Cruithne (3753 Cruithne) is in a near Earth orbit. Technically, a horseshoe orbit, because it orbits the sun so close to Earth's orbit that it almost, but never, passes us. When visible, which is rare, it appears to move in a horseshoe pattern.
https://en.wikipedia.org/wiki/3753_Cruithne
It also crosses the orbits of Mars and Venus and even Mercury.
Why, if it is so close and so large (over a mile and a half) haven't we tried to probe this small world?
3753 Cruinthe ranks as target number 16832 in JPL's list of near earth asteroids that might be worth visiting, someday. With 16381 better targets, that "someday" most likely will be a long time from now, if ever.
The problems with getting to Cruinthe are twofold. One issue is that its orbit is rather eccentric (0.51485). Changing an orbit's shape is an expensive operation in terms of delta V. The other issue is that its orbit is quite inclined with respect to the Earth's orbit about the Sun, by about 19.8°. Plane changes are also extremely expensive in terms of delta V. The combination of the two issues would make a Cruinthe rendezvous cost more than twice as much (in terms of delta V) as would a Moon landing or a Mars rendezvous.
It would be cheaper to send a lander to Ceres than it would be to rendezvous with Cruinthe.
