The roadster (I assume you mean Tesla's roadster) is in orbit of the sun (ie. It has escaped the earth gravitational pull). Unlike the space debris which orbits the earth. Therefore the debris is a threat and the roadster is not.
It's around Mars. Is not a threat, but surely is trash... thereis any reason to do it unless a wierd marketing for people that didn't care about throwing trash for no purpose around.
It was necessary to test the rocket with a payload. The usual test payload is a concrete block or something comparable. They wanted to do something more fun. The major debris problem is all the stuff we have put in orbit around Earth, not stuff farther out.
A garbage car is a single object, and it's large enough that earth-based radar can track its orbit, so it poses much less risk (since its orbit is known, the ISS and other satellites can move out of the way whenever a potential collision is predicted). Missile tests like this one generate debris that's too small to be tracked, but large enough to cause significant damage in case of a collision.
Also, to prove a rocket you need to have a dummy payload of some kind. Whatever you think about the stunt of using a Tesla as that dummy payload, there was going to be a payload of some kind however that decision went. The fact it was a car doesn't change the collision risk or debris amount compared to using a mass simulator.
The biggest risk from space garbage is that the small stuff is not trackable, so at any point it could slam into satellites or, god forbid, people or space stations. It's going fast enough that despite being small this would have dreadful consequences.
Larger items can be tracked, and therefore can be avoided, so they don't pose a risk any more than non-junk large items like other satellites and so on. There's quite a lot of room so if you know where things are it's not hard to avoid them.
Some amount of garbage is sadly unavoidable at this point in our development of space travel. For example many rockets are multi stage and jettison those stages, farings to protect satellites are jettisoned, and so on. That all falls into the "large and trackable" category so it's not a terrible problem, at least not yet. So the main current strategy for avoiding creating problems is to avoid creating small garbage, and people work very hard at that - being careful not to lose tools or even a single nut or bolt.
And yes, before you mention it, "lots of room" is a relative statement and this is not an infinitely sustainable strategy. But people are working on methods to capture and clean up garbage, and as those get more feasible we'll be able to go and clean up all this large garbage that we are tracking. So even with a long-term perspective, the large stuff is less of a problem.
Except the "starman" tesla was beyond Mars' orbit back in November.
But its coming back by 2091.
The Roadster and Starman will come within a few hundred thousand kilometers of our planet in 2091, according to an orbit-modeling study. The authors of that study determined that the car will slam into either Venus or Earth, likely within the next few tens of millions of years.
