escape manoeuvre. On April 22^{nd} 2035, given the uncertainties on the orbit, Starman could approach Mars to as little as 2.9 Lunar Distances, although the nominal calculated distance is 8.2 Lunar distances (3.1 million kilometres). It is this encounter that is the main source of uncertainty in future Earth encounters. This Mars encounter and an encounter with the Earth, on January 12^{th} 2047, to a calculated 9.2 Lunar Distances, have a significant randomising effect on the future orbit. Depending on the exact encounter distance, the Tesla will be subject to larger (if the encounter is closer than the nominal distance) or smaller perturbing forces (if further).

The way to get a definitive solution to the orbit would be to attempt recovery of the Tesla during the close approach to Earth in 2047. This would give a long-term solution for the average light pressure perturbation and would fix the orbit with high accuracy for centuries to come. The nominal date of encounter is January 12^{th} 2047, around 12UT, but with an error (3σ) of ±3 days. However, at around magnitude 21, the object would be picked-up by many of the asteroid survey programmes active today. The car will be in Leo, well-placed to be observed in the northern sky.