|1999||Flyby||9969 Braille||~1||Deep Space 1|
|2001||Landing||433 Eros||8.42||NEAR Shoemaker|
|2006||Flyby||132524 APL||~1.1||New Horizons|
This has led a number of people to express dismay that all the asteroids which have been identified for human exploration missions have significantly smaller estimated sizes.
Notice that the scale has changed from km to m. Of course, exactly why these rocks are small is most probably what makes them optimal for a human mission. Their close approaches to Earth most likely would have ended long ago if they were any bigger. Collecting samples from these asteroids will help us to understand why they are not in the main asteroid belt, which is very important to know as their larger cousins threaten the Earth.
Some mission planners at NASA and elsewhere have expressed a desire to exclude any target with a radius smaller than 25m, being referred to as "mere building sized" asteroids. This reduces the targets in the table above to just two, although the last typically scrapes by due to a lack of options. Exactly why this policy is being suggested is unknown. Some speculation includes:
- the belief that more targets will become available with greater funding directed to finding them, so it's best to downplay the available targets.
- the difficulty of approaching a small rapidly spinning body.
- lack of mass for sample collection.
- lack of surface area for exploration given extended mission duration.
- overall "spectacle".
The first reason is just bad politics, and it's completely unnecessary. No-one can reasonably say the asteroid surveys are getting "enough" money, but they're certainly getting some, and they're getting valuable priority time on telescopes, both optical and radio. Hopefully this is just an ugly rumor.
Approaching a spinning body is something astronauts have done before. Dale A. Gardner and others used the Manned Maneuvering Unit (aka, the astronaut jetpack) to match rotation with satellites in LEO to return them to Earth on the Space Shuttle. With this in mind, it would seem engineering and astronaut experience is more applicable to small spinning bodies than it is to large ones.
The lack of mass argument should be immediately recognized as a failure of imagination. Referring to these asteroids as "small" in the first place is suggestive of this. The smallest asteroid on the list above has an spherical radius of ~2.5m, about the size of this:
The biggest asteroid on the list above, 1991 VG, is about half the size of this:
And unlike these reference objects, asteroids are solid with few to no internal voids. So there's plenty of mass, and when it comes to surface area and mission duration, I actually think making the case for short mission duration makes a lot of sense, and besides, suggesting that just a few days is too much time to spend exploring one of those London bus sized objects is just silly. A meteorite of such a size would be investigated for years.
In regards to spectacle, I recommend watching the video I linked to above of Dale A. Gardner catching that satellite. Even with a London bus sized object you're in for one hell of a show.