It's not just mars and earth you have to worry about. The kuiper belt, Saturn, Jupiter, Scarlett Johansson; all of the stellar bodies orbiting the sun would be affected by the mass of the planetoid.
That said, internal heat is one thing for a planetoid to have. But will it be able to support life? Not as we know it.
Existing in space, the planet would have no atmosphere to protect the surface or any exposed area from vacuum. The surface would be extremely cold. Even inside the planetoid, miraculous coniditions would have to arise to create life. Conservatively, the mass of the planet would have to be three times earth so the upper crust could act as a shield to the living world inside it. Even then a self-sustaining ecosystem would be extremely taxing.
It would be cool to do a mission in a cave system that resembled Jules Verne's Journey to the Center of the Earth, but there's no need to complicate things by setting it on a collision course with Earth. If you want you can play the terrorist angle with Titan, one of Saturn's moons. We believe there might be life in the methane-based atmosphere there.