We learn that the code to get into the rebel camp is "ad astra" in Chapter 16 of Laurie Halse Anderson's novel Chains, just after Colonel Regan of the rebel forces promises to "do what is in [his] power" to help Isabel and her sister Ruth achieve their freedom in Rhode Island (p. 104). Colonel Regan has made this promise because Isabel has just shown him evidence of the loyalists' plot to assassinate General George Washington. Colonel Regan also informs Isabel that the words ad adstra are Latin for "to the stars" (p. 104).
The phrase "to the stars" is most definitely a perfect code for the rebel forces because it is essentially a promise to reach for the stars, to strive to achieve the impossible. The phrase can be attributed to the classic epic poet Virgil, who used the phrase several times throughout the Aeneid. In particular, in Book XII, lines 892-893, Aeneas says, "opta ardua pennis astra sequi," which translates to, "Desire to pursue the high stars on wings." The Aeneid is a story about Aeneas, a Trojan survivor of the Trojan War, founding Rome. Likewise, the rebels are striving to break free from the oppression of England and found their own country, like Rome. Just as Aeneas fought to establish his own free country, Isabel realizes, "We are all fighting for liberty," as she shouts out "Ad astra!" in Chapter 21 when she flees to the rebel camp to escape the wrath of her mistress.