The Helios probes reached those speeds by starting out much slower than that and then falling toward the sun, speeding up in the process. To actually hitch a r

The Helios probes reached those speeds by starting out much slower than that and then falling toward the sun, speeding up in the process. To actually hitch a ride with another object you have to match their velocity _and_ their location at the same time. If the object you are trying to catch started further from the sun than you, and was already moving faster than you, then you can't match its speed and location by falling toward the sun: when you arrive at the same location, it will have fallen further than you and hence picked up more kinetic energy per unit mass than you did, and it started off moving faster than you to start with, so it's still moving faster.