# What is the average distance to any asteroid nearer and larger than Phobos?

How much of the time, if it happens at all, is Phobos the nearest interplanetary object of its size (r=11.3 km) to the Earth-Moon system? Are there maybe larger NEAs always lurking closer by? I can't quite find how many asteroids larger than Phobos are crossing the orbits of Earth, Mars or Venus.

During Earth-Mars conjunctions, half of the time when Phobos is on the sunny side of Mars, and when Venus is not too close, Phobos is the nearest of the planets and their moons. (Same of course goes for Deimos when Phobos is out of the way).

• May I ask what is the purpose of this question? 'cause by now you surely know the distance measured in delta-V is much more important than line-of-sight distance when it comes to space travel...
– SF.
Jul 12, 2016 at 9:06
• Oh, just for the record. The nearest, the biggest the whatever gets popular attention. Jul 12, 2016 at 10:27
• Is there no easy way to find out which asteroid larger than X is closest to the Earth at the moment? Jul 12, 2016 at 10:30
• Maybe someone made it easier, but this should give you a picture of difficulty of the task. And at no point of time is Phobos the nearest of celestial bodies other than the Moon; there are simply too many asteroids.
– SF.
Jul 12, 2016 at 12:33

I haven't done an exhaustive search, but 433 Eros gets a lot closer and is larger (34.4×11.2×11.2 km).

There's a long list of Mars-crossers with 18 candidates that may be larger than Phobos. It doesn't identify which 18 though.

In this list of NEOs, 1036 Ganymed is the largest with a diameter of 32-34 km. Good luck tracing all their orbits and deciding how much of the time Phobos is nearer than all of them.

• Maybe one could run a planetarium software and keep track of all known large NEAs over a time. But asteroids with perihelion between 1.3 AU and Mars are not classified as NEA I think. Jul 12, 2016 at 10:29

JPL has a nice data base

Here's a screen shot:

Notice some of the options. You can sort by minimum possible distance. For time range you can choose All available.

They measure their distances in LD (Lunar Distance) or AU (Astronomical Unit). To get kilometers, multiply their LD number by 384,400 or their AU number by 150,000,000.

They also give Vinf. This rock's relative velocity to earth at close approach is of interest.

In many cases asteroid albedo is unknown. So the H (absolute magnitude) doesn't tell us the asteroid's size. Here is a JPL chart telling us the size range an H number might indicate.

This is one of the reasons I yearn for a WISE 2.0 parked at the SEL2 (Sun Earth L2). From SEL2's point of view, the sun, earth and moon all occupy the same small region of the sky. So a small shade suffices to block heat from 3 major heat sources. That gives most of the 3 K sky the infra red scope can radiate heat into.

When we do a visual and infra red observation at the same time, that tells how much light is being absorbed and how much reflected. When we have a handle on the rock's albedo, then H gives us a better idea of the size.

• Great! Within a year and within 0.5 AU, there are only 4 hits for magnitude <14 (4-9 km diameter i.e. from well less than half of Phobos) NEOs, and 3 of those have unknown(?) magnitude. So from that it seems as if at some, or many, conjunctions Phobos really is the nearest interplanetary object of its size. Jul 12, 2016 at 12:43
• @localfluff The time option I usually pick is "future only". Restricting close encounters to those within the next year gives a small fraction of the population. Jul 12, 2016 at 19:45