Astro is a nice system of defense against 1NT openings. Here's how it works:

(1NT) — 2:    Shows hearts and a minor.
(1NT) — 2:    Shows spades and a lower suit.

Both overcalls show at least 5-4 (or 4-5) in the two suits. The 2 and 2 overcalls retain their traditional meanings, and double is for penalty. With a long minor suit, you must choose between passing and overcalling at the 3-level.


        Responding to Astro.    Responder has the following choices after an Astro overcall:

(1NT) — 2 — (Pass) — 2:    Shows 3-card heart support.
2:    Asks overcaller to bid his second suit.
2NT:    Shows interest in game (forcing).

After the 2 response, overcaller has the following bids available:

(1NT) — 2 — 2Pass:    Shows diamonds.
2:    Shows 5 hearts.
2:    Shows 3 spades
    (in case responder has 5).
3:    Shows 5 clubs. 

Responder can bid 2NT after a 2 or 2 bid to ask for overcaller's minor.