‘‘Back to the future’’ major dynamical events in the local group 15 billion B.C. to A.D. 15 billion

Kahn and Woltjer (1959) assumed simple two‐body motion of M31 and our galaxy with initial recession and now approach of the two galaxies to estimate the total mass. However, recent measurements of recession speeds and distances of IC342 and Maffei 1 (Buta and McCall 1983, McCall 1989) indicate that each of these galaxies must once have been so close to M31 that there was a gravitational interaction stronger than with our galaxy, violating the two body assumption. We show with computer simulations and a generalized Kahn‐Woltjer calculation that a few‐body ejection/merger event involving M31 about 5⋅109 years ago can explain the present‐day distances and velocities of these galaxies. The smaller mass IC342 and Maffei were dynamically ejected rapidly away and the greater mass M31 slowly toward us in the opposite direction. In contrast, we find our galaxy to have been a ‘‘by‐stander’’ to these events. However, besides IC342 and Maffei 1, other much smaller mass objects undoubtedly were ejected, some toward our galaxy. One or more of these captured ejecta may confuse estimates of the mass of our galaxy using satellites. Looking toward the future, in about 10⋅109 years, our simulations predict a wide encounter with M31. Finally, although we find the initial assumption of Kahn‐Woltjer’s mass calculation of 4⋅1012 M⊙ for the Local Group invalid, we get a similar result with our generalized Kahn‐Woltjer method.Kahn and Woltjer (1959) assumed simple two‐body motion of M31 and our galaxy with initial recession and now approach of the two galaxies to estimate the total mass. However, recent measurements of recession speeds and distances of IC342 and Maffei 1 (Buta and McCall 1983, McCall 1989) indicate that each of these galaxies must once have been so close to M31 that there was a gravitational interaction stronger than with our galaxy, violating the two body assumption. We show with computer simulations and a generalized Kahn‐Woltjer calculation that a few‐body ejection/merger event involving M31 about 5⋅109 years ago can explain the present‐day distances and velocities of these galaxies. The smaller mass IC342 and Maffei were dynamically ejected rapidly away and the greater mass M31 slowly toward us in the opposite direction. In contrast, we find our galaxy to have been a ‘‘by‐stander’’ to these events. However, besides IC342 and Maffei 1, other much smaller mass objects undoubtedly were ejected, some toward ou...